Edge Function

The Edge Function runs small handlers in the CDN, next to the user, as Cloudflare Workers and Fastly Compute do.

When to place it#

• Place it when users are far from your origin and the handler is small: a redirect, a header rewrite, a cached read, a personalized page. It unlocks with Edge compute in Series C, after Content delivery, for 80,000 ◇.

• Skip it for CPU-heavy kinds, such as an upload's image resize, an export or a login's password hash. They belong on an App Server.

• Copy it when users in another region need it, since one Edge Function serves only its own region.

How it works#

It takes every request kind and serves it like an App Server, turning it into the backend calls it makes. A user in the Node's own region reaches it in 5 ms instead of the round trip to your origin, but a user in another region still pays that round trip. Cold starts barely register: a V8 isolate starts in under 5 ms, against the seconds a container or a JVM takes.

A live Edge Function also holds the edge key-value cache that a CDN answers read from. That cache answers 30% of API reads, which the CDN alone answers at 0%.

It bills per request, on the Requests line of the cloud bill, and nothing while it idles.

Options#

Two ceilings stand in for an instance type:

• CPU. A request may spend 10 ms of CPU. Past it the handler is killed and the request fails as a timeout.

• Memory. A handler runs in 128 MB of isolate, which fits a small handler and its response but not a working set.

It runs your Service's Stack, so the Stack has to compile to the edge: JavaScript and TypeScript, or Rust and Go built to WebAssembly. A PHP, Ruby, Python, Java, C#, Elixir, Zig or Haskell Stack is refused there, and the card says so.

How it fails#

• The CPU ceiling. A request that needs more than 10 ms of CPU is killed mid-flight, so an upload, a report or an export does not belong here.

• The Stack. A Company that runs PHP cannot use the edge at all. The fix is a migration to a Stack that compiles, not a second edge Node.

• The data tier. A handler that calls a Database on every request pays the round trip anyway, so it saves the user hop and not the query.

Certificate expired#

• Signs: The Certificate expired banner appears, and every request through the Edge Function fails, as through every other TLS Node.

• Do now: Click Renew in the banner.

• Prevent: Research Certificate automation and place a Secrets and Certificates Node, which renews the certificates before they run out.

Region outage#

• Signs: One region's board goes grey and the Edge Function in it goes down.

• Do now: Press Fail over on the banner. It needs Multiple zones and a second open region, and starts the Node again in the nearest other region, cold, for the price of a replacement. Otherwise it comes back when the incident ends: 180 sim seconds in the catalog, 60 once the error budget counts.

• Prevent: Research Multiple regions, keep a copy in a second region, and add GeoDNS to send users there.

Tips#

• Pair it with a CDN, so the edge answers what the CDN cannot cache and the CDN answers the rest.

• Keep the handler small. Treat the isolate's ceiling as a design constraint, not a bug to work around.