---
title: Generating Custom Metrics with Plugins
menu_order: 80
search_type: Documentation
---

The following topics are discussed:

 * [Official Plugins](#official-plugins)
 * [How Plugins Communicate with Scope](#plugins-internals)
  * [Plugin IDs](#plugin-id)
  * [Registering Plugins](#plugin-registration)
  * [Reporter Interface](#reporter-interface)
  * [Controller Interface](#controller-interface)
     * [Control](#control)
     * [How to Expose Controls](#expose-controls)
     * [Naming Nodes](#naming-nodes)
 * [A Guide to Developing Plugins](#plugins-developing-guide)
  * [Setting up the Structure](#structure)
  * [Defining the Reporter Interface](#defining-reporter-interface)
  * [Report Data Structures](#report-data-structures)

Any kind of metric can be generated and inserted in Scope with custom plugins. Metrics generated through a plugin are displayed in the user interface alongside the standard metrics found in Weave Scope.

![Custom Metrics With Plugins](images/plugin-features.png)

## <a id="official-plugins"></a>Official Plugins

Official Weave Scope plugins can be found at [Weaveworks Plugins](https://github.com/weaveworks-plugins).

* [IOWait](https://github.com/weaveworks-plugins/scope-iowait): is a Go plugin that uses [iostat](https://en.wikipedia.org/wiki/Iostat) to provide host-level CPU IO wait or idle metrics.

* [HTTP Statistics](https://github.com/weaveworks-plugins/scope-http-statistics): is a Python plugin that uses [bcc](http://iovisor.github.io/bcc/) to track multiple metrics about HTTP per process. It does this without any application-level instrumentation requirements and with a negligible performance toll. This plugin is a work in progress, and currently implements the following (for more information read the [plugin documentation](https://github.com/weaveworks-plugins/scope-http-statistics)):
	* Number of HTTP requests per seconds.
	* Number of HTTP responses code per second (per code).

>**Note:** The HTTP Statistics plugin requires a [recent kernel version with ebpf support](https://github.com/iovisor/bcc/blob/master/INSTALL.md#kernel-configuration) and it will not compile on [dlite](https://github.com/nlf/dlite) hosts.

* [Traffic Control](https://github.com/weaveworks-plugins/scope-traffic-control): This plugin allows you to modify latency and packet loss for a specific container via controls from the container's detailed view in the Scope user interface.

* [Volume Count](https://github.com/weaveworks-plugins/scope-volume-count): This plugin (written in Python) requests the number of mounted volumes for each container, and provides a container-level count.

>**Note:**Installed and running plugins are shown in the list of `PLUGINS` in the bottom right of the Scope UI.

## <a id="plugins-internals"></a>How Plugins Communicate with Scope

In this section how the different components of a plugin communicate with Scope are described. You can also find practical examples of how plugins work in the [Weaveworks Plugins](https://github.com/weaveworks-plugins) repositories.

### <a id="plugin-id"></a>Plugin IDs

Each plugin must have a unique ID and this ID must not change
during the plugin's lifetime. Scope probes retrieve the plugin's ID
from the plugin's socket filename. For example, if a socket is named
`my-plugin.sock`, the scope probe deduces the ID as
`my-plugin`. IDs may contain only alphanumeric sequences that are optionally
separated by a dash.

### <a id="plugin-registration"></a>Registering Plugins

All plugins 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 and looks for any added or removed sockets.

If you want to run permissions or store any other information with the socket, you can also put the plugin UNIX socket into a sub-directory.

When a new plugin is detected, the Scope probe begins requesting reports from it via `GET /report`. It is therefore important that **every plugin implements the report interface**. Implementing the report interface also means handling specific requests.

