175 lines
6.8 KiB
Markdown
175 lines
6.8 KiB
Markdown
---
|
||
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.
|