Planet Cassandra

Series

This blog entry is part of a series called Stream Processing With Spring, Kafka, Spark and Cassandra.

1. Part 1 - Overview
2. Part 2 - Setting up Kafka
3. Part 3 - Writing a Spring Boot Kafka Producer
4. Part 4 - Consuming Kafka data with Spark Streaming and Output to Cassandra
5. Part 5 - Displaying Cassandra Data With Spring Boot

Part 1 - Overview

Before starting any project I like to make a few drawings, just to keep everything in perspective. My main motivation for this series is to get better acquainted wit Apache Kafka. I just didn't have a chance to use it on some of the projects that I work on in my day to day life, but it's this new technology everybody is buzzing about so I wanted to give it a try. One other thing is that I also didn't get a chance to write Spark Streaming applications, so why not hit two birds with one stone? Here is 10 000 feet overview of the series:

Avoiding the tl;dr

Part of the motivation for splitting is in avoiding the tl;dr effect ;) Now, let's get back to the overview. We'll break down previous image box by box.

Using Spring Boot

We're basically just prototyping here, but to keep everything flexible and in the spirit of the newer architectural paradigms like Microservices the post will be split in 5 parts. The software will also be split so we won't use any specific container for our applications we'll just go with Spring Boot. In the posts we won't go much over the basic, you can always look it up in the official documentation.

Apache Kafka

This is the reason why I'm doing this in the first place. It's this new super cool messaging system that all the big players are using and I want to learn how to put it to everyday use.

Spark Streaming

For some time now I'm doing a lot of stuff with Apache Spark. But somehow I didn't get a chance to look into streaming a little bit better.

Cassandra

Why not?

What this series is about?

It's a year where everybody is talking about voting ... literary everywhere :) so let's make a voting app. In essence it will be a basic word count in the stream. But let's give some context to it while we're at it. We won't do anything complicated or useful. Basically the end result will be total count of token occurrence in the stream. We'll also break a lot of best practices in data modeling etc. in this series.

Series is for people oriented toward learning something new. I guess experienced and battle proven readers will find a ton of flaws in the concept but again most of them are deliberate. One thing I sometimes avoid in my posts is including source code. My opinion is that a lot more remains remembered and learners feel much more comfortable when faced with problems in practice. So I'll just copy paste crucial code parts. One more assumption from my side will be that the readers will be using IntelliJ IDEA. Let's got to Part 2 and see how to setup kafka.

Series

This blog entry is part of a series called Stream Processing With Spring, Kafka, Spark and Cassandra.

1. Part 1 - Overview
2. Part 2 - Setting up Kafka
3. Part 3 - Writing a Spring Boot Kafka Producer
4. Part 4 - Consuming Kafka data with Spark Streaming and Output to Cassandra
5. Part 5 - Displaying Cassandra Data With Spring Boot

Setting up Kafka

In this section we'll setup two kafka brokers. We'll also need a zookeeper. If you are reading this my guess is that you don't have one setup already so we'll use the one bundled with kafka. We won't cover everything here. Do read the official documentation for more in depth understanding.

Downloading

Download latest Apache Kafka. In this tutorial we'll use binary distribution. Pay attention to the version of scala if you attend to use kafka with specific scala version. In this tutorial we'll concentrate more on Java. But this will be more important in parts to come. In this section we'll use the tools that ship with Kafka distribution to test everything out. Once again download and extract the distribution of Apache Kafka from official pages.

Configuring brokers

Go into directory where you downloaded and extracted your kafka installation. There is a properties file template and we are going to use properties files to start the brokers. Make two copies of the file:

        $ cd your_kafka_installation_dir
        $ cp config/server.properties config/server0.properties
        $ cp config/server.properties config/server1.properties
    

        $ vi config/server0.properties
    

        broker.id=0
        listeners=PLAINTEXT://:9092
        num.partitions=2
        log.dirs=/var/tmp/kafka-logs-0
    

        $ vi config/server1.properties
    

        broker.id=1
        listeners=PLAINTEXT://:9093
        num.partitions=2
        log.dirs=/var/tmp/kafka-logs-1
    

Starting everything up

First you need to start the zookeeper, it will be used to store the offsets for topics. There are more advanced versions of using where you don't need it but for someone just starting out it's much easier to use zookeeper bundled with the downloaded kafka. I recommend opening one shell tab where you can hold all of the running processes. We didn't make any changes to the zookeeper properties, they are just fine for our example:

        $ bin/zookeeper-server-start.sh config/zookeeper.properties &
    

        $ bin/kafka-server-start.sh config/server0.properties &
        $ bin/kafka-server-start.sh config/server1.properties &
    

Creating a topic

Before producing and consuming messages we need to create a topic for now you can think of it as of queue name. We need to give a reference to the zookeeper. We'll name a topic "votes", topic will have 2 partitions and a replication factor of 2. Please read the official documentation for further explanation. You'll see additional output coming from broker logs because we are running the examples in the background.

        $ bin/kafka-topics.sh --zookeeper localhost:2181 --create --topic votes --partitions 2 --replication-factor 2
    

Sending and receiving messages with bundled command line tools

Open two additional shell tabs and position yourself in the directory where you installed kafka. We'll use one tab to produce messages. And second tab will consume the topic and will simply print out the stuff that we typed in in the first tab. Now this might be a bit funny, but imagine you are actually using kafka already!

In tab for producing messages run:

        $ bin/kafka-console-producer.sh --broker-list localhost:9092 --topic votes
    

In tab for consuming messages run:

        $ bin/kafka-console-consumer.sh --zookeeper localhost:2181 --topic votes
    

Next part

We covered a lot here but writing from one console window to another can be achieved wit far simpler combination of shell commands. In Part 3 we'll make an app that writes to a topic. We'll also use console reader just to verify that our app is actually sending something to topic.

Series

This blog entry is part of a series called Stream Processing With Spring, Kafka, Spark and Cassandra.

1. Part 1 - Overview
2. Part 2 - Setting up Kafka
3. Part 3 - Writing a Spring Boot Kafka Producer
4. Part 4 - Consuming Kafka data with Spark Streaming and Output to Cassandra
5. Part 5 - Displaying Cassandra Data With Spring Boot

Writing a Spring Boot Kafka Producer

We'll go over the steps necessary to write a simple producer for a kafka topic by using spring boot. The application will essentially be a simple proxy application and will receive a JSON containing the key that's going to be sent to kafka topic. Pretty simple but enough to get us going. We'll use IntelliJ IDEA to set everything up. The easiest way to get started is by using Spring Initializr.

