Prometheus Pushgateway

The Prometheus Pushgateway exists to allow ephemeral and batch jobs to expose their metrics to Prometheus. Since these kinds of jobs may not exist long enough to be scraped, they can instead push their metrics to a Pushgateway. The Pushgateway then exposes these metrics to Prometheus.

Non-goals

The Pushgateway is explicitly not an aggregator or distributed counter but rather a metrics cache. It does not have a statsd-like semantics. The metrics pushed are exactly the same as you would present for scraping in a permanently running program.

For machine-level metrics, the textfile collector of the Node exporter is usually more appropriate. The Pushgateway is intended for service-level metrics.

The Pushgateway is not an event store. While you can use Prometheus as a data source for Grafana annotations, tracking something like release events has to happen with some event-logging framework.

Run it

Download binary releases for your platform from the release page and unpack the tarball.

If you want to compile yourself from the sources, you need a working Go setup. Then use the provided Makefile (type make).

For the most basic setup, just start the binary. To change the address to listen on, use the -web.listen-address flag. By default, Pushgateway does not persist metrics. However, the -persistence.file flag allows you to specify a file in which the pushed metrics will be persisted (so that they survive restarts of the Pushgateway).

Use it

Configure the Pushgateway as a target to scrape

The Pushgateway has to be configured as a target to scrape by Prometheus, using one of the usual methods. However, you should always set honor_labels: true in the scrape config (see below for a detailed explanation).

Libraries

Prometheus client libraries should have a feature to push the registered metrics to a Pushgateway. Usually, a Prometheus client passively presents metric for scraping by a Prometheus server. A client library that supports pushing has a push function, which needs to be called by the client code. It will then actively push the metrics to a Pushgateway, using the API described below.

Command line

Using the Prometheus text protocol, pushing metrics is so easy that no separate CLI is provided. Simply use a command-line HTTP tool like curl. Your favorite scripting language has most likely some built-in HTTP capabilities you can leverage here as well.

Caveat: Note that in the text protocol, each line has to end with a line-feed character (aka 'LF' or '\n'). Ending a line in other ways, e.g. with 'CR' aka '\r', 'CRLF' aka '\r\n', or just the end of the packet, will result in a protocol error.

Examples:

Push a single sample into the group identified by {job="some_job"}:
```
  echo "some_metric 3.14" | curl --data-binary @- http://pushgateway.example.org:9091/metrics/job/some_job
```
Since no type information has been provided, some_metric will be of type untyped.

Push something more complex into the group identified by {job="some_job",instance="some_instance"}:

  cat <<EOF | curl --data-binary @- http://pushgateway.example.org:9091/metrics/job/some_job/instance/some_instance
  # TYPE some_metric counter
  some_metric{label="val1"} 42
  # This one even has a timestamp (but beware, see below).
  some_metric{label="val2"} 34 1398355504000
  # TYPE another_metric gauge
  # HELP another_metric Just an example.
  another_metric 2398.283
  EOF

Note how type information and help strings are provided. Those lines are optional, but strongly encouraged for anything more complex.

Delete all metrics grouped by job and instance:

  curl -X DELETE http://pushgateway.example.org:9091/metrics/job/some_job/instance/some_instance

Delete all metrics grouped by job only:

  curl -X DELETE http://pushgateway.example.org:9091/metrics/job/some_job

About the job and instance labels

The Prometheus server will attach a job label and an instance label to each scraped metric. The value of the job label comes from the scrape configuration. When you configure the Pushgateway as a scrape target for your Prometheus server, you will probably pick a job name like pushgateway. The value of the instance label is automatically set to the host and port of the target scraped. Hence, all the metrics scraped from the Pushgateway will have the host and port of the Pushgateway as the instance label and a job label like pushgateway. The conflict with the job and instance labels you might have attached to the metrics pushed to the Pushgateway is solved by renaming those labels to exported_job and exported_instance.

However, this behavior is usually undesired when scraping a Pushgateway. Generally, you would like to retain the job and instance labels of the metrics pushed to the Pushgateway. That's why you have set honor_labels: true in the scrape config for the Pushgateway. It enables the desired behavior. See the documentation for details.

This leaves us with the case where the metrics pushed to the Pushgateway do not feature an instance label. This case is quite commen as the pushed metrics are often on a service level and therefore not related to a particular instance. Even with honor_labels: true, the Prometheus server will attach an instance label if no instance label has been set in the first place. Therefore, if a metric is pushed to the Pushgateway without an instance label (and without instance label in the grouping key, see below), the Pushgateway will export it with an emtpy instance label ({instance=""}), which is equivalent to having no instance label at all but prevents the server from attaching one.

About timestamps

If you push metrics at time t₁, you might be tempted to believe that Prometheus will scrape them with that same timestamp t₁. Instead, what Prometheus attaches as a timestamp is the time when it scrapes the Pushgateway. Why so?

