ekg- Remote monitoring of processes

Safe HaskellNone




This module provides remote monitoring of a running process over HTTP. It can be used to run an HTTP server that provides both a web-based user interface and a machine-readable API (e.g. JSON.) The former can be used by a human to get an overview of what the program is doing and the latter can be used by automated monitoring tools.

Typical usage is to start the monitoring server at program startup

 main = do
     forkServer "localhost" 8000

and then periodically check the stats using a web browser or a command line tool (e.g. curl)

 $ curl -H "Accept: application/json" http://localhost:8000/


Required configuration

To make full use out of this this module you must first enable GC statistics collection in the run-time system. To enable GC statistics collection, either run your program with

 +RTS -T

or compile it with


The runtime overhead of -T is very small so it's safe to always leave it enabled.

Security considerations

Be aware that if the server started by forkServer is not bound to "localhost" (or equivalent) anyone on the network can access the monitoring server. Either make sure the network is secure or bind the server to "localhost".


To use the machine-readable REST API, send an HTTP GET request to the host and port passed to forkServer.

The API is versioned to allow for API evolution. This document is for version 1. To ensure you're using this version, append ?v=1 to your resource URLs. Omitting the version number will give you the latest version of the API.

The following resources (i.e. URLs) are available:

JSON object containing all metrics. Metrics are stored as nested objects, with one new object layer per "." in the metric name (see example below.) Content types: "text/html" (default), "application/json"
JSON object for a single metric. The metric name is created by converting all "/" to ".". Example: "/foo/bar" corresponds to the metric "foo.bar". Content types: "application/json"

Each metric is returned as an object containing a type field. Available types are:

In addition to the type field, there are metric specific fields:

  • Counters, gauges, and labels: the val field contains the actual value (i.e. an integer or a string).
  • Distributions: the mean, variance, count, sum, min, and max fields contain their statistical equivalents.

Example of a response containing the metrics "myapp.visitors" and "myapp.args":

   "myapp": {
     "visitors": {
       "val": 10,
       "type": "c"
     "args": {
       "val": "--a-flag",
       "type": "l"

The monitoring server

data Server Source

A handle that can be used to control the monitoring server. Created by forkServer.

serverThreadId :: Server -> ThreadIdSource

The thread ID of the server. You can kill the server by killing this thread (i.e. by throwing it an asynchronous exception.)

serverMetricStore :: Server -> StoreSource

The metric store associated with the server. If you want to add metric to the default store created by forkServer you need to use this function to retrieve it.



:: ByteString

Host to listen on (e.g. "localhost")

-> Int

Port to listen on (e.g. 8000)

-> IO Server 

Like forkServerWith, but creates a default metric store with some predefined metrics. The predefined metrics are those given in registerGcMetrics.



:: Store

Metric store

-> ByteString

Host to listen on (e.g. "localhost")

-> Int

Port to listen on (e.g. 8000)

-> IO Server 

Start an HTTP server in a new thread. The server replies to GET requests to the given host and port. The host argument can be either a numeric network address (dotted quad for IPv4, colon-separated hex for IPv6) or a hostname (e.g. "localhost".) The client can control the Content-Type used in responses by setting the Accept header. At the moment three content types are available: "application/json" and "text/html".

Registers the following counter, used by the UI:

The server time when the sample was taken, in milliseconds.

Note that this function, unlike forkServer, doesn't register any other predefined metrics. This allows other libraries to create and provide a metric store for use with this library. If the metric store isn't created by you and the creator doesn't register the metrics registered by forkServer, you might want to register them yourself.

Defining metrics

The monitoring server can store and serve integer-valued counters and gauges, string-valued labels, and statistical distributions. A counter is a monotonically increasing value (e.g. TCP connections established since program start.) A gauge is a variable value (e.g. the current number of concurrent connections.) A label is a free-form string value (e.g. exporting the command line arguments or host name.) A distribution is a statistic summary of events (e.g. processing time per request.) Each metric is associated with a name, which is used when it is displayed in the UI or returned in a JSON object.

Metrics share the same namespace so it's not possible to create e.g. a counter and a gauge with the same. Attempting to do so will result in an error.

To create and use a counter, simply call getCounter to create it and then call e.g. inc or add to modify its value. Example:

 main = do
     handle <- forkServer "localhost" 8000
     counter <- getCounter "iterations" handle
     let loop n = do
             inc counter

To create a gauge, use getGauge instead of getCounter and then call e.g. set. Similar for the other metric types.

It's also possible to register metrics directly using the System.Metrics module in the ekg-core package. This gives you a bit more control over how metric values are retrieved.



:: Text

Counter name

-> Server

Server that will serve the counter

-> IO Counter 

Return a new, zero-initialized counter associated with the given name and server. Multiple calls to getCounter with the same arguments will result in an error.



:: Text

Gauge name

-> Server

Server that will serve the gauge

-> IO Gauge 

Return a new, zero-initialized gauge associated with the given name and server. Multiple calls to getGauge with the same arguments will result in an error.



:: Text

Label name

-> Server

Server that will serve the label

-> IO Label 

Return a new, empty label associated with the given name and server. Multiple calls to getLabel with the same arguments will result in an error.



:: Text

Distribution name

-> Server

Server that will serve the distribution

-> IO Distribution 

Return a new distribution associated with the given name and server. Multiple calls to getDistribution with the same arguments will result in an error.