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A general purpose distributed locking library built for AWS DynamoDB.
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README.md

dynamodb-lock-client

Stability: 1 - Experimental

NPM version

A general purpose distributed locking library with fencing tokens built for AWS DynamoDB.

Contributors

@tristanls, @Jacob-Lynch, @simlu, Lukas Siemon, @tomyam1

Contents

Installation

npm install dynamodb-lock-client

Usage

To run the below example, run:

npm run readme
"use strict";

const AWS = require("aws-sdk");
const DynamoDBLockClient = require("../index.js");

const dynamodb = new AWS.DynamoDB.DocumentClient(
    {
        region: "us-east-1"
    }
);

// "fail closed": if process crashes and lock is not released, lock will
//                never be released (requires human intervention)
const failClosedClient = new DynamoDBLockClient.FailClosed(
    {
        dynamodb,
        lockTable: "my-lock-table-name",
        partitionKey: "mylocks",
        acquirePeriodMs: 1e4
    }
);

failClosedClient.acquireLock("my-fail-closed-lock", (error, lock) =>
    {
        if (error)
        {
            return console.error(error)
        }
        console.log("acquired fail closed lock");
        // do stuff
        lock.release(error => error ? console.error(error) : console.log("released fail closed lock"));
    }
);

// "fail open": if process crashes and lock is not released, lock will
//              eventually expire after leaseDurationMs from last heartbeat
//              sent
const failOpenClient = new DynamoDBLockClient.FailOpen(
    {
        dynamodb,
        lockTable: "my-lock-table-name",
        partitionKey: "mylocks",
        heartbeatPeriodMs: 3e3,
        leaseDurationMs: 1e4
    }
);

failOpenClient.acquireLock("my-fail-open-lock", (error, lock) =>
    {
        if (error)
        {
            return console.error(error)
        }
        console.log(`acquired fail open lock with fencing token ${lock.fencingToken}`);
        lock.on("error", error => console.error("failed to heartbeat!"));
        // do stuff
        lock.release(error => error ? console.error(error) : console.log("released fail open lock"));
    }
);

Tests

No tests at this time.

Documentation

Setting up the lock table in DynamoDB

The DynamoDB lock table needs to be created independently. The following is an example CloudFormation template that would create such a lock table:

AWSTemplateFormatVersion: "2010-09-09"

Resources:

  DistributedLocksStore:
    Type: AWS::DynamoDB::Table
    Properties:
      AttributeDefinitions:
        - AttributeName: id
          AttributeType: S
      KeySchema:
        - AttributeName: id
          KeyType: HASH
      ProvisionedThroughput:
        ReadCapacityUnits: !Ref DistributedLocksStoreReadCapacityUnits
        WriteCapacityUnits: !Ref DistributedLocksStoreWriteCapacityUnits
      TableName: "distributed-locks-store"

Parameters:

  DistributedLocksStoreReadCapacityUnits:
    Type: Number
    Default: 1
    Description: DistributedLocksStore ReadCapacityUnits

  DistributedLocksStoreWriteCapacityUnits:
    Type: Number
    Default: 1
    Description: DistributedLocksStore WriteCapacityUnits

Outputs:

  DistributedLocksStore:
    Value: !GetAtt DistributedLocksStore.Arn

The template above would make your config.partitionKey == "id" and your config.lockTable == "distributed-lock-store".

You can choose to call your config.partitionKey any valid string except guid or owner (these attribute names are reserved for use by DynamoDBLockClient library). Your config.partitionKey has to correspond to the partition key (HASH) of the Primary Key of your DynamoDB table.

DynamoDBLockClient

Public API

new DynamoDBLockClient.FailClosed(config)

  • config: Object
    • dynamodb: AWS.DynamoDB.DocumentClient Instance of AWS DynamoDB DocumentClient.
    • lockTable: String Name of lock table to use.
    • partitionKey: String Name of table partition key (hash key) to use.
    • acquirePeriodMs: Number How long to wait for the lock before giving up. Whatever operation this lock is protecting should take less time than acquirePeriodMs.
    • owner: String Customize owner name for lock (optional).
  • Return: Object Fail closed client.

