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118 lines
3.7 KiB
Markdown
118 lines
3.7 KiB
Markdown
# Docker Events Package
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[](https://godoc.org/github.com/docker/go-events)
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[](https://circleci.com/gh/docker/go-events)
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The Docker `events` package implements a composable event distribution package
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for Go.
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Originally created to implement the [notifications in Docker Registry
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2](https://github.com/docker/distribution/blob/master/docs/notifications.md),
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we've found the pattern to be useful in other applications. This package is
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most of the same code with slightly updated interfaces. Much of the internals
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have been made available.
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## Usage
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The `events` package centers around a `Sink` type. Events are written with
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calls to `Sink.Write(event Event)`. Sinks can be wired up in various
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configurations to achieve interesting behavior.
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The canonical example is that employed by the
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[docker/distribution/notifications](https://godoc.org/github.com/docker/distribution/notifications)
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package. Let's say we have a type `httpSink` where we'd like to queue
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notifications. As a rule, it should send a single http request and return an
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error if it fails:
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```go
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func (h *httpSink) Write(event Event) error {
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p, err := json.Marshal(event)
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if err != nil {
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return err
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}
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body := bytes.NewReader(p)
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resp, err := h.client.Post(h.url, "application/json", body)
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if err != nil {
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return err
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}
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defer resp.Body.Close()
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if resp.Status != 200 {
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return errors.New("unexpected status")
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}
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return nil
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}
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// implement (*httpSink).Close()
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```
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With just that, we can start using components from this package. One can call
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`(*httpSink).Write` to send events as the body of a post request to a
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configured URL.
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### Retries
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HTTP can be unreliable. The first feature we'd like is to have some retry:
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```go
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hs := newHTTPSink(/*...*/)
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retry := NewRetryingSink(hs, NewBreaker(5, time.Second))
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```
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We now have a sink that will retry events against the `httpSink` until they
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succeed. The retry will backoff for one second after 5 consecutive failures
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using the breaker strategy.
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### Queues
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This isn't quite enough. We we want a sink that doesn't block while we are
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waiting for events to be sent. Let's add a `Queue`:
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```go
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queue := NewQueue(retry)
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```
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Now, we have an unbounded queue that will work through all events sent with
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`(*Queue).Write`. Events can be added asynchronously to the queue without
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blocking the current execution path. This is ideal for use in an http request.
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### Broadcast
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It usually turns out that you want to send to more than one listener. We can
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use `Broadcaster` to support this:
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```go
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var broadcast = NewBroadcaster() // make it available somewhere in your application.
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broadcast.Add(queue) // add your queue!
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broadcast.Add(queue2) // and another!
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```
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With the above, we can now call `broadcast.Write` in our http handlers and have
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all the events distributed to each queue. Because the events are queued, not
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listener blocks another.
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### Extending
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For the most part, the above is sufficient for a lot of applications. However,
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extending the above functionality can be done implementing your own `Sink`. The
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behavior and semantics of the sink can be completely dependent on the
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application requirements. The interface is provided below for reference:
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```go
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type Sink {
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Write(Event) error
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Close() error
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}
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```
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Application behavior can be controlled by how `Write` behaves. The examples
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above are designed to queue the message and return as quickly as possible.
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Other implementations may block until the event is committed to durable
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storage.
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## Copyright and license
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Copyright © 2016 Docker, Inc. go-events is licensed under the Apache License,
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Version 2.0. See [LICENSE](LICENSE) for the full license text.
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