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docs Key/Value store (HTTP) docs-agent-http-kv The KV endpoint is used to expose a simple key/value store. This can be used to store service configurations or other meta data in a simple way.

Key/Value HTTP Endpoint

The KV endpoint is used to expose a simple key/value store. This can be used to store service configurations or other meta data in a simple way. It has only a single endpoint:

/v1/kv/<key>

This is the only endpoint that is used with the Key/Value store. Its use depends on the HTTP method. The GET, PUT and DELETE methods are all supported. It is important to note that each datacenter has its own K/V store, and that there is no replication between datacenters. By default the datacenter of the agent is queried, however the dc can be provided using the "?dc=" query parameter. If a client wants to write to all Datacenters, one request per datacenter must be made. The KV endpoint supports the use of ACL tokens.

If you are interested in Key/Value replication between datacenters, look at the consul-replicate project.

GET Method

When using the GET method, Consul will return the specified key, or if the "?recurse" query parameter is provided, it will return all keys with the given prefix.

Each object will look like:

[
  {
    "CreateIndex": 100,
    "ModifyIndex": 200,
    "LockIndex": 200,
    "Key": "zip",
    "Flags": 0,
    "Value": "dGVzdA==",
    "Session": "adf4238a-882b-9ddc-4a9d-5b6758e4159e"
  }
]

The CreateIndex is the internal index value that represents when the entry was created. The ModifyIndex is the last index that modified this key. This index corresponds to the X-Consul-Index header value that is returned. A blocking query can be used to wait for a value to change. If "?recurse" is used, the X-Consul-Index corresponds to the latest ModifyIndex and so a blocking query waits until any of the listed keys are updated. The LockIndex is the last index of a successful lock acquisition. If the lock is held, the Session key provides the session that owns the lock.

The Key is simply the full path of the entry. Flags are an opaque unsigned integer that can be attached to each entry. The use of this is left totally to the user. The Value is a base64 key value.

It is possible to also only list keys without their values by using the "?keys" query parameter along with a GET request. This will return a list of the keys under the given prefix. The optional "?separator=" can be used to list only up to a given separator.

For example, listing "/web/" with a "/" separator may return:

[
  "/web/bar",
  "/web/foo",
  "/web/subdir/"
]

Using the key listing method may be suitable when you do not need the values or flags, or want to implement a key-space explorer.

If the "?raw" query parameter is used with a non-recursive GET, then the response is just the raw value of the key, without any encoding.

If no entries are found, a 404 code is returned.

This endpoint supports blocking queries and all consistency modes.

PUT method

When using the PUT method, Consul expects the request body to be the value corresponding to the key. There are a number of parameters that can be used with a PUT request:

  • ?flags=<num> : This can be used to specify an unsigned value between 0 and 2^64-1. It is opaque to the user, but a client application may use it.

  • ?cas=<index> : This flag is used to turn the PUT into a Check-And-Set operation. This is very useful as it allows clients to build more complex synchronization primitives on top. If the index is 0, then Consul will only put the key if it does not already exist. If the index is non-zero, then the key is only set if the index matches the ModifyIndex of that key.

  • ?acquire=<session> : This flag is used to turn the PUT into a lock acquisition operation. This is useful as it allows leader election to be built on top of Consul. If the lock is not held and the session is valid, this increments the LockIndex and sets the Session value of the key in addition to updating the key contents. A key does not need to exist to be acquired.

  • ?release=<session> : This flag is used to turn the PUT into a lock release operation. This is useful when paired with "?acquire=" as it allows clients to yield a lock. This will leave the LockIndex unmodified but will clear the associated Session of the key. The key must be held by this session to be unlocked.

The return value is simply either true or false. If false is returned, then the update has not taken place.

DELETE method

The DELETE method can be used to delete a single key or all keys sharing a prefix. There are a number of query parameters that can be used with a DELETE request:

  • ?recurse : This is used to delete all keys which have the specified prefix. Without this, only a key with an exact match will be deleted.

  • ?cas=<index> : This flag is used to turn the DELETE into a Check-And-Set operation. This is very useful as it allows clients to build more complex synchronization primitives on top. If the index is 0, then Consul will only delete the key if it does not already exist (noop). If the index is non-zero, then the key is only deleted if the index matches the ModifyIndex of that key.