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https://issues.apache.org/jira/browse/SOLR-2765?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13123257#comment-13123257
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Ted Dunning commented on SOLR-2765:
-----------------------------------
I don't think that you need all nodes to watch all nodes.
What you need is a directory of assigned states that a central overseer sets.
Each node will watch their assignment to see which shards to serve and what
state to assume.
Then you need the live_nodes directory where all nodes can advertise an
ephemeral showing their current status including which shards served and any
other information. Since this is SOLR, XML is a natural format for that.
The only watcher on the live_nodes directory and the node files in that
directory is the overseer.
If a node dies, ZK will send notifications to all live query connections and
the overseer.
It is also nice to have the current serving state for query connections to be
in a file or in directory form. This file could be updated (atomically) by
nodes as they start or stop serving shards, but would have to be updated by the
overseer on node failure. Using a per shard directory allows the list of nodes
serving a shard to be handled as ephemerals, but it is a bit less desirable
because zknode names have to contain node references which is an example of
name/content confusion. Since atomic update is easy in ZK, the file
implementation is probably better.
The update to current shard state will ultimately cause a notification to be
sent to each live query connection.
> Shard/Node states
> -----------------
>
> Key: SOLR-2765
> URL: https://issues.apache.org/jira/browse/SOLR-2765
> Project: Solr
> Issue Type: Sub-task
> Components: SolrCloud, update
> Reporter: Yonik Seeley
> Fix For: 4.0
>
> Attachments: incremental_update.patch, shard-roles.patch
>
>
> Need state for shards that indicate they are recovering, active/enabled, or
> disabled.
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