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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-5051) Backwards compatibility for serializers in backend state
Date Thu, 08 Dec 2016 10:31:58 GMT

    [ https://issues.apache.org/jira/browse/FLINK-5051?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15731787#comment-15731787
] 

ASF GitHub Bot commented on FLINK-5051:
---------------------------------------

Github user StefanRRichter commented on the issue:

    https://github.com/apache/flink/pull/2863
  
    Replaced by PR #2962.


> Backwards compatibility for serializers in backend state
> --------------------------------------------------------
>
>                 Key: FLINK-5051
>                 URL: https://issues.apache.org/jira/browse/FLINK-5051
>             Project: Flink
>          Issue Type: Improvement
>          Components: State Backends, Checkpointing
>            Reporter: Stefan Richter
>            Assignee: Stefan Richter
>
> When a new state is register, e.g. in a keyed backend via `getPartitionedState`, the
caller has to provide all type serializers required for the persistence of state components.
Explicitly passing the serializers on state creation already allows for potentiall version
upgrades of serializers.
> However, those serializers are currently not part of any snapshot and are only provided
at runtime, when the state is registered newly or restored. For backwards compatibility, this
has strong implications: checkpoints are not self contained in that state is currently a blackbox
without knowledge about it's corresponding serializers. Most cases where we would need to
restructure the state are basically lost. We could only convert them lazily at runtime and
only once the user is registering the concrete state, which might happen at unpredictable
points.
> I suggest to adapt our solution as follows:
> - As now, all states are registered with their set of serializers.
> - Unlike now, all serializers are written to the snapshot. This makes savepoints self-contained
and also allows to create inspection tools for savepoints at some point in the future.
> - Introduce an interface {{Versioned}} with {{long getVersion()}} and {{boolean isCompatible(Versioned
v)}} which is then implemented by serializers. Compatible serializers must ensure that they
can deserialize older versions, and can then serialize them in their new format. This is how
we upgrade.
> We need to find the right tradeoff in how many places we need to store the serializers.
I suggest to write them once per parallel operator instance for each state, i.e. we have a
map with state_name -> tuple3<serializer<KEY>, serializer<NAMESPACE>, serializer<STATE>>.
This could go before all key-groups are written, right at the head of the file. Then, for
each file we see on restore, we can first read the serializer map from the head of the stream,
then go through the key groups by offset.



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