mirror of https://github.com/status-im/consul.git
99 lines
5.0 KiB
Markdown
99 lines
5.0 KiB
Markdown
---
|
|
layout: "docs"
|
|
page_title: "HTTP API"
|
|
sidebar_current: "docs-agent-http"
|
|
description: |-
|
|
The main interface to Consul is a RESTful HTTP API. The API can be used to perform CRUD operations on nodes, services, checks, configuration, and more. The endpoints are versioned to enable changes without breaking backwards compatibility.
|
|
---
|
|
|
|
# HTTP API
|
|
|
|
The main interface to Consul is a RESTful HTTP API. The API can be used to perform CRUD
|
|
operations on nodes, services, checks, configuration, and more. The endpoints are versioned
|
|
to enable changes without breaking backwards compatibility.
|
|
|
|
Each endpoint manages a different aspect of Consul:
|
|
|
|
* [acl](http/acl.html) - Access Control Lists
|
|
* [agent](http/agent.html) - Consul Agent
|
|
* [catalog](http/catalog.html) - Nodes and services
|
|
* [coordinate](http/coordinate.html) - Network coordinates
|
|
* [event](http/event.html) - User Events
|
|
* [health](http/health.html) - Health checks
|
|
* [kv](http/kv.html) - Key/Value store
|
|
* [query](http/query.html) - Prepared Queries
|
|
* [session](http/session.html) - Sessions
|
|
* [status](http/status.html) - Consul system status
|
|
|
|
Each of these is documented in detail at the links above. Consul also has a number
|
|
of internal APIs which are purposely undocumented and subject to change.
|
|
|
|
## Blocking Queries
|
|
|
|
Certain endpoints support a feature called a "blocking query." A blocking query
|
|
is used to wait for a potential change using long polling.
|
|
|
|
Not all endpoints support blocking, but those that do are clearly designated in the
|
|
documentation. Any endpoint that supports blocking will also set the HTTP header
|
|
`X-Consul-Index`, a unique identifier representing the current state of the
|
|
requested resource. On subsequent requests for this resource, the client can set the `index`
|
|
query string parameter to the value of `X-Consul-Index`, indicating that the client wishes
|
|
to wait for any changes subsequent to that index.
|
|
|
|
In addition to `index`, endpoints that support blocking will also honor a `wait`
|
|
parameter specifying a maximum duration for the blocking request. This is limited to
|
|
10 minutes. If not set, the wait time defaults to 5 minutes. This value can be specified
|
|
in the form of "10s" or "5m" (i.e., 10 seconds or 5 minutes, respectively).
|
|
|
|
A critical note is that the return of a blocking request is **no guarantee** of a change. It
|
|
is possible that the timeout was reached or that there was an idempotent write that does
|
|
not affect the result of the query.
|
|
|
|
## Consistency Modes
|
|
|
|
Most of the read query endpoints support multiple levels of consistency. Since no policy will
|
|
suit all clients' needs, these consistency modes allow the user to have the ultimate say in
|
|
how to balance the trade-offs inherent in a distributed system.
|
|
|
|
The three read modes are:
|
|
|
|
* default - If not specified, the default is strongly consistent in almost all cases. However,
|
|
there is a small window in which a new leader may be elected during which the old leader may
|
|
service stale values. The trade-off is fast reads but potentially stale values. The condition
|
|
resulting in stale reads is hard to trigger, and most clients should not need to worry about
|
|
this case. Also, note that this race condition only applies to reads, not writes.
|
|
|
|
* consistent - This mode is strongly consistent without caveats. It requires
|
|
that a leader verify with a quorum of peers that it is still leader. This
|
|
introduces an additional round-trip to all server nodes. The trade-off is
|
|
increased latency due to an extra round trip. Most clients should not use this
|
|
unless they cannot tolerate a stale read.
|
|
|
|
* stale - This mode allows any server to service the read regardless of whether
|
|
it is the leader. This means reads can be arbitrarily stale; however, results are generally
|
|
consistent to within 50 milliseconds of the leader. The trade-off is very fast and
|
|
scalable reads with a higher likelihood of stale values. Since this mode allows reads without
|
|
a leader, a cluster that is unavailable will still be able to respond to queries.
|
|
|
|
To switch these modes, either the `stale` or `consistent` query parameters
|
|
should be provided on requests. It is an error to provide both.
|
|
|
|
To support bounding the acceptable staleness of data, responses provide the `X-Consul-LastContact`
|
|
header containing the time in milliseconds that a server was last contacted by the leader node.
|
|
The `X-Consul-KnownLeader` header also indicates if there is a known leader. These can be used
|
|
by clients to gauge the staleness of a result and take appropriate action.
|
|
|
|
## Formatted JSON Output
|
|
|
|
By default, the output of all HTTP API requests is minimized JSON. If the client passes `pretty`
|
|
on the query string, formatted JSON will be returned.
|
|
|
|
## ACLs
|
|
|
|
Several endpoints in Consul use or require ACL tokens to operate. An agent
|
|
can be configured to use a default token in requests using the `acl_token`
|
|
configuration option. However, the token can also be specified per-request
|
|
by using the `X-Consul-Token` request header or the `token` querystring
|
|
parameter. The request header takes precedence over the default token, and
|
|
the querystring parameter takes precedence over everything.
|