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Measure process load with automatic handling of "Service Unavailable" plugin for Fastify.
It can check maxEventLoopDelay, maxHeapUsedBytes and maxRssBytes values.


npm i under-pressure --save


Require the plugin and register it into the Fastify instance.

const fastify = require('fastify')()
fastify.register(require('under-pressure'), {
  maxEventLoopDelay: 1000,
  maxHeapUsedBytes: 100000000,
  maxRssBytes: 100000000
fastify.get('/', (req, reply) => {
  reply.send({ hello: 'world'})
fastify.listen(3000, err => {
  if (err) throw err
  console.log(`server listening on ${fastify.server.address().port}`)

under-pressure will automatically handle for you the Service Unavailable error once one of the thresholds has been reached.
You can configure the error message and the Retry-After header.

fastify.register(require('under-pressure'), {
  maxEventLoopDelay: 1000,
  message: 'Under pressure!',
  retryAfter: 50

The default value for maxEventLoopDelay, maxHeapUsedBytes and maxRssBytes is 0.
If the value is 0 the check will not be performed.

Thanks to the encapsulation model of Fastify, you can selectively use this plugin in some subset of routes or even with different thresholds in different plugins.


This plugin also exposes a function that will tell you the current values of heapUsed, rssBytes and eventLoopDelay.


Status route

If needed you can pass { exposeStatusRoute: true } and under-pressure will expose a /status route for you that sends back a { status: 'ok' } object. This can be useful if you need to attach the server to an ELB on AWS for example.


This project is kindly sponsored by LetzDoIt.


Licensed under MIT.