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   this work for additional information regarding copyright ownership.
   The ASF licenses this file to You under the Apache License, Version 2.0
   (the "License"); you may not use this file except in compliance with
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======================================
Flume 1.7.0 Developer Guide
======================================

Introduction
============

Overview
--------

Apache Flume is a distributed, reliable, and available system for
efficiently collecting, aggregating and moving large amounts of log
data from many different sources to a centralized data store.

Apache Flume is a top-level project at the Apache Software Foundation.
There are currently two release code lines available, versions 0.9.x and 1.x.
This documentation applies to the 1.x codeline.
For the 0.9.x codeline, please see the `Flume 0.9.x Developer Guide
<http://archive.cloudera.com/cdh/3/flume/DeveloperGuide/>`_.

Architecture
------------

Data flow model
~~~~~~~~~~~~~~~

An ``Event`` is a unit of data that flows through a Flume agent. The ``Event``
flows from ``Source`` to ``Channel`` to ``Sink``, and is represented by an
implementation of the ``Event`` interface. An ``Event`` carries a payload (byte
array) that is accompanied by an optional set of headers (string attributes).
A Flume agent is a process (JVM) that hosts the components that allow
``Event``\ s to flow from an external source to a external destination.

.. figure:: images/DevGuide_image00.png
   :align: center
   :alt: Agent component diagram

A ``Source`` consumes ``Event``\ s having a specific format, and those
``Event``\ s are delivered to the ``Source`` by an external source like a web
server. For example, an ``AvroSource`` can be used to receive Avro ``Event``\ s
from clients or from other Flume agents in the flow. When a ``Source`` receives
an ``Event``, it stores it into one or more ``Channel``\ s.  The ``Channel`` is
a passive store that holds the ``Event`` until that ``Event`` is consumed by a
``Sink``. One type of ``Channel`` available in Flume is the ``FileChannel``
which uses the local filesystem as its backing store. A ``Sink`` is responsible
for removing an ``Event`` from the ``Channel`` and putting it into an external
repository like HDFS (in the case of an ``HDFSEventSink``) or forwarding it to
the ``Source`` at the next hop of the flow. The ``Source`` and ``Sink`` within
the given agent run asynchronously with the ``Event``\ s staged in the
``Channel``.

Reliability
~~~~~~~~~~~

An ``Event`` is staged in a Flume agent's ``Channel``. Then it's the
``Sink``\ 's responsibility to deliver the ``Event`` to the next agent or
terminal repository (like HDFS) in the flow. The ``Sink`` removes an ``Event``
from the ``Channel`` only after the ``Event`` is stored into the ``Channel`` of
the next agent or stored in the terminal repository. This is how the single-hop
message delivery semantics in Flume provide end-to-end reliability of the flow.
Flume uses a transactional approach to guarantee the reliable delivery of the
``Event``\ s. The ``Source``\ s and ``Sink``\ s encapsulate the
storage/retrieval of the ``Event``\ s in a ``Transaction`` provided by the
``Channel``. This ensures that the set of ``Event``\ s are reliably passed from
point to point in the flow. In the case of a multi-hop flow, the ``Sink`` from
the previous hop and the ``Source`` of the next hop both have their
``Transaction``\ s open to ensure that the ``Event`` data is safely stored in
the ``Channel`` of the next hop.

Building Flume
--------------

Getting the source
~~~~~~~~~~~~~~~~~~

Check-out the code using Git. Click here for
`the git repository root <https://git-wip-us.apache.org/repos/asf/flume.git>`_.

The Flume 1.x development happens under the branch "trunk" so this command line
can be used:

  git clone https://git-wip-us.apache.org/repos/asf/flume.git


Compile/test Flume
~~~~~~~~~~~~~~~~~~

The Flume build is mavenized. You can compile Flume using the standard Maven
commands:

#. Compile only: ``mvn clean compile``
#. Compile and run unit tests: ``mvn clean test``
#. Run individual test(s): ``mvn clean test -Dtest=<Test1>,<Test2>,... -DfailIfNoTests=false``
#. Create tarball package: ``mvn clean install``
#. Create tarball package (skip unit tests): ``mvn clean install -DskipTests``

Please note that Flume builds requires that the Google Protocol Buffers compiler
be in the path. You can download and install it by following the instructions
`here <https://developers.google.com/protocol-buffers/>`_.

