weave-scope/site/plugins.md

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Generating Custom Metrics with Plugins	80

The following topics are discussed:

• Listening Protocol
• Reporting
• Interfaces

With a Scope probe plugin, you can insert custom metrics into Scope and have them display in the user interface together with the Scope's standard set of metrics.

You can find some examples in the example plugins directory.

There are currently two different examples:

• A Python plugin using bcc to extract incoming HTTP request rates per process, without any application-level instrumentation requirements and negligible performance toll (metrics are obtained in-kernel without any packet copying to userspace).

Note: This plugin needs a recent kernel version with ebpf support. It will not compile on current dlite and boot2docker hosts.

• A Go plugin, using iostat to provide host-level CPU IO wait metrics.

The example plugins are run by calling make in their directory. This builds the plugin, and immediately runs it in the foreground. To run the plugin in the background, see the Makefile for examples of the docker run ... command.

If the running plugin was picked up by Scope, you will see it in the list of PLUGINS in the bottom right of the UI.

Listening Protocol

All plugins must listen for HTTP connections on a Unix socket in the /var/run/scope/plugins directory. The Scope probe recursively scans that directory every 5 seconds, to look for any sockets being added (or removed). It is also valid to put the plugin Unix socket into a sub-directory, in case you want to apply some permissions, or store any other information with the socket.

When a new plugin is detected, the scope probe begins requesting reports from it via GET /report.

All plugin endpoints are expected to respond within 500ms, and respond in the JSON format.

Reporting

When the Scope probe discovers a new plugin Unix socket, it begins to periodically make a GET request to the /report endpoint. The report data structure returned from this will be merged into the probe's report and sent to the app. An example of the report structure can be viewed at the /api/report endpoint of any Scope app.

In addition to any data about the topology nodes, the report returned from the plugin must include some metadata about the plugin itself.

For example:

{
  "Processes": {},
  "Plugins": [
    {
      "id":          "iowait",
      "label":       "IOWait",
      "description": "Adds a graph of CPU IO Wait to hosts",
      "interfaces":  ["reporter"],
      "api_version": "1",
    }
  ]
}

Note: The Plugins section includes exactly one plugin description. The plugin description fields are: interfaces including reporter.

The fields are:

• id is used to check for duplicate plugins. It is required.
• label is a human readable plugin label displayed in the UI. It is required.
• description is displayed in the UI
• interfaces is a list of interfaces which this plugin supports. It is required, and must equal ["reporter"].
• api_version is used to ensure both the plugin and the scope probe can speak to each other. It is required, and must match the probe.

Interfaces

Currently the only interface a plugin can fulfill is reporter.

See Also

• Building Scope