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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-12728) buffered writes substantially less useful after removal of HTablePool
Date Wed, 24 Dec 2014 04:09:13 GMT

    [ https://issues.apache.org/jira/browse/HBASE-12728?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14257936#comment-14257936
] 

stack commented on HBASE-12728:
-------------------------------

bq. It leads to an illusion of free speed gains when, in fact, you sacrifice durability guarantees,
a basic tenet of a database.

Yeah it does that. We should put this in flashing lights in the autoFlush javadoc.  Folks
will still want to buffer up writes though, in spite of the dangers.

bq. If this is a common enough use case, then HBase could even package this as a utility for
those who need it. 

It is a common use case and yes, we should have it as utillity.  Up to this the utility has
been in the Table implementation but the suggestion is that doing it this way "..is inherently
confusing ".

For your PutBuffer strawman, in a multithreaded context -- e.g. servlet -- where each thread
does small writes, PutBuffer would have to be threadsafe.  You'd create it at the start of
your map task or if a servlet, on servlet init, and keep it around until the map task was
done, or servlet destroyed?

You like it because it makes the buffering distinct, apart from Table? This could work.

On your less radical BufferedTable, again, we'd need it threadsafe -- or a threadsafe version
-- or else a pool of BufferedTables.  This could work also; would be closer to what we currently
have.

For both proposals, it sounds like Table would change to immediately flush any puts; it would
no longer keep any running 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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