Really good list to brush up basics.
Just one input, regarding
* An RDD's processing is scheduled by driver's jobscheduler as a job. At a
given point of time only one job is active. So, if one job is executing the
other jobs are queued.
We can have multiple jobs running in a given application at a point of time, if
they are submitted from different threads. So essentially in a single threaded
application, the above statement holds true.
Regards,
Sam
On May 24, 2015, at 1:14 PM, Tathagata Das
t...@databricks.commailto:t...@databricks.com wrote:
Blocks are replicated immediately, before the driver launches any jobs using
them.
On Thu, May 21, 2015 at 2:05 AM, Hemant Bhanawat
hemant9...@gmail.commailto:hemant9...@gmail.com wrote:
Honestly, given the length of my email, I didn't expect a reply. :-) Thanks for
reading and replying. However, I have a follow-up question:
I don't think if I understand the block replication completely. Are the blocks
replicated immediately after they are received by the receiver? Or are they
kept on the receiver node only and are moved only on shuffle? Has the
replication something to do with locality.wait?
Thanks,
Hemant
On Thu, May 21, 2015 at 2:21 AM, Tathagata Das
t...@databricks.commailto:t...@databricks.com wrote:
Correcting the ones that are incorrect or incomplete. BUT this is good list for
things to remember about Spark Streaming.
On Wed, May 20, 2015 at 3:40 AM, Hemant Bhanawat
hemant9...@gmail.commailto:hemant9...@gmail.com wrote:
Hi,
I have compiled a list (from online sources) of knobs/design considerations
that need to be taken care of by applications running on spark streaming. Is my
understanding correct? Any other important design consideration that I should
take care of?
* A DStream is associated with a single receiver. For attaining read
parallelism multiple receivers i.e. multiple DStreams need to be created.
* A receiver is run within an executor. It occupies one core. Ensure that
there are enough cores for processing after receiver slots are booked i.e.
spark.cores.max should take the receiver slots into account.
* The receivers are allocated to executors in a round robin fashion.
* When data is received from a stream source, receiver creates blocks of
data. A new block of data is generated every blockInterval milliseconds. N
blocks of data are created during the batchInterval where N =
batchInterval/blockInterval.
* These blocks are distributed by the BlockManager of the current executor
to the block managers of other executors. After that, the Network Input Tracker
running on the driver is informed about the block locations for further
processing.
* A RDD is created on the driver for the blocks created during the
batchInterval. The blocks generated during the batchInterval are partitions of
the RDD. Each partition is a task in spark. blockInterval== batchinterval would
mean that a single partition is created and probably it is processed locally.
The map tasks on the blocks are processed in the executors (one that received
the block, and another where the block was replicated) that has the blocks
irrespective of block interval, unless non-local scheduling kicks in (as you
observed next).
* Having bigger blockinterval means bigger blocks. A high value of
spark.locality.wait increases the chance of processing a block on the local
node. A balance needs to be found out between these two parameters to ensure
that the bigger blocks are processed locally.
* Instead of relying on batchInterval and blockInterval, you can define the
number of partitions by calling dstream.repartition(n). This reshuffles the
data in RDD randomly to create n number of partitions.
Yes, for greater parallelism. Though comes at the cost of a shuffle.
* An RDD's processing is scheduled by driver's jobscheduler as a job. At a
given point of time only one job is active. So, if one job is executing the
other jobs are queued.
* If you have two dstreams there will be two RDDs formed and there will be
two jobs created which will be scheduled one after the another.
* To avoid this, you can union two dstreams. This will ensure that a single
unionRDD is formed for the two RDDs of the dstreams. This unionRDD is then
considered as a single job. However the partitioning of the RDDs is not
impacted.
To further clarify, the jobs depend on the number of output operations (print,
foreachRDD, saveAsXFiles) and the number of RDD actions in those output
operations.
dstream1.union(dstream2).foreachRDD { rdd = rdd.count() }// one Spark job
per batch
dstream1.union(dstream2).foreachRDD { rdd = { rdd.count() ; rdd.count() } }
// TWO Spark jobs per batch
dstream1.foreachRDD { rdd = rdd.count } ; dstream2.foreachRDD { rdd =
rdd.count } // TWO Spark jobs per batch
*
* If the batch processing time is more than batchinterval then obviously
the receiver's memory will start filling
