» HTTP API

The main interface to Consul is a RESTful HTTP API. The API can perform basic CRUD operations on nodes, services, checks, configuration, and more.

» Version Prefix

All API routes are prefixed with /v1/. This documentation is only for the v1 API.

» 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 query string parameter. The request header takes precedence over the default token, and the query string parameter takes precedence over everything.

For more details about ACLs, please see the ACL Guide.

» Authentication

When authentication is enabled, a Consul token should be provided to API requests using the X-Consul-Token header. This reduces the probability of the token accidentally getting logged or exposed. When using authentication, clients should communicate via TLS.

Here is an example using curl:

$ curl \
    --header "X-Consul-Token: abcd1234" \
    https://consul.rocks/v1/agent/members

Previously this was provided via a ?token= query parameter. This functionality exists on many endpoints for backwards compatibility, but its use is highly discouraged, since it can show up in access logs as part of the URL.

» Blocking Queries

Many endpoints in Consul support a feature known as "blocking queries". A blocking query is used to wait for a potential change using long polling. Not all endpoints support blocking, but each endpoint uniquely documents its support for blocking queries in the documentation.

Endpoints that support blocking queries return an HTTP header named X-Consul-Index. This is 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.

When this is provided, the HTTP request will "hang" until a change in the system occurs, or the maximum timeout is reached. 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.

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 small random amount of additional wait time is added to the supplied maximum wait time to spread out the wake up time of any concurrent requests. This adds up to wait / 16 additional time to the maximum duration.

» 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.

» HTTP Methods

Consul's API aims to be RESTful, although there are some exceptions. The API responds to the standard HTTP verbs GET, PUT, and DELETE. Each API method will clearly document the verb(s) it responds to and the generated response. The same path with different verbs may trigger different behavior. For example:

PUT /v1/kv/foo
GET /v1/kv/foo

Even though these share a path, the PUT operation creates a new key whereas the GET operation reads an existing key.

Here is the same example using curl:

$ curl \
    --request PUT \
    --data 'hello consul' \
    https://consul.rocks/v1/kv/foo

» Translated Addresses

Consul 0.7 added the ability to translate addresses in HTTP response based on the configuration setting for translate_wan_addrs. In order to allow clients to know if address translation is in effect, the X-Consul-Translate-Addresses header will be added if translation is enabled, and will have a value of true. If translation is not enabled then this header will not be present.