dynamolock

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README

DynamoDB Lock Client for Go

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The dymanoDB Lock Client for Go is a general purpose distributed locking library built for DynamoDB. The dynamoDB 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.

It is a port in Go of Amazon's original dynamodb-lock-client.

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 DynamoDB.

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 leaseDuration that you set.

Getting Started

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

$ go get -u cirello.io/dynamolock

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/dynamolock@yourlocalcopy.

Then, you need to set up a DynamoDB table that has a hash key on a key with the name key. For your convenience, there is a function in the package called CreateTable that you can use to set up your table, but it is also possible to set up the table in the AWS Console. The table should be created in advance, since it takes a couple minutes for DynamoDB to provision your table for you. 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/dynamolock"
	"github.com/aws/aws-sdk-go/aws"
	"github.com/aws/aws-sdk-go/aws/session"
	"github.com/aws/aws-sdk-go/service/dynamodb"
)

func main() {
	svc := dynamodb.New(session.Must(session.NewSession(&aws.Config{
		Region: aws.String("us-west-2"),
	})))
	c, err := dynamolock.New(svc,
		"locks",
		dynamolock.WithLeaseDuration(3*time.Second),
		dynamolock.WithHeartbeatPeriod(1*time.Second),
	)
	if err != nil {
		log.Fatal(err)
	}
	defer c.Close()

	log.Println("ensuring table exists")
	c.CreateTable("locks",
		dynamolock.WithProvisionedThroughput(&dynamodb.ProvisionedThroughput{
			ReadCapacityUnits:  aws.Int64(5),
			WriteCapacityUnits: aws.Int64(5),
		}),
		dynamolock.WithCustomPartitionKeyName("key"),
	)

	data := []byte("some content a")
	lockedItem, err := c.AcquireLock("spock",
		dynamolock.WithData(data),
		dynamolock.ReplaceData(),
	)
	if err != nil {
		log.Fatal(err)
	}

	log.Println("lock content:", string(lockedItem.Data()))
	if got := string(lockedItem.Data()); string(data) != got {
		log.Println("losing information inside lock storage, wanted:", string(data), " got:", got)
	}

	log.Println("cleaning lock")
	success, err := c.ReleaseLock(lockedItem)
	if !success {
		log.Fatal("lost lock before release")
	}
	if err != nil {
		log.Fatal("error releasing lock:", err)
	}
	log.Println("done")
}

Selected Features

Send Automatic Heartbeats

When you create the lock client, you can specify WithHeartbeatPeriod(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 ReleaseLock() or lockItem.Close()

Read the data in a lock without acquiring it

You can read the data in the lock without acquiring it, and find out who owns the lock. Here's how:

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

Logic to avoid problems with clock skew

The lock client never stores absolute times in DynamoDB -- only the relative "lease duration" time is stored in DynamoDB. The way locks are expired is that a call to acquireLock reads in the current lock, checks the RecordVersionNumber of the lock (which is a GUID) and starts a timer. If the lock still has the same GUID 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.

Required DynamoDB Actions

For an IAM role to take full advantage of dynamolock, it must be allowed to perform all of the following actions on the DynamoDB table containing the locks:

  • GetItem
  • PutItem
  • UpdateItem
  • DeleteItem

Documentation

Overview

Package dynamolock provides a simple utility for using DynamoDB's consistent read/write feature to use it for managing distributed locks.

In order to use this package, the client must create a table in DynamoDB, although the client provides a convenience method for creating that table (CreateTable).

Basic usage: 

import (
	"log"

	"cirello.io/dynamolock"
	"github.com/aws/aws-sdk-go/aws"
	"github.com/aws/aws-sdk-go/aws/session"
	"github.com/aws/aws-sdk-go/service/dynamodb"
)

// ---

svc := dynamodb.New(session.Must(session.NewSession(&aws.Config{
	Region: aws.String("us-west-2"),
})))
c, err := dynamolock.New(svc,
	"locks",
	dynamolock.WithLeaseDuration(3*time.Second),
	dynamolock.WithHeartbeatPeriod(1*time.Second),
)
if err != nil {
	log.Fatal(err)
}
defer c.Close()

log.Println("ensuring table exists")
c.CreateTable("locks",
	dynamolock.WithProvisionedThroughput(&dynamodb.ProvisionedThroughput{
		ReadCapacityUnits:  aws.Int64(5),
		WriteCapacityUnits: aws.Int64(5),
	}),
	dynamolock.WithCustomPartitionKeyName("key"),
)

data := []byte("some content a")
lockedItem, err := c.AcquireLock("spock",
	dynamolock.WithData(data),
	dynamolock.ReplaceData(),
)
if err != nil {
	log.Fatal(err)
}

log.Println("lock content:", string(lockedItem.Data()))
if got := string(lockedItem.Data()); string(data) != got {
	log.Println("losing information inside lock storage, wanted:", string(data), " got:", got)
}

log.Println("cleaning lock")
success, err := c.ReleaseLock(lockedItem)
if !success {
	log.Fatal("lost lock before release")
}
if err != nil {
	log.Fatal("error releasing lock:", err)
}
log.Println("done")

This package is covered by this SLA: https://github.com/cirello-io/public/blob/master/SLA.md

Index

Constants

This section is empty.

