52 lines
2.6 KiB
Plaintext
52 lines
2.6 KiB
Plaintext
---
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layout: api
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page_title: Consistency Modes
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description: >-
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Most of the read query endpoints support multiple levels of consistency. Since
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no policy will suit all clients' needs, these consistency modes allow the user
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to have the ultimate say in how to balance the trade-offs inherent in a
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distributed system.
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---
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# Consistency Modes
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Most of the read query endpoints support multiple levels of consistency. Since
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no policy will suit all clients' needs, these consistency modes allow the user
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to have the ultimate say in how to balance the trade-offs inherent in a
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distributed system.
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The three read modes are:
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- `default` - If not specified, the default is strongly consistent in almost all
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cases. However, there is a small window in which a new leader may be elected
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during which the old leader may service stale values. The trade-off is fast
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reads but potentially stale values. The condition resulting in stale reads is
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hard to trigger, and most clients should not need to worry about this case.
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Also, note that this race condition only applies to reads, not writes.
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- `consistent` - This mode is strongly consistent without caveats. It requires
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that a leader verify with a quorum of peers that it is still leader. This
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introduces an additional round-trip to all server nodes. The trade-off is
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increased latency due to an extra round trip. Most clients should not use this
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unless they cannot tolerate a stale read.
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- `stale` - This mode allows any server to service the read regardless of
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whether it is the leader. This means reads can be arbitrarily stale; however,
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results are generally consistent to within 50 milliseconds of the leader. The
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trade-off is very fast and scalable reads with a higher likelihood of stale
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values. Since this mode allows reads without a leader, a cluster that is
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unavailable will still be able to respond to queries.
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To switch these modes, either the `stale` or `consistent` query parameters
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should be provided on requests. It is an error to provide both.
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Note that some endpoints support a `cached` parameter which has some of the same
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semantics as `stale` but different trade offs. This behavior is described in
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[agent caching feature documentation](/api/features/caching).
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To support bounding the acceptable staleness of data, responses provide the
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`X-Consul-LastContact` header containing the time in milliseconds that a server
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was last contacted by the leader node. The `X-Consul-KnownLeader` header also
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indicates if there is a known leader. These can be used by clients to gauge the
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staleness of a result and take appropriate action.
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