Apache Flume – Introduction

What is Flume?

Apache Flume is a tool/service/data ingestion mechanism for collecting aggregating and transporting large amounts of streaming data such as log files, events (etc…) from various sources to a centralized data store.

Flume is a highly reliable, distributed, and configurable tool. It is principally designed to copy streaming data (log data) from various web servers to HDFS.

Applications of Flume

Assume an e-commerce web application wants to analyze the customer behavior from a particular region. To do so, they would need to move the available log data in to Hadoop for analysis. Here, Apache Flume comes to our rescue.

Flume is used to move the log data generated by application servers into HDFS at a higher speed.

Advantages of Flume

Here are the advantages of using Flume −

• Using Apache Flume we can store the data in to any of the centralized stores (HBase, HDFS).

• When the rate of incoming data exceeds the rate at which data can be written to the destination, Flume acts as a mediator between data producers and the centralized stores and provides a steady flow of data between them.

• Flume provides the feature of contextual routing.

• The transactions in Flume are channel-based where two transactions (one sender and one receiver) are maintained for each message. It guarantees reliable message delivery.

• Flume is reliable, fault tolerant, scalable, manageable, and customizable.

Features of Flume

Some of the notable features of Flume are as follows −

• Flume ingests log data from multiple web servers into a centralized store (HDFS, HBase) efficiently.

• Using Flume, we can get the data from multiple servers immediately into Hadoop.

• Along with the log files, Flume is also used to import huge volumes of event data produced by social networking sites like Facebook and Twitter, and e-commerce websites like Amazon and Flipkart.

• Flume supports a large set of sources and destinations types.

• Flume supports multi-hop flows, fan-in fan-out flows, contextual routing, etc.

• Flume can be scaled horizontally.

Apache Flume – Data Transfer In Hadoop

Big Data, as we know, is a collection of large datasets that cannot be processed using traditional computing techniques. Big Data, when analyzed, gives valuable results. Hadoop is an open-source framework that allows to store and process Big Data in a distributed environment across clusters of computers using simple programming models.

Streaming / Log Data

Generally, most of the data that is to be analyzed will be produced by various data sources like applications servers, social networking sites, cloud servers, and enterprise servers. This data will be in the form of log files and events.

Log file − In general, a log file is a file that lists events/actions that occur in an operating system. For example, web servers list every request made to the server in the log files.

On harvesting such log data, we can get information about −

• the application performance and locate various software and hardware failures.
• the user behavior and derive better business insights.

The traditional method of transferring data into the HDFS system is to use the put command. Let us see how to use the put command.

HDFS put Command

The main challenge in handling the log data is in moving these logs produced by multiple servers to the Hadoop environment.

Hadoop File System Shell provides commands to insert data into Hadoop and read from it. You can insert data into Hadoop using the put command as shown below.

$ Hadoop fs –put /path of the required file  /path in HDFS where to save the file 

Problem with put Command

We can use the put command of Hadoop to transfer data from these sources to HDFS. But, it suffers from the following drawbacks −

• Using put command, we can transfer only one file at a time while the data generators generate data at a much higher rate. Since the analysis made on older data is less accurate, we need to have a solution to transfer data in real time.

• If we use put command, the data is needed to be packaged and should be ready for the upload. Since the webservers generate data continuously, it is a very difficult task.

What we need here is a solutions that can overcome the drawbacks of put command and transfer the “streaming data” from data generators to centralized stores (especially HDFS) with less delay.

Problem with HDFS

In HDFS, the file exists as a directory entry and the length of the file will be considered as zero till it is closed. For example, if a source is writing data into HDFS and the network was interrupted in the middle of the operation (without closing the file), then the data written in the file will be lost.

Therefore we need a reliable, configurable, and maintainable system to transfer the log data into HDFS.

Note − In POSIX file system, whenever we are accessing a file (say performing write operation), other programs can still read this file (at least the saved portion of the file). This is because the file exists on the disc before it is closed.

