---
layout: commands
page_title: 'Commands: Lock'
description: >-
  The lock command provides a mechanism for leader election, mutual exclusion,
  or worker pools. For example, this can be used to ensure a maximum number of
  services running at once across a cluster.
---

# Consul Lock

Command: `consul lock`

The `lock` command provides a mechanism for simple distributed locking.
A lock (or semaphore) is created at a given prefix in the KV store,
and only when held, is a child process invoked. If the lock is lost or
communication is disrupted, the child process is terminated.

The number of lock holders is configurable with the `-n` flag. By default,
a single holder is allowed, and a lock is used for mutual exclusion. This
uses the [leader election algorithm](https://learn.hashicorp.com/consul/developer-configuration/elections).

If the lock holder count is more than one, then a semaphore is used instead.
A semaphore allows more than a single holder, but this is less efficient than
a simple lock. This follows the [semaphore algorithm](https://learn.hashicorp.com/consul/developer-configuration/semaphore).

All locks using the same prefix must agree on the value of `-n`. If conflicting
values of `-n` are provided, an error will be returned.

An example use case is for highly-available N+1 deployments. In these
cases, if N instances of a service are required, N+1 are deployed and use
consul lock with `-n=N` to ensure only N instances are running. For singleton
services, a hot standby waits until the current leader fails to take over.

## Usage

Usage: `consul lock [options] prefix child...`

The only required options are the key prefix and the command to execute.
The prefix must be writable. The child is invoked only when the lock is held,
and the `CONSUL_LOCK_HELD` environment variable will be set to `true`.

If the lock is lost, communication is disrupted, or the parent process
interrupted, the child process will receive a `SIGTERM`. After a grace period
of 5 seconds, a `SIGKILL` will be used to force termination. For Consul agents
on Windows, the child process is always terminated with a `SIGKILL`, since
Windows has no POSIX compatible notion for `SIGTERM`.

#### API Options

@include 'http_api_options_client.mdx'

@include 'http_api_options_server.mdx'

#### Command Options

- `-child-exit-code` - Exit 2 if the child process exited with an error
  if this is true, otherwise this doesn't propagate an error from the
  child. The default value is false.

- `-monitor-retry` - Retry up to this number of times if Consul returns a 500 error
  while monitoring the lock. This allows riding out brief periods of unavailability
  without causing leader elections, but increases the amount of time required
  to detect a lost lock in some cases. Defaults to 3, with a 1s wait between retries.
  Set to 0 to disable.

- `-n` - Optional, limit of lock holders. Defaults to 1. The underlying
  implementation switches from a lock to a semaphore when increased past
  one. All locks on the same prefix must use the same value.

- `-name` - Optional name to associate with the underlying session.
  If not provided, one is generated based on the child command.

- `-shell` - Optional, use a shell to run the command (can set a custom shell via the
  SHELL environment variable). The default value is true.

- `-pass-stdin` - Pass stdin to child process.

- `-timeout` - Attempt to acquire the lock up to the given timeout. The timeout is a
  positive decimal number, with unit suffix, such as "500ms". Valid time units
  are "ns", "us" (or "µs"), "ms", "s", "m", "h". The default value is 0.

- `-verbose` - Enables verbose output.

## SHELL

Consul lock launches its children in a shell. By default, Consul will use the shell
defined in the environment variable `SHELL`. If `SHELL` is not defined, it will
default to `/bin/sh`. It should be noted that not all shells terminate child
processes when they receive `SIGTERM`. Under Ubuntu, `/bin/sh` is linked to `dash`,
which does **not** terminate its children. In order to ensure that child processes
are killed when the lock is lost, be sure to set the `SHELL` environment variable
appropriately, or run without a shell by setting `-shell=false`.