Setting up a project

1. Project SDK: Java 8
2. Initializr Service URL: https://start.spring.io
3. Next
4. Name: spring-boot-kafka-example
5. Type: Gradle Project
6. Packaging: Jar
7. Java Version: 1.8
8. Language: Java
9. Group: com.example
10. Artifact: spring-boot-kafka-example
11. Vesion: 0.0.1-SNAPSHOT
12. Description: Spring Boot Kafka Example
13. Package: com.example
14. Next
15. Spring Boot Version: 1.3
16. Core - Web
17. Next
18. Project name: spring-boot-kafka-example
19. The rest is just fine ...
20. Finish
21. After creating project check sdk setting, it should be java 8

build.gradle dependencies

        compile('org.apache.kafka:kafka_2.11:0.9.0.0')
        compile('org.apache.zookeeper:zookeeper:3.4.7')
    

application.properties

        brokerList=localhost:9092
        sync=sync
        topic=votes
    

SpringBootKafkaProducer

This is the class where all the important stuff is happening

package com.example;

import org.apache.kafka.clients.producer.KafkaProducer;
import org.apache.kafka.clients.producer.Producer;
import org.apache.kafka.clients.producer.ProducerRecord;
import org.apache.kafka.clients.producer.RecordMetadata;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Configuration;

import javax.annotation.PostConstruct;
import java.util.Properties;
import java.util.concurrent.ExecutionException;

@Configuration
public class SpringBootKafkaProducer {

    @Value("${brokerList}")
    private String brokerList;

    @Value("${sync}")
    private String sync;

    @Value("${topic}")
    private String topic;

    private Producer<String, String> producer;

    public SpringBootKafkaProducer() {
    }

    @PostConstruct
    public void initIt() {
        Properties kafkaProps = new Properties();

        kafkaProps.put("bootstrap.servers", brokerList);

        kafkaProps.put("key.serializer", 
            "org.apache.kafka.common.serialization.StringSerializer");
        kafkaProps.put("value.serializer", 
            "org.apache.kafka.common.serialization.StringSerializer");
        kafkaProps.put("acks", "1");

        kafkaProps.put("retries", "1");
        kafkaProps.put("linger.ms", 5);

        producer = new KafkaProducer<>(kafkaProps);

    }

    public void send(String value) throws ExecutionException, 
            InterruptedException {
        if ("sync".equalsIgnoreCase(sync)) {
            sendSync(value);
        } else {
            sendAsync(value);
        }
    }

    private void sendSync(String value) throws ExecutionException,
            InterruptedException {
        ProducerRecord<String, String> record = new ProducerRecord<>(topic, value);
        producer.send(record).get();

    }

    private void sendAsync(String value) {
        ProducerRecord<String, String> record = new ProducerRecord<>(topic, value);

        producer.send(record, (RecordMetadata recordMetadata, Exception e) -> {
            if (e != null) {
                e.printStackTrace();
            }
        });
    }
}
    

SpringBootKafkaExampleApplication

This one will be automatically generated.

package com.example;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class SpringBootKafkaExampleApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringBootKafkaExampleApplication.class, args);
    }
}
    

AppBeans

Setup beans for the controller.

package com.example;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class AppBeans {

    @Bean
    public SpringBootKafkaProducer initProducer() {
        return new SpringBootKafkaProducer();
    }
}
    

Helper beans

Status to return to clients, we'll just send "ok" every time.

package com.example;

public class Status {
    private String status;

    public Status(String status) {
        this.status = status;
    }

    public Status() {
    }

    public String getStatus() {
        return status;
    }

    public void setStatus(String status) {
        this.status = status;
    }
}
    

package com.example;

public class Vote {
    private String name;

    public Vote(String name) {
        this.name = name;
    }

    public Vote() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}
    

SpringBootKafkaController

This is the controller, after starting the app we should have an active endpoint available under http://localhost:8080/vote

package com.example;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.concurrent.ExecutionException;

@RestController
public class SpringBootKafkaController {

    @Autowired
    SpringBootKafkaProducer springBootKafkaProducer;

    @RequestMapping("/vote")
    public Status vote(@RequestBody Vote vote) throws ExecutionException, InterruptedException {

        springBootKafkaProducer.send(vote.getName());

        return new Status("ok");
    }

}
    

Checking everything

There should be an active console reader from previous post so we won't cover this. After running the SpringBootKafkaExampleApplication simply open a rest client application like Postman and try to send the following JSON to http://localhost:8080/vote

{
    "name": "Test"
}
    

Series

This blog entry is part of a series called Stream Processing With Spring, Kafka, Spark and Cassandra.

1. Part 1 - Overview
2. Part 2 - Setting up Kafka
3. Part 3 - Writing a Spring Boot Kafka Producer
4. Part 4 - Consuming Kafka data with Spark Streaming and Output to Cassandra
5. Part 5 - Displaying Cassandra Data With Spring Boot

Consuming Kafka data with Spark Streaming and Output to Cassandra

In this section we are going to use spark streaming to read the data in coming from kafka. We'll also combine it with the data already in cassandra, we're going to do some computation with it and we're going to put the results back to cassandra. The best practice would be to have a spark cluster running but for the sake of simplicity we are going to launch local spark context from a java application and do some processing there. We won't go into configuring Cassandra to run, there is plenty documentation there and it takes just minutes to setup.

Cassandra

Nothing fancy here, just a name of the entity for votes and a number of votes

CREATE KEYSPACE voting
    WITH REPLICATION = {
        'class' : 'SimpleStrategy',
        'replication_factor' : 1
    };

USE voting;

CREATE TABLE votes (name text PRIMARY KEY, votes int);
    

Let's create a simple java project with gradle for stream processing

1. File, New Project, Gradle
2. Project SDK: Java 8
3. Java
4. Next
5. GroupId: spark-kafka-streaming-example
6. ArtifactId: spark-kafka-streaming-example
7. Version: 1.0-SNAPSHOT
8. Next
9. Use default gradle wrapper
10. Next
11. Project name: spark-kafka-streaming-example
12. The rest is just fine ...
13. Finish
14. After creating project check sdk setting, it should be java 8

Let's have a look at the dependencies

group 'spark-kafka-streaming-example'
version '1.0-SNAPSHOT'

apply plugin: 'java'

sourceCompatibility = 1.8

repositories {
    mavenCentral()
}

dependencies {
    compile('org.apache.spark:spark-core_2.10:1.5.2')
    compile('org.apache.spark:spark-streaming_2.10:1.5.2')
    compile('org.apache.spark:spark-streaming-kafka_2.10:1.5.2')
    compile('com.datastax.spark:spark-cassandra-connector_2.10:1.5.0-M3')
    compile('com.datastax.spark:spark-cassandra-connector-java_2.10:1.5.0-M3')

    testCompile group: 'junit', name: 'junit', version: '4.11'
}
    

Simple Voting Class to go with Cassandra Table

We'll use this class for storing data into cassandra

import java.io.Serializable;

public class Vote implements Serializable {
    private String name;
    private Integer votes;

    public Vote(String name, Integer votes) {
        this.name = name;
        this.votes = votes;
    }

    public Vote() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getVotes() {
        return votes;
    }

    public void setVotes(Integer votes) {
        this.votes = votes;
    }
}
    

Spark streaming with kafka

And finally the code to accept tokens that come in, compare them with data in cassandra and then write them back to cassandra. I didn't spend much time around configuring the class for external parameters, but for the example it's good enough:

import com.datastax.spark.connector.japi.CassandraRow;
import com.datastax.spark.connector.japi.rdd.CassandraTableScanJavaRDD;
import kafka.serializer.StringDecoder;
import org.apache.spark.SparkConf;
import org.apache.spark.api.java.JavaRDD;
import org.apache.spark.api.java.JavaSparkContext;
import org.apache.spark.api.java.function.Function;
import org.apache.spark.api.java.function.Function2;
import org.apache.spark.api.java.function.PairFunction;
import org.apache.spark.streaming.Durations;
import org.apache.spark.streaming.api.java.JavaDStream;
import org.apache.spark.streaming.api.java.JavaPairDStream;
import org.apache.spark.streaming.api.java.JavaPairInputDStream;
import org.apache.spark.streaming.api.java.JavaStreamingContext;
import org.apache.spark.streaming.kafka.KafkaUtils;
import scala.Tuple2;

import java.io.IOException;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;

import static com.datastax.spark.connector.japi.CassandraJavaUtil.javaFunctions;
import static com.datastax.spark.connector.japi.CassandraJavaUtil.mapToRow;

public class SparkStreamingExample {

    public static JavaSparkContext sc;

    public static void main(String[] args) throws IOException {

        String brokers = "localhost:9092,localhost:9093";
        String topics = "votes";

