---
id: prometheus
title: "Prometheus Emitter"
---

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To use this Apache Druid extension, make sure to [include](../../development/extensions.md#loading-extensions) `prometheus-emitter` extension.

## Introduction

This extension exposes [Druid metrics](https://druid.apache.org/docs/latest/operations/metrics.html) for collection by a Prometheus server (https://prometheus.io/).
Emitter is enabled by setting `druid.emitter=prometheus` [configs](https://druid.apache.org/docs/latest/configuration/index.html#emitting-metrics) or include `prometheus` in the composing emitter list. 

## Configuration

All the configuration parameters for the Prometheus emitter are under `druid.emitter.prometheus`.

|property|description|required?|default|
|--------|-----------|---------|-------|
|`druid.emitter.prometheus.strategy`|The strategy to expose prometheus metrics. Default strategy `exporter` would expose metrics for scraping purpose. Only peon task (short-lived jobs) need to use `pushgateway` strategy.|yes|exporter|
|`druid.emitter.prometheus.port`|The port on which to expose the prometheus HTTPServer. Required if using exporter strategy.|no|none|
|`druid.emitter.prometheus.namespace`|Optional metric namespace. Must match the regex `[a-zA-Z_:][a-zA-Z0-9_:]*`|no|"druid"|
|`druid.emitter.prometheus.dimensionMapPath`|JSON file defining the Prometheus metric type, desired dimensions, help text, and conversionFactor for every Druid metric.|no|Default mapping provided. See below.|
|`druid.emitter.prometheus.pushGatewayAddress`|Pushgateway address. Required if using Pushgateway strategy|no|none|


### Metric names

All metric names and labels are reformatted to match Prometheus standards.
- For names: all characters which are not alphanumeric, underscores, or colons (matching `[^a-zA-Z_:][^a-zA-Z0-9_:]*`) are replaced with `_`
- For labels: all characters which are not alphanumeric or underscores (matching `[^a-zA-Z0-9_][^a-zA-Z0-9_]*`) are replaced with `_`

### Metric mapping

Each metric to be collected by Prometheus must specify a type, one of `[timer, counter, guage]`. Prometheus Emitter expects this mapping to
be provided as a JSON file.  Additionally, this mapping specifies which dimensions should be included for each metric.  Prometheus expects
histogram timers to use Seconds as the base unit.  Timers which do not use seconds as a base unit can use the `conversionFactor` to set
the base time unit. If the user does not specify their own JSON file, a default mapping is used.  All
metrics are expected to be mapped. Metrics which are not mapped will not be tracked.
Prometheus metric path is organized using the following schema:
`<druid metric name> : { "dimensions" : <dimension list>, "type" : <timer|counter|gauge>, conversionFactor: <conversionFactor>, "help" : <help text>,}`
e.g.
`query/time" : { "dimensions" : ["dataSource", "type"], "conversionFactor": 1000.0, "type" : "timer", "help": "Seconds taken to complete a query."}`

For metrics which are emitted from multiple services with different dimensions, the metric name is prefixed with
the service name.
e.g.
`"coordinator-segment/count" : { "dimensions" : ["dataSource"], "type" : "gauge" },
 "historical-segment/count" : { "dimensions" : ["dataSource", "tier", "priority"], "type" : "gauge" }`
 
For most use-cases, the default mapping is sufficient.