Available Solutions

To send streaming data (log files, events etc..,) from various sources to HDFS, we have the following tools available at our disposal −

Facebook’s Scribe

Scribe is an immensely popular tool that is used to aggregate and stream log data. It is designed to scale to a very large number of nodes and be robust to network and node failures.

Apache Kafka

Kafka has been developed by Apache Software Foundation. It is an open-source message broker. Using Kafka, we can handle feeds with high-throughput and low-latency.

Apache Flume

Apache Flume is a tool/service/data ingestion mechanism for collecting aggregating and transporting large amounts of streaming data such as log data, events (etc…) from various webserves to a centralized data store.

It is a highly reliable, distributed, and configurable tool that is principally designed to transfer streaming data from various sources to HDFS.

In this tutorial, we will discuss in detail how to use Flume with some examples.

Apache Flume – Architecture

The following illustration depicts the basic architecture of Flume. As shown in the illustration, data generators (such as Facebook, Twitter) generate data which gets collected by individual Flume agents running on them. Thereafter, a data collector (which is also an agent) collects the data from the agents which is aggregated and pushed into a centralized store such as HDFS or HBase.

Flume Event

An event is the basic unit of the data transported inside Flume. It contains a payload of byte array that is to be transported from the source to the destination accompanied by optional headers. A typical Flume event would have the following structure −

Flume Agent

An agent is an independent daemon process (JVM) in Flume. It receives the data (events) from clients or other agents and forwards it to its next destination (sink or agent). Flume may have more than one agent. Following diagram represents a Flume Agent

As shown in the diagram a Flume Agent contains three main components namely, source, channel, and sink.

Source

A source is the component of an Agent which receives data from the data generators and transfers it to one or more channels in the form of Flume events.

Apache Flume supports several types of sources and each source receives events from a specified data generator.

Example − Avro source, Thrift source, twitter 1% source etc.

Channel

A channel is a transient store which receives the events from the source and buffers them till they are consumed by sinks. It acts as a bridge between the sources and the sinks.

These channels are fully transactional and they can work with any number of sources and sinks.

Example − JDBC channel, File system channel, Memory channel, etc.

Sink

A sink stores the data into centralized stores like HBase and HDFS. It consumes the data (events) from the channels and delivers it to the destination. The destination of the sink might be another agent or the central stores.

Example − HDFS sink

Note − A flume agent can have multiple sources, sinks and channels. We have listed all the supported sources, sinks, channels in the Flume configuration chapter of this tutorial.

Additional Components of Flume Agent

What we have discussed above are the primitive components of the agent. In addition to this, we have a few more components that play a vital role in transferring the events from the data generator to the centralized stores.

Interceptors

Interceptors are used to alter/inspect flume events which are transferred between source and channel.

Channel Selectors

These are used to determine which channel is to be opted to transfer the data in case of multiple channels. There are two types of channel selectors −

• Default channel selectors − These are also known as replicating channel selectors they replicates all the events in each channel.

• Multiplexing channel selectors − These decides the channel to send an event based on the address in the header of that event.

Sink Processors

These are used to invoke a particular sink from the selected group of sinks. These are used to create failover paths for your sinks or load balance events across multiple sinks from a channel.

Apache Flume – Data Flow

Flume is a framework which is used to move log data into HDFS. Generally events and log data are generated by the log servers and these servers have Flume agents running on them. These agents receive the data from the data generators.

The data in these agents will be collected by an intermediate node known as Collector. Just like agents, there can be multiple collectors in Flume.

Finally, the data from all these collectors will be aggregated and pushed to a centralized store such as HBase or HDFS. The following diagram explains the data flow in Flume.

Multi-hop Flow

Within Flume, there can be multiple agents and before reaching the final destination, an event may travel through more than one agent. This is known as multi-hop flow.

Fan-out Flow

The dataflow from one source to multiple channels is known as fan-out flow. It is of two types −

• Replicating − The data flow where the data will be replicated in all the configured channels.

• Multiplexing − The data flow where the data will be sent to a selected channel which is mentioned in the header of the event.

Fan-in Flow

The data flow in which the data will be transferred from many sources to one channel is known as fan-in flow.