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

### <a id="reporter-interface"></a>Reporter Interface

When a 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 is 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:

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

>**Note:** The `Plugins` section includes exactly one plugin description that displays in the UI. The other plugin fields are:

* `id` - checks for duplicate plugins. It is required.
* `label` - a human readable label displayed in the UI. It is required.
* `description` - displayed in the UI. It is required.
* `interfaces` - a list of interfaces that the plugin supports. It is required, and must contain at least `["reporter"]`.
* `api_version` - ensure both the plugin and the scope probe can speak to each other. It is required, and must match the probe's value.

### <a id="controller-interface"></a>Controller Interface

Plugins _may_ also 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 receives them only
for the controls it exposed in its reports. All such requests come to the
`/control` endpoint.

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

#### <a id="control"></a> Control

The `POST` requests contain a JSON-encoded body with the following:

```json
{
  "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 JSON-encoded data. In most cases,
the body is an empty JSON object (so, "{}" after
serialization). If an error occurs when handling the control,
then the plugin sends a response with an `error` field set, for
example:

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

Sometimes the control activation can make the control obsolete, and 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 sends a shortcut report by filling the
`ShortcutReport` field in the response, like so:

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

#### <a id="expose-controls"></a>How to Expose Controls

Each topology in the report (be it host, pod, endpoint and so on) contains
a set of available controls that 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 with a different control defined:

```json
{
  "Host": {
    "controls": {
      "ctrl-one": {
        "id": "ctrl-one",
        "human": "Ctrl One",
        "icon": "far fa-futbol",
        "rank": 1
      },
      "ctrl-two": {
        "id": "ctrl-two",
        "human": "Ctrl Two",
        "icon": "fa 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 receives a
request as follows:

```json
{
  "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](http://fontawesome.io/cheatsheet/)

#### <a id="naming-nodes"></a>Naming Nodes

Often the controller plugin may want to add controls to already
existing nodes (for example add 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`

These are 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
have IDs formatted as `${host};${pid}`. The easiest way to
discover how the nodes are named are:

 1.  Read the code in
  [report/id.go](https://github.com/weaveworks/scope/blob/master/report/id.go).
 2.  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.
 3. 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`.


## <a id="plugins-developing-guide"></a>A Guide to Developing Plugins

This section explains how to develop a simple plugin in Go. The code used here is a simplified version of the [Scope IOWait](https://github.com/weaveworks-plugins/scope-iowait) plugin.

### <a id="structure"></a>Setting up the Structure

As stated in the previous section, plugins need to be put into the `/var/run/scope/plugins` socket directory to be able to communicate with Scope. The best practice is to put the socket into a sub-directory and name it with the plugin ID (for example, `/var/run/scope/plugins/plugins-id/plugins-id.sock`). This is useful because the plugin can set more restrictive permissions to avoid unauthorized access as well as store other information along with the socket if needed.

Example of a helper function for setting up the socket:

```go
func setupSocket(socketPath string) (net.Listener, error) {
	os.RemoveAll(filepath.Dir(socketPath))
	if err := os.MkdirAll(filepath.Dir(socketPath), 0700); err != nil {
		return nil, fmt.Errorf("failed to create directory %q: %v", filepath.Dir(socketPath), err)
	}
	listener, err := net.Listen("unix", socketPath)
	if err != nil {
		return nil, fmt.Errorf("failed to listen on %q: %v", socketPath, err)
	}

	log.Printf("Listening on: unix://%s", socketPath)
	return listener, nil
}
```

Because Scope detects running plugins by looking into the `/var/run/scope/plugins` directory, plugins should remove their socket and the directory (if created) when they exit. The side effect of not doing that is that the Scope UI will show that a plugin is running but that it is not reachable.

To remove the socket, and the directory, you can use the following helper function:

```go
func setupSignals(socketPath string) {
	interrupt := make(chan os.Signal, 1)
	signal.Notify(interrupt, os.Interrupt, syscall.SIGTERM)
	go func() {
		<-interrupt
		os.RemoveAll(filepath.Dir(socketPath))
		os.Exit(0)
	}()
}
```

Also add the following to the main function the following:

