74 lines
2.7 KiB
Markdown
74 lines
2.7 KiB
Markdown
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---
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layout: "intro"
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page_title: "Custom User Events"
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sidebar_current: "gettingstarted-userevents"
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---
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# Custom User Events
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In addition to the standard membership-related events that Serf fires,
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Serf is able to propagate custom events across the cluster. Custom events
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are useful for tasks such as: triggering deploys, telling the cluster to
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restart, etc.
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## Sending Custom Events
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First, start a Serf agent so we can see the event being sent. Since we'll
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just be running a single agent, running `serf agent` by itself is fine.
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Then, to send a custom event, use the `serf event` command:
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```
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$ serf event hello-there
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```
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If you look at the output of `serf agent`, you should see that it received
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the user event:
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```
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...
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2013/10/22 07:06:32 [INFO] agent: Received event: user-event: hello-there
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```
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If the cluster were made up of multiple members, all of the members
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would have received this event, eventually.
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Just like normal Serf events, event handlers can respond to user events.
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For example, if we had a "restart" custom event, we might create an
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event handler that restarts some server when it receives that event.
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## Event Payloads
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Events are not limited to just an event name. The event can also contain
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a payload: arbitrary data associated with the event. With our same agent
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running, let's deliver an event with a payload: `serf event my-name-is Mitchell`
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In practice, event payloads can contain information such as the git commit
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to deploy if you're using Serf as a deployment tool. Or perhaps it contains
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some updated configuration to modify on the nodes. It can contain anything
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you'd like; it is up to the event handler to use it in some meaningful way.
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## Custom Event Limitations
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Custom events are delivered using the Serf gossip layer. The benefits of
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this approach is that you get completely decentralized messaging across
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your entire cluster that is fault tolerant. Even if a node is down, it will
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eventually receive that event message.
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Due to the mechanics of gossip, custom
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events are highly scalable: Serf doesn't need to connect to each and every
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node to send the message, it only needs to connect to a handful, regardless
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of cluster size.
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Custom events come with some trade-offs, however:
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* Events are eventually consistent: Because events are delivered over
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gossip, the messages _eventually_ arrive at every node. In theory
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(and anecdotally in practice), the state of the cluster
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[converges rapidly](/docs/internals/simulator.html).
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* Payload size is limited: Serf gossips via UDP, so the payload must fit
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within a single UDP packet (alongside any other data Serf sends). This
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limits the potential size of a payload to less than 1 KB. In practice,
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Serf limits the payload to a much smaller size.
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