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etcd API

Running a Single Machine Cluster

These examples will use a single member cluster to show you the basics of the etcd REST API. Let's start etcd:

./bin/etcd

This will bring up etcd listening on the IANA assigned ports and listening on localhost. The IANA assigned ports for etcd are 2379 for client communication and 2380 for server-to-server communication.

Getting the etcd version

The etcd version of a specific instance can be obtained from the /version endpoint.

curl -L http://127.0.0.1:2379/version

Key Space Operations

The primary API of etcd is a hierarchical key space. The key space consists of directories and keys which are generically referred to as "nodes".

Setting the value of a key

Let's set the first key-value pair in the datastore. In this case the key is /message and the value is Hello world.

curl http://127.0.0.1:2379/v2/keys/message -XPUT -d value="Hello world"
{
    "action": "set",
    "node": {
        "createdIndex": 2,
        "key": "/message",
        "modifiedIndex": 2,
        "value": "Hello world"
    }
}

The response object contains several attributes:

  1. action: the action of the request that was just made. The request attempted to modify node.value via a PUT HTTP request, thus the value of action is set.

  2. node.key: the HTTP path to which the request was made. We set /message to Hello world, so the key field is /message. etcd uses a file-system-like structure to represent the key-value pairs, therefore all keys start with /.

  3. node.value: the value of the key after resolving the request. In this case, a successful request was made that attempted to change the node's value to Hello world.

  4. node.createdIndex: an index is a unique, monotonically-incrementing integer created for each change to etcd. This specific index reflects the point in the etcd state member at which a given key was created. You may notice that in this example the index is 2 even though it is the first request you sent to the server. This is because there are internal commands that also change the state behind the scenes, like adding and syncing servers.

  5. node.modifiedIndex: like node.createdIndex, this attribute is also an etcd index. Actions that cause the value to change include set, delete, update, create, compareAndSwap and compareAndDelete. Since the get and watch commands do not change state in the store, they do not change the value of node.modifiedIndex.

Response Headers

etcd includes a few HTTP headers in responses that provide global information about the etcd cluster that serviced a request:

X-Etcd-Index: 35
X-Raft-Index: 5398
X-Raft-Term: 1
  • X-Etcd-Index is the current etcd index as explained above. When request is a watch on key space, X-Etcd-Index is the current etcd index when the watch starts, which means that the watched event may happen after X-Etcd-Index.
  • X-Raft-Index is similar to the etcd index but is for the underlying raft protocol
  • X-Raft-Term is an integer that will increase whenever an etcd master election happens in the cluster. If this number is increasing rapidly, you may need to tune the election timeout. See the tuning section for details.

Get the value of a key

We can get the value that we just set in /message by issuing a GET request:

curl http://127.0.0.1:2379/v2/keys/message
{
    "action": "get",
    "node": {
        "createdIndex": 2,
        "key": "/message",
        "modifiedIndex": 2,
        "value": "Hello world"
    }
}

Changing the value of a key

You can change the value of /message from Hello world to Hello etcd with another PUT request to the key:

curl http://127.0.0.1:2379/v2/keys/message -XPUT -d value="Hello etcd"
{
    "action": "set",
    "node": {
        "createdIndex": 3,
        "key": "/message",
        "modifiedIndex": 3,
        "value": "Hello etcd"
    },
    "prevNode": {
    	"createdIndex": 2,
    	"key": "/message",
    	"value": "Hello world",
    	"modifiedIndex": 2
    }
}

Here we introduce a new field: prevNode. The prevNode field represents what the state of a given node was before resolving the request at hand. The prevNode field follows the same format as the node, and is omitted in the event that there was no previous state for a given node.