        SparkConf sparkConf = new SparkConf();
        sparkConf.setMaster("local[2]");
        sparkConf.setAppName("SparkStreamingExample");
        sparkConf.set("spark.cassandra.connection.host",
            "127.0.0.1");

        JavaStreamingContext jssc = new JavaStreamingContext(
            sparkConf,
            Durations.seconds(10));

        HashSet<String> topicsSet = new HashSet<>(
                Arrays.asList(topics.split(",")));
        HashMap<String, String> kafkaParams = new HashMap<>();
        kafkaParams.put("metadata.broker.list", brokers);

        JavaPairInputDStream<String, String> messages =
                KafkaUtils.createDirectStream(
                        jssc,
                        String.class,
                        String.class,
                        StringDecoder.class,
                        StringDecoder.class,
                        kafkaParams,
                        topicsSet
                );

        JavaDStream<String> lines =
                messages.map(
                        (Function<Tuple2
                        <String, String>,
                        String>) Tuple2::_2);

        JavaPairDStream<String, Integer> voteCount = lines
            .mapToPair(
                (PairFunction<String, String, Integer>) s ->
                        new Tuple2<>(s, 1)).reduceByKey(
                (Function2<Integer, Integer, Integer>)
                    (i1, i2) ->i1 + i2);

        sc = jssc.sparkContext();

        voteCount.foreachRDD((v1, v2) -> {
            v1.foreach((x) -> {
                CassandraTableScanJavaRDD<CassandraRow> previousVotes =
                    javaFunctions(sc)
                        .cassandraTable("voting", "votes")
                        .where("name = '" + x._1() + "'");

                Integer oldVotes = 0;
                if (previousVotes.count() > 0) {
                    oldVotes = 
                        previousVotes.first().getInt("votes");
                }

                Integer newVotes = oldVotes + x._2();

                List<Vote> votes = Arrays.asList(
                    new Vote(x._1(), newVotes));
                JavaRDD<Vote> rdd = sc.parallelize(votes);

                javaFunctions(rdd)
                    .writerBuilder("voting", "votes", mapToRow(Vote.class))
                    .saveToCassandra();
            });

            return null;
        });

        voteCount.print();

        jssc.start();
        jssc.awaitTermination();
    }
}
    

And that's it

You can check how data changes by running select statements from voting table. In Part 5 we are going to make a simple spring boot project that displays and sorts the voting data.

Series

This blog entry is part of a series called Stream Processing With Spring, Kafka, Spark and Cassandra.

1. Part 1 - Overview
2. Part 2 - Setting up Kafka
3. Part 3 - Writing a Spring Boot Kafka Producer
4. Part 4 - Consuming Kafka data with Spark Streaming and Output to Cassandra
5. Part 5 - Displaying Cassandra Data With Spring Boot

Displaying Cassandra Data With Spring Boot

Now that we have our voting data in Cassandra let's write a simple Spring Boot project that simply gathers all the data from cassandra sorts them and displays to user.

Setting up a project

1. Project SDK: Java 8
2. Initializr Service URL: https://start.spring.io
3. Next
4. Name: boot-cassandra-data-show
5. Type: Gradle Project
6. Packaging: Jar
7. Java Version: 1.8
8. Language: Java
9. Group: com.example
10. Artifact: boot-cassandra-data-show
11. Vesion: 0.0.1-SNAPSHOT
12. Description: Spring Boot Display Cassandra Data
13. Package: com.example
14. Next
15. Spring Boot Version: 1.3
16. Core - Web
17. Template Engines - Mustache
18. Next
19. Project name: boot-cassandra-data-show
20. The rest is just fine ...
21. Finish
22. After creating project check sdk setting, it should be java 8

Cassandra dependencies

compile('com.datastax.cassandra:cassandra-driver-core:2.1.9')
    

Vote class

We'll use this class to map rows from cassandra.

package com.example;

import java.io.Serializable;

public class Vote implements Serializable {
    private String name;
    private Integer votes;

    public Vote(String name, Integer votes) {
        this.name = name;
        this.votes = votes;
    }

    public Vote() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getVotes() {
        return votes;
    }

    public void setVotes(Integer votes) {
        this.votes = votes;
    }
}
    

application.properties

server.port = 8090
contactPoint = 127.0.0.1
keyspace = voting
    

CassandraSessionManager

package com.example;

import com.datastax.driver.core.Cluster;
import com.datastax.driver.core.Session;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Configuration;

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;

@Configuration
public class CassandraSessionManager {

    private Session session;
    private Cluster cluster;

    @Value("${contactPoint}")
    private String contactPoint;

    @Value("${keyspace}")
    private String keyspace;

    public CassandraSessionManager() {

    }

    public Session getSession() {
        return session;
    }

    @PostConstruct
    public void initIt() {
        cluster = Cluster.builder().addContactPoint(
            contactPoint).build();
        session = cluster.connect(keyspace);
    }

    @PreDestroy
    public void destroy() {
        if (session != null) {
            session.close();
        }
        if (cluster != null) {
            cluster.close();
        }
    }
}

    

BootCassandraDataShowApplication

package com.example;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class BootCassandraDataShowApplication {

    public static void main(String[] args) {
        SpringApplication.run(
        BootCassandraDataShowApplication.class, args);
    }
}
    

AppBeans

Bean for holding configured objects.

package com.example;

import com.datastax.driver.core.Session;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class AppBeans {

    @Bean
    public Session session() {
        return sessionManager().getSession();
    }

    @Bean
    public CassandraSessionManager sessionManager() {
        return new CassandraSessionManager();
    }
}

    

Web Controller

package com.example;

import com.datastax.driver.core.ResultSet;
import com.datastax.driver.core.Row;
import com.datastax.driver.core.Session;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Configuration;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Map;

@Configuration
@Controller
public class WelcomeController {

    @Autowired
    Session session;

    @RequestMapping("/")
    public String welcome(Map<String, Object> model) {

        final ResultSet rows = session.execute("SELECT * FROM votes");

        ArrayList results = new ArrayList<>();

        for (Row row : rows.all()) {
            results.add(new Vote(
                row.getString("name"),
                row.getInt("votes")
            ));
        }

        Collections.sort(results, (a, b) ->
        b.getVotes().compareTo(a.getVotes()));

        model.put("results", results);

        return "welcome";
    }
}
    

Template to show the results

<!DOCTYPE html>
<html lang="en">
<body>

<h1>Voting results:</h1>
<br/>
{{#results}}
    <strong>{{this.name}}</strong> {{this.votes}} <br/>
{{/results}}