Failure Handling

In Flume, for each event, two transactions take place: one at the sender and one at the receiver. The sender sends events to the receiver. Soon after receiving the data, the receiver commits its own transaction and sends a “received” signal to the sender. After receiving the signal, the sender commits its transaction. (Sender will not commit its transaction till it receives a signal from the receiver.)

Apache Flume – Environment

We already discussed the architecture of Flume in the previous chapter. In this chapter, let us see how to download and setup Apache Flume.

Before proceeding further, you need to have a Java environment in your system. So first of all, make sure you have Java installed in your system. For some examples in this tutorial, we have used Hadoop HDFS (as sink). Therefore, we would recommend that you go install Hadoop along with Java. To collect more information, follow the link −https://www.tutorialspoint.com/hadoop/hadoop_enviornment_setup.htm

Installing Flume

First of all, download the latest version of Apache Flume software from the website https://flume.apache.org/.

Step 1

Open the website. Click on the download link on the left-hand side of the home page. It will take you to the download page of Apache Flume.

Step 2

In the Download page, you can see the links for binary and source files of Apache Flume. Click on the link apache-flume-1.6.0-bin.tar.gz

You will be redirected to a list of mirrors where you can start your download by clicking any of these mirrors. In the same way, you can download the source code of Apache Flume by clicking on apache-flume-1.6.0-src.tar.gz.

Step 3

Create a directory with the name Flume in the same directory where the installation directories of Hadoop, HBase, and other software were installed (if you have already installed any) as shown below.

$ mkdir Flume 

Step 4

Extract the downloaded tar files as shown below.

$ cd Downloads/ 
$ tar zxvf apache-flume-1.6.0-bin.tar.gz  
$ tar zxvf apache-flume-1.6.0-src.tar.gz

Step 5

Move the content of apache-flume-1.6.0-bin.tar file to the Flume directory created earlier as shown below. (Assume we have created the Flume directory in the local user named Hadoop.)

$ mv apache-flume-1.6.0-bin.tar/* /home/Hadoop/Flume/

Configuring Flume

To configure Flume, we have to modify three files namely, flume-env.sh, flumeconf.properties, and bash.rc.

Setting the Path / Classpath

In the .bashrc file, set the home folder, the path, and the classpath for Flume as shown below.

conf Folder

If you open the conf folder of Apache Flume, you will have the following four files −

• flume-conf.properties.template,
• flume-env.sh.template,
• flume-env.ps1.template, and
• log4j.properties.

Now rename

• flume-conf.properties.template file as flume-conf.properties and

• flume-env.sh.template as flume-env.sh

flume-env.sh

Open flume-env.sh file and set the JAVA_Home to the folder where Java was installed in your system.

Verifying the Installation

Verify the installation of Apache Flume by browsing through the bin folder and typing the following command.

$ ./flume-ng 

If you have successfully installed Flume, you will get a help prompt of Flume as shown below.

Apache Flume – Configuration

After installing Flume, we need to configure it using the configuration file which is a Java property file having key-value pairs. We need to pass values to the keys in the file.

In the Flume configuration file, we need to −

• Name the components of the current agent.
• Describe/Configure the source.
• Describe/Configure the sink.
• Describe/Configure the channel.
• Bind the source and the sink to the channel.

Usually we can have multiple agents in Flume. We can differentiate each agent by using a unique name. And using this name, we have to configure each agent.

Naming the Components

First of all, you need to name/list the components such as sources, sinks, and the channels of the agent, as shown below.

agent_name.sources = source_name 
agent_name.sinks = sink_name 
agent_name.channels = channel_name 

Flume supports various sources, sinks, and channels. They are listed in the table given below.

You can use any of them. For example, if you are transferring Twitter data using Twitter source through a memory channel to an HDFS sink, and the agent name id TwitterAgent, then

TwitterAgent.sources = Twitter 
TwitterAgent.channels = MemChannel 
TwitterAgent.sinks = HDFS 

After listing the components of the agent, you have to describe the source(s), sink(s), and channel(s) by providing values to their properties.