Creates a "fail closed" client that acquires "fail closed" locks. If process crashes and lock is not released, lock will never be released. This means that some sort of intervention will be required to put the system back into operational state if lock is held and a process crashes while holding the lock.

new DynamoDBLockClient.FailOpen(config)

  • config: Object
    • dynamodb: AWS.DynamoDB.DocumentClient Instance of AWS DynamoDB DocumentClient.
    • lockTable: String Name of lock table to use.
    • partitionKey: String Name of table partition key (hash key) to use.
    • heartbeatPeriodMs: Number (Default: undefined) Optional period at which to send heartbeats in order to keep the lock locked. Providing this option will cause heartbeats to be sent.
    • leaseDurationMs: Number The length of lock lease duration. If the lock is not renewed via a heartbeat within leaseDurationMs it will be automatically released.
    • owner: String Customize owner name for lock (optional).
    • trustLocalTime: Boolean (Default: false) If set to true, when the client retrieves an existing lock, it will use local time to determine if leaseDurationMs has elapsed (and shorten its wait time accordingly) instead of always waiting the full leaseDurationMs milliseconds before making an acquisition attempt.
  • Return: Object Fail open client.

Creates a "fail open" client that acquires "fail open" locks. If process crashes and lock is not released, lock will eventually expire after leaseDurationMs from last heartbeat sent (if any). This means that if process acquires a lock, goes to sleep for more than leaseDurationMs, and then wakes up assuming it still has a lock, then it can perform an operation ignoring other processes that may assume they have a lock on the operation.

client.acquireLock(id, callback)

  • id: String|Buffer|Number Unique identifier for the lock. The type must correspond to lock table's partition key type.
  • callback: Function (error, lock) => {}
    • error: Error Error, if any.
    • lock: DynamoDBLockClient.Lock Successfully acquired lock object. Lock object is an instance of EventEmitter. If the lock is acquired via a fail open client configured to heartbeat, then the returned lock may emit an error event if a heartbeat operation fails.
      • fencingToken: Integer fail open locks only Integer monotonically incremented with every "fail open" lock acquisition to be used for fencing. Heartbeats do not increment fencingToken.

Attempts to acquire a lock. If lock acquisition fails, callback will be called with an error and lock will be falsy. If lock acquisition succeeds, callback will be called with lock, and error will be falsy.

Fail closed client will attempt to acquire a lock. On failure, client will retry after acquirePeriodMs. On another failure, client will fail lock acquisition. On successful acquisition, lock will be locked until lock.release() is called successfuly.

Fail open client will attempt to acquire a lock. On failure, if trustLocalTime is false (the default), client will retry after leaseDurationMs. If trustLocalTime is true, the client will retry after Math.max(0, leaseDurationMs - (localTimeMs - lockAcquiredTimeMs)) where localTimeMs is "now" and lockAcquiredTimeMs is the lock acquisition time recorded in the retrieved lock. On another failure, client will fail lock acquisition. On successful acquisition, if heartbeatPeriodMs option is not specified (heartbeats off), lock will expire after leaseDurartionMs. If heartbeatPeriodMs option is specified, lock will be renewed at heartbeatPeriodMs intervals until lock.release() is called successfuly. Additionally, if heartbeatPeriodMs option is specified, lock may emit an error event if it fails a heartbeat operation.

lock.release(callback)

  • callback: Function error => {}
    • error: Error Error, if any. No error implies successful lock release.

Releases previously acquired lock.

Fail closed lock is deleted, so that it can be acquired again.

Fail open lock heartbeats stop, and its leaseDurationMs is set to 1 millisecond so that it expires immediately. The datastructure is left in the datastore in order to provide continuity of fencingToken monotonicity guarantee.

Releases

We follow semantic versioning policy (see: semver.org):

Given a version number MAJOR.MINOR.PATCH, increment the:

MAJOR version when you make incompatible API changes,
MINOR version when you add functionality in a backwards-compatible manner, and
PATCH version when you make backwards-compatible bug fixes.

caveat: Major version zero is a special case indicating development version that may make incompatible API changes without incrementing MAJOR version.

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