</body>
</html>
    

That's all folks

Now this app might not seem as a lot, but there's a kafka cluster that receives messages comming in from a spring boot app that exposes REST interface. Messages that come in from kafka are then processed with Spark Streaming and then sent to Cassandra. There is another Spring Boot app that sorts and displays results to the users. This small tutorial covers most of the cool java/big data technologies now-days. Special thanks to the readers that went through all five parts of this tutorial ;)

Intro

Bucketing is one of the most important techniques when working with time series data in Cassandra. This post has it's roots in two very popular blog entries:

• Basic Time Series with Cassandra
• Advanced Time Series with Cassandra

The Case for Bucketing

For some time now I'm in the world if IoT and I find that explaining everything with a help of a simple temperature sensor is the best method to discuss the subject. Previously mentioned articles are also a good read. This section is sort of a warm up. Theoretically in most of the use cases we'll want to access temperature readings by some sensor Id and we know where this sensor is located. In the most simple case sensor id becomes the long row in cassandra and the readings are stored in it and kept sorted by time etc. However in some cases the temperature may be read very often and this could cause the wide row to grow to a proportion that is not manageable by cassandra so the data has to be split among multiple long rows. The easiest method to make this split is to make multiple long rows based on the measurement timestamp.

How big should my buckets be?

It may vary from project to project, but it depends on two important factors. How many readings are you storing per single measurement and how often the measurement is happening. For instance if you are recording a reading once per day you probably don't even need the bucketing. Also if you are recording it once per hour the project you are working on probably wont't last long enough for you to run into problem. It applies to seconds too, but only for the most trivial case where you are making a single reading. If you go into frequencies where something is happening on the milliseconds level you will most definetly need bucketing. The most complex project I worked up until now had time bucketing on a level of a single minute. meaning every minute, new bucket. But that project is not in the IoT world, In that world I'm using partitions on a month basis.

10 000 feet Bucketing View

Main problem is how to calculate the bucket based on measurement time stamp. Also keep in mind there might be differences between the timezones, in a distributed system a very advisable practice is to save everything in the UTC format. If we decided that wee need bucketing per day it could be something as simple as the following:

    FastDateFormat dateFormat = FastDateFormat.getInstance(
        "yyyy-MM-dd", TimeZone.getTimeZone("UTC"));

    public String dateBucket(Date date) {
        return dateFormat.format(date;
    }
    

Anything goes

Bare in mind that you should keep buckets in time series data easy to maintain. Also try to avoid having multiple implementation for the same thing in your code base. This section will not provide 100% implemented examples but will be more on a level of a pseudo code.

When you are fetching the data from the buckets, you will have two types of query. One is to fetch data out from the bucket without any restrictions on measurement time stamp. The other is when you will want to start from a certain position within the bucket. Again there is a question of ordering and sorting the retrieved data. I worked in systems having all sorts of practices there, most of the time reversing was done with a help of a specific boolean flag but my opinion is this should be avoided. It's best to stick to the from and to parameters and order the data according to them. i.e.

        from:   01/01/2016
        to:     02/02/2016
        returns: ascending

        from:   02/02/2016
        to:     01/01/2016
        returns: descending
    

Here is a bit of pseudo code:

        // constructor of your iterator object

        startPartition = dateBucket(from);
        endPartition = dateBucket(to);

        lastFetchedToken = null;

        bucketMoveCount = 0;

        String statement = "SELECT * FROM readings"

        // from past experience, somehow the driver takes out data the fastest
        // if it fetches 3000 items at once, would be interesting to drill down
        // why is this so :)

        int fetchSize = 3000;

        if (from.isBefore(to)) {
            select = statement + " ORDER BY measurement_timestamp ASC LIMIT " + fetchSize;
            selectFromBoundary = statement + " AND measurement_timestamp > ? ORDER BY measurement_timestamp ASC LIMIT " + fetchSize;

            partitionDiff = -1f;
        } else {
            selectNormal = statement + " LIMIT " + fetchSize;
            selectFromBoundary = statement + " AND measurement_timestamp < ? LIMIT " + fetchSize;

            partitionDiff = 1f;
        }
    

    public List<Row> getNextPage() {

        List<Row> resultOut = new ArrayList<>();

        boolean continueFromPreviousBucket = false;

        do {
            ResultSet resultSet =
                    lastFetchedToken == null ?
                            session.execute(new SimpleStatement(select, currentBucket)) :
                            session.execute(new SimpleStatement(selectFromBoundary, currentBucket, lastToken));

            List<Row> result = resultSet.all();

            if (result.size() == fetchSize) {
                if (continueFromPreviousBucket) {
                    resultOut.addAll(result.subList(0, fetchSize - resultOut.size()));
                } else {
                    resultOut = result;
                }

                lastFetchedToken = resultOut.get(resultOut.size() - 1).getUUID("measurement_timestamp");

            } else if (result.size() == 0) {
                currentBucket = calculateNextBucket();
                bucketMoveCount++;

            } else if (result.size() < fetchSize) {
                currentBucket = calculateNextBucket();
                bucketMoveCount++;

                lastFetchedToken = null;

                if (continueFromPreviousBucket) {
                    resultOut.addAll(result.subList(0, Math.min(result.size(), fetchSize - resultOut.size())));
                } else {
                    resultOut = result;
                }

                continueFromPreviousBucket = true;
            }

            if (resultOut.size() == fetchSize
                    || bucketMoveCount >= MAX_MOVE_COUNT
                    || Math.signum(currentBucket.compareTo(endPartition)) != okPartitionDiff) {
                break;
            }

        } while (true);

        return result;
    }
    

This is just a high level overview of how to move among the buckets. Actual implementation would actually be significantly different from project to project. My hope for this post is that you give the problems I faced a thought before you run into them.

Intro

For some time now spring data with cassandra is getting more and more popular. My main concern with the framework is performance characteristics when compared to native cql driver. After all with the driver everything is under your control and one can probably squeeze much more juice out of cluster. O.k. I admit it's not always about performance. If that would be the case we would all be writing software in C or assembler. But still I think it's a good practice to be aware of the drawbacks.

To be honest spring data cassandra is relatively new to me. I did the performance comparison on the lowest level without using repositories and other high level concepts that come with spring data cassandra. My focus in this post is more on the generics that decode the data that comes out from the driver. To make a comparison I'm going to use a simple cassandra table (skinny row), then I'm going to make query after query (5000 and 10000) towards cassandra and after that I'll decode results. Once again the focus in this post is not on performance characteristics of higher order functionalities like paged queries etc. I just wanted to know by a rule of thumb what can I expect from spring data cassandra.

