async-api-routes.md

title	Prevent Waterfall Chains in API Routes
impact	CRITICAL
impactDescription	2-10× improvement
tags	api-routes, server-actions, waterfalls, parallelization

Prevent Waterfall Chains in API Routes

In API routes and Server Actions, start independent operations immediately, even if you don't await them yet.

Incorrect (config waits for auth, data waits for both):

export async function GET(request: Request) {
  const session = await auth()
  const config = await fetchConfig()
  const data = await fetchData(session.user.id)
  return Response.json({ data, config })
}

Correct (auth and config start immediately):

export async function GET(request: Request) {
  const sessionPromise = auth()
  const configPromise = fetchConfig()
  const session = await sessionPromise
  const [config, data] = await Promise.all([
    configPromise,
    fetchData(session.user.id)
  ])
  return Response.json({ data, config })
}

For operations with more complex dependency chains, use better-all to automatically maximize parallelism (see Dependency-Based Parallelization).