Developing custom components
----------------------------

Client
~~~~~~

The client operates at the point of origin of events and delivers them to a
Flume agent. Clients typically operate in the process space of the application
they are consuming data from. Flume currently supports Avro, log4j, syslog,
and Http POST (with a JSON body) as ways to transfer data from a external
source. Additionally, there’s an ``ExecSource`` that can consume the output of a
local process as input to Flume.

It’s quite possible to have a use case where these existing options are not
sufficient. In this case you can build a custom mechanism to send data to
Flume. There are two ways of achieving this. The first option is to create a
custom client that communicates with one of Flume’s existing ``Source``\ s like
``AvroSource`` or ``SyslogTcpSource``. Here the client should convert its data
into messages understood by these Flume ``Source``\ s. The other option is to
write a custom Flume ``Source`` that directly talks with your existing client
application using some IPC or RPC protocol, and then converts the client data
into Flume ``Event``\ s to be sent downstream. Note that all events stored
within the ``Channel`` of a Flume agent must exist as Flume ``Event``\ s.


Client SDK
''''''''''

Though Flume contains a number of built-in mechanisms (i.e. ``Source``\ s) to
ingest data, often one wants the ability to communicate with Flume directly from
a custom application. The Flume Client SDK is a library that enables
applications to connect to Flume and send data into Flume’s data flow over RPC.


RPC client interface
''''''''''''''''''''

An implementation of Flume's RpcClient interface encapsulates the RPC mechanism
supported by Flume. The user's application can simply call the Flume Client
SDK's ``append(Event)`` or ``appendBatch(List<Event>)`` to send data and not
worry about the underlying message exchange details. The user can provide the
required ``Event`` arg by either directly implementing the ``Event`` interface,
by using a convenience implementation such as the SimpleEvent class, or by using
``EventBuilder``\ 's overloaded ``withBody()`` static helper methods.


RPC clients - Avro and Thrift
'''''''''''''''''''''''''''''

As of Flume 1.4.0, Avro is the default RPC protocol.  The
``NettyAvroRpcClient`` and ``ThriftRpcClient`` implement the ``RpcClient``
interface. The client needs to create this object with the host and port of
the target Flume agent, and can then use the ``RpcClient`` to send data into
the agent. The following example shows how to use the Flume Client SDK API
within a user's data-generating application:

.. code-block:: java

  import org.apache.flume.Event;
  import org.apache.flume.EventDeliveryException;
  import org.apache.flume.api.RpcClient;
  import org.apache.flume.api.RpcClientFactory;
  import org.apache.flume.event.EventBuilder;
  import java.nio.charset.Charset;

  public class MyApp {
    public static void main(String[] args) {
      MyRpcClientFacade client = new MyRpcClientFacade();
      // Initialize client with the remote Flume agent's host and port
      client.init("host.example.org", 41414);

      // Send 10 events to the remote Flume agent. That agent should be
      // configured to listen with an AvroSource.
      String sampleData = "Hello Flume!";
      for (int i = 0; i < 10; i++) {
        client.sendDataToFlume(sampleData);
      }

      client.cleanUp();
    }
  }

  class MyRpcClientFacade {
    private RpcClient client;
    private String hostname;
    private int port;

    public void init(String hostname, int port) {
      // Setup the RPC connection
      this.hostname = hostname;
      this.port = port;
      this.client = RpcClientFactory.getDefaultInstance(hostname, port);
      // Use the following method to create a thrift client (instead of the above line):
      // this.client = RpcClientFactory.getThriftInstance(hostname, port);
    }

    public void sendDataToFlume(String data) {
      // Create a Flume Event object that encapsulates the sample data
      Event event = EventBuilder.withBody(data, Charset.forName("UTF-8"));

      // Send the event
      try {
        client.append(event);
      } catch (EventDeliveryException e) {
        // clean up and recreate the client
        client.close();
        client = null;
        client = RpcClientFactory.getDefaultInstance(hostname, port);
        // Use the following method to create a thrift client (instead of the above line):
        // this.client = RpcClientFactory.getThriftInstance(hostname, port);
      }
    }

    public void cleanUp() {
      // Close the RPC connection
      client.close();
    }

  }

The remote Flume agent needs to have an ``AvroSource`` (or a
``ThriftSource`` if you are using a Thrift client) listening on some port.
Below is an example Flume agent configuration that's waiting for a connection
from MyApp:

.. code-block:: properties

  a1.channels = c1
  a1.sources = r1
  a1.sinks = k1

  a1.channels.c1.type = memory

  a1.sources.r1.channels = c1
  a1.sources.r1.type = avro
  # For using a thrift source set the following instead of the above line.
  # a1.source.r1.type = thrift
  a1.sources.r1.bind = 0.0.0.0
  a1.sources.r1.port = 41414

  a1.sinks.k1.channel = c1
  a1.sinks.k1.type = logger

For more flexibility, the default Flume client implementations
(``NettyAvroRpcClient`` and ``ThriftRpcClient``) can be configured with these
properties:

.. code-block:: properties

  client.type = default (for avro) or thrift (for thrift)

  hosts = h1                           # default client accepts only 1 host
                                       # (additional hosts will be ignored)

  hosts.h1 = host1.example.org:41414   # host and port must both be specified