Setup

    -- simple skinny row
    CREATE TABLE activities (
        activity_id uuid,
        activity_model_id bigint,
        activity_state text,
        asset_id text,
        attrs map<text, text>,
        creation_time timestamp,
        customer_id text,
        end_time timestamp,
        last_modified_time timestamp,
        person_id text,
        poi_id text,
        start_time timestamp,
        PRIMARY KEY (activity_id)
    );

    

    activity_id 72b493f0-e59d-11e3-9bd6-0050568317c1
    activity_model_id 66
    activity_state DONE
    asset_id 8400848739855200000
    attrs {
        'businessDrive': '1:1',
        'customer': '4:test_test_test',
        'distance': '3:180', 
        'endLocation': '6:15.7437466839,15.9846853333,0.0000000000',
        'fromAddress': '4:XX1', 
        'locked': '1:0', 
        'reason': '4:Some reason 2', 
        'startLocation': 
        '6:15.7364385831,15.0071729736,0.0000000000', 
        'toAddress': '4:YY2'
        }
    creation_time 2014-05-27 14:50:14+0200
    customer_id 8400768435301400000
    end_time 2014-05-27 12:15:40+0200
    last_modified_time 2014-05-29 21:30:44+0200
    person_id 8401111750365200000
    poi_id null
    start_time 2014-05-27 12:13:05+0200
    

The code

To separate all possible interfering effects I used two separate projects. I had a situation where I used an old thrift api together with cql driver and it significantly affected performance. And it required additional configuration parameters etc. The main code snippets are located on gist. This is not the focus here, but if somebody is interested:

spring-data
native-drivers

Results in milliseconds

    3 fields - 5000 items
        spring-data
        5381
        5282
        5385
        avg: 5339

        driver
        4426
        4280
        4469
        avg: 4390

        result: driver faster 21.6%

    3 fields - 10000 items
        spring-data
        8560
        8133
        8144
        avg: 8279

        driver
        6822
        6770
        6875
        avg: 6822
        
        result: driver faster 21.3%

    12 fields - 5000 items
        spring-data
        5911
        5920
        5928
        avg: 5920 - 10.88 % slower than with 3 fields!

        driver
        4687
        4669
        4606
        avg: 4654 - 6 % slower than with 3 fields

        result: driver faster 27%

Conclusions

Spring data cassandra may be very interesting if you are interested to learn something new. It might also have very positive development effects when prototyping or doing something similar. I didn't test the higher order functionalities like pagination etc. This was just a rule of a thumb test to see what to expect. Basically the bigger the classes that you have to decode the bigger the deserialization cost. At least this is the effect I'm noticing in my basic tests.

Follow up with Object Mapping available in Cassandra driver 2.1

There was an interesting follow up disuccion on reddit. By a proposal from reddit user v_krishna another candidate was added to comparison Object-mapping API.

Let's see the results:

    3 fields - 5000 items
        spring-data
        5438
        5453
        5576
        avg: 5489

        object-map
        5390
        5299
        5476
        avg: 5388

        driver
        4382
        4410
        4249
        avg: 4347

    conclusion
        - driver 26% faster than spring data
        - object map just under 2% faster than spring data

    3 fields - 10000 items
        spring-data
        8792
        8507
        8473
        avg: 8591

        object-map
        8435
        8494
        8365
        avg: 8431

        driver
        6632
        6760
        6646
        avg: 6679

    conclusion
        - driver faster 28.6% than spring data
        - object mapping just under 2% faster than spring data

    12 fields 5000 items
        spring-data
        6193
        5999
        5938
        avg: 6043

        object-map
        6062
        5936
        5911
        avg: 5970

        driver
        4910
        4955
        4596
        avg: 4820

    conclusion
        - driver 25% faster than spring data
        - object mapping 1.2% faster than spring data

To keep everything fair, there was some deviation in test runs when compared to previous test, here are deviations:

comparison with first run:
    3 fields - 5000 items
        spring-data
        avg1: 5339
        avg2: 5489
        2.7% deviation

        driver
        avg1: 4390
        avg2: 4347
        1% deviation

    3 fields - 10000 items
        spring-data
        avg1: 8279
        avg2: 8591
        3.6% deviation

        driver
        avg1: 6822
        avg2: 6679
        2.1% deviation

    12 fields 5000 items
        spring-data
        avg1: 5920
        avg2: 6043
        2% deviation

        driver
        avg1: 4654
        avg2: 4820
        3.4% deviation

Intro

In the previous post we went over the steps for gathering the data on the Rasperry pi.

1. Gather Data on Raspberry Pi with Cassandra and Arduino
2. Arduino Greenhouse

Cassandra queries

    -- we'll keep the data on just one node
    CREATE KEYSPACE home
    WITH REPLICATION = {
        'class' : 'SimpleStrategy',
        'replication_factor' : 1
    };
    

    -- create statement, bucketed by date
    CREATE TABLE greenhouse (
        source text,
        day text,
        time timestamp,
        temperaturein decimal,
        temperatureout decimal,
        temperaturecheck decimal,
        humidity decimal,
        light int,
        PRIMARY KEY ((source, day), time)
    )
    WITH CLUSTERING ORDER BY (time DESC);
    

    -- example insert, just to check everything out
    INSERT INTO greenhouse (
        source, day, time, temperaturein,
        temperatureout, temperaturecheck,
        humidity, light)
    VALUES ('G', '2015-04-04', dateof(now()), 0,
        0, 0, 0, 0);
    

    -- check if everything is inserted
    SELECT * FROM greenhouse WHERE source = 'G' AND day = '2015-04-19';
    

Analysis results

I wanted to keep the partitions relatively small because I didn't know how RaspberryPi is going to handle the data. Timeout is possible if the rows get to big so I went with the partitioning the data by day. The analysis of the April showed that the project paid off. Here are the results of analysis:

Total Data points(not much, but it's a home DIY solution after all)
172651

First record
Measurement{source='G', day='2015-04-04', time=Sat Apr 04 17:04:41 CEST 2015, temperaturein=11.77, temperatureout=10.43, temperaturecheck=15.0, humidity=46.0, light=57}

Last record
Measurement{source='G', day='2015-05-04', time=Mon May 04 09:37:35 CEST 2015, temperaturein=22.79, temperatureout=20.49, temperaturecheck=23.0, humidity=31.0, light=68}

Cold nights(bellow 2 C outside)
2015-04-06
2015-04-07
2015-04-10
2015-04-16
2015-04-17
2015-04-18
2015-04-19
2015-04-20

Lowest In
Measurement{source='G', day='2015-04-06', time=Mon Apr 06 06:22:25 CEST 2015, temperaturein=2.28, temperatureout=2.39, temperaturecheck=4.0, humidity=41.0, light=8}

Highest In
Measurement{source='G', day='2015-04-22', time=Wed Apr 22 14:52:26 CEST 2015, temperaturein=75.53, temperatureout=43.53, temperaturecheck=71.0, humidity=21.0, light=84}

Average In
19.45

Lowest Out
Measurement{source='G', day='2015-04-20', time=Mon Apr 20 04:42:16 CEST 2015, temperaturein=4.48, temperatureout=-2.88, temperaturecheck=6.0, humidity=31.0, light=0}

Highest Out
Measurement{source='G', day='2015-04-22', time=Wed Apr 22 15:58:32 CEST 2015, temperaturein=57.69, temperatureout=45.07, temperaturecheck=56.0, humidity=24.0, light=71}

Average Out
14.71

Average Difference
4.75

Biggest Diff
Measurement{source='G', day='2015-04-20', time=Mon Apr 20 15:11:53 CEST 2015, temperaturein=69.93, temperatureout=28.36, temperaturecheck=62.0, humidity=21.0, light=83}

The code

1. Spark analysis code

Intro

In the previous post we went over the steps necessary to make a sensor for a small greenhouse for the balcony.