Describing the Source

Each source will have a separate list of properties. The property named “type” is common to every source, and it is used to specify the type of the source we are using.

Along with the property “type”, it is needed to provide the values of all the required properties of a particular source to configure it, as shown below.

agent_name.sources. source_name.type = value 
agent_name.sources. source_name.property2 = value 
agent_name.sources. source_name.property3 = value 

For example, if we consider the twitter source, following are the properties to which we must provide values to configure it.

TwitterAgent.sources.Twitter.type = Twitter (type name) 
TwitterAgent.sources.Twitter.consumerKey =  
TwitterAgent.sources.Twitter.consumerSecret = 
TwitterAgent.sources.Twitter.accessToken =   
TwitterAgent.sources.Twitter.accessTokenSecret = 

Describing the Sink

Just like the source, each sink will have a separate list of properties. The property named “type” is common to every sink, and it is used to specify the type of the sink we are using. Along with the property “type”, it is needed to provide values to all the required properties of a particular sink to configure it, as shown below.

agent_name.sinks. sink_name.type = value 
agent_name.sinks. sink_name.property2 = value 
agent_name.sinks. sink_name.property3 = value

For example, if we consider HDFS sink, following are the properties to which we must provide values to configure it.

TwitterAgent.sinks.HDFS.type = hdfs (type name)  
TwitterAgent.sinks.HDFS.hdfs.path = HDFS directory’s Path to store the data

Describing the Channel

Flume provides various channels to transfer data between sources and sinks. Therefore, along with the sources and the channels, it is needed to describe the channel used in the agent.

To describe each channel, you need to set the required properties, as shown below.

agent_name.channels.channel_name.type = value 
agent_name.channels.channel_name. property2 = value 
agent_name.channels.channel_name. property3 = value 

For example, if we consider memory channel, following are the properties to which we must provide values to configure it.

TwitterAgent.channels.MemChannel.type = memory (type name)

Binding the Source and the Sink to the Channel

Since the channels connect the sources and sinks, it is required to bind both of them to the channel, as shown below.

agent_name.sources.source_name.channels = channel_name 
agent_name.sinks.sink_name.channels = channel_name 

The following example shows how to bind the sources and the sinks to a channel. Here, we consider twitter source, memory channel, and HDFS sink.

TwitterAgent.sources.Twitter.channels = MemChannel
TwitterAgent.sinks.HDFS.channels = MemChannel 

Starting a Flume Agent

After configuration, we have to start the Flume agent. It is done as follows −

$ bin/flume-ng agent --conf ./conf/ -f conf/twitter.conf 
Dflume.root.logger=DEBUG,console -n TwitterAgent 

where −

• agent − Command to start the Flume agent

• –conf ,-c<conf> − Use configuration file in the conf directory

• -f<file> − Specifies a config file path, if missing

• –name, -n <name> − Name of the twitter agent

• -D property =value − Sets a Java system property value.

Apache Flume – Fetching Twitter Data

Using Flume, we can fetch data from various services and transport it to centralized stores (HDFS and HBase). This chapter explains how to fetch data from Twitter service and store it in HDFS using Apache Flume.

As discussed in Flume Architecture, a webserver generates log data and this data is collected by an agent in Flume. The channel buffers this data to a sink, which finally pushes it to centralized stores.

In the example provided in this chapter, we will create an application and get the tweets from it using the experimental twitter source provided by Apache Flume. We will use the memory channel to buffer these tweets and HDFS sink to push these tweets into the HDFS.

To fetch Twitter data, we will have to follow the steps given below −

• Create a twitter Application
• Install / Start HDFS
• Configure Flume

Creating a Twitter Application

In order to get the tweets from Twitter, it is needed to create a Twitter application. Follow the steps given below to create a Twitter application.

Step 1

To create a Twitter application, click on the following link https://apps.twitter.com/. Sign in to your Twitter account. You will have a Twitter Application Management window where you can create, delete, and manage Twitter Apps.