```golang
defer func() {
		os.RemoveAll(filepath.Dir(socketPath))
	}()
```

This ensures that when the plugin terminates because of an error or an interrupt command, the `/var/run/scope/plugins/plugins-id` directory will be removed.

A bare minimum boilerplate can be the following:

```go
package main

import (
	syscall
)

func main() {
	const socketPath = "/var/run/scope/plugins/my-plugin/my-plugin.sock"
	setupSignals(socketPath)

	listener, err := setupSocket(socketPath)

	plugin := &Plugin{}
	http.HandleFunc("/report", plugin.Report)

	defer func() {
		listener.Close()
		os.RemoveAll(filepath.Dir(socketPath))
	}()
}

```

### <a id="defining-reporter-interface"></a>Defining the Reporter Interface

As stated in the [How Plugins Communicate with Scope](#plugins-internals) section, the reporter interface is mandatory.
Implementing the reporter interface means handling `GET /report` requests.

The following code snippet is sufficient to implement it:

```go
// Plugin groups the methods a plugin needs
type Plugin struct {
	lock       sync.Mutex
}

type report struct {
	Plugins []pluginSpec
}

func (p *Plugin) makeReport() (*report, error) {
	rpt := &report{
		Plugins: []pluginSpec{
			{
				ID:          "plugin-id",
				Label:       "Plugin Name",
				Description: "Plugin short description",
				Interfaces:  []string{"reporter"},
				APIVersion:  "1",
			},
		},
	}
	return rpt, nil
}

// Report is called by scope when a new report is needed. It is part of the
// "reporter" interface, which all plugins must implement.
func (p *Plugin) Report(w http.ResponseWriter, r *http.Request) {
	p.lock.Lock()
	defer p.lock.Unlock()
	log.Println(r.URL.String())
	rpt, err := p.makeReport()
	if err != nil {
		log.Printf("error: %v", err)
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
	raw, err := json.Marshal(*rpt)
	if err != nil {
		log.Printf("error: %v", err)
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
	w.WriteHeader(http.StatusOK)
	w.Write(raw)
}

```

### <a id="report-data-structures"></a>Report Data structures
A report can contain many types of information.
If you go back to the [Reporter Interface](#reporter-interface) section, you will see the top-level `Plugins` attribute.
Along with that, a report may contain multiple topologies.
An example of a report containing a few topologies is the following:

```json
{
  "Host": {},
  "Container": {},
  "Process": {},
  ...,
  "Plugins": [...,]
}
```

### Topologies
A topology consists of a list of nodes along with controls and templates described below.
These are the available topologies:

- `Endpoint` nodes are `(address, port)` tuples on each host.
- `Process` nodes are processes on each host.
- `Container` nodes represent all Docker containers on hosts running probes.
- `Pods` nodes represent all Kubernetes pods running on hosts running probes.
- `Service` nodes represent all Kubernetes services running on hosts running probes.
- `Deployment` nodes represent all Kubernetes deployments running on hosts running probes.
- `ReplicaSet` nodes represent all Kubernetes ReplicaSets running on hosts running probes.
- `ContainerImage` nodes represent all Docker container images on hosts running probes.
- `Host` nodes are physical hosts that run probes.
- `ECSTask` nodes represent [AWS ECS](https://aws.amazon.com/ecs/) [tasks](http://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_definitions.html).
- `ECSService` nodes represent [AWS ECS services](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs_services.html).
- `Overlay` nodes are active peers in any software-defined network that's overlaid on the infrastructure.

The topology structure consists of the following attributes:

- `nodes` - is the list of the nodes that compose the topology.
- `controls` - contains the list of IDs of the active controls at a particular time.
- `metadata_templates` - contains the templates used to render data into the Scope UI.
- `table_templates` - contains the templates used to render tables into the Scope UI.
- `metric_templates` - contains the templates used to render metrics into the Scope UI.

**Note**: These attribute are not required. But a topology with no `nodes` does not have any information to render.
`metadata_templates`, as well as `table_templates`, are needed to know how to render the information carried by `nodes` in the Scope UI.

### Nodes
A Node contains information about a specific element of a topology.
For example, the Host topology will contain nodes describing all the hosts in it.
The same applies for containers and the Container topology, pods and the Pod topology and so on.
Nodes are represented as follows:

