98 lines
4.9 KiB
Plaintext
98 lines
4.9 KiB
Plaintext
---
|
|
layout: api
|
|
page_title: Agent Caching
|
|
sidebar_title: Agent Caching
|
|
description: |-
|
|
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`](#simple-caching) or
|
|
[`background refresh`](#background-refresh-caching) 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 if 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.
|