Hi,
As part of the discussion about schema, Romain mentioned hint. I think it's
worth to have an explanation about that and especially it could be wider than
schema.
Today, to give information to the runner, we use PipelineOptions. The runner can
use these options, and apply for all inner representation of the PCollection in
the runner.
For instance, for the Spark runner, the persistence storage level (memory, disk,
...) can be defined via pipeline options.
Then, the Spark runner automatically defines if RDDs have to be persisted (using
the storage level defined in the pipeline options), for instance if the same
POutput/PCollection is read several time.
However, the user doesn't have any way to provide indication to the runner to
deal with a specific PCollection.
Imagine, the user has a pipeline like this: pipeline.apply().apply().apply(). We
have three PCollections involved in this pipeline. It's not currently possible
to give indications how the runner should "optimized" and deal with the second
PCollection only.
The idea is to add a method on the PCollection:
PCollection.addHint(String key, Object value);
For instance:
collection.addHint("spark.persist", StorageLevel.MEMORY_ONLY);
I see three direct usage of this:
1. Related to schema: the schema definition could be a hint
2. Related to the IO: add headers for the IO and the runner how to specifically
process a collection. In Apache Camel, we have headers on the message and
properties on the exchange similar to this. It allows to give some indication
how to process some messages on the Camel component. We can imagine the same of
the IO (using the PCollection hints to react accordingly).
3. Related to runner optimization: I see for instance a way to use RDD or
dataframe in Spark runner, or even specific optimization like persist. I had lot
of questions from Spark users saying: "in my Spark job, I know where and how I
should use persist (rdd.persist()), but I can't do such optimization using
Beam". So it could be a good improvements.
Thoughts ?
Regards
JB
--
Jean-Baptiste Onofré
jbono...@apache.org
http://blog.nanthrax.net
Talend - http://www.talend.com