Step 2

Click on the Create New App button. You will be redirected to a window where you will get an application form in which you have to fill in your details in order to create the App. While filling the website address, give the complete URL pattern, for example, http://example.com.

Step 3

Fill in the details, accept the Developer Agreement when finished, click on the Create your Twitter application button which is at the bottom of the page. If everything goes fine, an App will be created with the given details as shown below.

Step 4

Under keys and Access Tokens tab at the bottom of the page, you can observe a button named Create my access token. Click on it to generate the access token.

Step 5

Finally, click on the Test OAuth button which is on the right side top of the page. This will lead to a page which displays your Consumer key, Consumer secret, Access token, and Access token secret. Copy these details. These are useful to configure the agent in Flume.

Starting HDFS

Since we are storing the data in HDFS, we need to install / verify Hadoop. Start Hadoop and create a folder in it to store Flume data. Follow the steps given below before configuring Flume.

Step 1: Install / Verify Hadoop

Install Hadoop. If Hadoop is already installed in your system, verify the installation using Hadoop version command, as shown below.

$ hadoop version 

If your system contains Hadoop, and if you have set the path variable, then you will get the following output −

Hadoop 2.6.0 
Subversion https://git-wip-us.apache.org/repos/asf/hadoop.git -r 
e3496499ecb8d220fba99dc5ed4c99c8f9e33bb1 
Compiled by jenkins on 2014-11-13T21:10Z 
Compiled with protoc 2.5.0 
From source with checksum 18e43357c8f927c0695f1e9522859d6a 
This command was run using /home/Hadoop/hadoop/share/hadoop/common/hadoop-common-2.6.0.jar

Step 2: Starting Hadoop

Browse through the sbin directory of Hadoop and start yarn and Hadoop dfs (distributed file system) as shown below.

cd /$Hadoop_Home/sbin/ 
$ start-dfs.sh 
localhost: starting namenode, logging to
   /home/Hadoop/hadoop/logs/hadoop-Hadoop-namenode-localhost.localdomain.out 
localhost: starting datanode, logging to 
   /home/Hadoop/hadoop/logs/hadoop-Hadoop-datanode-localhost.localdomain.out 
Starting secondary namenodes [0.0.0.0] 
starting secondarynamenode, logging to 
   /home/Hadoop/hadoop/logs/hadoop-Hadoop-secondarynamenode-localhost.localdomain.out
  
$ start-yarn.sh 
starting yarn daemons 
starting resourcemanager, logging to 
   /home/Hadoop/hadoop/logs/yarn-Hadoop-resourcemanager-localhost.localdomain.out 
localhost: starting nodemanager, logging to 
   /home/Hadoop/hadoop/logs/yarn-Hadoop-nodemanager-localhost.localdomain.out 

Step 3: Create a Directory in HDFS

In Hadoop DFS, you can create directories using the command mkdir. Browse through it and create a directory with the name twitter_data in the required path as shown below.

$cd /$Hadoop_Home/bin/ 
$ hdfs dfs -mkdir hdfs://localhost:9000/user/Hadoop/twitter_data 

Configuring Flume

We have to configure the source, the channel, and the sink using the configuration file in the conf folder. The example given in this chapter uses an experimental source provided by Apache Flume named Twitter 1% Firehose Memory channel and HDFS sink.

Twitter 1% Firehose Source

This source is highly experimental. It connects to the 1% sample Twitter Firehose using streaming API and continuously downloads tweets, converts them to Avro format, and sends Avro events to a downstream Flume sink.

We will get this source by default along with the installation of Flume. The jar files corresponding to this source can be located in the lib folder as shown below.

Setting the classpath

Set the classpath variable to the lib folder of Flume in Flume-env.sh file as shown below.

export CLASSPATH=$CLASSPATH:/FLUME_HOME/lib/* 

This source needs the details such as Consumer key, Consumer secret, Access token, and Access token secret of a Twitter application. While configuring this source, you have to provide values to the following properties −

• Channels

• Source type : org.apache.flume.source.twitter.TwitterSource

• consumerKey − The OAuth consumer key

• consumerSecret − OAuth consumer secret

• accessToken − OAuth access token

• accessTokenSecret − OAuth token secret

• maxBatchSize − Maximum number of twitter messages that should be in a twitter batch. The default value is 1000 (optional).

