Added information on backpressure

Author: tdas

docs/configuration.md

@@ -1444,6 +1444,19 @@ Apart from these, the following properties are also available, and may be useful
 #### Spark Streaming
 <table class="table">
 <tr><th>Property Name</th><th>Default</th><th>Meaning</th></tr>
+<tr>
+  <td><code>spark.streaming.backpressure.enabled</code></td>
+  <td>false</td>
+  <td>
+    Enables or disables Spark Streaming's internal backpressure mechanism (since 1.5).
+    This enables the Spark Streaming to control the receiving rate based on the 
+    current batch scheduling delays and processing times so that the system receives
+    only as fast as the system can process. Internally, this dynamically sets the 
+    maximum receiving rate of receivers. This rate is upper bounded by the values
+    `spark.streaming.receiver.maxRate` and `spark.streaming.kafka.maxRatePerPartition`
+    if they are set (see below).
+  </td>
+</tr>
 <tr>
   <td><code>spark.streaming.blockInterval</code></td>
   <td>200ms</td>

docs/streaming-programming-guide.md

@@ -1813,7 +1813,7 @@ To run a Spark Streaming applications, you need to have the following.
     + *Mesos* - [Marathon](https://github.com/mesosphere/marathon) has been used to achieve this
       with Mesos.
 
-- *[Since Spark 1.2] Configuring write ahead logs* - Since Spark 1.2,
+- *Configuring write ahead logs* - Since Spark 1.2,
   we have introduced _write ahead logs_ for achieving strong
   fault-tolerance guarantees. If enabled,  all the data received from a receiver gets written into
   a write ahead log in the configuration checkpoint directory. This prevents data loss on driver
@@ -1828,6 +1828,17 @@ To run a Spark Streaming applications, you need to have the following.
   stored in a replicated storage system. This can be done by setting the storage level for the
   input stream to `StorageLevel.MEMORY_AND_DISK_SER`.
 
+- *Setting the max receiving rate* - If the cluster resources is not large enough for the streaming
+  application to process data as fast as it is being received, the receivers can be rate limited
+  by setting a maximum rate limit in terms of records / sec.
+  See the [configuration parameters](configuration.html#spark-streaming)
+  `spark.streaming.receiver.maxRate` for receivers and `spark.streaming.kafka.maxRatePerPartition`
+  for Direct Kafka approach. In Spark 1.5, we have introduced a feature called *backpressure* that
+  eliminate the need to set this rate limit, as Spark Streaming automatically figures out the
+  rate limits and dynamically adjusts them if the processing conditions change. This backpressure
+  can be enabled by setting the [configuration parameter](configuration.html#spark-streaming)
+  `spark.streaming.backpressure.enabled` to `true`.
+
 ### Upgrading Application Code
 {:.no_toc}