# Load shedding

> Reject some work early and cheaply so the rest still finishes on time.

Concept · Reliability · https://levelcode.org/concepts/load-shedding

When demand exceeds capacity, some requests won't be served well. The choice is between **rejecting some quickly** or **serving all of them slowly** — and "slowly" usually means everything times out, so nothing is served. Load shedding makes the first choice deliberately.

## Shed early and cheaply

A rejection should cost microseconds and happen **before** any expensive work: before parsing large bodies, querying the database or calling dependencies. A request that does 2 seconds of work and then times out at the client wasted all of it.

Common signals to shed on:

- **Concurrency:** more than N requests in flight ([Little's law](https://levelcode.org/concepts/littles-law) tells you N for your latency target).
- **Queue wait time:** a request has already waited longer than it could usefully wait (CoDel-style).
- **Resource pressure:** CPU or memory beyond a threshold.

Respond with `503 Service Unavailable` (or `429` for per-client limits) and a `Retry-After` header.

## Shed by priority

Not all traffic is equal. Decide the order in advance:

1. Background jobs, prefetches, analytics beacons.
2. Anonymous and crawler traffic.
3. Non-critical features of logged-in sessions.
4. Last: checkout, login, payments.

Tag requests with a priority at the edge so every layer can make the same decision.

## Related mechanisms

- **Rate limiting** caps each client ([rate-limiting algorithms](https://levelcode.org/concepts/rate-limiting-algorithms)); load shedding caps the server whatever the client mix. You want both.
- **Backpressure** pushes the overload signal upstream so producers slow down ([backpressure](https://levelcode.org/concepts/backpressure)).
- **Autoscaling** adds capacity, but takes minutes; shedding protects you in the meantime.
- **Graceful degradation** reduces the cost per request (turn off recommendations) so more requests fit ([graceful degradation](https://levelcode.org/concepts/graceful-degradation)).

## Watch for retry storms

Shed requests come back as retries. Clients must back off with jitter, and `Retry-After` helps them do it ([retries, backoff and jitter](https://levelcode.org/concepts/retries-with-backoff)).