• maxBatchDurationMillis − Maximum number of milliseconds to wait before closing a batch. The default value is 1000 (optional).

Channel

We are using the memory channel. To configure the memory channel, you must provide value to the type of the channel.

• type − It holds the type of the channel. In our example, the type is MemChannel.

• Capacity − It is the maximum number of events stored in the channel. Its default value is 100 (optional).

• TransactionCapacity − It is the maximum number of events the channel accepts or sends. Its default value is 100 (optional).

HDFS Sink

This sink writes data into the HDFS. To configure this sink, you must provide the following details.

• Channel

• type − hdfs

• hdfs.path − the path of the directory in HDFS where data is to be stored.

And we can provide some optional values based on the scenario. Given below are the optional properties of the HDFS sink that we are configuring in our application.

• fileType − This is the required file format of our HDFS file. SequenceFile, DataStream and CompressedStream are the three types available with this stream. In our example, we are using the DataStream.

• writeFormat − Could be either text or writable.

• batchSize − It is the number of events written to a file before it is flushed into the HDFS. Its default value is 100.

• rollsize − It is the file size to trigger a roll. It default value is 100.

• rollCount − It is the number of events written into the file before it is rolled. Its default value is 10.

Example – Configuration File

Given below is an example of the configuration file. Copy this content and save as twitter.conf in the conf folder of Flume.

# Naming the components on the current agent. 
TwitterAgent.sources = Twitter 
TwitterAgent.channels = MemChannel 
TwitterAgent.sinks = HDFS
  
# Describing/Configuring the source 
TwitterAgent.sources.Twitter.type = org.apache.flume.source.twitter.TwitterSource
TwitterAgent.sources.Twitter.consumerKey = Your OAuth consumer key
TwitterAgent.sources.Twitter.consumerSecret = Your OAuth consumer secret 
TwitterAgent.sources.Twitter.accessToken = Your OAuth consumer key access token 
TwitterAgent.sources.Twitter.accessTokenSecret = Your OAuth consumer key access token secret 
TwitterAgent.sources.Twitter.keywords = tutorials point,java, bigdata, mapreduce, mahout, hbase, nosql
  
# Describing/Configuring the sink 

TwitterAgent.sinks.HDFS.type = hdfs 
TwitterAgent.sinks.HDFS.hdfs.path = hdfs://localhost:9000/user/Hadoop/twitter_data/
TwitterAgent.sinks.HDFS.hdfs.fileType = DataStream 
TwitterAgent.sinks.HDFS.hdfs.writeFormat = Text 
TwitterAgent.sinks.HDFS.hdfs.batchSize = 1000
TwitterAgent.sinks.HDFS.hdfs.rollSize = 0 
TwitterAgent.sinks.HDFS.hdfs.rollCount = 10000 
 
# Describing/Configuring the channel 
TwitterAgent.channels.MemChannel.type = memory 
TwitterAgent.channels.MemChannel.capacity = 10000 
TwitterAgent.channels.MemChannel.transactionCapacity = 100
  
# Binding the source and sink to the channel 
TwitterAgent.sources.Twitter.channels = MemChannel
TwitterAgent.sinks.HDFS.channel = MemChannel 

Execution

Browse through the Flume home directory and execute the application as shown below.

$ cd $FLUME_HOME 
$ bin/flume-ng agent --conf ./conf/ -f conf/twitter.conf 
Dflume.root.logger=DEBUG,console -n TwitterAgent

If everything goes fine, the streaming of tweets into HDFS will start. Given below is the snapshot of the command prompt window while fetching tweets.

Verifying HDFS

You can access the Hadoop Administration Web UI using the URL given below.

http://localhost:50070/ 

Click on the dropdown named Utilities on the right-hand side of the page. You can see two options as shown in the snapshot given below.

