Lock Exec

lock-exec is a CLI that makes it easy to run at-most-once commands in a distributed environment. At Loom we run multiple identical Kubernetes clusters and we use lock-exec to ensure that Kubernetes cron jobs only run in a single cluster. lock-exec uses dynamodb to lock on a user specified key, run the input command, and then unlock the key.

Requirements

You must already be authenticated to AWS with a default region. lock-exec uses config.LoadDefaultConfig to load AWS credentials using the standard credential chain and does not currently support any direct method of authentication.

Additionally, lock-exec requires a dynamodb table to use for locking. This table must have a partition key named key that stores the lock keys. The default table name is lock-exec. If you use a different table name you must specify it explictly using the --table flag.

Required IAM Permissions

The following IAM permissions are required on the DynamoDB table containing the locks:

• dynamodb:GetItem
• dynamodb:PutItem
• dynamodb:UpdateItem
• dynamodb:DeleteItem

Creating The Table

AWS CLI

aws dynamodb create-table --table-name lock-exec \
  --table-class STANDARD \
  --billing-mode PAY_PER_REQUEST \
  --key-schema AttributeName=key,KeyType=HASH
  --attribute-definitions AttributeName=key,AttributeType=S

Terraform

resource "aws_dynamodb_table" "lock_exec" {
  name = "lock-exec"

  table_class  = "STANDARD"
  billing_mode = "PAY_PER_REQUEST"

  hash_key = "key"

  attribute {
    name = "key"
    type = "S"
  }
}

Installation

Download the lock-exec binary from the releases page. You can also install using Go or Docker.

# Using Go
go install github.com/loomhq/lock-exec@latest

# Using Docker
docker run ghcr.io/loomhq/lock-exec --help

Usage

Basic usage is lock-exec run <key> <command>.

$ go run main.go run testkey 'echo "hello world"' -t loomctl-locks
{"level":"info","ts":1651558192.655782,"caller":"cmd/run.go:22","msg":"running command","key":"testkey","command":"echo \"hello world\""}
{"level":"info","ts":1651558192.944402,"caller":"cmd/run.go:33","msg":"command succeeded","key":"testkey","command":"echo \"hello world\"","output":"\"hello world\"\n"}

Once the command finishes running lock-exec will unlock the key. It also listens for os interrupts and unlocks the key before exiting. In the rare case where lock-exec exits and fails to unlock the key will remain locked for the next 24 hours. The key can manually be unlocked earlier using lock-exec unlock <key>.