                                       # (neither has a default)

  batch-size = 100                     # Must be >=1 (default: 100)

  connect-timeout = 20000              # Must be >=1000 (default: 20000)

  request-timeout = 20000              # Must be >=1000 (default: 20000)

Secure RPC client - Thrift
''''''''''''''''''''''''''

As of Flume 1.6.0, Thrift source and sink supports kerberos based authentication.
The client needs to use the getThriftInstance method of ``SecureRpcClientFactory``
to get hold of a ``SecureThriftRpcClient``. ``SecureThriftRpcClient`` extends
``ThriftRpcClient`` which implements the ``RpcClient`` interface. The kerberos
authentication module resides in flume-ng-auth module which is
required in classpath, when using the ``SecureRpcClientFactory``. Both the client
principal and the client keytab should be passed in as parameters through the
properties and they reflect the credentials of the client to authenticate
against the kerberos KDC. In addition, the server principal of the destination
Thrift source to which this client is connecting to, should also be provided.
The following example shows how to use the ``SecureRpcClientFactory``
within a user's data-generating application:

.. code-block:: java

  import org.apache.flume.Event;
  import org.apache.flume.EventDeliveryException;
  import org.apache.flume.event.EventBuilder;
  import org.apache.flume.api.SecureRpcClientFactory;
  import org.apache.flume.api.RpcClientConfigurationConstants;
  import org.apache.flume.api.RpcClient;
  import java.nio.charset.Charset;
  import java.util.Properties;

  public class MyApp {
    public static void main(String[] args) {
      MySecureRpcClientFacade client = new MySecureRpcClientFacade();
      // Initialize client with the remote Flume agent's host, port
      Properties props = new Properties();
      props.setProperty(RpcClientConfigurationConstants.CONFIG_CLIENT_TYPE, "thrift");
      props.setProperty("hosts", "h1");
      props.setProperty("hosts.h1", "client.example.org"+":"+ String.valueOf(41414));

      // Initialize client with the kerberos authentication related properties
      props.setProperty("kerberos", "true");
      props.setProperty("client-principal", "flumeclient/client.example.org@EXAMPLE.ORG");
      props.setProperty("client-keytab", "/tmp/flumeclient.keytab");
      props.setProperty("server-principal", "flume/server.example.org@EXAMPLE.ORG");
      client.init(props);

      // Send 10 events to the remote Flume agent. That agent should be
      // configured to listen with an AvroSource.
      String sampleData = "Hello Flume!";
      for (int i = 0; i < 10; i++) {
        client.sendDataToFlume(sampleData);
      }

      client.cleanUp();
    }
  }

  class MySecureRpcClientFacade {
    private RpcClient client;
    private Properties properties;

    public void init(Properties properties) {
      // Setup the RPC connection
      this.properties = properties;
      // Create the ThriftSecureRpcClient instance by using SecureRpcClientFactory
      this.client = SecureRpcClientFactory.getThriftInstance(properties);
    }

    public void sendDataToFlume(String data) {
      // Create a Flume Event object that encapsulates the sample data
      Event event = EventBuilder.withBody(data, Charset.forName("UTF-8"));

      // Send the event
      try {
        client.append(event);
      } catch (EventDeliveryException e) {
        // clean up and recreate the client
        client.close();
        client = null;
        client = SecureRpcClientFactory.getThriftInstance(properties);
      }
    }

    public void cleanUp() {
      // Close the RPC connection
      client.close();
    }
  }

The remote ``ThriftSource`` should be started in kerberos mode.
Below is an example Flume agent configuration that's waiting for a connection
from MyApp:

.. code-block:: properties

  a1.channels = c1
  a1.sources = r1
  a1.sinks = k1

  a1.channels.c1.type = memory

  a1.sources.r1.channels = c1
  a1.sources.r1.type = thrift
  a1.sources.r1.bind = 0.0.0.0
  a1.sources.r1.port = 41414
  a1.sources.r1.kerberos = true
  a1.sources.r1.agent-principal = flume/server.example.org@EXAMPLE.ORG
  a1.sources.r1.agent-keytab = /tmp/flume.keytab


  a1.sinks.k1.channel = c1
  a1.sinks.k1.type = logger

Failover Client
'''''''''''''''

This class wraps the default Avro RPC client to provide failover handling
capability to clients. This takes a whitespace-separated list of <host>:<port>
representing the Flume agents that make-up a failover group. The Failover RPC
Client currently does not support thrift. If there’s a
communication error with the currently selected host (i.e. agent) agent,
then the failover client automatically fails-over to the next host in the list.
For example:

.. code-block:: java

  // Setup properties for the failover
  Properties props = new Properties();
  props.put("client.type", "default_failover");

  // List of hosts (space-separated list of user-chosen host aliases)
  props.put("hosts", "h1 h2 h3");

  // host/port pair for each host alias
  String host1 = "host1.example.org:41414";
  String host2 = "host2.example.org:41414";
  String host3 = "host3.example.org:41414";
  props.put("hosts.h1", host1);
  props.put("hosts.h2", host2);
  props.put("hosts.h3", host3);

  // create the client with failover properties
  RpcClient client = RpcClientFactory.getInstance(props);

For more flexibility, the failover Flume client implementation
(``FailoverRpcClient``) can be configured with these properties:

.. code-block:: properties

  client.type = default_failover

  hosts = h1 h2 h3                     # at least one is required, but 2 or
                                       # more makes better sense

