[jira] [Commented] (HBASE-17009) Revisiting the removement of managed connection and connection caching

["Enis Soztutar (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-17009?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15633686#comment-15633686 ] 

Enis Soztutar commented on HBASE-17009:
---------------------------------------

Did you see HBASE-16713? 

I think the removal of HTable was the correct thing. Yes, it makes the life of the user a little bit harder, but the lifecycle should be managed explicitly. Otherwise, it is super hard to manage the resources and ensure that connections are closed, etc. 

I have a prototype patch for HBASE-16713. It does not bring HTable(Configuration) back, but it introduces a new {{CachingConnectionFactory}} which does the caching for you if you do not want to deal with managing the lifecycle yourself. The difference is that the new Factory has its own lifecycle as well which is still managed explicitly. I'll attach it over there, and let me know whether this is the approach you want to take. 

> Revisiting the removement of managed connection and connection caching
> ----------------------------------------------------------------------
>
>                 Key: HBASE-17009
>                 URL: https://issues.apache.org/jira/browse/HBASE-17009
>             Project: HBase
>          Issue Type: Task
>          Components: Operability
>            Reporter: Yu Li
>            Assignee: Yu Li
>            Priority: Critical
>
> In HBASE-13197 we have done lots of good cleanups for Connection API, but among which HBASE-13252 dropped the feature of managed connection and connection caching, and this JIRA propose to have a revisit on this decision for below reasons.
> Assume we have a long running process with multiple threads accessing HBase (a common case for streaming application), let's see what happens previously and now.
> Previously:
> User could create an HTable instance whenever they want w/o worrying about the underlying connections because HBase client will mange it automatically, say no matter how many threads there will be only one Connection instance
> {code}
>   @Deprecated
>   public HTable(Configuration conf, final TableName tableName)
>   throws IOException {
>     ...
>     this.connection = ConnectionManager.getConnectionInternal(conf);
>     ...
>   }
>   static ClusterConnection getConnectionInternal(final Configuration conf)
>     throws IOException {
>     HConnectionKey connectionKey = new HConnectionKey(conf);
>     synchronized (CONNECTION_INSTANCES) {
>       HConnectionImplementation connection = CONNECTION_INSTANCES.get(connectionKey);
>       if (connection == null) {
>         connection = (HConnectionImplementation)createConnection(conf, true);
>         CONNECTION_INSTANCES.put(connectionKey, connection);
>       } else if (connection.isClosed()) {
>         ConnectionManager.deleteConnection(connectionKey, true);
>         connection = (HConnectionImplementation)createConnection(conf, true);
>         CONNECTION_INSTANCES.put(connectionKey, connection);
>       }
>       connection.incCount();
>       return connection;
>     }
>   }
> {code}
> Now:
> User has to create the connection by themselves, using below codes like indicated in our recommendations
> {code}
>     Connection connection = ConnectionFactory.createConnection(conf);
>     Table table = connection.getTable(tableName);
> {code}
> And they must make sure *only one* single connection created in one *process* instead of creating HTable instance freely, or else there might be many connections setup to zookeeper/RS with multiple threads. Also user might ask "when I should close the connection I close?" and the answer is "make sure don't close it until the *process* shutdown"
> So now we have much more things for user to "Make sure", but custom is something hard to change. User used to create table instance in each thread (according to which table to access per requested) so probably they will still create connections everywhere, and then operators will have to crazily resolve all kinds of problems...
> So I'm proposing to add back the managed connection and connection caching support. IMHO it's something good and ever existed in our implementation, so let's bring it back and save the workload for operators when they decided to upgrade from 1.x to 2.x
> Thoughts?



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