Click on Browse the file system and enter the path of the HDFS directory where you have stored the tweets. In our example, the path will be /user/Hadoop/twitter_data/. Then, you can see the list of twitter log files stored in HDFS as given below.

Apache Flume – Sequence Generator Source

In the previous chapter, we have seen how to fetch data from twitter source to HDFS. This chapter explains how to fetch data from Sequence generator.

Prerequisites

To run the example provided in this chapter, you need to install HDFS along with Flume. Therefore, verify Hadoop installation and start the HDFS before proceeding further. (Refer the previous chapter to learn how to start the HDFS).

Configuring Flume

We have to configure the source, the channel, and the sink using the configuration file in the conf folder. The example given in this chapter uses a sequence generator source, a memory channel, and an HDFS sink.

Sequence Generator Source

It is the source that generates the events continuously. It maintains a counter that starts from 0 and increments by 1. It is used for testing purpose. While configuring this source, you must provide values to the following properties −

• Channels

• Source type − seq

Channel

We are using the memory channel. To configure the memory channel, you must provide a value to the type of the channel. Given below are the list of properties that you need to supply while configuring the memory channel −

• type − It holds the type of the channel. In our example the type is MemChannel.

• Capacity − It is the maximum number of events stored in the channel. Its default value is 100. (optional)

• TransactionCapacity − It is the maximum number of events the channel accepts or sends. Its default is 100. (optional).

HDFS Sink

This sink writes data into the HDFS. To configure this sink, you must provide the following details.

• Channel

• type − hdfs

• hdfs.path − the path of the directory in HDFS where data is to be stored.

And we can provide some optional values based on the scenario. Given below are the optional properties of the HDFS sink that we are configuring in our application.

• fileType − This is the required file format of our HDFS file. SequenceFile, DataStream and CompressedStream are the three types available with this stream. In our example, we are using the DataStream.

• writeFormat − Could be either text or writable.

• batchSize − It is the number of events written to a file before it is flushed into the HDFS. Its default value is 100.

• rollsize − It is the file size to trigger a roll. It default value is 100.

• rollCount − It is the number of events written into the file before it is rolled. Its default value is 10.

Example – Configuration File

Given below is an example of the configuration file. Copy this content and save as seq_gen .conf in the conf folder of Flume.

# Naming the components on the current agent 

SeqGenAgent.sources = SeqSource   
SeqGenAgent.channels = MemChannel 
SeqGenAgent.sinks = HDFS 
 
# Describing/Configuring the source 
SeqGenAgent.sources.SeqSource.type = seq
  
# Describing/Configuring the sink
SeqGenAgent.sinks.HDFS.type = hdfs 
SeqGenAgent.sinks.HDFS.hdfs.path = hdfs://localhost:9000/user/Hadoop/seqgen_data/
SeqGenAgent.sinks.HDFS.hdfs.filePrefix = log 
SeqGenAgent.sinks.HDFS.hdfs.rollInterval = 0
SeqGenAgent.sinks.HDFS.hdfs.rollCount = 10000
SeqGenAgent.sinks.HDFS.hdfs.fileType = DataStream 
 
# Describing/Configuring the channel 
SeqGenAgent.channels.MemChannel.type = memory 
SeqGenAgent.channels.MemChannel.capacity = 1000 
SeqGenAgent.channels.MemChannel.transactionCapacity = 100 
 
# Binding the source and sink to the channel 
SeqGenAgent.sources.SeqSource.channels = MemChannel
SeqGenAgent.sinks.HDFS.channel = MemChannel 

Execution

Browse through the Flume home directory and execute the application as shown below.

$ cd $FLUME_HOME 
$./bin/flume-ng agent --conf $FLUME_CONF --conf-file $FLUME_CONF/seq_gen.conf 
   --name SeqGenAgent 

If everything goes fine, the source starts generating sequence numbers which will be pushed into the HDFS in the form of log files.

Given below is a snapshot of the command prompt window fetching the data generated by the sequence generator into the HDFS.

Verifying the HDFS

You can access the Hadoop Administration Web UI using the following URL −

http://localhost:50070/

Click on the dropdown named Utilities on the right-hand side of the page. You can see two options as shown in the diagram given below.

