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From "Solomon Duskis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-12728) buffered writes substantially less useful after removal of HTablePool
Date Fri, 02 Jan 2015 02:31:13 GMT

    [ https://issues.apache.org/jira/browse/HBASE-12728?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14262718#comment-14262718
] 

Solomon Duskis commented on HBASE-12728:
----------------------------------------

bq. 2. is a BufferedConnection really necessary? The connection isn't buffering, it's just
handing back the same BufferedTable instance to each caller, correct? That makes for little/no
code change for a consumer to convert to the buffered implementation, but I think that isn't
idea. Better to have the decorator accept the Table instance (or TableName + Connection instance?)
and allow for setting up buffers/queues/&c for async flushing.

Are HTable.delete(), .batch(), coprocessorService() and others thread-safe?  If not, then
in the multithreaded case, we likely need to create new (or a pooled) BufferredTable per thread.
 The "BufferredConnection" would perform that functionality.  We could implement it as a group
of BufferTables that use the same underlying write buffer.

> 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
>
> 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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