open-consul/website/source/intro/getting-started/kv.html.markdown

layout	page_title	sidebar_current
intro	Key/Value Data	gettingstarted-kv

Key/Value Data

In addition to providing service discovery and integrated health checking, Consul provides an easy to use Key/Value store. This can be used to hold dynamic configuration, assist in service coordination, build leader election, and anything else a developer can think to build. The HTTP API fully documents the features of the K/V store.

This page assumes you have at least one Consul agent already running.

Simple Usage

To demonstrate how simple it is to get started, we will manipulate a few keys in the K/V store.

Querying the agent we started in a prior page, we can first verify that there are no existing keys in the k/v store:

$ curl -v http://localhost:8500/v1/kv/?recurse
* About to connect() to localhost port 8500 (#0)
*   Trying 127.0.0.1... connected
> GET /v1/kv/?recurse HTTP/1.1
> User-Agent: curl/7.22.0 (x86_64-pc-linux-gnu) libcurl/7.22.0 OpenSSL/1.0.1 zlib/1.2.3.4 libidn/1.23 librtmp/2.3
> Host: localhost:8500
> Accept: */*
>
< HTTP/1.1 404 Not Found
< X-Consul-Index: 1
< Date: Fri, 11 Apr 2014 02:10:28 GMT
< Content-Length: 0
< Content-Type: text/plain; charset=utf-8
<
* Connection #0 to host localhost left intact
* Closing connection #0

Since there are no keys, we get a 404 response back. Now, we can put a few example keys:

$ curl -X PUT -d 'test' http://localhost:8500/v1/kv/web/key1
true
$ curl -X PUT -d 'test' http://localhost:8500/v1/kv/web/key2?flags=42
true
$ curl -X PUT -d 'test'  http://localhost:8500/v1/kv/web/sub/key3
true
$ curl http://localhost:8500/v1/kv/?recurse
[{"CreateIndex":97,"ModifyIndex":97,"Key":"web/key1","Flags":0,"Value":"dGVzdA=="},
 {"CreateIndex":98,"ModifyIndex":98,"Key":"web/key2","Flags":42,"Value":"dGVzdA=="},
 {"CreateIndex":99,"ModifyIndex":99,"Key":"web/sub/key3","Flags":0,"Value":"dGVzdA=="}]

Here we have created 3 keys, each with the value of "test". Note that the Value field returned is base64 encoded to allow non-UTF8 characters. For the "web/key2" key, we set a flag value of 42. All keys support setting a 64bit integer flag value. This is opaque to Consul but can be used by clients for any purpose.

After setting the values, we then issued a GET request to retrieve multiple keys using the ?recurse parameter.

You can also fetch a single key just as easily:

$ curl http://localhost:8500/v1/kv/web/key1
[{"CreateIndex":97,"ModifyIndex":97,"Key":"web/key1","Flags":0,"Value":"dGVzdA=="}]

Deleting keys is simple as well. We can delete a single key by specifying the full path, or we can recursively delete all keys under a root using "?recurse":

$ curl -X DELETE http://localhost:8500/v1/kv/web/sub?recurse
$ curl http://localhost:8500/v1/kv/web?recurse
[{"CreateIndex":97,"ModifyIndex":97,"Key":"web/key1","Flags":0,"Value":"dGVzdA=="},
 {"CreateIndex":98,"ModifyIndex":98,"Key":"web/key2","Flags":42,"Value":"dGVzdA=="}]

A key can be updated by setting a new value by issuing the same PUT request. Additionally, Consul provides a Check-And-Set operation, enabling atomic key updates. This is done by providing the ?cas= parameter with the last ModifyIndex value from the GET request. For example, suppose we wanted to update "web/key1":

$ curl -X PUT -d 'newval' http://localhost:8500/v1/kv/web/key1?cas=97
true
$ curl -X PUT -d 'newval' http://localhost:8500/v1/kv/web/key1?cas=97
false

In this case, the first CAS update succeeds because the last modify time is 97. However the second operation fails because the ModifyIndex is no longer 97.

We can also make use of the ModifyIndex to wait for a key's value to change. For example, suppose we wanted to wait for key2 to be modified:

$ curl "http://localhost:8500/v1/kv/web/key2?index=101&wait=5s"
[{"CreateIndex":98,"ModifyIndex":101,"Key":"web/key2","Flags":42,"Value":"dGVzdA=="}]

By providing "?index=" we are asking to wait until the key has a ModifyIndex greater than 101. However the "?wait=5s" parameter restricts the query to at most 5 seconds, returning the current, unchanged value. This can be used to efficiently wait for key modifications. Additionally, this same technique can be used to wait for a list of keys, waiting only until any of the keys has a newer modification time.

This is only a few example of what the API supports. For full documentation, please reference the HTTP API.