Variables

View Source 
var (
	ErrSessionMonitorNotSet  = errors.New("session monitor is not set")
	ErrLockAlreadyReleased   = errors.New("lock is already released")
	ErrCannotReleaseNullLock = errors.New("cannot release null lock item")
	ErrOwnerMismatched       = errors.New("lock owner mismatched")
)

Errors related to session manager life-cycle.

View Source 
var ErrClientClosed = errors.New("client already closed")

ErrClientClosed reports the client cannot be used because it is already closed.

Functions

This section is empty.

Types

type AcquireLockOption 

type AcquireLockOption func(*acquireLockOptions)

AcquireLockOption allows to change how the lock is actually held by the client.

func FailIfLocked 

func FailIfLocked() AcquireLockOption

FailIfLocked will not retry to acquire the lock, instead returning.

func ReplaceData 

func ReplaceData() AcquireLockOption

ReplaceData will force the new content to be stored in the key.

func WithAdditionalAttributes 

func WithAdditionalAttributes(attr map[string]*dynamodb.AttributeValue) AcquireLockOption

WithAdditionalAttributes stores some additional attributes with each lock. This can be used to add any arbitrary parameters to each lock row.

func WithAdditionalTimeToWaitForLock 

func WithAdditionalTimeToWaitForLock(d time.Duration) AcquireLockOption

WithAdditionalTimeToWaitForLock defines how long to wait in addition to the lease duration (if set to 10 minutes, this will try to acquire a lock for at least 10 minutes before giving up and returning an error).

func WithData 

func WithData(b []byte) AcquireLockOption

WithData stores the content into the lock itself.

func WithDeleteLockOnRelease 

func WithDeleteLockOnRelease() AcquireLockOption

WithDeleteLockOnRelease defines whether or not the lock should be deleted when Close() is called on the resulting LockItem will force the new content to be stored in the key.

func WithRefreshPeriod 

func WithRefreshPeriod(d time.Duration) AcquireLockOption

WithRefreshPeriod defines how long to wait before trying to get the lock again (if set to 10 seconds, for example, it would attempt to do so every 10 seconds).

func WithSessionMonitor 

func WithSessionMonitor(safeTime time.Duration, callback func()) AcquireLockOption

WithSessionMonitor registers a callback that is triggered if the lock is about to expire.

The purpose of this construct is to provide two abilities: provide the ability to determine if the lock is about to expire, and run a user-provided callback when the lock is about to expire. The advantage this provides is notification that your lock is about to expire before it is actually expired, and in case of leader election will help in preventing that there are no two leaders present simultaneously.

If due to any reason heartbeating is unsuccessful for a configurable period of time, your lock enters into a phase known as "danger zone." It is during this "danger zone" that the callback will be run.

Bear in mind that the callback may be null. In this case, no callback will be run upon the lock entering the "danger zone"; yet, one can still make use of the Lock.IsAlmostExpired() call. Furthermore, non-null callbacks can only ever be executed once in a lock's lifetime. Independent of whether or not a callback is run, the client will attempt to heartbeat the lock until the lock is released or obtained by someone else.

Consider an example which uses this mechanism for leader election. One way to make use of this SessionMonitor is to register a callback that kills the instance in case the leader's lock enters the danger zone:

type Client 

type Client struct {
	// contains filtered or unexported fields
}

Client is a dynamoDB based distributed lock client.

func New 

func New(dynamoDB dynamodbiface.DynamoDBAPI, tableName string, opts ...ClientOption) (*Client, error)

New creates a new dynamoDB based distributed lock client.

func (*Client) AcquireLock 

func (c *Client) AcquireLock(key string, opts ...AcquireLockOption) (*Lock, error)

AcquireLock holds the defined lock.

func (*Client) AcquireLockWithContext added in v1.4.0 

func (c *Client) AcquireLockWithContext(ctx context.Context, key string, opts ...AcquireLockOption) (*Lock, error)

AcquireLockWithContext holds the defined lock. The given context is passed down to the underlying dynamoDB call.