  hosts.h1 = host1.example.org:41414

  hosts.h2 = host2.example.org:41414

  hosts.h3 = host3.example.org:41414

  max-attempts = 3                     # Must be >=0 (default: number of hosts
                                       # specified, 3 in this case). A '0'
                                       # value doesn't make much sense because
                                       # it will just cause an append call to
                                       # immmediately fail. A '1' value means
                                       # that the failover client will try only
                                       # once to send the Event, and if it
                                       # fails then there will be no failover
                                       # to a second client, so this value
                                       # causes the failover client to
                                       # degenerate into just a default client.
                                       # It makes sense to set this value to at
                                       # least the number of hosts that you
                                       # specified.

  batch-size = 100                     # Must be >=1 (default: 100)

  connect-timeout = 20000              # Must be >=1000 (default: 20000)

  request-timeout = 20000              # Must be >=1000 (default: 20000)

LoadBalancing RPC client
''''''''''''''''''''''''

The Flume Client SDK also supports an RpcClient which load-balances among
multiple hosts. This type of client takes a whitespace-separated list of
<host>:<port> representing the Flume agents that make-up a load-balancing group.
This client can be configured with a load balancing strategy that either
randomly selects one of the configured hosts, or selects a host in a round-robin
fashion. You can also specify your own custom class that implements the
``LoadBalancingRpcClient$HostSelector`` interface so that a custom selection
order is used. In that case, the FQCN of the custom class needs to be specified
as the value of the ``host-selector`` property. The LoadBalancing RPC Client
currently does not support thrift.

If ``backoff`` is enabled then the client will temporarily blacklist
hosts that fail, causing them to be excluded from being selected as a failover
host until a given timeout. When the timeout elapses, if the host is still
unresponsive then this is considered a sequential failure, and the timeout is
increased exponentially to avoid potentially getting stuck in long waits on
unresponsive hosts.

The maximum backoff time can be configured by setting ``maxBackoff`` (in
milliseconds). The maxBackoff default is 30 seconds (specified in the
``OrderSelector`` class that's the superclass of both load balancing
strategies). The backoff timeout will increase exponentially with each
sequential failure up to the maximum possible backoff timeout.
The maximum possible backoff is limited to 65536 seconds (about 18.2 hours).
For example:

.. code-block:: java

  // Setup properties for the load balancing
  Properties props = new Properties();
  props.put("client.type", "default_loadbalance");

  // List of hosts (space-separated list of user-chosen host aliases)
  props.put("hosts", "h1 h2 h3");

  // host/port pair for each host alias
  String host1 = "host1.example.org:41414";
  String host2 = "host2.example.org:41414";
  String host3 = "host3.example.org:41414";
  props.put("hosts.h1", host1);
  props.put("hosts.h2", host2);
  props.put("hosts.h3", host3);

  props.put("host-selector", "random"); // For random host selection
  // props.put("host-selector", "round_robin"); // For round-robin host
  //                                            // selection
  props.put("backoff", "true"); // Disabled by default.

  props.put("maxBackoff", "10000"); // Defaults 0, which effectively
                                    // becomes 30000 ms

  // Create the client with load balancing properties
  RpcClient client = RpcClientFactory.getInstance(props);

For more flexibility, the load-balancing Flume client implementation
(``LoadBalancingRpcClient``) can be configured with these properties:

.. code-block:: properties

  client.type = default_loadbalance

  hosts = h1 h2 h3                     # At least 2 hosts are required

  hosts.h1 = host1.example.org:41414

  hosts.h2 = host2.example.org:41414

  hosts.h3 = host3.example.org:41414

  backoff = false                      # Specifies whether the client should
                                       # back-off from (i.e. temporarily
                                       # blacklist) a failed host
                                       # (default: false).

  maxBackoff = 0                       # Max timeout in millis that a will
                                       # remain inactive due to a previous
                                       # failure with that host (default: 0,
                                       # which effectively becomes 30000)

  host-selector = round_robin          # The host selection strategy used
                                       # when load-balancing among hosts
                                       # (default: round_robin).
                                       # Other values are include "random"
                                       # or the FQCN of a custom class
                                       # that implements
                                       # LoadBalancingRpcClient$HostSelector

  batch-size = 100                     # Must be >=1 (default: 100)

  connect-timeout = 20000              # Must be >=1000 (default: 20000)

  request-timeout = 20000              # Must be >=1000 (default: 20000)

Embedded agent
~~~~~~~~~~~~~~