1. Arduino Greenhouse

• Arduino Uno
• USB cable
• Raspberry PI
• nRF24L01+
• 7 Wires

Persisting the data

To persist the data I opted for Apache Cassandra. It's a good fit even for a low powered Raspberry Pi. Cassandra is java technology. So before installing Cassandra you have to install java. It's all written up nicely in the following posts:

1. Install Java
2. Installing Cassandra

Overview of the process

The code

1. Data Gathering in Arduino
2. Python serial to Cassandra bridge

Intro

Recently I got an assignment to prove that Cassandra cluster can hold up to 100 000 requests per second. Also all this had to be done on the budget and with not so much time spent on development of the whole application. This setup had to be as close to the real thing as possible. We will go trough the details soon. Here is just the basic overview of the experiment:

Amazon

Generating and handling the load on this scale requires the infrastructure that is usually not available within a personal budget so I turned to Amazon EC2. I listened about the EC2 for quite some time now and It turned out really easy to use. Basically All you have to do is to setup a security group and store the "pem" file for that security group. Really easy and if anybody didn't try it yet there is a free micro instance available for a whole year after registering. I won't go into details of how to setup the security group. It's all described in the DataStax documentation. Note that the security definition is a bit extensive and that defining the port range from 1024-65535 is sufficient for an inter group communication and I didn't expose any ports to the public as described in the documentation. The second part is generating the key pair. In the rest of the document I'll reference this file as "cassandra.pem".

Load

Generating the load on that scale is not as easy as it might seem. After some searching I've stumbled upon the following. So I came to a conclusion that the best solution is to use Tsung. I've setup the load generating machines with the following snippet. Note that I've placed the "cassandra.pem" file on the node from which I'll start running tsung. Read the node addresses from the aws console. The rest is pretty much here:

        # do this only for the machine from which you'll initiate tsung
        scp -i cassandra.pem cassandra.pem ec2-user@tsung_machine:~

        # connect to every load machine and install erlang and tsung
        ssh -i cassandra.pem ec2-user@every_load_machine

        # repeat this on every node
        sudo yum install erlang

        wget http://tsung.erlang-projects.org/dist/tsung-1.5.1.tar.gz
        tar -xvzf tsung-1.5.1.tar.gz
        cd tsung-1.5.1
        ./configure
        make
        sudo make install

        # you can close other load nodes now
        # go back to the first node. and move cassandra.pem to id_rsa
        mv cassandra.pem .ssh/id_rsa

        # now make an ssh connection from first tsung node to every
        # load generating machine (to add the host key) so that
        # the first tsung node won't have any problem connecting to
        # other nodes and issuing erlang commands to them
        ssh ip-a-b-c-d
        exit

        # create the basic.xml file on the first tsung node
        vi basic.xml
    

The second part with the load generating machines is to edit the basic.xml file. To make it more interesting we are going to send various kinds of messages with a timestamp. The users list will be predefined in a file userlist.csv. Note that the password is the same for all the users, you can adapt this to your own needs or completely remove the password:

        0000000001;pass
        0000000002;pass
        0000000003;pass
        ...
        ...
        ...
    

The tsung tool is well documented, the configuration I used is similar to this:

        <?xml version="1.0" encoding="utf-8"?>
        <!DOCTYPE tsung SYSTEM "/usr/share/tsung/tsung-1.0.dtd" []>
        <tsung loglevel="warning">

        <clients>
            <client host="ip-a-b-c-d0" cpu="8" maxusers="25"/>
            <client host="ip-a-b-c-d1" cpu="8" maxusers="25"/>
            <client host="ip-a-b-c-d2" cpu="8" maxusers="25"/>
            <client host="ip-a-b-c-d3" cpu="8" maxusers="25"/>
        </clients>

        <servers>
            <server host="app-servers-ip-addresses-internal" port="8080" type="tcp"/>
            <!-- enter the rest of the app servers here-->
        </servers>

        <load>
            <arrivalphase phase="1" duration="11" unit="minute">
                <users maxnumber="100" arrivalrate="100" unit="second"/>
            </arrivalphase>
        </load>

        <options>
            <option name="file_server" id='id' value="userlist.csv"/>
        </options>

        <sessions>
            <session probability="100" name="load_session" type="ts_http">
                <setdynvars sourcetype="file" fileid="id" delimiter=";" order="iter">
                    <var name="username" />
                    <var name="pass" />
                </setdynvars>
                <setdynvars sourcetype="eval"
                            code="fun({Pid,DynVars}) -&gt;
                            {Mega, Sec, Micro} = os:timestamp(),
                            (Mega*1000000 + Sec)*1000 + round(Micro/1000)
                            end.
                            ">
                    <var name="millis" />
                </setdynvars>
                <for from="1" to="10000000" var="i">
                    <request subst="true">
                        <http url="/m?c=%%_username%%%%_millis%%ABC41.7127837,42.71278370000.0"  method="GET"/>
                    </request>
                    <request subst="true">
                        <http url="/m?c=%%_username%%%%_millis%%DEF43.7127837,44.71278370000.0"  method="GET"/>
                    </request>
                    <request subst="true">
                        <http url="/m?c=%%_username%%%%_millis%%GHI45.7127837,46.71278370000.0"  method="GET"/>
                    </request>
                    <request subst="true">
                        <http url="/m?c=%%_username%%%%_millis%%JKL47.7127837,48.71278370000.0"  method="GET"/>
                    </request>
                    <request subst="true">
                        <http url="/m?c=%%_username%%%%_millis%%MNO49.7127837,50.71278370000.0"  method="GET"/>
                    </request>
                </for>
            </session>
        </sessions>
        </tsung>
    

Resources

• 3x c3.xlarge
• 1x c4.xlarge

App

I've spent most of the time on the app part when developing. The basics for the component handling the requests was netty listener. In one of my previous posts I described how to use netty to handle http requests and acknowledge them with HELLO message. Here I acknowledged them with OK.

The most complicated part with the messages was sending them to cassandra as fast as possible. The fastest way to send them is to use executeAsync. Initially I had trouble with it where I was loosing messages. Some of the issues were due to concurrency. Some were due to poor understanding of the DataStax driver.

Concurrency - Basically what I was doing was that I tried to save on instantiating the BoundStatement instances because of the overal speed. The BoundStatement is not thread safe and after calling the bind method it returns "this". It took me some time to figure this out because when used in loops this behavior is not dangerous. Anyway, thanks to colleague I figured it out.

        // always instantiate new in concurrent code
        // don't reuse and make multiple calls with .bind()!

        BoundStatement bs = new BoundStatement(insertStatement);
    

Asynchronous execution - also a bit tricky. The executeAsync returns a future. Initially I was just adding it to Futures.

        // don't do this under heavy load with the result of executeAsync
        // in Cassandra you will start to loose data

        Futures.addCallback(future, ...
    

