The missing piece for adding update and delete to the streaming API is a
primary key. Updates and deletes in SQL work by scanning the table or
partition where the record resides. This is assumed to be ok since we
are not supporting transactional workloads and thus update/deletes are
assumed to be infrequent. But a need to scan for each update or delete
will not perform adequately in the streaming case.
I've had a few discussions with others recently who are thinking of
adding merge like functionality, where you would upload all changes to a
temp table and then in one scan/transaction apply those changes. This
is a common way to handle these situations for data warehouses, and is
much easier than adding a primary key concept to Hive.
Alan.
Elliot West <mailto:tea...@gmail.com>
March 26, 2015 at 14:08
Hi,
I'd like to ascertain if it might be possible to add 'update' and
'delete' operations to the hive-hcatalog-streaming API. I've been
looking at the API with interest for the last week as it appears to
have the potential to help with some general data processing patterns
that are prevalent where I work. Ultimately, we continuously load
large amounts of data into Hadoop which is partitioned by some time
interval - usually hour, day, or month depending on the data size.
However, the records that reside in this data can change. We often
receive some new information that mutates part of an existing record
already stored in a partition in HDFS. Typically the amount of
mutations is very small compared to the number of records in each
partitions.
To handle this currently we re-read and re-write all partitions that
could potentially be affected by new data. In practice a single hour's
worth of new data can require the reading and writing of 1 month's
worth of partitions. By storing the data in a transactional Hive table
I believe that we can instead issue updates and deletes for only the
affected rows. Although we do use Hive for analytics on this data,
much of the processing that generates and consumes the data is
performed using Cascading. Therefore I'd like to be able to read and
write the data via an API which we'd aim to integrate into a Cascading
Tap of some description. Our Cascading processes could determine the
new, updated, and deleted records and then use the API to stream these
changes to the transactional Hive table.
We have most of this working in a proof of concept, but as
hive-hcatalog-streaming does not expose the delete/update methods of
the OrcRecordUpdater we've had to hack together something unpleasant
based on the original API.
As a first step I'd like to check if there is any appetite for adding
such functionality to the API or if this goes against the original
motivations of the project? If this suggestion sounds reasonable then
I'd be keen to help move this forward.
Thanks - Elliot.
