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 Bearer header in Authorization 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" \

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.

» Implementation Details

While the mechanism is relatively simple to work with, there are a few edge cases that must be handled correctly.

  • Reset the index if it goes backwards. While indexes in general are monotonically increasing(i.e. they should only ever increase as time passes), there are several real-world scenarios in which they can go backwards for a given query. Implementations must check to see if a returned index is lower than the previous value, and if it is, should reset index to 0 - effectively restarting their blocking loop. Failure to do so may cause the client to miss future updates for an unbounded time, or to use an invalid index value that causes no blocking and increases load on the servers. Cases where this can occur include:

    • If a raft snapshot is restored on the servers with older version of the data.
    • KV list operations where an item with the highest index is removed.
    • A Consul upgrade changes the way watches work to optimize them with more granular indexes.
  • Sanity check index is greater than zero. After the initial request (or a reset as above) the X-Consul-Index returned should always be greater than zero. It is a bug in Consul if it is not, however this has happened a few times and can still be triggered on some older Consul versions. It's especially bad because it causes blocking clients that are not aware to enter a busy loop, using excessive client CPU and causing high load on servers. It is always safe to use an index of 1 to wait for updates when the data being requested doesn't exist yet, so clients should sanity check that their index is at least 1 after each blocking response is handled to be sure they actually block on the next request.

  • Rate limit. The blocking query mechanism is reasonably efficient when updates are relatively rare (order of tens of seconds to minutes between updates). In cases where a result gets updated very fast however - possibly during an outage or incident with a badly behaved client - blocking query loops degrade into busy loops that consume excessive client CPU and cause high server load. While it's possible to just add a sleep to every iteration of the loop, this is not recommended since it causes update delivery to be delayed in the happy case, and it can exacerbate the problem since it increases the chance that the index has changed on the next request. Clients should instead rate limit the loop so that in the happy case they proceed without waiting, but when values start to churn quickly they degrade into polling at a reasonable rate (say every 15 seconds). Ideally this is done with an algorithm that allows a couple of quick successive deliveries before it starts to limit rate - a token bucket with burst of 2 is a simple way to achieve this.

» Hash-based Blocking Queries

A limited number of agent endpoints also support blocking however because the state is local to the agent and not managed with a consistent raft index, their blocking mechanism is different.

Since there is no monotonically increasing index, each response instead contains a header X-Consul-ContentHash which is an opaque hash digest generated by hashing over all fields in the response that are relevant.

Subsequent requests may be sent with a query parameter hash=<value> where value is the last hash header value seen, and this will block until the wait timeout is passed or until the local agent's state changes in such a way that the hash would be different.

Other than the different header and query parameter names, the biggest difference is that hash values are opaque and can't be compared to see if one result is older or newer than another. In general hash-based blocking will not return too early due to an idempotent update since the hash will remain the same unless the result actually changes, however as with index-based blocking there is no strict guarantee that clients will never observe the same result delivered before the full timeout has elapsed.

» 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.

Note that some endpoints support a cached parameter which has some of the same semantics as stale but different trade offs. This behaviour is described in Agent Caching.

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.

» 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 get's 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.

» 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' \

» 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.

» UUID Format

UUID-format identifiers generated by the Consul API use the hashicorp/go-uuid library.

These UUID-format strings are generated using high quality, purely random bytes. It is not intended to be RFC compliant, merely to use a well-understood string representation of a 128-bit value.