Deleting a key

You can remove the /message key with a DELETE request:

curl http://127.0.0.1:2379/v2/keys/message -XDELETE
{
    "action": "delete",
    "node": {
        "createdIndex": 3,
        "key": "/message",
        "modifiedIndex": 4
    },
    "prevNode": {
    	"key": "/message",
    	"value": "Hello etcd",
    	"modifiedIndex": 3,
    	"createdIndex": 3
    }
}

Using key TTL

Keys in etcd can be set to expire after a specified number of seconds. You can do this by setting a TTL (time to live) on the key when sending a PUT request:

curl http://127.0.0.1:2379/v2/keys/foo -XPUT -d value=bar -d ttl=5
{
    "action": "set",
    "node": {
        "createdIndex": 5,
        "expiration": "2013-12-04T12:01:21.874888581-08:00",
        "key": "/foo",
        "modifiedIndex": 5,
        "ttl": 5,
        "value": "bar"
    }
}

Note the two new fields in response:

  1. The expiration is the time at which this key will expire and be deleted.

  2. The ttl is the specified time to live for the key, in seconds.

NOTE: Keys can only be expired by a cluster leader, so if a member gets disconnected from the cluster, its keys will not expire until it rejoins.

Now you can try to get the key by sending a GET request:

curl http://127.0.0.1:2379/v2/keys/foo

If the TTL has expired, the key will have been deleted, and you will be returned a 100.

{
    "cause": "/foo",
    "errorCode": 100,
    "index": 6,
    "message": "Key not found"
}

The TTL can be unset to avoid expiration through update operation:

curl http://127.0.0.1:2379/v2/keys/foo -XPUT -d value=bar -d ttl= -d prevExist=true
{
    "action": "update",
    "node": {
        "createdIndex": 5,
        "key": "/foo",
        "modifiedIndex": 6,
        "value": "bar"
    },
    "prevNode": {
        "createdIndex": 5,
        "expiration": "2013-12-04T12:01:21.874888581-08:00",
        "key": "/foo",
        "modifiedIndex": 5,
        "ttl": 3,
        "value": "bar"
    }
}

Waiting for a change

We can watch for a change on a key and receive a notification by using long polling. This also works for child keys by passing recursive=true in curl.

In one terminal, we send a GET with wait=true :

curl http://127.0.0.1:2379/v2/keys/foo?wait=true

Now we are waiting for any changes at path /foo.

In another terminal, we set a key /foo with value bar:

curl http://127.0.0.1:2379/v2/keys/foo -XPUT -d value=bar

The first terminal should get the notification and return with the same response as the set request:

{
    "action": "set",
    "node": {
        "createdIndex": 7,
        "key": "/foo",
        "modifiedIndex": 7,
        "value": "bar"
    },
    "prevNode": {
        "createdIndex": 6,
        "key": "/foo",
        "modifiedIndex": 6,
        "value": "bar"
    }
}

However, the watch command can do more than this. Using the index, we can watch for commands that have happened in the past. This is useful for ensuring you don't miss events between watch commands. Typically, we watch again from the modifiedIndex + 1 of the node we got.

Let's try to watch for the set command of index 7 again:

curl 'http://127.0.0.1:2379/v2/keys/foo?wait=true&waitIndex=7'

The watch command returns immediately with the same response as previously.

If we were to restart the watch from index 8 with:

curl 'http://127.0.0.1:2379/v2/keys/foo?wait=true&waitIndex=8'

Then even if etcd is on index 9 or 800, the first event to occur to the /foo key between 8 and the current index will be returned.

Note: etcd only keeps the responses of the most recent 1000 events across all etcd keys. It is recommended to send the response to another thread to process immediately instead of blocking the watch while processing the result.

Watch from cleared event index

If we miss all the 1000 events, we need to recover the current state of the watching key space through a get and then start to watch from the X-Etcd-Index + 1.

For example, we set /other="bar" for 2000 times and try to wait from index 8.

curl 'http://127.0.0.1:2379/v2/keys/foo?wait=true&waitIndex=8'

We get the index is outdated response, since we miss the 1000 events kept in etcd.

{"errorCode":401,"message":"The event in requested index is outdated and cleared","cause":"the requested history has been cleared [1008/8]","index":2007}

To start watch, first we need to fetch the current state of key /foo:

curl 'http://127.0.0.1:2379/v2/keys/foo' -vv
< HTTP/1.1 200 OK
< Content-Type: application/json
< X-Etcd-Cluster-Id: 7e27652122e8b2ae
< X-Etcd-Index: 2007
< X-Raft-Index: 2615
< X-Raft-Term: 2
< Date: Mon, 05 Jan 2015 18:54:43 GMT
< Transfer-Encoding: chunked
< 
{"action":"get","node":{"key":"/foo","value":"bar","modifiedIndex":7,"createdIndex":7}}

Unlike watches we use the X-Etcd-Index + 1 of the response as a waitIndex instead of the node's modifiedIndex + 1 for two reasons:

  1. The X-Etcd-Index is always greater than or equal to the modifiedIndex when getting a key because X-Etcd-Index is the current etcd index, and the modifiedIndex is the index of an event already stored in etcd.
  2. None of the events represented by indexes between modifiedIndex and X-Etcd-Index will be related to the key being fetched.

