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146 lines
5.7 KiB
Markdown
146 lines
5.7 KiB
Markdown
# DynamoDB Lock Client for Go
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[](https://godoc.org/cirello.io/dynamolock)
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[](https://travis-ci.org/cirello-io/dynamolock)
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[](https://coveralls.io/github/cirello-io/dynamolock?branch=master)
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[](https://goreportcard.com/report/github.com/cirello-io/dynamolock)
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[](https://github.com/cirello-io/public/blob/master/SLA.md)
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This repository is covered by this [SLA](https://github.com/cirello-io/public/blob/master/SLA.md).
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The dymanoDB Lock Client for Go is a general purpose distributed locking library
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built for DynamoDB. The dynamoDB Lock Client for Go supports both fine-grained
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and coarse-grained locking as the lock keys can be any arbitrary string, up to a
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certain length. Please create issues in the GitHub repository with questions,
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pull request are very much welcome.
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It is a port in Go of Amazon's original [dynamodb-lock-client](https://github.com/awslabs/dynamodb-lock-client).
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## Use cases
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A common use case for this lock client is:
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let's say you have a distributed system that needs to periodically do work on a
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given campaign (or a given customer, or any other object) and you want to make
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sure that two boxes don't work on the same campaign/customer at the same time.
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An easy way to fix this is to write a system that takes a lock on a customer,
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but fine-grained locking is a tough problem. This library attempts to simplify
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this locking problem on top of DynamoDB.
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Another use case is leader election. If you only want one host to be the leader,
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then this lock client is a great way to pick one. When the leader fails, it will
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fail over to another host within a customizable leaseDuration that you set.
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## Getting Started
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To use the DynamoDB Lock Client for Go, you must make it sure it is present in
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`$GOPATH` or in your vendor directory.
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```sh
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$ go get -u cirello.io/dynamolock
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```
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This package has the `go.mod` file to be used with Go's module system. If you
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need to work on this package, use `go mod edit -replace=cirello.io/dynamolock@yourlocalcopy`.
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Then, you need to set up a DynamoDB table that has a hash key on a key with the
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name `key`. For your convenience, there is a function in the package called
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`CreateTable` that you can use to set up your table, but it is also possible to
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set up the table in the AWS Console. The table should be created in advance,
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since it takes a couple minutes for DynamoDB to provision your table for you.
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The package level documentation comment has an example of how to use this
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package. Here is some example code to get you started:
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```Go
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package main
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import (
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"log"
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"cirello.io/dynamolock"
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"github.com/aws/aws-sdk-go/aws"
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"github.com/aws/aws-sdk-go/aws/session"
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"github.com/aws/aws-sdk-go/service/dynamodb"
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)
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func main() {
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svc := dynamodb.New(session.Must(session.NewSession(&aws.Config{
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Region: aws.String("us-west-2"),
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})))
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c, err := dynamolock.New(svc,
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"locks",
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dynamolock.WithLeaseDuration(3*time.Second),
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dynamolock.WithHeartbeatPeriod(1*time.Second),
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)
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if err != nil {
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log.Fatal(err)
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}
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defer c.Close()
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log.Println("ensuring table exists")
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c.CreateTable("locks",
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dynamolock.WithProvisionedThroughput(&dynamodb.ProvisionedThroughput{
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ReadCapacityUnits: aws.Int64(5),
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WriteCapacityUnits: aws.Int64(5),
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}),
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dynamolock.WithCustomPartitionKeyName("key"),
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)
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data := []byte("some content a")
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lockedItem, err := c.AcquireLock("spock",
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dynamolock.WithData(data),
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dynamolock.ReplaceData(),
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)
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if err != nil {
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log.Fatal(err)
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}
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log.Println("lock content:", string(lockedItem.Data()))
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if got := string(lockedItem.Data()); string(data) != got {
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log.Println("losing information inside lock storage, wanted:", string(data), " got:", got)
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}
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log.Println("cleaning lock")
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success, err := c.ReleaseLock(lockedItem)
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if !success {
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log.Fatal("lost lock before release")
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}
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if err != nil {
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log.Fatal("error releasing lock:", err)
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}
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log.Println("done")
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}
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```
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## Selected Features
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### Send Automatic Heartbeats
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When you create the lock client, you can specify `WithHeartbeatPeriod(time.Duration)`
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like in the above example, and it will spawn a background goroutine that
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continually updates the record version number on your locks to prevent them from
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expiring (it does this by calling the `SendHeartbeat()` method in the lock
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client.) This will ensure that as long as your application is running, your
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locks will not expire until you call `ReleaseLock()` or `lockItem.Close()`
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### Read the data in a lock without acquiring it
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You can read the data in the lock without acquiring it, and find out who owns
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the lock. Here's how:
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```Go
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lock, err := lockClient.Get("kirk");
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```
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## Logic to avoid problems with clock skew
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The lock client never stores absolute times in DynamoDB -- only the relative
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"lease duration" time is stored in DynamoDB. The way locks are expired is that a
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call to acquireLock reads in the current lock, checks the RecordVersionNumber of
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the lock (which is a GUID) and starts a timer. If the lock still has the same
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GUID after the lease duration time has passed, the client will determine that
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the lock is stale and expire it.
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What this means is that, even if two different machines disagree about what time
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it is, they will still avoid clobbering each other's locks.
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## Required DynamoDB Actions
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For an IAM role to take full advantage of `dynamolock`, it must be allowed to
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perform all of the following actions on the DynamoDB table containing the locks:
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* `GetItem`
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* `PutItem`
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* `UpdateItem`
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* `DeleteItem`
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