fastify-metrics

Prometheus metrics exporter for Fastify.

This plugin uses prom-client under the hood with optional support for @platformatic/prom-client as a high-performance drop-in replacement.

This plugin also adds two http metrics for your routes:

• Requests duration histogram
• Requests duration summary

ToC

• fastify-metrics
  • ToC
  • Fastify support
  • Notable changes
    • v12.x.x
    • v11.x.x
    • v10.x.x
    • v9.x.x
    • v6.x.x
  • Installation
  • Using @platformatic/prom-client
  • Features and requirements
  • Usage
    • Registry clear
    • Plugin options
      • Route metrics
        • Route metrics enabled
        • Route metrics overrides
          • Labels
          • Request durations summary
          • Request durations histogram
    • HTTP routes metrics in Prometheus
  • API Docs
  • Changelog
  • See also
  • License

Fastify support

• v3.x.x - supports fastify-1.x
• v4.x.x - supports fastify-2.x prom-client-11.x
• v5.x.x - supports fastify-2.x prom-client-12.x
• v6.x.x - supports fastify-3.x
• v9.x.x - supports fastify-4.x prom-client-14.x
• v11.x.x - supports fastify-4.x prom-client-15.x
• v12.x.x - supports fastify-5.x prom-client-15.x

Notable changes

v12.x.x

• Fastify v5 support.
• Drop node.js 18 support.

v11.x.x

• Drop node.js 16 support.
• Upgrade to prom-client 15.1.

v10.x.x

• Replace route context.config with routeConfig due to deprecation in fastify v4 and removal in fastify v5. If you had disableMetrics option in you route config, update fastify to latest version.
• Prefer request.routeOptions.method over deprecated request.routerMethod.

v9.x.x

• Fastify v4 support.
• Complete config rewrite, default behaviour changed.
• Support disabling metrics in route config.
• Now collects metrics only for registered routes by default.
• Unknown routes metrics collection disabled by default.
• Removed metrics from request. Now it uses WeakMap and not exposed.
• Add balcklisting possibility for request methods.
• Registry overrides moved to metric configuration.
• Support overriding all Summary and Histogram options for default route metrics.

v6.x.x

• Fastify v3 support.
• Drop node.js 8 support.
• enableDefaultMetrics - now enables only default prom-client metrics. Set to true by default.
• enableRouteMetrics - additional flag that enables route metrics. Set to true by default.

Installation

npm i fastify-metrics --save
pnpm i fastify-metrics --save

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Using @platformatic/prom-client

This plugin supports @platformatic/prom-client as an optional drop-in replacement for prom-client. It is a performance-focused fork by Platformatic that provides the same API with lower overhead — optimized internal data structures, reduced memory allocations, and faster metric serialization.

If @platformatic/prom-client is installed, the plugin will use it automatically. No configuration changes needed. If it's not installed, the plugin falls back to standard prom-client.

Why use it

| | prom-client | @platformatic/prom-client | |---|---|---| | API | Standard | Same (drop-in compatible) | | Internal storage | hashMap | Optimized LabelMap | | Memory allocations | Standard | Reduced | | Metric serialization | Standard | Faster | | Node.js support | >=16 | ^20 || ^22 || >=24 |

Installation

Just install it alongside fastify-metrics:

npm i @platformatic/prom-client
pnpm i @platformatic/prom-client

The plugin auto-detects it on startup. No code changes required.

Explicit client override

You can also pass the client explicitly via the promClient option. This takes precedence over auto-detection:

import fastify from 'fastify';
import metricsPlugin from 'fastify-metrics';
import client from '@platformatic/prom-client';

const app = fastify();
await app.register(metricsPlugin, {
  endpoint: '/metrics',
  promClient: client,
});

Verifying which client is active

After registration, fastify.metrics.client holds the resolved prom-client instance. You can check which one is being used:

await app.ready();
console.log(app.metrics.client); // the active prom-client module

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Features and requirements

• Collects default server metrics (see prom-client);
• Collects route response timings
• Adds metrics to fastify instance for your custom metrics.

• Requires fastify >=4.0.0.
• Node.js >=20.0.0.

