If a telemetry option is provided, ContainerPilot will expose a Prometheus HTTP client interface that can be used to scrape performance telemetry. The telemetry interface is advertised as a service to the discovery service similar to services configured via the services block. Each sensor for the telemetry service will run periodically and record values in the Prometheus client library. A Prometheus server can then make HTTP requests to the telemetry endpoint.

Configuration details follow, but this blog post offers a usage example and narrative for it.

Configuring telemetry

The top-level telemetry configuration defines the telemetry HTTP endpoint. This endpoint will be advertised to Consul (or other discovery service) just as a typical ContainerPilot service block is. The service will be called containerpilot and will be served on the path /metrics. The telemetry service will send periodic heartbeats to the discovery service to identify that it is still operating. Unlike a typical ContainerPilot service, there is no user-defined health check for the telemetry service endpoint, and you don't need to configure the poll/TTL; it will send a 15 second heartbeat every 5 seconds.

A minimal configuration for ContainerPilot including telemetry might look like this:

  "consul": "consul:8500",
  "telemetry": {
    "port": 9090,
    "interfaces": ["eth0"],
    "tags": ["tag1"],
    "sensors": [
        "namespace": "my_namespace",
        "subsystem": "my_subsystem",
        "name": "my_events_count",
        "help": "help text",
        "type": "counter",
        "poll": 5,
        "check": ["/bin/"],
        "timeout": "5s"

The fields are as follows:

  • port is the port the telemetry service will advertise to the discovery service. (Default value is 9090.)
  • interfaces is an optional single or array of interface specifications. If given, the IP of the service will be obtained from the first interface specification that matches. (Default value is ["eth0:inet"])
  • tags is an optional array of tags. If the discovery service supports it (Consul does), the service will register itself with these tags.
  • sensors is an optional array of sensor configurations (see below). If no sensors are provided, then the telemetry endpoint will still be exposed and will show only telemetry about ContainerPilot internals.

Configuring sensors

The sensors field is a list of user-defined sensors that the telemetry service will use to collect telemetry. Each time a sensor is polled, the user-defined check executable will be run. If the value that the check returns from stdout can be parsed as a 64-bit float, then the telemetry collector will receive that value.

The protocol between ContainerPilot and the sensors is a preview and may change. Although we'll attempt to keep backwards compatibility with sensors as described below, as real-world use cases are developed we may discover that the current format is more limited than we'd like. If you're using telemetry, be sure to review the changelogs of the next few releases.

The fields for a sensor are as follows:

  • namespace, subsystem, and name are the names that the Prometheus client library will use to construct the name for the telemetry. These three names are concatenated with underscores _ to become the final name that is scraped recorded by Prometheus. In the example above the metric recorded would be named my_namespace_my_subsystem_my_event_count. You can leave off the namespace and subsystem values and put everything into the name field if desired; the option to provide these other fields is simply for convenience of those who might be generating ContainerPilot configurations programmatically. Please see the Prometheus documents on naming for best practices on how to name your telemetry.
  • help is the help text that will be associated with the metric recorded by Prometheus. This is useful for debugging by giving a more verbose description.
  • type is the type of collector Prometheus will use (one of counter, gauge, histogram or summary). See below for details.
  • poll is the time in seconds between running the check.
  • check is the executable (and its arguments) that is called when it is time to perform a telemetry collection.
  • timeout an optional value to wait before forcibly killing the check handler. Handlers killed in this way are terminated immediately (SIGKILL) without an opportunity to clean up their state. The minimum timeout is 1ms. Omitting this field means that ContainerPilot will wait indefinitely for the check handler. Deprecation warning: in ContainerPilot 3.0 this will default to the poll time.

The check executable is expected to return via stdout a value that can be parsed as a single 64-bit float number. Whitespace will be trimmed, but any other text in the stdout of the executable will cause the metric to be dropped. If you need to return additional information for logging, you should return this via stderr (which ContainerPilot will pass along to the Docker engine).

For example, a check field like "check": ["/usr/bin/free"] is not a working check because the output contains multiple fields as well as text.

An example of a good check script might be:

# check free memory
echo "checked free memory sensor" 1>&2
free | awk -F' +' '/Mem/{print $3}'

This check script will return exactly one numeric value on stdout, and sends additional logging info to stderr where it can be safely handled.

Collector types

ContainerPilot supports all four of the metric types available in the Prometheus API. Briefly these are:


A cumulative metric that represents a single numerical value that only ever goes up. A typical use case for a counter is a count of the number of of certain events. The value returned by the sensor will be added to the counter for that metric.


A metric that represents a single numerical value that can arbitrarily go up and down. A typical use case for a gauge might be a measurement of the current memory usage. The value returned by the sensor script will be set as the new value for the gauge metric.


A count of observations in "buckets", along with the sum of all observed values. A typical use case might be request durations or response sizes. When the Prometheus server scrapes this telemetry endpoint, it will receive a list of buckets and their counts. For example:

namespace_subsystem_response_bucket{le="1"} 0
namespace_subsystem_response_bucket{le="2.5"} 0
namespace_subsystem_response_bucket{le="5"} 1
namespace_subsystem_response_bucket{le="10"} 2
namespace_subsystem_response_bucket{le="+Inf"} 2

This indicates that the collector has seen 2 events in total. One event had a value less than 5 (le="5"), whereas a second was less than 10.


A summary is similar to a histogram, but while it also provides a total count of observations and a sum of all observed values, it calculates quantiles over a sliding time window. For example:

namespace_subsystem_response_seconds_summary{quantile="0.5"} 0.3
namespace_subsystem_response_seconds_summary{quantile="0.9"} 0.5
namespace_subsystem_response_seconds_summary{quantile="0.99"} 2

This indicates that the 50th percentile response time is 0.3 seconds, the 90th percentile is 0.5 seconds, and the 99th percentile is 2 seconds.

Please see the Prometheus docs on histograms for best practices on when you should choose histograms vs summaries.