advisory-lock

Distributed* locking using PostgreSQL advisory locks.

Some use cases:

• You have a clock process and want to make absolutely sure there will never be more than one process active at any given time.

  This sort of situation can otherwise arise if the clock process is scaled up by accident or during a deployment which keeps the old version running until the new version responds to a health check.

• Running a database migration at server startup. If your app is scaled, multiple processes will simultaneously try to run the database migration which can lead to problems.

• Leader election. Let's say you have a web app and want to post a message to Slack every 30 mins containing some statistic (e.g. new registrations in the last 30 mins). You might have 10 processes running but don't want to get 10 identical messages in Slack. You can use this library to elect a "master" process which is responsible for sending the message.

• etc.

* Your PostgreSQL database being a central point of failure. For a high available distributed lock, have a look at ZooKeeper.

Install

npm install --save advisory-lock

CLI Usage

A withlock command line utility is provided to make to facilitate the common use case of ensuring only one instance of a process is running at any time.

withlock <lockName> [--db <connectionString>] -- <command>

Where <lockName> is the name of the lock, <command> (everything after --) is the command to run exclusively, once the lock is acquired. --db <connectionString> is optional and if not specified, the PG_CONNECTION_STRING environment variable will be used.

Example:

export PG_CONNECTION_STRING="postgres://postgres@127.0.0.1/mydb"
withlock dbmigration -- npm run knex migrate:latest

Usage

advisoryLock(connectionString)

• connectionString must be a Postgres connection string

Returns a createMutex function.

The createMutex function also exposes a client property that can be used to terminate the database connection if necessary.

PS: Each call to advisoryLock(connectionString) creates a new PostgreSQL connection which is not automatically terminated, so if that is an issue for you, you can use createMutex.client.end() to end the connection when appropriate (e.g. after releasing a lock). This is however typically not needed since usually, advisoryLock() only needs to be called once.

createMutex(lockName)

• lockName must be a unique identifier for the lock

Returns a mutex object containing the functions listed below. All object methods are really just functions attached to the object and are not bound to this so they can be safely destructured, e.g. const { withLock } = createMutext(lockName).

For a better understanding of what each functions does, see PosgtreSQL's manual.

mutex.withLock(fn)

• fn Promise returning function or regular function to be executed once the lock is acquired

Like lock() but automatically release the lock after fn() resolves.

Returns a promise which resolves to the value fn resolves to.

Throws an error if the Postgres connection closes unexpectedly.

mutex.tryLock()

Returns a promise which resolves to true if the lock is free and false if the lock is taken. Doesn't "block".

mutex.lock()

Wait until we get exclusive lock.

mutex.unlock()

Release the exclusive lock.

mutex.tryLockShared()

Like tryLock() but for shared lock.

mutex.lockShared()

While held, this blocks any attempt to obtain an exclusive lock. (e.g.: calls to .lock() or .withLock())

mutex.unlockShared()

Release shared lock.

mutex.withLockShared(fn)

Same as withLock() but using a shared lock.

Example

import advisoryLock from 'advisory-lock'
const mutex = advisoryLock('postgres://user:pass@localhost:3475/dbname')('some-lock-name')

const doSomething = () => {
  // doSomething
  return Promise.resolve()
}

mutex
  .withLock(doSomething) // "blocks" until lock is free
  .catch((err) => {
    // this gets executed if the postgres connection closes unexpectedly, etc.
  })
  .then(() => {
    // lock is released now...
  })

// doesn't "block"
mutex.tryLock().then((obtainedLock) => {
  if (obtainedLock) {
    return doSomething().then(() => mutex.unlock())
  } else {
    throw new Error('failed to obtain lock')
  }
})

See ./test for more usage examples.