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From "Nick Dimiduk (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HBASE-12728) buffered writes substantially less useful after removal of HTablePool
Date Thu, 22 Jan 2015 00:36:37 GMT

     [ https://issues.apache.org/jira/browse/HBASE-12728?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Nick Dimiduk updated HBASE-12728:
---------------------------------
    Release Note: 
In our pre-1.0 API, HTable is considered a light-weight object that consumed by a single thread
at a time. The HTablePool class provided a means of sharing multiple HTable instances across
a number of threads. As an optimization, HTable managed a "write buffer", accumulating edits
and sending a "batch" all at once. By default the batch was sent as the last step in invocations
of put(Put) and put(List<Put>). The user could disable the automatic flushing of the
write buffer, retaining edits locally and only sending the whole "batch" once the write buffer
has filled or when the flushCommits() method in invoked explicitly. Explicit or implicit batch
writing was controlled by the setAutoFlushTo(boolean) method. A value of true (the default)
had the write buffer flushed at the completion of a call to put(Put) or put(List<Put>).
A value of false allowed for explicit buffer management. HTable also exposed the buffer to
consumers via getWriteBuffer().

The combination of HTable with setAutoFlushTo(false) and the HTablePool provided a convenient
mechanism by which multiple "Put-producing" threads could share a common write buffer. Both
HTablePool and HTable are deprecated, and they are officially replaced in The new 1.0 API
by Table and BufferedMutator. Table, which replaces HTable, no longer exposes explicit write-buffer
management. Instead, explicit buffer management is exposed via BufferedMutator. BufferedMutator
is made safe for concurrent use. Where code would previously retrieve and return HTables from
an HTablePool, now that code creates and shares a single BufferedMutator instance across all
threads.

> 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
>            Assignee: Nick Dimiduk
>            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-12728-3.patch, HBASE-12728-4.patch, HBASE-12728-5.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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