cirello.io/pglock

PostgreSQL Lock Client for Go

The PostgreSQL Lock Client for Go is a general purpose distributed locking library built for PostgreSQL. The PostgreSQL Lock Client for Go supports both fine-grained and coarse-grained locking as the lock keys can be any arbitrary string, up to a certain length. Please create issues in the GitHub repository with questions, pull request are very much welcome.

Recommended PostgreSQL version: 11 or newer

Use cases

A common use case for this lock client is: let's say you have a distributed system that needs to periodically do work on a given campaign (or a given customer, or any other object) and you want to make sure that two boxes don't work on the same campaign/customer at the same time. An easy way to fix this is to write a system that takes a lock on a customer, but fine-grained locking is a tough problem. This library attempts to simplify this locking problem on top of PostgreSQL.

Another use case is leader election. If you only want one host to be the leader, then this lock client is a great way to pick one. When the leader fails, it will fail over to another host within a customizable lease duration that you set.

Getting Started

To use the PostgreSQL Lock Client for Go, you must make it sure it is present in $GOPATH or in your vendor directory.

$ go get -u cirello.io/pglock

This package has the go.mod file to be used with Go's module system. If you need to work on this package, use go mod edit -replace=cirello.io/pglock@yourlocalcopy.

For your convenience, there is a function in the package called CreateTable that you can use to set up your table, or you may use the schema.sql file. The package level documentation comment has an example of how to use this package. Here is some example code to get you started:

package main

import (
	"log"

	"cirello.io/pglock"
)

func main() {
	db, err := sql.Open("postgres", *dsn)
	if err != nil {
		log.Fatal("cannot connect to test database server:", err)
	}
	c, err := pglock.New(db,
		pglock.WithLeaseDuration(3*time.Second),
		pglock.WithHeartbeatFrequency(1*time.Second),
	)
	if err != nil {
		log.Fatal("cannot create lock client:", err)
	}
	if err := c.CreateTable(); err != nil {
		log.Fatal("cannot create table:", err)
	}
	l, err := c.Acquire("lock-name")
	if err != nil {
		log.Fatal("unexpected error while acquiring 1st lock:", err)
	}
	defer l.Close()
	// execute the logic
}

Selected Features

Send Automatic Heartbeats

When you create the lock client, you can specify WithHeartbeatFrequency(time.Duration) like in the above example, and it will spawn a background goroutine that continually updates the record version number on your locks to prevent them from expiring (it does this by calling the SendHeartbeat() method in the lock client.) This will ensure that as long as your application is running, your locks will not expire until you call Release() or lockItem.Close()

Read the data in a lock without acquiring it

You can read the data in the lock without acquiring it. Here's how:

lock, err := lockClient.Get("kirk");

Logic to avoid problems with clock skew

The lock client never stores absolute times in PostgreSQL. The way locks are expired is that a call to tryAcquire reads in the current lock, checks the record version number of the lock and starts a timer. If the lock still has the same after the lease duration time has passed, the client will determine that the lock is stale and expire it.

What this means is that, even if two different machines disagree about what time it is, they will still avoid clobbering each other's locks.