```json
{
  "Host": {
    "nodes" : {
      "hostID;<host>": {...}
    }
  },
  "Container": {
    "nodes" : {
      "containerID;<container>": {...}
    }
  },
  ...
}
```

Nodes are stored in a dictionary.
The ID of nodes is different depending on the node topology. For instance, nodes representing hosts or containers have the format `ID;<type>`, where `ID` is the alphanumeric identifier of the nodes and type is the literal string `host` or `container` respectively. A node representing an endpoint could look like `pc-4026531969;127.0.0.1;36238`.

A node contains all the information about the represented object (e.g. host, container, pod, etc.).
In particular, a node may contain:

- `latest` - an id-value map containing the latest values. Each id has only one value.
- `latestControls` - the latest available controls.
- `metrics` - the collection of metrics to display in the UI. Each metric has multiple timestamped values.
- `sets` - a string->set-of-strings map, for example a list of local networks.
- `counters` - a string->int map.

### Controls
Controls describe interfaces that expose actions that the user can perform on different objects (e.g. host, container, etc.).
Controls are an element of nodes. In this way, each control in a node is attached to it and performs an action on the object described by the node itself. Below is an example of how controls are represented in the JSON report.
In the report, the attribute `latest_controls` contains all the controls exposed by scope and/or plugins, but only those alive will be listed in the attribute `controls`.

```json
"controls": {
	"timestamp": "2016-11-17T08:53:04.567890059Z",
	"controls": [
		"switchToIOWait"
	]
},
"latestControls": {
	"switchToIOWait": {
		"timestamp": "2016-11-17T08:53:03.171438309Z",
		"value": {
			"dead": false
		}
	},
	"switchToIdle": {
		"timestamp": "2016-11-17T08:53:03.171438309Z",
		"value": {
			"dead": true
		}
	}
}
```

- `timestamp` specifies when the control was exposed.
- `value` is an object containing the control value. At the moment, only the state is available.
 - `dead` is a boolean to know the state (active, dead) of a control. It is useful to show controls only when they are in a usable state.

### Metadata
All metadata entries are placed within nodes in the section named `latest`.
This section contains the latest values to display and consists of `timestamp` and `value`.
Both should be written as JSON strings (with double quotes).
Scope uses `metadata_templates` to display this data.
To pair metadata with its template, it is necessary to use the `metadata-template-id` as a key to identify that particular piece of data. Example:

```json
"metadata_templates": {
      "metadata-templates-id": {
        "id": "metadata-templates-id",
        "label": "Human-readable description",
        "priority": 1.6,
        "from": "latest"
      }
}

"latest": {
	"metadata-templates-id": {
		"timestamp": "2016-11-17T08:53:02.189193735Z",
		"value": "42"
	}
}
```

### Metadata Templates
A metadata template describes a kind of metadata that is present in zero, one or several nodes of the topology and specifies how to display such metadata in Scope.
Metadata templates are not placed within nodes but in the `metadata_templates` section of the JSON file.

```json
"metadata_templates": {
      "traffic-control-pktloss": {
        "id": "traffic-control-pktloss",
        "label": "Human-readable description",
        "dataType": "number",
        "priority": 13.5,
        "from": "latest"
      },
      "another-plugins-id": {...}
}
```

- `id` is a string identifying the particular metadata template (here `traffic-control-pktloss`) and is also used as a key to the template value.
- `label` contains the label used by the Scope UI.
- `dataType` specifies the type of data, and determines how the value is displayed. Possible values for this attribute are: "number", "ip", "datetime", "link" and "" for strings.
- `priority` is a floating point value used to decide the display ordering (lower values are displayed before higher ones). If omitted, the UI displays it last.
- `from` indicates where to look for the metadata. The possible values are:
  - `latest`
  - `sets`
  - `counters`

### Table Templates
Table Templates describe a table and also how to identify the metadata templates that belong to the table.

