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From "Marshall McMullen (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-965) Need a multi-update command to allow multiple znodes to be updated safely
Date Wed, 04 May 2011 04:33:03 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-965?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13028590#comment-13028590

Marshall McMullen commented on ZOOKEEPER-965:

The C client is just calling the new zoo_multi API. One of the unit tests does a zoo_multi
which purposefully fails (due to duplicate nodes). The error is correctly detected and propagated
from server back to client. The *NEXT* unit tests which causes the server to start up again
during it's init phase reads in the database off of the disk (ZKDatabase.loadDatabase) which
then ultimately ends up in DataTree.processTxn(DataTree.java:818). That line of code is new
code I've added to processTxn that essentially says:

  case OpCode.multi:
      MultiTxn multiTxn = (MultiTxn) txn ;                                               
      List<Txn> txns = multiTxn.getTxns(); // <---- THAT CAUSES THE NULL POINTER
EXCEPTION B/C 'txn' is null.

So this bug only happens after a failed multi op. The normal multi op gets written to disk
and loaded correctly. I don't understand all of how this works, but I presume we take a snapshot
before we actually apply a transaction so we can roll back if it fails. So I *think* the snapshot
we're taking is putting a null txn into the snapshot ... I'm not sure... Any ideas?

> Need a multi-update command to allow multiple znodes to be updated safely
> -------------------------------------------------------------------------
>                 Key: ZOOKEEPER-965
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-965
>             Project: ZooKeeper
>          Issue Type: Bug
>    Affects Versions: 3.3.3
>            Reporter: Ted Dunning
>            Assignee: Ted Dunning
>             Fix For: 3.4.0
>         Attachments: ZOOKEEPER-965.patch, ZOOKEEPER-965.patch, ZOOKEEPER-965.patch, ZOOKEEPER-965.patch
> The basic idea is to have a single method called "multi" that will accept a list of create,
delete, update or check objects each of which has a desired version or file state in the case
of create.  If all of the version and existence constraints can be satisfied, then all updates
will be done atomically.
> Two API styles have been suggested.  One has a list as above and the other style has
a "Transaction" that allows builder-like methods to build a set of updates and a commit method
to finalize the transaction.  This can trivially be reduced to the first kind of API so the
list based API style should be considered the primitive and the builder style should be implemented
as syntactic sugar.
> The total size of all the data in all updates and creates in a single transaction should
be limited to 1MB.
> Implementation-wise this capability can be done using standard ZK internals.  The changes
> - update to ZK clients to all the new call
> - additional wire level request
> - on the server, in the code that converts transactions to idempotent form, the code
should be slightly extended to convert a list of operations to idempotent form.
> - on the client, a down-rev server that rejects the multi-update should be detected gracefully
and an informative exception should be thrown.
> To facilitate shared development, I have established a github repository at https://github.com/tdunning/zookeeper
 and am happy to extend committer status to anyone who agrees to donate their code back to
Apache.  The final patch will be attached to this bug as normal.

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