weave-scope/examples/plugins

Scope Probe Plugins

Scope probe plugins let you insert your own custom data and controls into Scope and display them in the UI. The list of the current running plugins is displayed next to the label PLUGINS in the bottom right of the UI.

Official Plugins

You can find all the official plugins at Weaveworks Plugins.

• IOWait: a Go plugin using iostat to provide host-level CPU IO wait or idle metrics.

• HTTP Statistics: A Python plugin using bcc to track multiple metrics about HTTP per process, without any application-level instrumentation requirements and negligible performance toll. This plugin is a work in progress, as of now it implements two metrics (for more information read the plugin documentation):

  • Number of HTTP requests per seconds.
  • Number of HTTP responses code per second (per code).

• Traffic Control: This plugin allows the user to modify latency and packet loss for a specific container via buttons in the UI's container detailed view.

Plugins Internals

This section explains the fundamental parts of the plugins structure necessary to understand how a plugin communicates with Scope. You can find more practical examples in Weaveworks Plugins repositories.

Plugin ID

Each plugin should have an unique ID. It is forbidden to change it during the plugin's lifetime. The scope probe will get the plugin's ID from the plugin's socket filename. For example, the socket named my-plugin.sock, the scope probe will deduce the ID as my-plugin. IDs can only contain alphanumeric sequences, optionally separated with a dash.

Plugin registration

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

Protocol

There are several interfaces a plugin may (or must) implement. Usually implementing an interface means handling specific requests. These requests are described below.

Reporter interface

Plugins must implement the reporter interface because Scope uses it to discover which other interfaces the plugin implements. Implementing this interface means listening for HTTP requests at /report.

Note: Plugins must add the "reporter" string to the interfaces field in the plugin specification even though this interface is implicitly implemented.

Report

When the scope probe discovers a new plugin unix socket it will begin periodically making 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:

{
  ...,
  "Plugins": [
    {
      "id":          "plugin-id",
      "label":       "Human Friendly Name",
      "description": "Plugin's brief description",
      "interfaces":  ["reporter"],
      "api_version": "1",
    }
  ]
}

Note that the Plugins section includes exactly one plugin description. The plugin description fields are:

• id is used to check for duplicate plugins. It is required. Described in the Plugin ID section.
• 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 contain at least ["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.

Controller interface

Plugins may implement the controller interface. Implementing the controller interface means that the plugin can react to HTTP POST control requests sent by the app. The plugin will receive them only for controls it exposed in its reports. The requests will come to the /control endpoint.

Add the "controller" string to the interfaces field in the plugin specification.

Control

The POST requests will have a JSON-encoded body with the following contents:

{
  "AppID": "some ID of an app",
  "NodeID": "an ID of the node that had the control activated",
  "Control": "the name of the activated control"
}

The body of the response should also be a JSON-encoded data. Usually the body would be an empty JSON object (so, "{}" after serialization). If some error happens during handling the control, then the plugin can send a response with an error field set, for example:

{
  "error": "An error message here"
}

Sometimes the control activation can make the control obsolete, so the plugin may want to hide it (for example, control for stopping the container should be hidden after the container is stopped). For this to work, the plugin can send a shortcut report by filling the ShortcutReport field in the response, like for example:

{
  "ShortcutReport": { body of the report here }
}

How to expose controls

Each topology in the report (be it host, pod, endpoint and so on) has a set of available controls a node in the topology may want to show. The following (rather artificial) example shows a topology with two controls (ctrl-one and ctrl-two) and two nodes, each having a different control from the two:

{
  "Host": {
    "controls": {
      "ctrl-one": {
        "id": "ctrl-one",
        "human": "Ctrl One",
        "icon": "fa-futbol-o",
        "rank": 1
      },
      "ctrl-two": {
        "id": "ctrl-two",
        "human": "Ctrl Two",
        "icon": "fa-beer",
        "rank": 2
      }
    },
    "nodes": {
      "host1": {
        "latestControls": {
          "ctrl-one": {
            "timestamp": "2016-07-20T15:51:05Z01:00",
            "value": {
              "dead": false
            }
          }
        }
      },
      "host2": {
        "latestControls": {
          "ctrl-two": {
            "timestamp": "2016-07-20T15:51:05Z01:00",
            "value": {
              "dead": false
            }
          }
        }
      }
    }
  }
}

When control "ctrl-one" is activated, the plugin will receive a request like:

{
  "AppID": "some ID of an app",
  "NodeID": "host1",
  "Control": "ctrl-one"
}

A short note about the "icon" field of the topology control - the value for it can be taken from Font Awesome Cheatsheet

Node naming

Very often the controller plugin wants to add some controls to already existing nodes (like controls for network traffic management to nodes representing the running Docker container). To achieve that, it is important to make sure that the node ID in the plugin's report matches the ID of the node created by the probe. The ID is a semicolon-separated list of strings.

For containers, images, hosts and others the ID is usually formatted as ${name};<${tag}>. The ${name} variable is usually a name of a thing the node represents, like an ID of the Docker container or the hostname. The ${tag} denotes the type of the node. There is a fixed set of tags used by the probe:

• host
• container
• container_image
• pod
• service
• deployment
• replica_set

The examples of "tagged" node names:

• The Docker container with full ID 2299a2ca59dfd821f367e689d5869c4e568272c2305701761888e1d79d7a6f51: 2299a2ca59dfd821f367e689d5869c4e568272c2305701761888e1d79d7a6f51;<container>
• The Docker image with name docker.io/alpine: docker.io/alpine;<container_image>
• The host with name example.com: example.com:<host>

The fixed set of tags listed above is not a complete set of names a node can have though. For example, nodes representing processes are have ID formatted as ${host};${pid}. Probably the easiest ways to discover how the nodes are named are:

• Read the code in report/id.go.
• Browse the Weave Scope GUI, select some node and search for an id key in the nodeDetails array in the address bar.
  • For example in the http://localhost:4040/#!/state/{"controlPipe":null,"nodeDetails":[{"id":"example.com;<host>","label":"example.com","topologyId":"hosts"}],… URL, you can find the example.com;<host> which is an ID of the node representing the host.
  • Mentally substitute the <SLASH> with /. This can appear in Docker image names, so docker.io/alpine in the address bar will be docker.io<SLASH>alpine.