```json
"table_templates": {
      "table-template-id": {
        "id": "table-template-id",
        "label": "Human-readable description",
        "prefix": "table-id-"
      },
      "another-table-template-id": {...}
}
```

- `table-template-identifier` and `id` identify a particular table template.
- `label` contains the label used by the Scope UI.
- `prefix` is used to identify which metadata templates belong to the table.

To display data in a table, define a table template and prepend the table prefix to all of the metadata templates that identify the data you want to put into the table.

#### Multicolumn table

In order to display tabular data with multiple columns, additional fields are needed.

```json
"table_templates": {
      "table-template-id": {
        "id": "table-template-id",
        "label": "Human-readable description",
        "prefix": "table-id-",
        "type": "multicolumn-table"
        "columns": [
          {
            "id": "table-column-id-1",
            "label": "Label 1",
            "dataType": ""
          },
          {...}
        ]
      },
      "another-table-template-id": {...}
}
```

- `type` specifies the table type. Defaults to 'property-list' if absent.
- `columns` defines the table's columns
  - `id` the column id
  - `label` the column header
  - `dataType`: specifies the column type. Can be "" (for a string), "number", "link", or "ip". Defaults to string if absent.

Values are placed in their respective columns and rows by specially formatting the entries in latest (`prefix`{unique row id}___`column-id`). _**Note:** that's 3 underscores._

```json
"latest": {
	"table-id-{unique-row-id}___table-column-id-1": {
		"timestamp": "2017-05-05T08:53:23.183293735Z",
		"value": "X"
	}...
}

````

### Metrics
Metrics are a particular kind of data that can be plotted on the UI as a graph.
Scope uses `metric_templates` to display graph data in Scope. To pair a metric with its template, use the `metric-template-id` as the key for identifying a particular metric.
Metrics can be used to display CPU and memory usage, HTTP requests rate, I/O operations, etc.
The following is an example of a report with a metric preceded by its metric template:

```json
"metric_templates": {
      "metric-template-id": {
        "id": "metric-id",
        "label": "Human-readable description",
        "format": "percent",
        "priority": 1.6
      },
}

"metrics": {
	"metric-template-id": {
		"samples": [
			{
				"date": "2016-11-17T08:53:03.171424664Z",
				"value": 98.24
			},
			{
				"date": "2016-11-17T08:53:04.171789887Z",
				"value": 80.11
			}
		],
		"min": 0,
		"max": 100
	}
}
```

- `samples` is the list of the samples for this report.
- `min` is the minimum value possible.
- `max` is the maximum value possible.

### Metric Templates
Metric Templates describe a particular metric.
The following is an example of metric template:

```json
"metric_templates": {
      "metric-template-id": {
        "id": "metric-id",
        "label": "Human-readable description",
        "format": "percent",
        "priority": 1.6
      },
      "another-metric-template-id": {...}
}
```

- `metric-template-id` and `id` identify a specific metric template.
- `label` contains the label used by Scope UI.
- `format` describes how the metrics are formatted and can be:
  - `percent` the metric value is a percentage.
  - `filesize` the metric value is a file size (e.g. memory usage), it is displayed with the suffix KB, MB, GB, etc.
  - `integer` the metric value is an integer.
- `priority` is a floating point value used to decide the display ordering (lower values are displayed before higher ones).

### Time Window
The `Window` attribute is a time window, expressed as a duration and it defines the period from which data in the report is considered valid.
The default window is 15 seconds.
You may change the window value using the option `-app.window <SECONDS>` when launching scope.
However, using values smaller than 15 seconds increases the chance of information not being correctly displayed.

**See Also**

  * [Building Scope](/site/building.md)