After some trial and error I found a pattern where I didn't loose any data:

        // here we are going to keep the futures
        private ArrayBlockingQueue<ResultSetFuture> queue = 
            new ArrayBlockingQueue<>(10000);

        // in the handling code
        queue.add(session.executeAsync(bs));

        // when reaching 1000th element in the queue
        // start emptying it
        if (queue.size() % 1000 == 0) {
            ResultSetFuture elem;
            do {
                elem = queue.poll();
                if (elem != null) {
                    elem.getUninterruptibly();
                }
            } while (elem != null);
        }

        // this will make your insertions around
        // 4x faster when compared to normal execute
    

App setup

The instances come with Open JDK installed. This doesn't guarantee the best performance so I installed the Oracle java. In order not to loose the time on firewall setup I simply copied the "cassandra.pem" file to every node.

        # copy ".jar" and "cassandra.pem" file to a single app node
        # copy the two files from single node to other nodes
        # it's a lot faster then uploading to every node (at least on my connection)

        # setup the machine
        wget --no-check-certificate --no-cookies - --header "Cookie: oraclelicense=accept-securebackup-cookie" "http://download.oracle.com/otn-pub/java/jdk/7u71-b14/jdk-7u71-linux-x64.tar.gz"
        
        tar -xvzf jdk-7u71-linux-x64.tar.gz

        sudo update-alternatives --install "/usr/bin/java" "java" "/home/ec2-user/jdk1.7.0_71/jre/bin/java" 1
        
        # pick the new java number in this step
        sudo update-alternatives --config java

        # check with this
        java -version
    

Resources

• 2x c4.xlarge
• 2x c4.2xlarge
• 4x c3.xlarge

Cassandra

Setting up the Cassandra is the easiest part of the whole undertaking. All I did was following this guide by DataStax.

Resources

• 7x c3.2xlarge

Results

In the end it took me around 30$ to reach the 100k limit. I'm afraid to calculate how much this setup would cost on a monthly or yearly basis.

The successful run looked like this:

Total messages: 31 145 914 messages
Checked number: 31 145 914 messages
Average: 103 809 req/s

Intro

From time to time having just one Cassandra instance installed on your machine is not enough because you want to test certain behaviors when Cassandra cluster is up and running. Having extra spare hardware on the side or processing time on amazon is not always an option. So it's a good idea to setup a simple cluster on your own machine with instances in virtual machines. This post is going to show you how to do it with VirtualBox.

Getting VirtualBox Images

The reason why I chose VirtualBox is that there are lot of free virtual images available. Most of the time you'll be installing Cassandra on a Linux machine. I decided to go with the CentOS. Head over to http://virtualboxes.org/images/centos/ and download CentOS-6.6-x86_64-minimal. The default settings are fine for every machine. Create couple of them, give them names so that you can differentiate between them (Node1, Node2, etc. ...).

Perhaps the best idea would be for you to setup one node first and then make copies afterwards. Do not forget to set the network to bridged adapter. The username and password for the virtual machines are probably set to "root/reverse" but check those options when downloading the virtual box image. To keep it short I'll just continue with using the root user. When doing things in production it's an extremely bad practice.

Setup networking

When importing .ova file virtual box is going to ask you if you want to reinitialize mac address. Check that option. There is a certain amount of buggy behavior when it comes down to networking. So to prevent those errors run the following command when logging in to the virtual machine (root/reverse):

        rm  /etc/udev/rules.d/70-persistant-net.rules
    

        cat /etc/sysconfig/network-scripts/ifcfg-eth0
    

Install Java

Just to make things a bit easier we're going to install wget:

        yum install wget
    

        $ cd /opt/
        $ wget --no-cookies --no-check-certificate --header "Cookie: gpw_e24=http%3A%2F%2Fwww.oracle.com%2F; oraclelicense=accept-securebackup-cookie" "http://download.oracle.com/otn-pub/java/jdk/7u72-b14/jdk-7u72-linux-x64.tar.gz"
        $ tar xzf jdk-7u72-linux-x64.tar.gz
        $ rm jdk-7u72-linux-x64.tar.gz

        $ cd /opt/jdk1.7.0_72/

        $ alternatives --install /usr/bin/java java /opt/jdk1.7.0_72/bin/java 2
        $ alternatives --config java

        $ alternatives --install /usr/bin/jar jar /opt/jdk1.7.0_72/bin/jar 2
        $ alternatives --install /usr/bin/javac javac /opt/jdk1.7.0_72/bin/javac 2
        $ alternatives --set jar /opt/jdk1.7.0_72/bin/jar
        $ alternatives --set javac /opt/jdk1.7.0_72/bin/javac

        $ vi /etc/profile.d/java.sh
        export JAVA_HOME=/opt/jdk1.7.0_72
        export JRE_HOME=/opt/jdk1.7.0_72/jre
        export PATH=$PATH:/opt/jdk1.7.0_72/bin:/opt/jdk1.7.0_72/jre/bin
    

Install Cassandra

Cassandra is installed and run by the following commands:

        $ cd /opt/
        $ wget http://downloads.datastax.com/community/dsc-cassandra-2.1.2-bin.tar.gz
        $ tar xzf dsc-cassandra-2.1.2-bin.tar.gz
        $ rm dsc-cassandra-2.1.2-bin.tar.gz

        [check ip address with ifconfig]

        $ cd conf

        $ vi cassandra.yaml
            rpc_address: ip address of the node
            broadcast_address: ip address of the node
            - seeds: ip_address of the first node

        $ cd ../bin
        $ ./cassandra
    

Firewall settings

The cluster will not work out of the box because of the firewall settings. To start everything you will need to enable the following ports:

        $ iptables -I INPUT -p tcp -m tcp --dport 9042 -j ACCEPT
        $ iptables -I INPUT -p tcp -m tcp --dport 7000 -j ACCEPT
        $ iptables -I INPUT -p tcp -m tcp --dport 7001 -j ACCEPT
        $ iptables -I INPUT -p tcp -m tcp --dport 7199 -j ACCEPT

        $ /etc/init.d/iptables save

        $ service iptables restart
    

Intro

I started doing some DIY home automation projects. Although I have the mega popular Raspberry Pi available I decided to use the MINIX NEO X5 mini because I felt this device could be used a lot better if it served me as some sort of home automation server. The first part in this story is getting a more server oriented OS on the device. I decided to go with the linux. After a lot of searching and trial and error I decided to deploy an application called Linux deploy and described it in my previous blog post. Trough the rest of the tutorial I'll assume you managed to install a linux instance on your MINIX. I am going to gather a lot of telemetry data with the solution I am building so installing Cassandra seems as a natural choice to me. There will be a lot of writes and Cassandra is good at writing at an incredible scale.

Installing Java

        $ echo "deb http://ppa.launchpad.net/webupd8team/java/ubuntu trusty main" | sudo tee /etc/apt/sources.list.d/webupd8team-java.list
        $ echo "deb-src http://ppa.launchpad.net/webupd8team/java/ubuntu trusty main" | sudo tee -a /etc/apt/sources.list.d/webupd8team-java.list
        $ sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys EEA14886
        $ sudo apt-get update
        $ sudo apt-get install oracle-java8-installer
        # you'll need to accept license agreement

        # set environment variables
        $ sudo apt-get install oracle-java8-set-default

        # login once again just in case
        $ exit
    

Installing python

Cassandra comes with a very nice tool called cqlsh. The version of linux we currently have installed will not run it without a python available on the system. So we have to install it first.

