Added more information about writing reliable receivers in the custom…

… receiver guide.

docs/configuration.md

@@ -8,7 +8,7 @@ title: Spark Configuration
 Spark provides three locations to configure the system:
 
 * [Spark properties](#spark-properties) control most application parameters and can be set by using
-  a [SparkConf](api/core/index.html#org.apache.spark.SparkConf) object, or through Java
+  a [SparkConf](api/scala/index.html#org.apache.spark.SparkConf) object, or through Java
   system properties.
 * [Environment variables](#environment-variables) can be used to set per-machine settings, such as
   the IP address, through the `conf/spark-env.sh` script on each node.

docs/streaming-custom-receivers.md

@@ -7,25 +7,29 @@ Spark Streaming can receive streaming data from any arbitrary data source beyond
 the one's for which it has in-built support (that is, beyond Flume, Kafka, Kinesis, files, sockets, etc.).
 This requires the developer to implement a *receiver* that is customized for receiving data from
 the concerned data source. This guide walks through the process of implementing a custom receiver
-and using it in a Spark Streaming application.
+and using it in a Spark Streaming application. Note that custom receivers can be implemented
+in Scala or Java.
 
-### Implementing a Custom Receiver
+## Implementing a Custom Receiver
 
-This starts with implementing a [Receiver](api/scala/index.html#org.apache.spark.streaming.receiver.Receiver).
+This starts with implementing a **Receiver**
+([Scala doc](api/scala/index.html#org.apache.spark.streaming.receiver.Receiver),
+[Java doc](api/java/org/apache/spark/streaming/receiver/Receiver.html)).
 A custom receiver must extend this abstract class by implementing two methods
 - `onStart()`: Things to do to start receiving data.
 - `onStop()`: Things to do to stop receiving data.
 
-Note that `onStart()` and `onStop()` must not block indefinitely. Typically, onStart() would start the threads
+Both `onStart()` and `onStop()` must not block indefinitely. Typically, `onStart()` would start the threads
 that responsible for receiving the data and `onStop()` would ensure that the receiving by those threads
 are stopped. The receiving threads can also use `isStopped()`, a `Receiver` method, to check whether they
 should stop receiving data.
 
 Once the data is received, that data can be stored inside Spark
-by calling `store(data)`, which is a method provided by the
-[Receiver](api/scala/index.html#org.apache.spark.streaming.receiver.Receiver) class.
+by calling `store(data)`, which is a method provided by the Receiver class.
 There are number of flavours of `store()` which allow you store the received data
-record-at-a-time or as whole collection of objects / serialized bytes.
+record-at-a-time or as whole collection of objects / serialized bytes. Note that the flavour of
+`store()` used to implemented a receiver affects its reliability and fault-tolerance semantics.
+This is discussed [later](#receiver-reliability) in more detail.
 
 Any exception in the receiving threads should be caught and handled properly to avoid silent
 failures of the receiver. `restart(<exception>)` will restart the receiver by
@@ -158,7 +162,7 @@ public class JavaCustomReceiver extends Receiver<String> {
 </div>
 
 
-### Using the custom receiver in a Spark Streaming application
+## Using the custom receiver in a Spark Streaming application
 
 The custom receiver can be used in a Spark Streaming application by using
 `streamingContext.receiverStream(<instance of custom receiver>)`. This will create
@@ -191,9 +195,68 @@ The full source code is in the example [JavaCustomReceiver.java](https://github.
 </div>
 </div>
 
-
-
-### Implementing and Using a Custom Actor-based Receiver
+## Receiver Reliability
+As discussed in brief in the
+[Spark Streaming Programming Guide](streaming-programming-guide.html#receiver-reliability),
+there are two kinds of receivers based on their reliability and fault-tolerance semantics.
+
+1. *Reliable Receiver* - For *reliable sources* that allow sent data to be acknowledged, a
+  *reliable receiver* correctly acknowledges to the source that the data has been received
+  and stored in Spark reliably (that is, replicated successfully). Usually,
+  implementing this receiver involves careful consideration of the semantics of source
+  acknowledgements.
+1. *Unreliable Receiver* - These are receivers for unreliable sources that do not support
+  acknowledging. Even for reliable sources, one may implement an unreliable receiver that
+  do not go into the complexity of acknowledging correctly.
+
+To implement a *reliable receiver*, you have to use `store(multiple-records)` to store data.
+This flavour of `store` is a blocking call which returns only after all the given records have
+been stored inside Spark. If replication is enabled receiver's configured storage level
+(enabled by default), then this call returns after replication has completed.
+Thus it ensures that the data is reliably stored, and the receiver can now acknowledge the
+source appropriately. This ensures that no data is caused when the receiver fails in the middle
+of replicating data -- the buffered data will not be acknowledged and hence will be later resent
+by the source.
+
+An *unreliable receiver* does not have to implement any of this logic. It can simply receive
+records from the source and insert them one-at-a-time using `store(single-record)`. While it does
+not get the reliability guarantees of `store(multiple-records)`, it has the following advantages.
+
+- The system takes care of chunking that data into appropriate sized blocks (look for block
+interval in the [Spark Streaming Programming Guide](streaming-programming-guide.html)).
+- The system takes care of controlling the receiving rates if the rate limits have been specified.
+- Because of these two, *unreliable receivers are simpler to implement than reliable receivers.
+
+The following table summarizes the characteristics of both types of receivers
+
+<table class="table">
+<tr>
+  <th>Receiver Type</th>
+  <th>Characteristics</th>
+</tr>
+<tr>
+  <td><b>Unreliable Receivers</b></td>
+  <td>
+    Simple to implement.<br>
+    System takes care of block generation and rate control.
+    No fault-tolerance guarantees, can loose data on receiver failure.
+  </td>
+</tr>
+<tr>
+  <td><b>Reliable Receivers</b></td>
+  <td>
+    Strong fault-tolerance guarantees, can ensure zero data loss.<br/>
+    Block generation and rate control to be handled by the receiver implementation.<br/>
+    Implementation complexity depends on the acknowledgement mechanisms of the source.
+  </td>
+</tr>
+<tr>
+  <td></td>
+  <td></td>
+</tr>
+</table>
+
+## Implementing and Using a Custom Actor-based Receiver
 
 Custom [Akka Actors](http://doc.akka.io/docs/akka/2.2.4/scala/actors.html) can also be used to
 receive data. The [`ActorHelper`](api/scala/index.html#org.apache.spark.streaming.receiver.ActorHelper)
@@ -217,5 +280,3 @@ val lines = ssc.actorStream[String](Props(new CustomActor()), "CustomReceiver")
 
 See [ActorWordCount.scala](https://github.com/apache/spark/blob/master/examples/src/main/scala/org/apache/spark/examples/streaming/ActorWordCount.scala)
 for an end-to-end example.
-
-