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---
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layout: "docs"
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page_title: "Semaphore"
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sidebar_current: "docs-guides-semaphore"
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description: |-
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This guide demonstrates how to implement a distributed semaphore using the Consul Key/Value store.
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---
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# Semaphore
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2015-03-01 05:18:21 +00:00
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This guide demonstrates how to implement a distributed semaphore using the Consul
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Key/Value store. This is useful when you want to coordinate many services while
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restricting access to certain resources.
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~> If you only need mutual exclusion or leader election,
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[this guide](/docs/guides/leader-election.html)
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provides a simpler algorithm that can be used instead.
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There are a number of ways that a semaphore can be built, so our goal is not to
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cover all the possible methods. Instead, we will focus on using Consul's support for
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[sessions](/docs/internals/sessions.html). Sessions allow us to build a system that
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can gracefully handle failures.
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Note that JSON output in this guide has been pretty-printed for easier
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reading. Actual values returned from the API will not be formatted.
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## Contending Nodes
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Let's imagine we have a set of nodes who are attempting to acquire a slot in the
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semaphore. All nodes that are participating should agree on three decisions: the
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prefix in the Key/Value store used to coordinate, a single key to use as a lock,
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and a limit on the number of slot holders.
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For the prefix we will be using for coordination, a good pattern is simply:
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```text
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service/<service name>/lock/
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```
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We'll abbreviate this pattern as simply `<prefix>` for the rest of this guide.
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The first step is to create a session. This is done using the
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[Session HTTP API](/docs/agent/http/session.html#session_create):
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```text
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curl -X PUT -d '{"Name": "dbservice"}' \
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http://localhost:8500/v1/session/create
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```
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This will return a JSON object contain the session ID:
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```text
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{
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"ID": "4ca8e74b-6350-7587-addf-a18084928f3c"
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}
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```
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Next, we create a contender entry. Each contender creates an entry that is tied
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to a session. This is done so that if a contender is holding a slot and fails,
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it can be detected by the other contenders.
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Create the contender key by doing an `acquire` on `<prefix>/<session>` via `PUT`.
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This is something like:
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```text
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curl -X PUT -d <body> http://localhost:8500/v1/kv/<prefix>/<session>?acquire=<session>
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```
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The `<session>` value is the ID returned by the call to
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[`/v1/session/create`]((/docs/agent/http/session.html#session_create).
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`body` can be used to associate a meaningful value with the contender. This is opaque
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to Consul but can be useful for human operators.
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The call will either return `true` or `false`. If `true`, the contender entry has been
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created. If `false`, the contender node was not created; it'slikely that this indicates
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a session invalidation.
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The next step is to use a single key to coordinate which holders are currently
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reserving a slot. A good choice for this lock key is simply `<prefix>/.lock`. We will
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refer to this special coordinating key as `<lock>`.
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The current state of the semaphore is read by doing a `GET` on the entire `<prefix>`:
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```text
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curl http://localhost:8500/v1/kv/<prefix>?recurse
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```
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Within the list of the entries, we should find the `<lock>`. That entry should hold
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both the slot limit and the current holders. A simple JSON body like the following works:
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```text
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{
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"Limit": 3,
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"Holders": {
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"4ca8e74b-6350-7587-addf-a18084928f3c": true,
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"adf4238a-882b-9ddc-4a9d-5b6758e4159e": true
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}
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}
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```
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When the `<lock>` is read, we can verify the remote `Limit` agrees with the local value. This
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is used to detect a potential conflict. The next step is to determine which of the current
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slot holders are still alive. As part of the results of the `GET`, we have all the contender
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entries. By scanning those entries, we create a set of all the `Session` values. Any of the
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`Holders` that are not in that set are pruned. In effect, we are creating a set of live contenders
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based on the list results and doing a set difference with the `Holders` to detect and prune
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any potentially failed holders.
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If the number of holders (after pruning) is less than the limit, a contender attempts acquisition
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by adding its own session to the `Holders` and doing a Check-And-Set update of the `<lock>`. This
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performs an optimistic update.
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This is done by:
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```text
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curl -X PUT -d <Updated Lock> http://localhost:8500/v1/kv/<lock>?cas=<lock-modify-index>
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```
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If this suceeds with `true`, the contender now holds a slot in the semaphore. If this fails
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with `false`, then likely there was a race with another contender to acquire the slot.
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Both code paths now go into an idle waiting state. In this state, we watch for changes
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on `<prefix>`. This is because a slot may be released, a node may fail, etc.
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Slot holders must also watch for changes since the slot may be released by an operator
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or automatically released due to a false positive in the failure detector.
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Note that the session by default makes use of only the gossip failure detector. That
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is, the session is considered held by a node as long as the default Serf health check
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has not declared the node unhealthy. Additional checks can be specified if desired.
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Watching for changes is done via a blocking query against `<prefix>`. If a contender
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holds a slot, then on any change the `<lock>` should be re-checked to ensure the slot is
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still held. If no slot is held, then the same acquisition logic is triggered to check
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and potentially re-attempt acquisition. This allows a contender to steal the slot from
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a failed contender or one that has voluntarily released its slot.
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If a slot holder ever wishes to release voluntarily, this should be done by doing a
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Check-And-Set operation against `<lock>` to remove its session from the `Holders` object.
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Once that is done, the contender entry at `<prefix>/<session>` should be deleted. Finally,
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the session should be destroyed.
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