        $ sudo apt-get install python2.7
    

Let's start the Cassandra

Configuring the Cassandra is a chapter on it's own. We'll make minimal adjustments before starting. We'll configure the Cassandra to respond to queries from other hosts and while we are at it we'll enable the virtual nodes. (Will be easier to scale later).

        $ cd CASSANDRA_INSTALL_DIRECTORY
        $ nano conf/cassandra.yaml

        # uncomment
        num_tokens: 256

        # change to 0.0.0.0
        # this will enable you to contact the cassandra
        # from other computers etc.
        rpc_address: 0.0.0.0

        #save file

        $ cd ..
        $ ./bin/cassandra

        # after seeing something like
        # Startup completed! Now serving reads.
        # press ^C (don't be afraid cassandra still runs)

        $ bin/cqlsh

        Connected to Test Cluster at localhost:9160.
        [cqlsh 3.1.8 | Cassandra 1.2.18 | CQL spec 3.0.5 | 
        Thrift protocol 19.36.2]
        Use HELP for help.
        cqlsh>
    

Shutting cassandra down:

        # find PID of a cassandra process

        $ ps -ef | grep cassandra

        # run kill -9 [the PID number ... i.e. 8212]
    

Running Cassandra on android multimedia player is fun :)

Intro

This post continues where this post stopped. The Cassandra setup used for this post is more or less the same so please read this post if you are interested in cassandra setup before continuing with the rest of the post.

Arduino

Learning big data stuff is most exciting when the data represents something from the real world and not something generated with a help of big loop and then randomized data in it. To create data for this example I've used the following components:

1. arduino uno
2. Photoresistor GL5528 LDR
3. 10K OHM NTC Thermistor 5mm
4. 2x 10k resistor
5. Protoboard
6. Wires

The Arduino sketch will be on the gitHub, so we'll concentrate on the important parts. The light level in this example is read at analog 0. Reading analog values in arduino results in values ranging from 0-1023. We'll define light level as a mapping from 0-1023 into 0-100. Arduino already has a built in function for this called map. Also, I had some trouble in my initial experiments with Arduino serial communication and reading pin values. The data written to the serial port simply got corrupted after a while. I've read a couple of forums on this subject and found out that it actually helps when one delays execution after reading a pin value for 1ms. Also to keep the things as stable as possible we'll pause the execution for 1 second after writing to serial port as shown here:

  int light = map(analogRead(0), 0, 1023, 0, 100);
  delay(1);

    ....

  sprintf(sOut, "%d,%s", light, deblank(sTemp));

  Serial.println(sOut);
  delay(1000);
 

Node.js and Cassandra

Parsing the messages that come from the measuring devices is pretty repetitive stuff that causes pretty ugly code. I've learned that the hard way. To make parsing of this messages as easy as possible I've written a small utility package for parsing the messages that come from the measuring devices and it's available on npm.

Using serial ports in node.js doesn't take a lot of steps to setup:

  var serial = require( "serialport" );
  var SerialPort = serial.SerialPort;

  var portName = "/dev/tty.usb-something";

  var sp = new SerialPort(portName, {
      baudrate:9600,
      parser:serial.parsers.readline("\n")
  });

  sp.on("data", function ( data ) {
  var arduinoData = translator.parse(data);
  //...
 

To make the data handling easier and more in accordance with cassandra best practices the readings will be partitioned by date when they were recorded.

  CREATE TABLE room_data (
    day text,
    measurementtime timestamp,
    light int,
    temperature float,
    PRIMARY KEY (day, measurementtime)
  ) WITH CLUSTERING ORDER BY (measurementtime DESC);
 

Also the data will probably be more often fetched for recent time stamps with queries that have limits set on them. To make this fetching easier we've added a clustering statement above. Also to get the current light and temperature level we would just have to run the following query (no where combined with now function):

  SELECT * FROM room_data LIMIT 1;
 

After setting up the cassandra and reading the data from the serial port and parsing the data it's time to write this data into the cassandra. Analyzing the data and doing something useful with it will be in some future posts that I'll make but for now I'll stop with writing the data into cassandra:

  client.execute('INSERT INTO room_data ' + 
   '(day, measurementtime, light, temperature)' + 
   ' VALUES (?, dateof(now()), ?, ?)',
   [
    moment().format('YYYY-MM-DD'),
    arduinoData.light,
    arduinoData.temperature
   ],
   function(err, result) {
    if (err) {
     console.log('insert failed', err);
    }
   }
  );
 

On the fifth line I've used moment.js to format current time into string representation of current date used for partitioning in cassandra. The rest of the code is pretty much the usual sql stuff found in other database environments.

I recorder couple of hours worth of data here. Just in case anybody wants a sneak peak without having to setup everything up. I've exported the data out from cassandra trought cql using this command:

  COPY room_data (day, measurementtime, light, temperature) 
   TO 'room_data.csv';
 

The rest of the example is located on gitHub.

Intro

Since I started to work in a team that deals with BigData stuff I came into contact with Apache Cassandra. After years in the relational world it took me some getting used to the many concepts that the Cassandra relies on. Actually in the relational world the concepts would be heavy anti patterns. I went over a couple of tutorials etc. for intro into the Cassandra data model I would recommend this video by Patrick McFadin:

C* Summit 2013: The World's Next Top Data Model

Basic setup

The easiest way to get the Cassandra is to download it from here: http://planetcassandra.org/Download/StartDownload

I somehow dislike when various applications write to /var/something and having to use the root access to install something unless it's absolutely necessary. So I followed this manual to avoid this problem.

cassandra.yaml

The Cassandra is setup out of the box to support queries coming from cql shell ("cqlsh"). The goal of this blog entry is to show how to make a simple connection from node.js to the Cassandra, so there is a bit of tweaking that has to be done in order to get all this working. The necessary configuration is located in this file:

 install_dir/conf/cassandra.yaml

 authenticator: PasswordAuthenticator
 authorizer: CassandraAuthorizer

 ./cqlsh -u cassandra -p cassandra

Cassandra keyspace setup

 CREATE KEYSPACE test WITH REPLICATION = 
  {'class': 'SimpleStrategy', 'replication_factor': 1};

 --check if it's created with this
 DESCRIBE KEYSPACES;

 USE test;

 CREATE TABLE test_table (
  id text,
  test_value text,
  PRIMARY KEY (id)
 );

 INSERT INTO test_table (id, test_value) VALUES ('1', 'a');

 INSERT INTO test_table (id, test_value) VALUES ('2', 'b');

 INSERT INTO test_table (id, test_value) VALUES ('3', 'c');

 SELECT * FROM test_table;
 

  id  | test_value
 ----+------------
   3 |          c
   2 |          b
   1 |          a

 (3 rows)

  create user testuser with password 'testuser';

  grant all on test.test_table to testuser;

node-cassandra-cql

I tried several Cassandra connection libraries from gitHub for the node.js and the one that I found most easy to work with (and setup) was node-cassandra-cql by jorgebay. The story with the project is pretty much standard. Going into new project empty directory and initializing it with init and then installing module with npm.

 npm init

 npm install node-cassandra-cql

 #copy hellocassandra.js from
 #https://github.com/msval/hellocassandrainnodejs

 node hellocassandra.js
 

Anyway here's my example on gitHub.