func (*Client) Close 

func (c *Client) Close() error

Close releases all of the locks.

func (*Client) CloseWithContext added in v1.4.0 

func (c *Client) CloseWithContext(ctx context.Context) error

CloseWithContext releases all of the locks. The given context is passed down to the underlying dynamoDB calls.

func (*Client) CreateTable 

func (c *Client) CreateTable(tableName string, opts ...CreateTableOption) (*dynamodb.CreateTableOutput, error)

CreateTable prepares a DynamoDB table with the right schema for it to be used by this locking library. The table should be set up in advance, because it takes a few minutes for DynamoDB to provision a new instance. Also, if the table already exists, it will return an error.

func (*Client) CreateTableWithContext added in v1.4.0 

func (c *Client) CreateTableWithContext(ctx context.Context, tableName string, opts ...CreateTableOption) (*dynamodb.CreateTableOutput, error)

CreateTableWithContext prepares a DynamoDB table with the right schema for it to be used by this locking library. The table should be set up in advance, because it takes a few minutes for DynamoDB to provision a new instance. Also, if the table already exists, it will return an error. The given context is passed down to the underlying dynamoDB call.

func (*Client) Get 

func (c *Client) Get(key string) (*Lock, error)

Get finds out who owns the given lock, but does not acquire the lock. It returns the metadata currently associated with the given lock. If the client currently has the lock, it will return the lock, and operations such as releaseLock will work. However, if the client does not have the lock, then operations like releaseLock will not work (after calling Get, the caller should check lockItem.isExpired() to figure out if it currently has the lock.)

func (*Client) GetWithContext added in v1.4.0 

func (c *Client) GetWithContext(ctx context.Context, key string) (*Lock, error)

GetWithContext finds out who owns the given lock, but does not acquire the lock. It returns the metadata currently associated with the given lock. If the client currently has the lock, it will return the lock, and operations such as releaseLock will work. However, if the client does not have the lock, then operations like releaseLock will not work (after calling Get, the caller should check lockItem.isExpired() to figure out if it currently has the lock.) If the context is canceled, it is going to return the context error on local cache hit. The given context is passed down to the underlying dynamoDB call.

func (*Client) ReleaseLock 

func (c *Client) ReleaseLock(lockItem *Lock, opts ...ReleaseLockOption) (bool, error)

ReleaseLock releases the given lock if the current user still has it, returning true if the lock was successfully released, and false if someone else already stole the lock or a problem happened. Deletes the lock item if it is released and deleteLockItemOnClose is set.

func (*Client) ReleaseLockWithContext added in v1.4.0 

func (c *Client) ReleaseLockWithContext(ctx context.Context, lockItem *Lock, opts ...ReleaseLockOption) (bool, error)

ReleaseLockWithContext releases the given lock if the current user still has it, returning true if the lock was successfully released, and false if someone else already stole the lock or a problem happened. Deletes the lock item if it is released and deleteLockItemOnClose is set.

func (*Client) SendHeartbeat 

func (c *Client) SendHeartbeat(lockItem *Lock, opts ...SendHeartbeatOption) error

SendHeartbeat indicates that the given lock is still being worked on. If using WithHeartbeatPeriod > 0 when setting up this object, then this method is unnecessary, because the background thread will be periodically calling it and sending heartbeats. However, if WithHeartbeatPeriod = 0, then this method must be called to instruct DynamoDB that the lock should not be expired.

func (*Client) SendHeartbeatWithContext added in v1.4.0 

func (c *Client) SendHeartbeatWithContext(ctx context.Context, lockItem *Lock, opts ...SendHeartbeatOption) error

SendHeartbeatWithContext indicates that the given lock is still being worked on. If using WithHeartbeatPeriod > 0 when setting up this object, then this method is unnecessary, because the background thread will be periodically calling it and sending heartbeats. However, if WithHeartbeatPeriod = 0, then this method must be called to instruct DynamoDB that the lock should not be expired. The given context is passed down to the underlying dynamoDB call.

type ClientOption 

type ClientOption func(*Client)

ClientOption reconfigure the lock client creation.

func DisableHeartbeat 

func DisableHeartbeat() ClientOption

DisableHeartbeat disables automatic hearbeats. Use SendHeartbeat to freshen up the lock.

func WithHeartbeatPeriod 

func WithHeartbeatPeriod(d time.Duration) ClientOption

WithHeartbeatPeriod defines the frequency of the heartbeats. Set to zero to disable it. Heartbeats should have no more than half of the duration of the lease.

func WithLeaseDuration 

func WithLeaseDuration(d time.Duration) ClientOption

WithLeaseDuration defines how long should the lease be held.

