Prerequisites

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🚀 Feature Proposal

Currently, we support genReqId initial config when creating a fastify app.
This affects the root instance, and all its child afterwards.

But currently, there's no way to customize it after creation. e.g. through a plugin, or just through an encapsulated context.

Having fastify#setGenReqId (similar to the work done for fastify#setChildLoggerFactory) can enable different use cases at fastify at different levels.

e.g.

Attach a function from a plugin
Maybe you have an observability plugin standardised for your company, and it handles custom ways of inferring or creating request ids.

function ObsPlugin(instance, config, done) {
  // do magic
  instance.setGenReqId((req) => {
    // infer it or create it in a custom way
  })
}

This will affect the parent context.

Create different request ids based on resources

// I might want to generate one per resource
fastify.register((instance, opts, done) => {
  instance.setGenReqId((req) => { /* one way for admin */})
}, { prefix: '/admin' })

fastify.register((instance, opts, done) => {
  instance.setGenReqId((req) => { /* one way for common users */})
}, { prefix: '/users' })

I believe the benefit of the encapsulated context can be a key differentiator.

Motivation

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Example

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