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page_title: "HTTP API" page_title: "HTTP API"
sidebar_current: "docs-agent-http" sidebar_current: "docs-agent-http"
description: |- description: |-
The main interface to Consul is a RESTful HTTP API. The API can be used for CRUD for nodes, services, checks, and configuration. The endpoints are versioned to enable changes without breaking backwards compatibility. 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 # HTTP API
The main interface to Consul is a RESTful HTTP API. The API can be The main interface to Consul is a RESTful HTTP API. The API can be used to perform CRUD
used for CRUD for nodes, services, checks, and configuration. The endpoints are operations on nodes, services, checks, configuration, and more. The endpoints are versioned
versioned to enable changes without breaking backwards compatibility. to enable changes without breaking backwards compatibility.
All endpoints fall into one of several categories: Each endpoint manages a different aspect of Consul:
* [kv](http/kv.html) - Key/Value store * [kv](http/kv.html) - Key/Value store
* [agent](http/agent.html) - Agent control * [agent](http/agent.html) - Consul Agent
* [catalog](http/catalog.html) - Manages nodes and services * [catalog](http/catalog.html) - Nodes and services
* [health](http/health.html) - Manages health checks * [health](http/health.html) - Health checks
* [session](http/session.html) - Session manipulation * [session](http/session.html) - Sessions
* [acl](http/acl.html) - ACL creations and management * [acl](http/acl.html) - Access Control Lists
* [event](http/event.html) - User Events * [event](http/event.html) - User Events
* [status](http/status.html) - Consul system status * [status](http/status.html) - Consul system status
* internal - Internal APIs. Purposely undocumented, subject to change. * internal - Internal APIs. Purposely undocumented, subject to change.
Each of the categories and their respective endpoints are documented below. Each of these is documented in detail at the links above.
## Blocking Queries ## Blocking Queries
Certain endpoints support a feature called a "blocking query." A blocking query Certain endpoints support a feature called a "blocking query." A blocking query
is used to wait for a change to potentially take place using long polling. is used to wait for a potential change using long polling.
Queries that support this will mention it specifically, however the use of this Not all endpoints support blocking, but those that do are clearly designated in the
feature is the same for all. If supported, the query will set an HTTP header documentation. Any endpoint that supports blocking will also set the HTTP header
"X-Consul-Index". This is an opaque handle that the client will use. `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.
To cause a query to block, the query parameters "?wait=\<interval\>&index=\<idx\>" are added In addition to `index`, endpoints that support blocking will also honor a `wait`
to a request. The "?wait=" query parameter limits how long the query will potentially parameter specifying a maximum duration for the blocking request. If not set, it will
block for. It not set, it will default to 10 minutes. It can be specified in the form of default to 10 minutes. This value can be specified in the form of "10s" or "5m" (i.e.,
"10s" or "5m", which is 10 seconds or 5 minutes respectively. The "?index=" parameter is an 10 seconds or 5 minutes, respectively).
opaque handle, which is used by Consul to detect changes. The "X-Consul-Index" header for a
query provides this value, and can be used to wait for changes since the query was run.
When provided, Consul blocks sending a response until there is an update that A critical note is that the return of a blocking request is **no guarantee** of a change. It
could have cause the output to change, and thus advancing the index. A critical is possible that the timeout was reached or that there was an idempotent write that does
note is that when the query returns there is **no guarantee** of a change. It is not affect the result of the query.
possible that the timeout was reached, or that there was an idempotent write that
does not affect the result.
## Consistency Modes ## Consistency Modes
Most of the read query endpoints support multiple levels of consistency. Most of the read query endpoints support multiple levels of consistency. Since no policy will
These are to provide a tuning knob that clients can be used to find a happy suit all clients' needs, these consistency modes allow the user to have the ultimate say in
medium that best matches their needs. how to balance the trade-offs inherent in a distributed system.
The three read modes are: The three read modes are:
* default - If not specified, this mode is used. It is strongly consistent * default - If not specified, the default is strongly consistent in almost all cases. However,
in almost all cases. However, there is a small window in which an new there is a small window in which a new leader may be elected during which the old leader may
leader may be elected, and the old leader may service stale values. The service stale values. The trade-off is fast reads but potentially stale values. The condition
trade off is fast reads, but potentially stale values. This condition is resulting in stale reads is hard to trigger, and most clients should not need to worry about
hard to trigger, and most clients should not need to worry about the stale read. this case. Also, note that this race condition only applies to reads, not writes.
This only applies to reads, and a split-brain is not possible on writes.
* consistent - This mode is strongly consistent without caveats. It requires * 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 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 introduces an additional round-trip to all server nodes. The trade-off is
always consistent reads, but increased latency due to an extra round trip. increased latency due to an extra round trip. Most clients should not use this
Most clients should not use this unless they cannot tolerate a stale read. unless they cannot tolerate a stale read.
* stale - This mode allows any server to service the read, regardless of if * stale - This mode allows any server to service the read regardless of whether
it is the leader. This means reads can be arbitrarily stale, but are generally it is the leader. This means reads can be arbitrarily stale; however, results are generally
within 50 milliseconds of the leader. The trade off is very fast and scalable consistent to within 50 milliseconds of the leader. The trade-off is very fast and
reads but values will be stale. This mode allows reads without a leader, meaning scalable reads with a higher likelihood of stale values. Since this mode allows reads without
a cluster that is unavailable will still be able to respond. 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 To switch these modes, either the `stale` or `consistent` query parameters
are provided. It is an error to provide both. should be provided on requests. It is an error to provide both.
To support bounding how stale data is, there is an "X-Consul-LastContact" To support bounding the acceptable staleness of data, responses provide the `X-Consul-LastContact`
which is the last time a server was contacted by the leader node in header containing the time in milliseconds that a server was last contacted by the leader node.
milliseconds. The "X-Consul-KnownLeader" also indicates if there is a known The `X-Consul-KnownLeader` header also indicates if there is a known leader. These can be used
leader. These can be used to gauge if a stale read should be used. by clients to gauge the staleness of a result and take appropriate action.
## Formatted JSON Output ## Formatted JSON Output
By default, the output of all HTTP API requests return minimized JSON with all By default, the output of all HTTP API requests is minimized JSON. If the client passes `pretty`
whitespace removed. By adding "?pretty" to the HTTP request URL, on the query string, formatted JSON will be returned.
formatted JSON will be returned.
## ACLs ## ACLs
Several endpoints in Consul use or require ACL tokens to operate. An agent 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` 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 configuration option. However, the token can also be specified per-request
by using the "?token=" query parameter. This will take precedence over the by using the `token` query parameter. This will take precedent over the
default token. default token.

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