func WithLogger 

func WithLogger(l Logger) ClientOption

WithLogger injects a logger into the client, so its internals can be recorded.

func WithOwnerName 

func WithOwnerName(s string) ClientOption

WithOwnerName changes the owner linked to the client, and by consequence to locks.

func WithPartitionKeyName 

func WithPartitionKeyName(s string) ClientOption

WithPartitionKeyName defines the key name used for asserting keys uniqueness.

type CreateTableOption 

type CreateTableOption func(*createDynamoDBTableOptions)

CreateTableOption is an options type for the CreateTable method in the lock client. This allows the user to create a DynamoDB table that is lock client-compatible and specify optional parameters such as the desired throughput and whether or not to use a sort key.

func WithCustomPartitionKeyName 

func WithCustomPartitionKeyName(s string) CreateTableOption

WithCustomPartitionKeyName changes the partition key name of the table. If not specified, the default "key" will be used.

func WithProvisionedThroughput 

func WithProvisionedThroughput(provisionedThroughput *dynamodb.ProvisionedThroughput) CreateTableOption

WithProvisionedThroughput changes the billing mode of DynamoDB and tells DynamoDB to operate in a provisioned throughput mode instead of pay-per-request

func WithTags added in v1.4.0 

func WithTags(tags []*dynamodb.Tag) CreateTableOption

WithTags changes the tags of the table. If not specified, the table will have empty tags.

type Lock 

type Lock struct {
	// contains filtered or unexported fields
}

Lock item properly speaking.

func (*Lock) AdditionalAttributes 

func (l *Lock) AdditionalAttributes() map[string]*dynamodb.AttributeValue

AdditionalAttributes returns the lock's additional data stored during acquisition.

func (*Lock) Close 

func (l *Lock) Close() error

Close releases the lock.

func (*Lock) Data 

func (l *Lock) Data() []byte

Data returns the content of the lock, if any is available.

func (*Lock) IsAlmostExpired 

func (l *Lock) IsAlmostExpired() (bool, error)

IsAlmostExpired returns whether or not the lock is entering the "danger zone" time period.

It returns if the lock has been released or the lock's lease has entered the "danger zone". It returns false if the lock has not been released and the lock has not yet entered the "danger zone"

func (*Lock) IsExpired 

func (l *Lock) IsExpired() bool

IsExpired returns if the lock is expired, released, or neither.

func (*Lock) OwnerName 

func (l *Lock) OwnerName() string

OwnerName returns the lock's owner.

type LockNotGrantedError 

type LockNotGrantedError struct {
	// contains filtered or unexported fields
}

LockNotGrantedError indicates that an AcquireLock call has failed to establish a lock because of its current lifecycle state.

func (*LockNotGrantedError) Error 

func (e *LockNotGrantedError) Error() string

func (*LockNotGrantedError) Unwrap added in v1.3.3 

func (e *LockNotGrantedError) Unwrap() error

Unwrap reveals the underlying cause why the lock was not granted.

type Logger 

type Logger interface {
	Println(v ...interface{})
}

Logger defines the minimum desired logger interface for the lock client.

type ReleaseLockOption 

type ReleaseLockOption func(*releaseLockOptions)

ReleaseLockOption provides options for releasing a lock when calling the releaseLock() method. This class contains the options that may be configured during the act of releasing a lock.

func WithDataAfterRelease 

func WithDataAfterRelease(data []byte) ReleaseLockOption

WithDataAfterRelease is the new data to persist to the lock (only used if deleteLock=false.) If the data is null, then the lock client will keep the data as-is and not change it.

func WithDeleteLock 

func WithDeleteLock(deleteLock bool) ReleaseLockOption

WithDeleteLock defines whether or not to delete the lock when releasing it. If set to false, the lock row will continue to be in DynamoDB, but it will be marked as released.

type SendHeartbeatOption 

type SendHeartbeatOption func(*sendHeartbeatOptions)

SendHeartbeatOption allows to proceed with Lock content changes in the heartbeat cycle.

func DeleteData 

func DeleteData() SendHeartbeatOption

DeleteData removes the Lock data on heartbeat.

func ReplaceHeartbeatData 

func ReplaceHeartbeatData(data []byte) SendHeartbeatOption

ReplaceHeartbeatData overrides the content of the Lock in the heartbeat cycle.

type TimeoutError added in v1.3.3 

type TimeoutError struct {
	Age time.Duration
}

TimeoutError indicates that the dynamolock gave up acquiring the lock. It holds the length of the attempt that resulted in the error.

func (*TimeoutError) Error added in v1.3.3 

func (e *TimeoutError) Error() string

Source Files

View all Source files

Directories

Path Synopsis
_examples
cmd/lock Module

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