Click on Browse the file system and enter the path of the HDFS directory where you have stored the data generated by the sequence generator.

In our example, the path will be /user/Hadoop/ seqgen_data /. Then, you can see the list of log files generated by the sequence generator, stored in the HDFS as given below.

Verifying the Contents of the File

All these log files contain numbers in sequential format. You can verify the contents of these file in the file system using the cat command as shown below.

Apache Flume – NetCat Source

This chapter takes an example to explain how you can generate events and subsequently log them into the console. For this, we are using the NetCat source and the logger sink.

Prerequisites

To run the example provided in this chapter, you need to install Flume.

Configuring Flume

We have to configure the source, the channel, and the sink using the configuration file in the conf folder. The example given in this chapter uses a NetCat Source, Memory channel, and a logger sink.

NetCat Source

While configuring the NetCat source, we have to specify a port while configuring the source. Now the source (NetCat source) listens to the given port and receives each line we entered in that port as an individual event and transfers it to the sink through the specified channel.

While configuring this source, you have to provide values to the following properties −

• channels

• Source type − netcat

• bind − Host name or IP address to bind.

• port − Port number to which we want the source to listen.

Channel

We are using the memory channel. To configure the memory channel, you must provide a value to the type of the channel. Given below are the list of properties that you need to supply while configuring the memory channel −

• type − It holds the type of the channel. In our example, the type is MemChannel.

• Capacity − It is the maximum number of events stored in the channel. Its default value is 100. (optional)

• TransactionCapacity − It is the maximum number of events the channel accepts or sends. Its default value is 100. (optional).

Logger Sink

This sink logs all the events passed to it. Generally, it is used for testing or debugging purpose. To configure this sink, you must provide the following details.

• Channel

• type − logger

Example Configuration File

Given below is an example of the configuration file. Copy this content and save as netcat.conf in the conf folder of Flume.

# Naming the components on the current agent
NetcatAgent.sources = Netcat   
NetcatAgent.channels = MemChannel 
NetcatAgent.sinks = LoggerSink  

# Describing/Configuring the source 
NetcatAgent.sources.Netcat.type = netcat 
NetcatAgent.sources.Netcat.bind = localhost
NetcatAgent.sources.Netcat.port = 56565  

# Describing/Configuring the sink 
NetcatAgent.sinks.LoggerSink.type = logger  

# Describing/Configuring the channel 
NetcatAgent.channels.MemChannel.type = memory 
NetcatAgent.channels.MemChannel.capacity = 1000 
NetcatAgent.channels.MemChannel.transactionCapacity = 100 
 
# Bind the source and sink to the channel 
NetcatAgent.sources.Netcat.channels = MemChannel
NetcatAgent.sinks. LoggerSink.channel = MemChannel

Execution

Browse through the Flume home directory and execute the application as shown below.

$ cd $FLUME_HOME
$ ./bin/flume-ng agent --conf $FLUME_CONF --conf-file $FLUME_CONF/netcat.conf 
   --name NetcatAgent -Dflume.root.logger=INFO,console

If everything goes fine, the source starts listening to the given port. In this case, it is 56565. Given below is the snapshot of the command prompt window of a NetCat source which has started and listening to the port 56565.

Passing Data to the Source

To pass data to NetCat source, you have to open the port given in the configuration file. Open a separate terminal and connect to the source (56565) using the curl command. When the connection is successful, you will get a message “connected” as shown below.

$ curl telnet://localhost:56565 
connected 

Now you can enter your data line by line (after each line, you have to press Enter). The NetCat source receives each line as an individual event and you will get a received message “OK”.

Whenever you are done with passing data, you can exit the console by pressing (Ctrl&plus;C). Given below is the snapshot of the console where we have connected to the source using the curl command.

Each line that is entered in the above console will be received as an individual event by the source. Since we have used the Logger sink, these events will be logged on to the console (source console) through the specified channel (memory channel in this case).

The following snapshot shows the NetCat console where the events are logged.