279 lines
13 KiB
Plaintext
279 lines
13 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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Every Consul HTTP API read request uses one of several "consistency modes" that offer
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different levels of response consistency and performance. By default, Consul uses a
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consistency mode that is suitable for most use cases. Users can override consistency modes
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to fine-tune these trade-offs for their own use case.
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---
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# Consistency Modes
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Every Consul HTTP API read request uses one of several _consistency modes_ that offer
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different levels of response consistency and performance. By default, Consul uses a
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consistency mode that is suitable for most use cases. Users can override consistency modes
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to fine-tune these trade-offs for their own use case at two levels:
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- Per interface (HTTP API, DNS)
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- Per request (HTTP API requests only)
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## What is Consistency?
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Consul servers are responsible for maintaining state information like the registration and
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health status of services and nodes. To protect this state against the potential failure of
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Consul servers, this state is replicated across three or more Consul servers in production using a
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[consensus protocol](/docs/architecture/consensus).
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One Consul server is elected leader and acts as the ultimate authority on Consul's state.
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If a majority of Consul servers agree to a state change, the leader is responsible for recording
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the change and then distributing it to the non-leader Consul servers—known as followers.
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Because it takes time for a state change on the leader to propagate to the followers,
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followers may have a slightly outdated, or "stale", view of Consul's state.
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If a read request is handled by the current leader, the response is guaranteed to be
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fully _consistent_ (as up-to-date as possible).
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If the same request were handled by a follower, the response may be less consistent:
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based on a _stale_ (outdated) copy of the leader's state.
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Consistency is highest if the response comes from the leader.
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But ensuring only the leader can respond to the request prevents spreading
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read request load across all Consul servers.
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The consistency mode controls which Consul servers can respond to read requests,
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enabling control over this inherent trade-off between consistency and performance.
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## Available Consistency Modes
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Each HTTP API endpoint documents its support for the three read consistency modes:
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- `stale` -
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[Consul DNS queries use `stale` mode by default](#consul-dns-queries).
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This mode allows any server to handle the read regardless of whether it is the leader.
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The trade-off is very fast and scalable reads with a higher likelihood of stale values.
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Results are generally consistent to within 50 milliseconds of the leader,
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though there is no upper limit on this staleness.
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Since this mode allows reads without a leader,
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a cluster that is unavailable (no quorum) can still respond to queries.
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- `default` -
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[Consul HTTP API queries use `default` mode by default](#consul-http-api-queries).
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It is strongly consistent in almost all cases.
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However, there is a small window in which a new leader may be elected
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during which the old leader may respond with stale values.
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The trade-off is fast reads but potentially stale values.
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The condition resulting in stale reads is hard to trigger,
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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` -
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This mode is strongly consistent without caveats.
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It requires that a leader verify with a quorum of peers that it is still leader.
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This introduces an additional round-trip to all server nodes.
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The trade-off is increased latency due to an extra round trip.
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Most clients should not use this unless they cannot tolerate a stale read.
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~> **Scaling read requests**: The most effective way to increase read scalability
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is to convert non-`stale` reads to `stale` reads. If most requests are already
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`stale` reads and additional load reduction is desired, use Consul Enterprise
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[redundancy zones](/docs/enterprise/redundancy) or
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[read replicas](/docs/enterprise/read-scale)
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to spread `stale` reads across additional, _non-voting_ Consul servers.
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Non-voting servers enhance read scalability without increasing the number
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of voting servers; adding more then 5 voting servers is not recommended because
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it decreases Consul's performance.
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### Intra-Datacenter Request Behavior
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The following diagrams show how the intra-datacenter request path varies
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per consistency mode and the relative trade-offs between level of consistency and performance.
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<Tabs>
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<Tab heading="Stale reads">
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[](/img/consistency-modes/consistency-mode-stale.png)
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</Tab>
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<Tab heading="Default reads">
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[](/img/consistency-modes/consistency-mode-default.png)
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</Tab>
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<Tab heading="Consistent reads, all writes">
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[](/img/consistency-modes/consistency-mode-consistent.png)
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</Tab>
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</Tabs>
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### Cross-Datacenter Request Behavior
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When making a request across federated Consul datacenters, requests are forwarded from
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a local server to any remote server. Once in the remote datacenter, the request path
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is the same as a [local request with the same consistency mode](#intra-datacenter-request-behavior).
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The following diagrams show the cross-datacenter request paths when Consul servers in datacenters are
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[federated either directly or via mesh gateways](/docs/connect/gateways/mesh-gateway/wan-federation-via-mesh-gateways).
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<Tabs>
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<Tab heading="Traditional WAN federation">
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[](/img/consistency-modes/consistency-mode-cross-datacenter-federation-traditional.png)
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</Tab>
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<Tab heading="WAN federation via mesh gateways">
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[](/img/consistency-modes/consistency-mode-cross-datacenter-federation-mesh-gateways.png)
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</Tab>
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</Tabs>
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## Usage
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### Consul DNS Queries
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When DNS queries are issued to [Consul's DNS interface](/docs/discovery/dns),
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Consul uses the `stale` consistency mode by default when interfacing with its
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underlying Consul service discovery HTTP APIs
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([Catalog](/api-docs/catalog), [Health](/api-docs/health), and [Prepared Query](/api-docs/query)).
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The consistency mode underlying Consul DNS queries cannot be overridden on a
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per-request basis.
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#### Changing the Default Consistency Mode
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If stronger consistency is required at the expense of increased latency,
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set [`dns_config.allow_stale`] to `false` to make Consul use the
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`default` consistency mode instead of `stale`
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when interfacing with the underlying Consul service discovery HTTP APIs.
