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From "Josh Elser (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-3842) [UMBRELLA] Remove non-transient data from ZooKeeper
Date Fri, 22 May 2015 20:33:17 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-3842?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14556790#comment-14556790
] 

Josh Elser commented on ACCUMULO-3842:
--------------------------------------

{quote}
bq.    ZooKeeper doesn't keep us from accomplishing this. We would need to write code to actually
get the strong consensus for ourselves.

Isn't this statement is true for HDFS also? Wether using zookeeper or HDFS, config needs to
be cached on tservers for efficiency. For most operations config is not changing, do not want
to make a synchronous read to ZK of HDFS before servicing each RPC. I think we need something
at the API level to address this, regardless of implementation.
{quote}

I haven't put nearly enough thought into this (nor reading necessary literature), but my original
thought was to use Accumulo tables as much as possible for the persistence (and wrap the access
in some classes to make everything more natural).

If we keep a notion of monotonically increasing versions for configs, a FATE op could wait
for each server to report at least a minimal version. Discerning the "end" state might be
difficult in the face of servers dying and starting...

> [UMBRELLA] Remove non-transient data from ZooKeeper
> ---------------------------------------------------
>
>                 Key: ACCUMULO-3842
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-3842
>             Project: Accumulo
>          Issue Type: Improvement
>          Components: client, tserver
>            Reporter: Josh Elser
>             Fix For: 1.8.0
>
>
> Wanted to start brainstorming about this.
> We store a lot of persistent data in ZooKeeper that would better stored in something
backed by HDFS. ZooKeeper can be a very convenient place to store persisted data so that it's
available to all nodes, but it comes at a price and often must be asynchronously accessed
to achieve good performance.
> * Table/Namespace configuration
> * Users/Authorizations
> * Problem reports (maybe?)
> * System configuration overrides (maybe?)
> Some benefits we'd see from this:
> * Loss of ZooKeeper doesn't lose table configuration and users.
> * Greatly reduce zookeeper watchers (assume watchers=50*num_tables*num_tservers)
> * Consistent updates of table constraints and all other table properties
> The last note is the most important one IMO. The number of test issues alone that we've
had with constraints not being seen on all servers are bound to affect users.



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