etcd/Documentation/api.md

etcd API

Running a Single Machine Cluster

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

./etcd -data-dir machine0 -name machine0

This will bring up etcd listening on port 4001 for client communication and on port 7001 for server-to-server communication. The -data-dir machine0 argument tells etcd to write machine configuration, logs and snapshots to the ./machine0/ directory. The -name machine tells the rest of the cluster that this machine is named machine0.

Setting the value to a key

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

curl -L http://127.0.0.1:4001/v2/keys/message -X PUT -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 the 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 at which point in the etcd state machine 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 and compareAndSwap. 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 that provide global information about the etcd cluster that serviced a request:

X-Etcd-Index: 35
X-Raft-Index: 5398
X-Raft-Term: 0

• X-Etcd-Index is the current etcd index as explained above.
• X-Raft-Index is similar to the etcd index but is for the underlying raft protocol
• X-Raft-Term this number will increase when an etcd master election happens. If this number is increasing rapdily 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 -L http://127.0.0.1:4001/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 -L http://127.0.0.1:4001/v2/keys/message -XPUT -d value="Hello etcd"

{
    "action": "set",
    "node": {
        "createdIndex": 3,
        "key": "/message",
        "modifiedIndex": 3,
        "value": "Hello etcd"
    }
}

Deleting a key

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

curl -L http://127.0.0.1:4001/v2/keys/message -XDELETE

{
    "action": "delete",
    "node": {
        "createdIndex": 3,
        "key": "/message",
        "modifiedIndex": 4
    }
}

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 send a PUT request:

curl -L http://127.0.0.1:4001/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 that this key will expire and be deleted.

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

NOTE: Keys can only be expired by a cluster leader so if a machine 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 -L http://127.0.0.1:4001/v2/keys/foo

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

{
    "cause": "/foo",
    "errorCode": 100,
    "index": 6,
    "message": "Key Not Found"
}

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 request with wait=true :

curl -L http://127.0.0.1:4001/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 -L http://127.0.0.1:4001/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"
    }
}

However, the watch command can do more than this. Using the the index we can watch for commands that has happened in the past. This is useful for ensuring you don't miss events between watch commands.

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

curl -L http://127.0.0.1:4001/v2/keys/foo?wait=true\&waitIndex=7

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

Atomically Creating In-Order Keys

Using the 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 that need to be processed in strict order. An example use case is the locking module which uses it to ensure clients get fair access to a mutex.

Creating an in-order key is easy

curl -X POST http://127.0.0.1:4001/v2/keys/queue -d value=Job1

{
    "action": "create",
    "node": {
        "createdIndex": 6,
        "key": "/queue/6",
        "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 -X POST http://127.0.0.1:4001/v2/keys/queue -d value=Job2

{
    "action": "create",
    "node": {
        "createdIndex": 29,
        "key": "/queue/29",
        "modifiedIndex": 29,
        "value": "Job2"
    }
}

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

curl -s -X GET 'http://127.0.0.1:4001/v2/keys/queue?recursive=true&sorted=true'

{
    "action": "get",
    "node": {
        "createdIndex": 2,
        "dir": true,
        "key": "/queue",
        "modifiedIndex": 2,
        "nodes": [
            {
                "createdIndex": 2,
                "key": "/queue/2",
                "modifiedIndex": 2,
                "value": "Job1"
            },
            {
                "createdIndex": 3,
                "key": "/queue/3",
                "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 -L http://127.0.0.1:4001/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": "/newdir",
        "modifiedIndex": 17,
        "ttl": 30
    }
}

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

curl -L http://127.0.0.1:4001/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 -X GET http://127.0.0.1:4001/v2/keys/dir/asdf\?consistent\=true\&wait\=true

{
    "action": "expire",
    "node": {
        "createdIndex": 8,
        "key": "/dir",
        "modifiedIndex": 15
    }
}

Atomic Compare-and-Swap (CAS)

Etcd can be used as a centralized coordination service in a cluster and CompareAndSwap 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 index of the key.

3. prevExist - checks existence of the key: if prevExist is true, it is a 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 -L http://127.0.0.1:4001/v2/keys/foo -XPUT -d value=one

Let's try some invalid CompareAndSwap commands first.

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

curl -L http://127.0.0.1:4001/v2/keys/foo?prevExist=false -XPUT -d value=three

The error code explains the problem:

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

Now lets provide a prevValue parameter:

curl -L http://127.0.0.1:4001/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] [0 != 8]",
    "errorCode": 101,
    "index": 8,
    "message": "Test Failed"
}

which means CompareAndSwap failed.

Let's try a valid condition:

curl -L http://127.0.0.1:4001/v2/keys/foo?prevValue=one -XPUT -d value=two

The response should be

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

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

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 only you cannot provide a value and must add the dir=true parameter.

curl -L http://127.0.0.1:4001/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 -L http://127.0.0.1:4001/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 -L http://127.0.0.1:4001/v2/keys/

We should see the response as an array of items:

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

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 -L http://127.0.0.1:4001/v2/keys/?recursive=true

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

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 -L -X DELETE 'http://127.0.0.1:4001/v2/keys/dir?dir=true'

{
    "action": "delete",
    "node": {
        "createdIndex": 30,
        "dir": true,
        "key": "/dir",
        "modifiedIndex": 31
    }
}

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

curl -L http://127.0.0.1:4001/v2/keys/dir?recursive=true -XDELETE

{
    "action": "delete",
    "node": {
        "createdIndex": 10,
        "dir": true,
        "key": "/dir",
        "modifiedIndex": 11
    }
}

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 -L http://127.0.0.1:4001/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 -L http://127.0.0.1:4001/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 -L http://127.0.0.1:4001/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.