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#### Limiting Staleness (Advanced Usage)
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~> **Caution**:
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Configuring [`dns_config.max_stale`] may result in a prolonged outage
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if your Consul servers become overloaded.
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We do not recommend using this setting to limit staleness
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unless it would be preferable for your services to be unavailable
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than to receive results that are too stale.
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If bounded result consistency is required by a service, consider
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modifying the service to use `consistent` service discovery HTTP API queries
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instead of DNS lookups.
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If Consul is configured to respond to DNS queries in `stale` consistency mode
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([`dns_config.allow_stale`] is `true`),
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you can set an upper limit on how stale a response is allowed to be by
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configuring [`dns_config.max_stale`].
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When the Consul agent handling the response is behind the leader by more than [`dns_config.max_stale`] time,
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the request will be forwarded to the leader for handling as in `default` consistency mode.
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A common cause for response staleness exceeding [`dns_config.max_stale`] is that
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Consul servers are overloaded by requests. Under these circumstances, [`dns_config.max_stale`]
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will make the overloading worse by converting cheap `stale` reads to more expensive `default` reads.
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### Consul HTTP API Queries
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All HTTP API read requests use the `default` consistency mode by default
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unless [overridden on a per-request basis](#overriding-a-request-s-consistency-mode).
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We do [not recommend changing the consistency mode used by default](#changing-the-default-consistency-mode-advanced-usage).
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Write requests do not support defining a consistency mode.
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All writes follow the behavior of the `consistent` consistency mode.
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#### Overriding a Request's Consistency Mode
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The HTTP API documentation for each endpoint lists which consistency modes can be used
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to override the default on a per-request basis. To use a supported consistency mode
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other than the default, include the desired consistency mode as a URL query parameter
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when calling the endpoint:
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- `stale`: Use the `stale` query parameter
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- `consistent`: Use the `consistent` query parameter
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- `default`: Use the `leader` query parameter;
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only relevant [if the default consistency mode is changed to `stale`](#changing-the-default-consistency-mode-advanced-usage)
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#### Changing the Default Consistency Mode (Advanced Usage)
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~> **Caution**:
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Configuring [`discovery_max_stale`] may result in a prolonged outage
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if your Consul servers become overloaded.
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We do not recommend using this setting to limit staleness
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unless it would be preferable for your services to be unavailable
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than to receive results that are too stale.
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If bounded result consistency is required by a service, consider
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modifying the service to use `consistent` mode
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[on a per-request basis](#overriding-a-request-s-consistency-mode).
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The default consistency mode can be changed to `stale` for service discovery HTTP API endpoints,
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including:
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- [Catalog API](/api-docs/catalog)
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- [Health API](/api-docs/health)
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- [Prepared Query API](/api-docs/query)
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This allows Consul operators to spread service discovery read load across Consul servers
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with a centralized configuration change, avoiding the need to modify every service to
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explicitly set `stale` consistency mode in their requests.
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Services can still override this default on a per-request basis by
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[specifying a supported consistency mode as a query parameter in the request](#overriding-a-request-s-consistency-mode).
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To configure Consul servers that receive service discovery requests to use `stale`
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consistency mode unless overridden,
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set [`discovery_max_stale`] to a value greater than zero in their agent configuration.
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The `stale` consistency mode will be used by default unless the data is sufficiently stale:
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its Raft log's index is more than [`discovery_max_stale`] indices behind the leader's.
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Setting a positive value for [`discovery_max_stale`]
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is analogous to [limiting the staleness of Consul DNS queries](#limiting-staleness-advanced-usage)
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by setting [`dns_config.max_stale`].
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A common cause for response staleness exceeding [`discovery_max_stale`] is that
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Consul servers are overloaded by requests. Under these circumstances, [`discovery_max_stale`]
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will make the overloading worse by converting cheap `stale` reads to more expensive `default` reads.
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### Visibility into Response Staleness
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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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### Visibility into Consistency Mode Used
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The Consul DNS and HTTP API interfaces allow setting the consistency mode
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at different levels (interface and per-request).
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Advanced settings for the [DNS](#limiting-staleness-advanced-usage) and
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[HTTP API](#changing-the-default-consistency-mode-advanced-usage) interfaces can even
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cause the consistency mode to change from `stale` to `default` if results are sufficiently stale.
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To understand the consistency mode used for a particular HTTP API request,
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check the `X-Consul-Effective-Consistency` response header.
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The value of the header matches the
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[name of the query parameter associated with the consistency mode](#overriding-a-request-s-consistency-mode).
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In the following example, the header shows that the `default` consistency mode was used.
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```shell-session
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$ curl --include $CONSUL_HTTP_ADDR/v1/catalog/services
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HTTP/1.1 200 OK
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Content-Type: application/json
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...
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X-Consul-Effective-Consistency: leader
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X-Consul-KnownLeader: true
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X-Consul-LastContact: 0
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{
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"redis": [],
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"postgresql": ["primary", "secondary"]
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}
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```
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The DNS interface does not support viewing the consistency mode used for a particular query.
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### Relationship to Request Caching
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Note that some HTTP API endpoints support a `cached` parameter which has some of the same
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semantics as `stale` consistency mode but different trade offs. This behavior is described in
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[agent caching feature documentation](/api-docs/features/caching)
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<!-- Common links references -->
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[`dns_config.allow_stale`]: /docs/agent/config/config-files#allow_stale
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[`dns_config.max_stale`]: /docs/agent/config/config-files#max_stale
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[`discovery_max_stale`]: /docs/agent/config/config-files#discovery_max_stale
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