v.23.5Experimental Feature

When Parallel Replicas are Enabled, Unavailable Servers are Skipped by Default

Official CH changelog text
When parallel replicas are enabled they will always skip unavailable servers (the behavior is controlled by the setting skip_unavailable_shards, enabled by default and can be only disabled). This closes: #48565. #50293 (Nikita Mikhaylov).
When parallel replicas are enabled, the system will automatically skip unavailable servers during query execution to improve reliability and performance.

Why it matters

This feature ensures that queries do not fail or hang due to unreachable replicas by skipping unavailable shards. It enhances fault tolerance and query efficiency in distributed setups with parallel replicas.

How to use it

The behavior is controlled by the setting skip_unavailable_shards, which is enabled by default. Users can keep it enabled to skip unavailable servers automatically or disable it to change this behavior.

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