Flume has an embedded agent api which allows users to embed an agent in their
application. This agent is meant to be lightweight and as such not all
sources, sinks, and channels are allowed. Specifically the source used
is a special embedded source and events should be send to the source
via the put, putAll methods on the EmbeddedAgent object. Only File Channel
and Memory Channel are allowed as channels while Avro Sink is the only
supported sink. Interceptors are also supported by the embedded agent.

Note: The embedded agent has a dependency on hadoop-core.jar.

Configuration of an Embedded Agent is similar to configuration of a
full Agent. The following is an exhaustive list of configration options:

Required properties are in **bold**.

=====================  ================  ======================================================================
Property Name          Default           Description
=====================  ================  ======================================================================
source.type            embedded          The only available source is the embedded source.
**channel.type**       --                Either ``memory`` or ``file`` which correspond 
		         		 to MemoryChannel and FileChannel respectively.
channel.*              --                Configuration options for the channel type requested,
					 see MemoryChannel or FileChannel user guide for an exhaustive list.
**sinks**              --                List of sink names
**sink.type**          --                Property name must match a name in the list of sinks. 
					 Value must be ``avro``
sink.*                 --                Configuration options for the sink. 
					 See AvroSink user guide for an exhaustive list,
					 however note AvroSink requires at least hostname and port.
**processor.type**     --                Either ``failover`` or ``load_balance`` which correspond
		            		 to FailoverSinksProcessor and LoadBalancingSinkProcessor respectively.
processor.*            --                Configuration options for the sink processor selected.
					 See FailoverSinksProcessor and LoadBalancingSinkProcessor 
					 user guide for an exhaustive list.
source.interceptors    --                Space-separated list of interceptors
source.interceptors.*  --                Configuration options for individual interceptors 
					 specified in the source.interceptors property
=====================  ================  ======================================================================