Using the modifiedIndex + 1 is functionally equivalent for subsequent watches, but since it is smaller than the X-Etcd-Index + 1, we may receive a 401 EventIndexCleared error immediately.

So the first watch after the get should be:

curl 'http://127.0.0.1:2379/v2/keys/foo?wait=true&waitIndex=2008'

Connection being closed prematurely

The server may close a long polling connection before emitting any events. This can happen due to a timeout or the server being shutdown. Since the HTTP header is sent immediately upon accepting the connection, the response will be seen as empty: 200 OK and empty body. The clients should be prepared to deal with this scenario and retry the watch.

Atomically Creating In-Order Keys

Using POST on a directory, you can create keys with key names that are created in-order. This can be used in a variety of useful patterns, like implementing queues of keys which need to be processed in strict order. An example use case would be ensuring clients get fair access to a mutex.

Creating an in-order key is easy:

curl http://127.0.0.1:2379/v2/keys/queue -XPOST -d value=Job1
{
    "action": "create",
    "node": {
        "createdIndex": 6,
        "key": "/queue/00000000000000000006",
        "modifiedIndex": 6,
        "value": "Job1"
    }
}

If you create another entry some time later, it is guaranteed to have a key name that is greater than the previous key. Also note the key names use the global etcd index, so the next key can be more than previous + 1.

curl http://127.0.0.1:2379/v2/keys/queue -XPOST -d value=Job2
{
    "action": "create",
    "node": {
        "createdIndex": 29,
        "key": "/queue/00000000000000000029",
        "modifiedIndex": 29,
        "value": "Job2"
    }
}

To enumerate the in-order keys as a sorted list, use the "sorted" parameter.

curl -s 'http://127.0.0.1:2379/v2/keys/queue?recursive=true&sorted=true'
{
    "action": "get",
    "node": {
        "createdIndex": 2,
        "dir": true,
        "key": "/queue",
        "modifiedIndex": 2,
        "nodes": [
            {
                "createdIndex": 2,
                "key": "/queue/00000000000000000002",
                "modifiedIndex": 2,
                "value": "Job1"
            },
            {
                "createdIndex": 3,
                "key": "/queue/00000000000000000003",
                "modifiedIndex": 3,
                "value": "Job2"
            }
        ]
    }
}

Using a directory TTL

Like keys, directories in etcd can be set to expire after a specified number of seconds. You can do this by setting a TTL (time to live) on a directory when it is created with a PUT:

curl http://127.0.0.1:2379/v2/keys/dir -XPUT -d ttl=30 -d dir=true
{
    "action": "set",
    "node": {
        "createdIndex": 17,
        "dir": true,
        "expiration": "2013-12-11T10:37:33.689275857-08:00",
        "key": "/dir",
        "modifiedIndex": 17,
        "ttl": 30
    }
}

The directory's TTL can be refreshed by making an update. You can do this by making a PUT with prevExist=true and a new TTL.

curl http://127.0.0.1:2379/v2/keys/dir -XPUT -d ttl=30 -d dir=true -d prevExist=true

Keys that are under this directory work as usual, but when the directory expires, a watcher on a key under the directory will get an expire event:

curl 'http://127.0.0.1:2379/v2/keys/dir?wait=true'
{
    "action": "expire",
    "node": {
        "createdIndex": 8,
        "key": "/dir",
        "modifiedIndex": 15
    },
    "prevNode": {
        "createdIndex": 8,
        "key": "/dir",
        "dir":true,
        "modifiedIndex": 17,
        "expiration": "2013-12-11T10:39:35.689275857-08:00"
    }
}

Atomic Compare-and-Swap

etcd can be used as a centralized coordination service in a cluster, and CompareAndSwap (CAS) is the most basic operation used to build a distributed lock service.

