---
layout: "docs"
page_title: "Semaphore"
sidebar_current: "docs-guides-semaphore"
description: |-
  This guide demonstrates how to implement a distributed semaphore using the Consul KV store.
---

# Semaphore

This guide demonstrates how to implement a distributed semaphore using the Consul
KV store. This is useful when you want to coordinate many services while
restricting access to certain resources.

~>  If you only need mutual exclusion or leader election,
[this guide](/docs/guides/leader-election.html)
provides a simpler algorithm that can be used instead.

There are a number of ways that a semaphore can be built, so our goal is not to
cover all the possible methods. Instead, we will focus on using Consul's support for
[sessions](/docs/internals/sessions.html). Sessions allow us to build a system that
can gracefully handle failures.

-> **Note:** JSON output in this guide has been pretty-printed for easier reading. Actual values returned from the API will not be formatted.

## Contending Nodes

Let's imagine we have a set of nodes who are attempting to acquire a slot in the
semaphore. All nodes that are participating should agree on three decisions: the
prefix in the KV store used to coordinate, a single key to use as a lock,
and a limit on the number of slot holders.

For the prefix we will be using for coordination, a good pattern is simply:

```text
service/<service name>
```

We'll abbreviate this pattern as simply `<prefix>` for the rest of this guide.

The first step is for each contender to create a session. This is done using the
[Session HTTP API](/api/session.html#session_create):

```text
curl  -X PUT -d '{"Name": "db-semaphore"}' \
  http://localhost:8500/v1/session/create
 ```

This will return a JSON object contain the session ID:

```text
{
  "ID": "4ca8e74b-6350-7587-addf-a18084928f3c"
}
```

->  **Note:** Sessions by default only make use of the gossip failure detector. That is, the session is considered held by a node as long as the default Serf health check has not declared the node unhealthy. Additional checks can be specified at session creation if desired.

Next, we create a lock contender entry. Each contender creates a kv entry that is tied
to a session. This is done so that if a contender is holding a slot and fails, its session
is detached from the key, which can then be detected by the other contenders.

Create the contender key by doing an `acquire` on `<prefix>/<session>` via `PUT`.
This is something like:

```text
curl -X PUT -d <body> http://localhost:8500/v1/kv/<prefix>/<session>?acquire=<session>
 ```

`body` can be used to associate a meaningful value with the contender, such as its node’s name. 
This body is opaque to Consul but can be useful for human operators.

The `<session>` value is the ID returned by the call to
[`/v1/session/create`](/api/session.html#session_create).

The call will either return `true` or `false`. If `true`, the contender entry has been
created. If `false`, the contender node was not created; it's likely that this indicates
a session invalidation.

The next step is to create a single key to coordinate which holders are currently
reserving a slot. A good choice for this lock key is simply `<prefix>/.lock`. We will
refer to this special coordinating key as `<lock>`.

This is done with:

```text
curl -X PUT -d <body> http://localhost:8500/v1/kv/<lock>?cas=0
 ```

Since the lock is being created, a `cas` index of 0 is used so that the key is only put if it does not exist.

`body` should contain both the intended slot limit for the semaphore and the session ids
of the current holders (initially only of the creator). A simple JSON body like the following works:

```text
{
    "Limit": 2,
    "Holders": [
      "<session>"
    ]
}
```

The current state of the semaphore is read by doing a `GET` on the entire `<prefix>`:

```text
curl http://localhost:8500/v1/kv/<prefix>?recurse
 ```

Within the list of the entries, we should find two keys: the `<lock>` and the
contender key ‘<prefix>/<session>’. 

```text
[
  {
    "LockIndex": 0,
    "Key": "<lock>",
    "Flags": 0,
    "Value": "eyJMaW1pdCI6IDIsIkhvbGRlcnMiOlsiPHNlc3Npb24+Il19",
    "Session": "",
    "CreateIndex": 898,
    "ModifyIndex": 901
  },
  {
    "LockIndex": 1,
    "Key": "<prefix>/<session>",
    "Flags": 0,
    "Value": null,
    "Session": "<session>",
    "CreateIndex": 897,
    "ModifyIndex": 897
  }
]
```
Note that the `Value` we embedded into `<lock>` is Base64 encoded when returned by the API.

When the `<lock>` is read and its `Value` is decoded, we can verify the `Limit` agrees with the `Holders` count. 
This is used to detect a potential conflict. The next step is to determine which of the current
slot holders are still alive. As part of the results of the `GET`, we also have all the contender
entries. By scanning those entries, we create a set of all the `Session` values. Any of the
`Holders` that are not in that set are pruned. In effect, we are creating a set of live contenders
based on the list results and doing a set difference with the `Holders` to detect and prune
any potentially failed holders. In this example `<session>` is present in `Holders` and 
is attached to the key `<prefix>/<session>`, so no pruning is required.

If the number of holders after pruning is less than the limit, a contender attempts acquisition
by adding its own session to the `Holders` list and doing a Check-And-Set update of the `<lock>`. 
This performs an optimistic update.

This is done with:

```text
curl -X PUT -d <Updated Lock Body> http://localhost:8500/v1/kv/<lock>?cas=<lock-modify-index>
 ```
`lock-modify-index` is the latest `ModifyIndex` value known for `<lock>`, 901 in this example.

If this request succeeds with `true`, the contender now holds a slot in the semaphore. 
If this fails with `false`, then likely there was a race with another contender to acquire the slot.

To re-attempt the acquisition, we watch for changes on `<prefix>`. This is because a slot
may be released, a node may fail, etc. Watching for changes is done via a blocking query
against `/kv/<prefix>?recurse`. 

Slot holders **must** continously watch for changes to `<prefix>` since their slot can be 
released by an operator or automatically released due to a false positive in the failure detector. 
On changes to `<prefix>` the lock’s `Holders` list must be re-checked to ensure the slot
is still held. Additionally, if the watch fails to connect the slot should be considered lost. 

This semaphore system is purely *advisory*. Therefore it is up to the client to verify
that a slot is held before (and during) execution of some critical operation.

Lastly, if a slot holder ever wishes to release its slot voluntarily, it should be done by doing a
Check-And-Set operation against `<lock>` to remove its session from the `Holders` object.
Once that is done, both its contender key `<prefix>/<session>` and session should be deleted.
