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Web / Apache Solr Interview questions

How does ZooKeeper coordinate a SolrCloud cluster?

ZooKeeper acts as the single source of truth for a SolrCloud cluster's metadata, using a hierarchy of znodes that every node watches for changes.

  • /live_nodes - ephemeral znodes listing which Solr nodes are currently up; removed automatically if a node disconnects.
  • /collections/<name>/state.json - shard and replica layout, including which replica is the leader for each shard.
  • /configs - shared config sets (schema, solrconfig) that collections reference.
  • Overseer - a designated node that processes cluster-state changes like collection creation, sequentially.

Because these znodes are watched, changes propagate quickly: if a node crashes, its ephemeral /live_nodes entry disappears, triggering leader election for any shard it was leading, without any manual intervention.

What kind of znode is used for /live_nodes, and why?
Where does SolrCloud store which replica is the current leader for a shard?

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