This command will set the value of a key only if the client-provided conditions are equal to the current conditions.

The current comparable conditions are:

  1. prevValue - checks the previous value of the key.

  2. prevIndex - checks the previous modifiedIndex of the key.

  3. prevExist - checks existence of the key: if prevExist is true, it is an update request; if prevExist is false, it is a create request.

Here is a simple example. Let's create a key-value pair first: foo=one.

curl http://127.0.0.1:2379/v2/keys/foo -XPUT -d value=one

Now let's try some invalid CompareAndSwap commands.

Trying to set this existing key with prevExist=false fails as expected:

curl http://127.0.0.1:2379/v2/keys/foo?prevExist=false -XPUT -d value=three

The error code explains the problem:

{
    "cause": "/foo",
    "errorCode": 105,
    "index": 39776,
    "message": "Key already exists"
}

Now let's provide a prevValue parameter:

curl http://127.0.0.1:2379/v2/keys/foo?prevValue=two -XPUT -d value=three

This will try to compare the previous value of the key and the previous value we provided. If they are equal, the value of the key will change to three.

{
    "cause": "[two != one]",
    "errorCode": 101,
    "index": 8,
    "message": "Compare failed"
}

which means CompareAndSwap failed. cause explains why the test failed. Note: the condition prevIndex=0 always passes.

Let's try a valid condition:

curl http://127.0.0.1:2379/v2/keys/foo?prevValue=one -XPUT -d value=two

The response should be:

{
    "action": "compareAndSwap",
    "node": {
        "createdIndex": 8,
        "key": "/foo",
        "modifiedIndex": 9,
        "value": "two"
    },
    "prevNode": {
    	"createdIndex": 8,
    	"key": "/foo",
    	"modifiedIndex": 8,
    	"value": "one"
    }
}

We successfully changed the value from "one" to "two" since we gave the correct previous value.

Atomic Compare-and-Delete

This command will delete a key only if the client-provided conditions are equal to the current conditions.

The current comparable conditions are:

  1. prevValue - checks the previous value of the key.

  2. prevIndex - checks the previous modifiedIndex of the key.

Here is a simple example. Let's first create a key: foo=one.

curl http://127.0.0.1:2379/v2/keys/foo -XPUT -d value=one

Now let's try some CompareAndDelete commands.

Trying to delete the key with prevValue=two fails as expected:

curl http://127.0.0.1:2379/v2/keys/foo?prevValue=two -XDELETE

The error code explains the problem:

{
	"errorCode": 101,
	"message": "Compare failed",
	"cause": "[two != one]",
	"index": 8
}

As does a CompareAndDelete with a mismatched prevIndex:

curl http://127.0.0.1:2379/v2/keys/foo?prevIndex=1 -XDELETE
{
	"errorCode": 101,
	"message": "Compare failed",
	"cause": "[1 != 8]",
	"index": 8
}

And now a valid prevValue condition:

curl http://127.0.0.1:2379/v2/keys/foo?prevValue=one -XDELETE

The successful response will look something like:

{
	"action": "compareAndDelete",
	"node": {
		"key": "/foo",
		"modifiedIndex": 9,
		"createdIndex": 8
	},
	"prevNode": {
		"key": "/foo",
		"value": "one",
		"modifiedIndex": 8,
		"createdIndex": 8
	}
}

Creating Directories

In most cases, directories for a key are automatically created. But there are cases where you will want to create a directory or remove one.

Creating a directory is just like a key except you cannot provide a value and must add the dir=true parameter.

curl http://127.0.0.1:2379/v2/keys/dir -XPUT -d dir=true
{
    "action": "set",
    "node": {
        "createdIndex": 30,
        "dir": true,
        "key": "/dir",
        "modifiedIndex": 30
    }
}

Listing a directory

In etcd we can store two types of things: keys and directories. Keys store a single string value. Directories store a set of keys and/or other directories.

