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https://issues.apache.org/jira/browse/SOLR-6249?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14097720#comment-14097720
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Timothy Potter commented on SOLR-6249:
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Just jotting down more notes ...
Forgot that a managed-schema can be shared across many collections (potentially
hundreds) so that complicates the blocking idea. We could have the API call
block until a random active replica returns the expected zkversion using a new
endpoint I'm adding /schema/zkversion (with some timeout built-in to avoid
blocking too long). The idea here is that if 1 other replica has the update,
then likely all do, which is a little loose but this is just to alleviate the
need for a client application to add any polling code in the client side. In
other words, we'll block for a short amount of time (just enough that another
replica has the update) which should be sufficient and allow the client to
proceed with using the update.
Looked into the two-phase commit recipe in ZK and that seems like overkill for
this.
Seems to me like less coordination here is best? In other words, instead of the
core that accepts the update request worrying about all the other cores that
are going to see the update, it just assumes it will be successful. If any of
the remote cores fail in processing the update, then they mark themselves as
"down". This seems to solve the root of the problem raised by this ticket,
namely a replica using the wrong version of a managed schema. It's likely that
a replica that can't process a good update successfully will need some manual
intervention anyway (such as missing a JAR file or something).
> Schema API changes return success before all cores are updated
> --------------------------------------------------------------
>
> Key: SOLR-6249
> URL: https://issues.apache.org/jira/browse/SOLR-6249
> Project: Solr
> Issue Type: Improvement
> Components: Schema and Analysis, SolrCloud
> Reporter: Gregory Chanan
>
> See SOLR-6137 for more details.
> The basic issue is that Schema API changes return success when the first core
> is updated, but other cores asynchronously read the updated schema from
> ZooKeeper.
> So a client application could make a Schema API change and then index some
> documents based on the new schema that may fail on other nodes.
> Possible fixes:
> 1) Make the Schema API calls synchronous
> 2) Give the client some ability to track the state of the schema. They can
> already do this to a certain extent by checking the Schema API on all the
> replicas and verifying that the field has been added, though this is pretty
> cumbersome. Maybe it makes more sense to do this sort of thing on the
> collection level, i.e. Schema API changes return the zk version to the
> client. We add an API to return the current zk version. On a replica, if
> the zk version is >= the version the client has, the client knows that
> replica has at least seen the schema change. We could also provide an API to
> do the distribution and checking across the different replicas of the
> collection so that clients don't need ot do that themselves.
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