Below is an example of how to use the agent:

.. code-block:: java

    Map<String, String> properties = new HashMap<String, String>();
    properties.put("channel.type", "memory");
    properties.put("channel.capacity", "200");
    properties.put("sinks", "sink1 sink2");
    properties.put("sink1.type", "avro");
    properties.put("sink2.type", "avro");
    properties.put("sink1.hostname", "collector1.apache.org");
    properties.put("sink1.port", "5564");
    properties.put("sink2.hostname", "collector2.apache.org");
    properties.put("sink2.port",  "5565");
    properties.put("processor.type", "load_balance");
    properties.put("source.interceptors", "i1");
    properties.put("source.interceptors.i1.type", "static");
    properties.put("source.interceptors.i1.key", "key1");
    properties.put("source.interceptors.i1.value", "value1");

    EmbeddedAgent agent = new EmbeddedAgent("myagent");

    agent.configure(properties);
    agent.start();

    List<Event> events = Lists.newArrayList();

    events.add(event);
    events.add(event);
    events.add(event);
    events.add(event);

    agent.putAll(events);

    ...

    agent.stop();


Transaction interface
~~~~~~~~~~~~~~~~~~~~~

The ``Transaction`` interface is the basis of reliability for Flume. All the
major components (ie. ``Source``\ s, ``Sink``\ s and ``Channel``\ s) must use a
Flume ``Transaction``.

.. figure:: images/DevGuide_image01.png
   :align: center
   :alt: Transaction sequence diagram

A ``Transaction`` is implemented within a ``Channel`` implementation. Each
``Source`` and ``Sink`` that is connected to a ``Channel`` must obtain a
``Transaction`` object. The ``Source``\ s use a ``ChannelProcessor``
to manage the ``Transaction``\ s, the ``Sink``\ s manage them explicitly via
their configured ``Channel``. The operation to stage an
``Event`` (put it into a ``Channel``) or extract an ``Event`` (take it out of a
``Channel``) is done inside an active ``Transaction``. For example:

.. code-block:: java

  Channel ch = new MemoryChannel();
  Transaction txn = ch.getTransaction();
  txn.begin();
  try {
    // This try clause includes whatever Channel operations you want to do

    Event eventToStage = EventBuilder.withBody("Hello Flume!",
                         Charset.forName("UTF-8"));
    ch.put(eventToStage);
    // Event takenEvent = ch.take();
    // ...
    txn.commit();
  } catch (Throwable t) {
    txn.rollback();

    // Log exception, handle individual exceptions as needed

    // re-throw all Errors
    if (t instanceof Error) {
      throw (Error)t;
    }
  } finally {
    txn.close();
  }

Here we get hold of a ``Transaction`` from a ``Channel``. After ``begin()``
returns, the ``Transaction`` is now active/open and the ``Event`` is then put
into the ``Channel``. If the put is successful, then the ``Transaction`` is
committed and closed.

Sink
~~~~

The purpose of a ``Sink`` to extract ``Event``\ s from the ``Channel`` and
forward them to the next Flume Agent in the flow or store them in an external
repository. A ``Sink`` is associated with exactly one ``Channel``\ s, as
configured in the Flume properties file. There’s one ``SinkRunner`` instance
associated with every configured ``Sink``, and when the Flume framework calls
``SinkRunner.start()``, a new thread is created to drive the ``Sink`` (using
``SinkRunner.PollingRunner`` as the thread's ``Runnable``). This thread manages
the ``Sink``\ ’s lifecycle. The ``Sink`` needs to implement the ``start()`` and
``stop()`` methods that are part of the ``LifecycleAware`` interface. The
``Sink.start()`` method should initialize the ``Sink`` and bring it to a state
where it can forward the ``Event``\ s to its next destination.  The
``Sink.process()`` method should do the core processing of extracting the
``Event`` from the ``Channel`` and forwarding it. The ``Sink.stop()`` method
should do the necessary cleanup (e.g. releasing resources). The ``Sink``
implementation also needs to implement the ``Configurable`` interface for
processing its own configuration settings. For example:

.. code-block:: java

  public class MySink extends AbstractSink implements Configurable {
    private String myProp;

    @Override
    public void configure(Context context) {
      String myProp = context.getString("myProp", "defaultValue");

      // Process the myProp value (e.g. validation)

      // Store myProp for later retrieval by process() method
      this.myProp = myProp;
    }

    @Override
    public void start() {
      // Initialize the connection to the external repository (e.g. HDFS) that
      // this Sink will forward Events to ..
    }

    @Override
    public void stop () {
      // Disconnect from the external respository and do any
      // additional cleanup (e.g. releasing resources or nulling-out
      // field values) ..
    }

    @Override
    public Status process() throws EventDeliveryException {
      Status status = null;

      // Start transaction
      Channel ch = getChannel();
      Transaction txn = ch.getTransaction();
      txn.begin();
      try {
        // This try clause includes whatever Channel operations you want to do

        Event event = ch.take();

        // Send the Event to the external repository.
        // storeSomeData(e);

        txn.commit();
        status = Status.READY;
      } catch (Throwable t) {
        txn.rollback();

        // Log exception, handle individual exceptions as needed

        status = Status.BACKOFF;

        // re-throw all Errors
        if (t instanceof Error) {
          throw (Error)t;
        }
      }
      return status;
    }
  }

Source
~~~~~~

The purpose of a ``Source`` is to receive data from an external client and store
it into the configured ``Channel``\ s. A ``Source`` can get an instance of its own
``ChannelProcessor`` to process an ``Event``, commited within a ``Channel``
local transaction, in serial. In the case of an exception, required
``Channel``\ s will propagate the exception, all ``Channel``\ s will rollback their
transaction, but events processed previously on other ``Channel``\ s will remain
committed.

Similar to the ``SinkRunner.PollingRunner`` ``Runnable``, there’s
a ``PollingRunner`` ``Runnable`` that executes on a thread created when the
Flume framework calls ``PollableSourceRunner.start()``. Each configured
``PollableSource`` is associated with its own thread that runs a
``PollingRunner``. This thread manages the ``PollableSource``\ ’s lifecycle,
such as starting and stopping. A ``PollableSource`` implementation must
implement the ``start()`` and ``stop()`` methods that are declared in the
``LifecycleAware`` interface. The runner of a ``PollableSource`` invokes that
``Source``\ 's ``process()`` method. The ``process()`` method should check for
new data and store it into the ``Channel`` as Flume ``Event``\ s.

Note that there are actually two types of ``Source``\ s. The ``PollableSource``
was already mentioned. The other is the ``EventDrivenSource``.  The
``EventDrivenSource``, unlike the ``PollableSource``, must have its own callback
mechanism that captures the new data and stores it into the ``Channel``. The
``EventDrivenSource``\ s are not each driven by their own thread like the
``PollableSource``\ s are. Below is an example of a custom ``PollableSource``:

.. code-block:: java

  public class MySource extends AbstractSource implements Configurable, PollableSource {
    private String myProp;

    @Override
    public void configure(Context context) {
      String myProp = context.getString("myProp", "defaultValue");

      // Process the myProp value (e.g. validation, convert to another type, ...)

      // Store myProp for later retrieval by process() method
      this.myProp = myProp;
    }

    @Override
    public void start() {
      // Initialize the connection to the external client
    }

    @Override
    public void stop () {
      // Disconnect from external client and do any additional cleanup
      // (e.g. releasing resources or nulling-out field values) ..
    }

    @Override
    public Status process() throws EventDeliveryException {
      Status status = null;

      try {
        // This try clause includes whatever Channel/Event operations you want to do

        // Receive new data
        Event e = getSomeData();

        // Store the Event into this Source's associated Channel(s)
        getChannelProcessor().processEvent(e);

        status = Status.READY;
      } catch (Throwable t) {
        // Log exception, handle individual exceptions as needed

        status = Status.BACKOFF;

        // re-throw all Errors
        if (t instanceof Error) {
          throw (Error)t;
        }
      } finally {
        txn.close();
      }
      return status;
    }
  }

Channel
~~~~~~~

TBD
