mirror of https://github.com/status-im/consul.git
98 lines
4.9 KiB
Plaintext
98 lines
4.9 KiB
Plaintext
---
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layout: api
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page_title: Agent Caching
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sidebar_title: Agent Caching
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description: |-
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Some read endpoints support agent caching. They are clearly marked in the
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documentation.
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---
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# Agent Caching
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Some read endpoints support agent caching. They are clearly marked in the
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documentation. Agent caching can take two forms, [`simple`](#simple-caching) or
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[`background refresh`](#background-refresh-caching) depending on the endpoint's
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semantics. The documentation for each endpoint clearly identify which if any
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form of caching is supported. The details for each are described below.
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Where supported, caching can be enabled though the `?cached` parameter.
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Combining `?cached` with `?consistent` is an error.
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## Simple Caching
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Endpoints supporting simple caching may return a result directly from the local
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agent's cache without a round trip to the servers. By default the agent caches
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results for a relatively long time (3 days) such that it can still return a
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result even if the servers are unavailable for an extended period to enable
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"fail static" semantics.
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That means that with no other arguments, `?cached` queries might receive a
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response which is days old. To request better freshness, the HTTP
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`Cache-Control` header may be set with a directive like `max-age=<seconds>`. In
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this case the agent will attempt to re-fetch the result from the servers if the
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cached value is older than the given `max-age`. If the servers can't be reached
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a 500 is returned as normal.
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To allow clients to maintain fresh results in normal operation but allow stale
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ones if the servers are unavailable, the `stale-if-error=<seconds>` directive
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may be additionally provided in the `Cache-Control` header. This will return the
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cached value anyway even if it's older than `max-age` (provided it's not older
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than `stale-if-error`) rather than a 500. It must be provided along with a
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`max-age` or `must-revalidate`. The `Age` response header, if larger than
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`max-age` can be used to determine if the server was unreachable and a cached
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version returned instead.
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For example, assuming there is a cached response that is 65 seconds old, and
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that the servers are currently unavailable, `Cache-Control: max-age=30` will
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result in a 500 error, while `Cache-Control: max-age=30 stale-if-error=259200`
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will result in the cached response being returned.
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A request setting either `max-age=0` or `must-revalidate` directives will cause
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the agent to always re-fetch the response from servers. Either can be combined
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with `stale-if-error=<seconds>` to ensure fresh results when the servers are
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available, but falling back to cached results if the request to the servers
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fails.
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Requests that do not use `?cached` currently bypass the cache entirely so the
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cached response returned might be more stale than the last uncached response
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returned on the same agent. If this causes problems, it is possible to make
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requests using `?cached` and setting `Cache-Control: must-revalidate` to have
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always-fresh results yet keeping the cache populated with the most recent
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result.
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In all cases the HTTP `X-Cache` header is always set in the response to either
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`HIT` or `MISS` indicating whether the response was served from cache or not.
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For cache hits, the HTTP `Age` header is always set in the response to indicate
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how many seconds since that response was fetched from the servers.
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## Background Refresh Caching
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Endpoints supporting background refresh caching may return a result directly
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from the local agent's cache without a round trip to the severs. The first fetch
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that is a miss will cause an initial fetch from the servers, but will also
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trigger the agent to begin a background blocking query that watches for any
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changes to that result and updates the cached value if changes occur.
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Following requests will _always_ be a cache hit until there has been no request
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for the resource for the TTL (which is typically 3 days).
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Clients can perform blocking queries against the local agent which will be
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served from the cache. This allows multiple clients to watch the same resource
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locally while only a single blocking watch for that resource will be made to the
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servers from a given client agent.
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HTTP `Cache-Control` headers are ignored in this mode since the cache is being
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actively updated and has different semantics to a typical passive cache.
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In all cases the HTTP `X-Cache` header is always set in the response to either
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`HIT` or `MISS` indicating whether the response was served from cache or not.
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For cache hits, the HTTP `Age` header is always set in the response to indicate
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how many seconds since that response was fetched from the servers. As long as
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the local agent has an active connection to the servers, the age will always be
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`0` since the value is up-to-date. If the agent gets disconnected, the cached
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result is still returned but with an `Age` that indicates how many seconds have
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elapsed since the local agent got disconnected from the servers, during which
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time updates to the result might have been missed.
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