In the world view of Prometheus, a metric can be scraped at any time. A metric that cannot be scraped has basically ceased to exist. Prometheus is somewhat tolerant, but if it cannot get any samples for a metric in 5min, it will behave as if that metric does not exist anymore. Preventing that is actually one of the reasons to use a push gateway. The push gateway will make the metrics of your ephemeral job scrapable at any time. Attaching the time of pushing as a timestamp would defeat that purpose because 5min after the last push, your metric will look as stale to Prometheus as if it could not be scraped at all anymore. (Prometheus knows only one timestamp per sample, there is no way to distinguish a 'time of pushing' and a 'time of scraping'.)

You can still force Prometheus to attach a different timestamp by using the optional timestamp field in the exchange format. However, there are very few use cases where that would make sense. (Essentially, if you push more often than every 5min, you could attach the time of pushing as a timestamp.)

API

All pushes are done via HTTP. The interface is vaguely REST-like.

URL

The default port the push gateway is listening to is 9091. The path looks like

/metrics/job/<JOBNAME>{/<LABEL_NAME>/<LABEL_VALUE>}

<JOBNAME> is used as the value of the job label, followed by any number of other label pairs (which might or might not include an instance label). The label set defined by the URL path is used as a grouping key. Any of those labels already set in the body of the request (as regular labels, e.g. name{job="foo"} 42) will be overwritten to match the labels defined by the URL path!

Note that / cannot be used as part of a label value or the job name, even if escaped as %2F. (The decoding happens before the path routing kicks in, cf. the Go documentation of URL.Path.)

Deprecated URL

There is a deprecated version of the URL path, using jobs instead of job:

/metrics/jobs/<JOBNAME>[/instances/<INSTANCENAME>]

If this version of the URL path is used with the instances part, it is equivalent to the URL path above with an instance label, i.e.

/metrics/jobs/foo/instances/bar

is equivalent to

/metrics/job/foo/instance/bar

(Note the missing pluralizations.)

However, if the instances part is missing, the Pushgateway will automatically use the IP number of the pushing host as the 'instance' label, and grouping happens by 'job' and 'instance' labels.

Example: Pushing metrics from host 1.2.3.4 using the deprecated URL path

/metrics/jobs/foo

is equivalent to pushing using the URL path

/metrics/job/foo/instance/1.2.3.4

`PUT` method

PUT is used to push a group of metrics. All metrics with the grouping key specified in the URL are replaced by the metrics pushed with PUT.

The body of the request contains the metrics to push either as delimited binary protocol buffers or in the simple flat text format (both in version 0.0.4, see the data exposition format specification). Discrimination between the two variants is done via the Content-Type header. (Use the value application/vnd.google.protobuf; proto=io.prometheus.client.MetricFamily; encoding=delimited for protocol buffers, otherwise the text format is tried as a fall-back.)

The response code upon success is always 202 (even if the same grouping key has never been used before, i.e. there is no feedback to the client if the push has replaced an existing group of metrics or created a new one).

If using the protobuf format, do not send duplicate MetricFamily proto messages (i.e. more than one with the same name) in one push, as they will overwrite each other.

A successfully finished request means that the pushed metrics are queued for an update of the storage. Scraping the push gateway may still yield the old results until the queued update is processed. Neither is there a guarantee that the pushed metrics are persisted to disk. (A server crash may cause data loss. Or the push gateway is configured to not persist to disk at all.)

`POST` method

POST works exactly like the PUT method but only metrics with the same name as the newly pushed metrics are replaced (among those with the same grouping key).

`DELETE` method

DELETE is used to delete metrics from the push gateway. The request must not contain any content. All metrics with the grouping key specified in the URL are deleted.

The response code upon success is always 202. The delete request is merely queued at that moment. There is no guarantee that the request will actually be executed or that the result will make it to the persistence layer (e.g. in case of a server crash). However, the order of PUT/POST and DELETE request is guaranteed, i.e. if you have successfully sent a DELETE request and then send a PUT, it is guaranteed that the DELETE will be processed first (and vice versa).

Deleting a grouping key without metrics is a no-op and will not result in an error.

Caution: Up to version 0.1.1 of the Pushgateway, a DELETE request using the following path in the URL would delete all metrics with the job label 'foo':

/metrics/jobs/foo

Newer versions will interpret the above URL path as deprecated (see above). Consequently, they will add the IP number of the client as an instance label and then delete all metrics with the corresponding job and instance label as their grouping key.

Development

The normal binary embeds the files in resources. For development purposes, it is handy to have a running binary use those files directly (so that you can see the effect of changes immediately). To switch to direct usage, type make bindata-debug just before compiling the binary. Switch back to "normal" mode by typing make bindata-embed. (Just make after a resource has changed will result in the same.)

Contributing

Relevant style guidelines are the Go Code Review Comments and the Formatting and style section of Peter Bourgon's Go: Best Practices for Production Environments.

Using Docker

You can deploy the Pushgateway using the prom/pushgateway Docker image.

For example:

docker pull prom/pushgateway

docker run -d -p 9091:9091 prom/pushgateway

ihr/pushgateway