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From "Carter (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-12728) buffered writes substantially less useful after removal of HTablePool
Date Tue, 30 Dec 2014 18:12:13 GMT

    [ https://issues.apache.org/jira/browse/HBASE-12728?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14261316#comment-14261316
] 

Carter commented on HBASE-12728:
--------------------------------

[~sduskis] and I have been discussing this offline for a couple of days and have come up with
the following proposal.  It will require a few JIRA sub-tasks, but it's probably only a week's
worth of work, plus reviews.  In a nutshell:

# Deprecate autoFlush methods (ie Put buffering) in {{HTable}}
# Remove all autoFlush methods from {{Table}}
# Create {{BufferedTable}} (outlined below), which will buffer Puts using {{HTableMultiplexer}}
# Create {{BufferedConnection}} as a new factory class
# Have {{HTableMultiplexer}} implement {{Closeable}} (just fixing bad behavior)
# _Behavior change_: {{HTableMultiplexer}} flushes Puts by having {{FlushWorker}} threads...
#* _OLD_: implement low-level logic against {{AsyncProcess}}
#* _NEW_: call {{Table#put<List<Put>>}}, thus removing duplicate code and improving
encapsulation
# _Behavior change_: When its queue is full, {{HTableMultiplexer}}...
#* _OLD_: immediately rejects all Puts
#* _NEW_: blocks for a configurable time in ms (can be 0) before rejecting Puts
# _Behavior change_: When an async Put fails...
#* _OLD_: the exception is thrown during a unrelated future Put operation (confusing)
#* _NEW_: an exception is sent to the a listener provided by the client, following the Observer
pattern

These are the new classes:

{code:java}
public class BufferedConnection implements Connection {
    private Connection c;
    private HTableMultiplexer htm;

    /* If listener is null, will log but won't notify an async exceptions */
    public BufferedConnection(Connection c, ExceptionListener l) {
        this.c = c;
        this.htm = new HTableMultiplexer(..., c, l);
    }
    public BufferedTable getTable(TableName tn) {
        return new BufferedTable(c.getTable(tn), htm);
    }

    /* getAdmin() and getRegionLocator(...) methods delegate to Connection */
}

public class BufferedTable implements Table {
    private Table t;
    private HTableMultiplexer htm;

    public BufferedTable(Table t, HTableMultiplexer htm) { ... }

    /* Puts go to htm.doPut(...), all methods delegate to t */
}

public interface ExceptionListener {
    public void onException(RetriesExhaustedWithDetailsException e);
}
{code}

>From a user standpoint it looks like this:

*Before*
{code:java}
Connection conn = ConnectionFactory.createConnection();
Table t = conn.getTable(TableName.valueOf("mytable"));
t.setAutoFlushTo(false);

/* do stuff */

t.close();
conn.close();
{code}

*After*
{code:java}
Connection conn = new BufferedConnection(ConnectionFactory.createConnection());
Table t = conn.getTable(TableName.valueOf("mytable"));

/* do stuff */

t.close();
conn.close();
{code}

In essence, a few new classes, a moderate amount of work in HTableMultiplexer, and a few deprecation
annotations in HTable.  Let us know if this looks acceptable and we'll create some subtasks
and make it so.


> 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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