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Usage

Add it to your project like regular fastify plugin. Use register method and pass options to it.

const fastify = require('fastify');
const app = fastify();

const metricsPlugin = require('fastify-metrics');
await app.register(metricsPlugin, { endpoint: '/metrics' });

It also exports client to fastify instance fastify.metrics.client which you may use it in your routes.

You may create your metrics when app starts and store it in fastify.metrics object and reuse them in multiple routes.

Registry clear

After calling registry.clear() all metrics are removed from registry. In order to add them again to the registry, call fastify.metrics.initMetricsInRegistry.

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Plugin options

See for details docs

| Property | Type | Default Value | | ----------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------- | ------------------- | | clearRegisterOnInit? | boolean | false | | defaultMetrics? | IDefaultMetricsConfig | { enabled: true } | | endpoint? | string | null | Fastify.RouteOptions | '/metrics' | | name? | string | 'metrics' | | promClient? | prom-client instance | null | null | | routeMetrics? | IRouteMetricsConfig | { enabled: true } |

Route metrics

| Property | Type | Default Value | | -------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------- | --------------------------------------- | | enabled? | boolean | { histogram: boolean, summary: boolean } | true | | groupStatusCodes? | boolean | false | | invalidRouteGroup? | string | '__unknown__' | | methodBlacklist? | readonly string[] | ['HEAD','OPTIONS','TRACE','CONNECT'] | | customLabels? | Record<string, string | ((request: FastifyRequest, reply: FastifyReply) => string)> | undefined | | overrides? | IRouteMetricsOverrides | | | registeredRoutesOnly? | boolean | true | | routeBlacklist? | readonly (string | RegExp)[] | undefined |

Route metrics enabled

The enabled configuration option can be either a boolean which enables/disables generation of both histograms and summaries, or it can be set to an object that allows you to pick individually whether you want histograms or summaries to be generated, for example:

{
  ...
  routeMetrics: {
    enabled: {
      histogram: true,
      summary: false
    }
  }
}

would result in the library only generating histograms.

Route metrics overrides

You may override default metrics settings. You may provide overrides for two metrics tracking http request durations: histogram and summary.

const fastify = require('fastify');
const app = fastify();
const metricsPlugin = require('fastify-metrics');

await app.register(metricsPlugin, {
  endpoint: '/metrics',
  routeMetrics: {
    overrides: {
      histogram: {
        name: 'my_custom_http_request_duration_seconds',
        buckets: [0.1, 0.5, 1, 3, 5],
      },
      summary: {
        help: 'custom request duration in seconds summary help',
        labelNames: ['status_code', 'method', 'route'],
        percentiles: [0.5, 0.75, 0.9, 0.95, 0.99],
      },
    },
  },
});

Labels

| Property | Type | Default value | | ------------------------------------------------------------------------------------- | -------------------------------------- | --------------- | | getRouteLabel? | (request: FastifyRequest) => string | undefined | | method? | string | 'method' | | route? | string | 'route' | | status? | string | 'status_code' |

Request durations summary

| Property | Type | Default value | | ----------------------------------------------------------------------- | ---------- | --------------------------------------- | | name? | string | 'http_request_summary_seconds' | | help? | string | 'request duration in seconds summary' | | percentiles? | number[] | [0.5, 0.9, 0.95, 0.99] |

Request durations histogram

| Property | Type | Default value | | --------------------------------------------------------------------- | ---------- | --------------------------------- | | name? | string | 'http_request_duration_seconds' | | help? | string | 'request duration in seconds' | | buckets? | number[] | [0.05, 0.1, 0.5, 1, 3, 5, 10] |

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HTTP routes metrics in Prometheus

The following table shows what metrics will be available in Prometheus (subject to the enabled configuration option). Note suffixes like _bucket, _sum, _count are added automatically.

| metric | labels | description | | -------------------------------------- | -------------------------------- | ----------------------------- | | http_request_duration_seconds_count | method, route, status_code | Requests total count | | http_request_duration_seconds_bucket | method, route, status_code | Requests durations by bucket | | http_request_summary_seconds | method, route, status_code | Requests duration percentiles | | http_request_summary_seconds_count | method, route, status_code | Requests total count |

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API Docs

See docs.

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Changelog

See changelog.

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See also

• fastify-prom-client - just simple client that adds aggregated http requests metric.

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License

Licensed under MIT.

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