bc42074f57
Fix spelling errors, API doc inconsistencies, and formatting issues. * Fix several spelling errors. * Prepend / to v1/event/list path in Watches. * Rename script handlers to match Watch type. * Remove /v1 path prefix on service health API endpoints. Makes request path consistent with the rest of the HTTP API documentation which does not include the /v1 prefix. * Fix bracket formatting issue on Telemetry page. The HTML codes used for brackets inside of the code block are not interpolated, and are shown as literal strings. Replace the numeric HTML codes with the intended character value to fix display formatting. Also placed variable reference on agent/options.html inside code block for consistency with the presentation of other options on the page. * Add missing word to Coordinate.Node docstring. Resolves #6014
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| layout | page_title | sidebar_current | description |
|---|---|---|---|
| api | Agent Caching | api-features-caching | Some read endpoints support agent caching. They are clearly marked in the documentation. |
Agent Caching
Some read endpoints support agent caching. They are clearly marked in the
documentation. Agent caching can take two forms, simple or
background refresh depending on the endpoint's
semantics. The documentation for each endpoint clearly identify which if any
form of caching is supported. The details for each are described below.
Where supported, caching can be enabled though the ?cached parameter.
Combining ?cached with ?consistent is an error.
Simple Caching
Endpoints supporting simple caching may return a result directly from the local agent's cache without a round trip to the servers. By default the agent caches results for a relatively long time (3 days) such that it can still return a result even if the servers are unavailable for an extended period to enable "fail static" semantics.
That means that with no other arguments, ?cached queries might receive a
response which is days old. To request better freshness, the HTTP
Cache-Control header may be set with a directive like max-age=<seconds>. In
this case the agent will attempt to re-fetch the result from the servers if the
cached value is older than the given max-age. If the servers can't be reached
a 500 is returned as normal.
To allow clients to maintain fresh results in normal operation but allow stale
ones if the servers are unavailable, the stale-if-error=<seconds> directive
may be additionally provided in the Cache-Control header. This will return the
cached value anyway even it it's older than max-age (provided it's not older
than stale-if-error) rather than a 500. It must be provided along with a
max-age or must-revalidate. The Age response header, if larger than
max-age can be used to determine if the server was unreachable and a cached
version returned instead.
For example, assuming there is a cached response that is 65 seconds old, and
that the servers are currently unavailable, Cache-Control: max-age=30 will
result in a 500 error, while Cache-Control: max-age=30 stale-if-error=259200
will result in the cached response being returned.
A request setting either max-age=0 or must-revalidate directives will cause
the agent to always re-fetch the response from servers. Either can be combined
with stale-if-error=<seconds> to ensure fresh results when the servers are
available, but falling back to cached results if the request to the servers
fails.
Requests that do not use ?cached currently bypass the cache entirely so the
cached response returned might be more stale than the last uncached response
returned on the same agent. If this causes problems, it is possible to make
requests using ?cached and setting Cache-Control: must-revalidate to have
always-fresh results yet keeping the cache populated with the most recent
result.
In all cases the HTTP X-Cache header is always set in the response to either
HIT or MISS indicating whether the response was served from cache or not.
For cache hits, the HTTP Age header is always set in the response to indicate
how many seconds since that response was fetched from the servers.
Background Refresh Caching
Endpoints supporting background refresh caching may return a result directly from the local agent's cache without a round trip to the severs. The first fetch that is a miss will cause an initial fetch from the servers, but will also trigger the agent to begin a background blocking query that watches for any changes to that result and updates the cached value if changes occur.
Following requests will always be a cache hit until there has been no request for the resource for the TTL (which is typically 3 days).
Clients can perform blocking queries against the local agent which will be served from the cache. This allows multiple clients to watch the same resource locally while only a single blocking watch for that resource will be made to the servers from a given client agent.
HTTP Cache-Control headers are ignored in this mode since the cache is being
actively updated and has different semantics to a typical passive cache.
In all cases the HTTP X-Cache header is always set in the response to either
HIT or MISS indicating whether the response was served from cache or not.
For cache hits, the HTTP Age header is always set in the response to indicate
how many seconds since that response was fetched from the servers. As long as
the local agent has an active connection to the servers, the age will always be
0 since the value is up-to-date. If the agent gets disconnected, the cached
result is still returned but with an Age that indicates how many seconds have
elapsed since the local agent got disconnected from the servers, during which
time updates to the result might have been missed.