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DEVELOPMENT.md

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Development

This document aims to describe the architecture of this library and how one might want to expand on this library.

High level

@fastly/serve-vercel-build-output is a runtime library for Fastly Compute, which aims to execute an application that conforms to Vercel's Build Output API.

There are some specific goals here:

Primitive Type Support Notes
Static Files Yes Similar to Vercel, all these files will be served from the Edge.
Serverless Functions No At the current time we do not directly support Serverless Functions. (*1)
Edge Functions Yes These are run against the Compute JavaScript runtime.
Prerender Functions Yes These use the KV store to store the generated versions. (*2)

*1 - Next.js running with runtime: "nodejs" will normally compile to a Serverless function. @fastly/next-compute-js provides a replacement runtime to convert this into an Edge function so that it is possible to use with Compute.

*2 - Vercel describes this as a "Serverless Function that will be cached by the Vercel Edge Network", but we support this using an Edge function.

There are some non-goals:

  • At this point, we do not plan to support Serverless functions.
    • A future option may be a way to delegate this to a node runtime running on an origin server.

Architecture

Classes

Infrastructure

VercelBuildOutputServer - The "server" application that adapter code should call into. This class orchestrates instances of the classes below and dispatches a request to the EdgeMiddlewareStep, where processing begins.

EdgeMiddlewareStep - Simulates Vercel's "Edge Middleware Step":

  • Performs routing - see RouteMatcher below
  • Calls middleware if present
  • Applies wildcards
  • Calls into the Edge Network Cache step to serve static and function responses

EdgeNetworkCacheStep - Simulates Vercel's "Edge Network Cache Step":

  • Checks if current request has a cached response
  • If cache entry exists and is not expired, responds with cached copy
  • Otherwise, calls into Function step to generate function response
  • Always serves static assets as though they are cached

FunctionsStep - Executes an Edge function that corresponds to the current request and returns the response.

VercelExecLayer - Used by EdgeMiddlewareStep and FunctionsStep to prepare the request for calling a function asset.

Routing

RouteMatcher - Performs actual routing of the current request against the Vercel routing rules. See README.md in routing for implementation details.

RoutesCollection - Holds an abstraction of the routes defined in the Vercel config file.

RouteSrcMatcher - Tests a single route's "src" against a path string.