1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
|
# Docker Events Package
[](https://godoc.org/github.com/docker/go-events)
[](https://circleci.com/gh/docker/go-events)
The Docker `events` package implements a composable event distribution package
for Go.
Originally created to implement the [notifications in Docker Registry
2](https://github.com/docker/distribution/blob/master/docs/notifications.md),
we've found the pattern to be useful in other applications. This package is
most of the same code with slightly updated interfaces. Much of the internals
have been made available.
## Usage
The `events` package centers around a `Sink` type. Events are written with
calls to `Sink.Write(event Event)`. Sinks can be wired up in various
configurations to achieve interesting behavior.
The canonical example is that employed by the
[docker/distribution/notifications](https://godoc.org/github.com/docker/distribution/notifications)
package. Let's say we have a type `httpSink` where we'd like to queue
notifications. As a rule, it should send a single http request and return an
error if it fails:
```go
func (h *httpSink) Write(event Event) error {
p, err := json.Marshal(event)
if err != nil {
return err
}
body := bytes.NewReader(p)
resp, err := h.client.Post(h.url, "application/json", body)
if err != nil {
return err
}
defer resp.Body.Close()
if resp.Status != 200 {
return errors.New("unexpected status")
}
return nil
}
// implement (*httpSink).Close()
```
With just that, we can start using components from this package. One can call
`(*httpSink).Write` to send events as the body of a post request to a
configured URL.
### Retries
HTTP can be unreliable. The first feature we'd like is to have some retry:
```go
hs := newHTTPSink(/*...*/)
retry := NewRetryingSink(hs, NewBreaker(5, time.Second))
```
We now have a sink that will retry events against the `httpSink` until they
succeed. The retry will backoff for one second after 5 consecutive failures
using the breaker strategy.
### Queues
This isn't quite enough. We we want a sink that doesn't block while we are
waiting for events to be sent. Let's add a `Queue`:
```go
queue := NewQueue(retry)
```
Now, we have an unbounded queue that will work through all events sent with
`(*Queue).Write`. Events can be added asynchronously to the queue without
blocking the current execution path. This is ideal for use in an http request.
### Broadcast
It usually turns out that you want to send to more than one listener. We can
use `Broadcaster` to support this:
```go
var broadcast = NewBroadcaster() // make it available somewhere in your application.
broadcast.Add(queue) // add your queue!
broadcast.Add(queue2) // and another!
```
With the above, we can now call `broadcast.Write` in our http handlers and have
all the events distributed to each queue. Because the events are queued, not
listener blocks another.
### Extending
For the most part, the above is sufficient for a lot of applications. However,
extending the above functionality can be done implementing your own `Sink`. The
behavior and semantics of the sink can be completely dependent on the
application requirements. The interface is provided below for reference:
```go
type Sink {
Write(Event) error
Close() error
}
```
Application behavior can be controlled by how `Write` behaves. The examples
above are designed to queue the message and return as quickly as possible.
Other implementations may block until the event is committed to durable
storage.
## Copyright and license
Copyright © 2016 Docker, Inc. go-events is licensed under the Apache License,
Version 2.0. See [LICENSE](LICENSE) for the full license text.
|