In this example, let's first create some keys:

We already have /foo=two so now we'll create another one called /foo_dir/foo with the value of bar:

curl http://127.0.0.1:2379/v2/keys/foo_dir/foo -XPUT -d value=bar
{
    "action": "set",
    "node": {
        "createdIndex": 2,
        "key": "/foo_dir/foo",
        "modifiedIndex": 2,
        "value": "bar"
    }
}

Now we can list the keys under root /:

curl http://127.0.0.1:2379/v2/keys/

We should see the response as an array of items:

{
    "action": "get",
    "node": {
        "key": "/",
        "dir": true,
        "nodes": [
            {
                "key": "/foo_dir",
                "dir": true,
                "modifiedIndex": 2,
                "createdIndex": 2
            },
            {
                "key": "/foo",
                "value": "two",
                "modifiedIndex": 1,
                "createdIndex": 1
            }
        ]
    }
}

Here we can see /foo is a key-value pair under / and /foo_dir is a directory. We can also recursively get all the contents under a directory by adding recursive=true.

curl http://127.0.0.1:2379/v2/keys/?recursive=true
{
    "action": "get",
    "node": {
        "key": "/",
        "dir": true,
        "nodes": [
            {
                "key": "/foo_dir",
                "dir": true,
                "nodes": [
                    {
                        "key": "/foo_dir/foo",
                        "value": "bar",
                        "modifiedIndex": 2,
                        "createdIndex": 2
                    }
                ],
                "modifiedIndex": 2,
                "createdIndex": 2
            },
            {
                "key": "/foo",
                "value": "two",
                "modifiedIndex": 1,
                "createdIndex": 1
            }
        ]
    }
}

Deleting a Directory

Now let's try to delete the directory /foo_dir.

You can remove an empty directory using the DELETE verb and the dir=true parameter.

curl 'http://127.0.0.1:2379/v2/keys/foo_dir?dir=true' -XDELETE
{
    "action": "delete",
    "node": {
        "createdIndex": 30,
        "dir": true,
        "key": "/foo_dir",
        "modifiedIndex": 31
    },
    "prevNode": {
    	"createdIndex": 30,
    	"key": "/foo_dir",
    	"dir": true,
    	"modifiedIndex": 30
    }
}

To delete a directory that holds keys, you must add recursive=true.

curl http://127.0.0.1:2379/v2/keys/dir?recursive=true -XDELETE
{
    "action": "delete",
    "node": {
        "createdIndex": 10,
        "dir": true,
        "key": "/dir",
        "modifiedIndex": 11
    },
    "prevNode": {
    	"createdIndex": 10,
    	"dir": true,
    	"key": "/dir",
    	"modifiedIndex": 10
    }
}

Creating a hidden node

We can create a hidden key-value pair or directory by add a _ prefix. The hidden item will not be listed when sending a GET request for a directory.

First we'll add a hidden key named /_message:

curl http://127.0.0.1:2379/v2/keys/_message -XPUT -d value="Hello hidden world"
{
    "action": "set",
    "node": {
        "createdIndex": 3,
        "key": "/_message",
        "modifiedIndex": 3,
        "value": "Hello hidden world"
    }
}

Next we'll add a regular key named /message:

curl http://127.0.0.1:2379/v2/keys/message -XPUT -d value="Hello world"
{
    "action": "set",
    "node": {
        "createdIndex": 4,
        "key": "/message",
        "modifiedIndex": 4,
        "value": "Hello world"
    }
}

Now let's try to get a listing of keys under the root directory, /:

curl http://127.0.0.1:2379/v2/keys/
{
    "action": "get",
    "node": {
        "dir": true,
        "key": "/",
        "nodes": [
            {
                "createdIndex": 2,
                "dir": true,
                "key": "/foo_dir",
                "modifiedIndex": 2
            },
            {
                "createdIndex": 4,
                "key": "/message",
                "modifiedIndex": 4,
                "value": "Hello world"
            }
        ]
    }
}

Here we see the /message key but our hidden /_message key is not returned.

Setting a key from a file

You can also use etcd to store small configuration files, JSON documents, XML documents, etc directly. For example you can use curl to upload a simple text file and encode it:

echo "Hello\nWorld" > afile.txt
curl http://127.0.0.1:2379/v2/keys/afile -XPUT --data-urlencode [email protected]

{
    "action": "get",
    "node": {
        "createdIndex": 2,
        "key": "/afile",
        "modifiedIndex": 2,
        "value": "Hello\nWorld\n"
    }
}

Read Linearization

If you want a read that is fully linearized you can use a quorum=true GET. The read will take a very similar path to a write and will have a similar speed. If you are unsure if you need this feature feel free to email etcd-dev for advice.

