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From "Nick Dimiduk (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-12728) buffered writes substantially less useful after removal of HTablePool
Date Thu, 15 Jan 2015 22:11:36 GMT

    [ https://issues.apache.org/jira/browse/HBASE-12728?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14279373#comment-14279373
] 

Nick Dimiduk commented on HBASE-12728:
--------------------------------------

I think it's better to cover the multi-threaded coordination on behalf of the user than expect
them to do the synchronizing themselves. The train rolling here is a good one -- it's nice
cleanup, it's consistent with previous behaviors, and it makes things more obvious for users.
Accompany this with thoughtful javadoc review and a fat example that we can dump into the
online book and this will be a fine resolution.

I still like better having a {{Table}},{{BufferedTable}} instead of {{Table}},{{BufferedMutator}}.
I think having a drop-in buffering option will make the most sense for a usable API. I hear
the argument of maybe it's not the place of our client out-of-the-box, but we have a solution
to this today that some folks depend on, so I think it's irresponsible to omit it for 1.0.
If [~sduskis] is truly fed up with us ( ::smile:: ) I'm happy to pick up the patch in this
direction.

I also think splitting the {{Table}} concept into a reader and a writer is something worth
exploring, but not for 1.0. I'm hoping by 2.0 we'll have a valid story for an async (or [reactive|http://www.reactivemanifesto.org]?)
client and maybe even something that operates on top of a C/native implementation so we can
close the gap for folks who aren't on the JVM. For now, let's get 1.0 release unblocked.

> buffered writes substantially less useful after removal of HTablePool
> ---------------------------------------------------------------------
>
>                 Key: HBASE-12728
>                 URL: https://issues.apache.org/jira/browse/HBASE-12728
>             Project: HBase
>          Issue Type: Bug
>          Components: hbase
>    Affects Versions: 0.98.0
>            Reporter: Aaron Beppu
>            Priority: Blocker
>             Fix For: 1.0.0, 2.0.0, 1.1.0
>
>         Attachments: 12728.connection-owns-buffers.example.branch-1.0.patch, HBASE-12728-2.patch,
HBASE-12728.patch, bulk-mutator.patch
>
>
> In previous versions of HBase, when use of HTablePool was encouraged, HTable instances
were long-lived in that pool, and for that reason, if autoFlush was set to false, the table
instance could accumulate a full buffer of writes before a flush was triggered. Writes from
the client to the cluster could then be substantially larger and less frequent than without
buffering.
> However, when HTablePool was deprecated, the primary justification seems to have been
that creating HTable instances is cheap, so long as the connection and executor service being
passed to it are pre-provided. A use pattern was encouraged where users should create a new
HTable instance for every operation, using an existing connection and executor service, and
then close the table. In this pattern, buffered writes are substantially less useful; writes
are as small and as frequent as they would have been with autoflush=true, except the synchronous
write is moved from the operation itself to the table close call which immediately follows.
> More concretely :
> ```
> // Given these two helpers ...
> private HTableInterface getAutoFlushTable(String tableName) throws IOException {
>   // (autoflush is true by default)
>   return storedConnection.getTable(tableName, executorService);
> }
> private HTableInterface getBufferedTable(String tableName) throws IOException {
>   HTableInterface table = getAutoFlushTable(tableName);
>   table.setAutoFlush(false);
>   return table;
> }
> // it's my contention that these two methods would behave almost identically,
> // except the first will hit a synchronous flush during the put call,
> and the second will
> // flush during the (hidden) close call on table.
> private void writeAutoFlushed(Put somePut) throws IOException {
>   try (HTableInterface table = getAutoFlushTable(tableName)) {
>     table.put(somePut); // will do synchronous flush
>   }
> }
> private void writeBuffered(Put somePut) throws IOException {
>   try (HTableInterface table = getBufferedTable(tableName)) {
>     table.put(somePut);
>   } // auto-close will trigger synchronous flush
> }
> ```
> For buffered writes to actually provide a performance benefit to users, one of two things
must happen:
> - The writeBuffer itself shouldn't live, flush and die with the lifecycle of it's HTableInstance.
If the writeBuffer were managed elsewhere and had a long lifespan, this could cease to be
an issue. However, if the same writeBuffer is appended to by multiple tables, then some additional
concurrency control will be needed around it.
> - Alternatively, there should be some pattern for having long-lived HTable instances.
However, since HTable is not thread-safe, we'd need multiple instances, and a mechanism for
leasing them out safely -- which sure sounds a lot like the old HTablePool to me.
> See discussion on mailing list here : http://mail-archives.apache.org/mod_mbox/hbase-user/201412.mbox/%3CCAPdJLkEzmUQZ_kvD%3D8mrxi4V%3DhCmUp3g9MUZsddD%2Bmon%2BAvNtg%40mail.gmail.com%3E



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