Statistics

An etcd cluster keeps track of a number of statistics including latency, bandwidth and uptime. These are exposed via the statistics endpoint to understand the internal health of a cluster.

Leader Statistics

The leader has a view of the entire cluster and keeps track of two interesting statistics: latency to each peer in the cluster, and the number of failed and successful Raft RPC requests. You can grab these statistics from the /v2/stats/leader endpoint:

curl http://127.0.0.1:2379/v2/stats/leader
{
    "followers": {
        "6e3bd23ae5f1eae0": {
            "counts": {
                "fail": 0,
                "success": 745
            },
            "latency": {
                "average": 0.017039507382550306,
                "current": 0.000138,
                "maximum": 1.007649,
                "minimum": 0,
                "standardDeviation": 0.05289178277920594
            }
        },
        "a8266ecf031671f3": {
            "counts": {
                "fail": 0,
                "success": 735
            },
            "latency": {
                "average": 0.012124141496598642,
                "current": 0.000559,
                "maximum": 0.791547,
                "minimum": 0,
                "standardDeviation": 0.04187900156583733
            }
        }
    },
    "leader": "924e2e83e93f2560"
}

Self Statistics

Each node keeps a number of internal statistics:

  • id: the unique identifier for the member
  • leaderInfo.leader: id of the current leader member
  • leaderInfo.uptime: amount of time the leader has been leader
  • name: this member's name
  • recvAppendRequestCnt: number of append requests this node has processed
  • recvBandwidthRate: number of bytes per second this node is receiving (follower only)
  • recvPkgRate: number of requests per second this node is receiving (follower only)
  • sendAppendRequestCnt: number of requests that this node has sent
  • sendBandwidthRate: number of bytes per second this node is sending (leader only). This value is undefined on single member clusters.
  • sendPkgRate: number of requests per second this node is sending (leader only). This value is undefined on single member clusters.
  • state: either leader or follower
  • startTime: the time when this node was started

This is an example response from a follower member:

curl http://127.0.0.1:2379/v2/stats/self
{
    "id": "eca0338f4ea31566",
    "leaderInfo": {
        "leader": "8a69d5f6b7814500",
        "startTime": "2014-10-24T13:15:51.186620747-07:00",
        "uptime": "10m59.322358947s"
    },
    "name": "node3",
    "recvAppendRequestCnt": 5944,
    "recvBandwidthRate": 570.6254930219969,
    "recvPkgRate": 9.00892789741075,
    "sendAppendRequestCnt": 0,
    "startTime": "2014-10-24T13:15:50.072007085-07:00",
    "state": "StateFollower"
}

And this is an example response from a leader member:

curl http://127.0.0.1:2379/v2/stats/self
{
    "id": "924e2e83e93f2560",
    "leaderInfo": {
        "leader": "924e2e83e93f2560",
        "startTime": "2015-02-09T11:38:30.177534688-08:00",
        "uptime": "9m33.891343412s"
    },
    "name": "infra3",
    "recvAppendRequestCnt": 0,
    "sendAppendRequestCnt": 6535,
    "sendBandwidthRate": 824.1758351191694,
    "sendPkgRate": 11.111234716807138,
    "startTime": "2015-02-09T11:38:28.972034204-08:00",
    "state": "StateLeader"
}

Store Statistics

The store statistics include information about the operations that this node has handled.

Operations that modify the store's state like create, delete, set and update are seen by the entire cluster and the number will increase on all nodes. Operations like get and watch are node local and will only be seen on this node.

curl http://127.0.0.1:2379/v2/stats/store
{
    "compareAndSwapFail": 0,
    "compareAndSwapSuccess": 0,
    "createFail": 0,
    "createSuccess": 2,
    "deleteFail": 0,
    "deleteSuccess": 0,
    "expireCount": 0,
    "getsFail": 4,
    "getsSuccess": 75,
    "setsFail": 2,
    "setsSuccess": 4,
    "updateFail": 0,
    "updateSuccess": 0,
    "watchers": 0
}

Cluster Config

See the other etcd APIs for details on the cluster management.