# FixedStepService

> Documentation for Redweb 0.13.0. Install that exact version when following these examples.

Route-scoped simulation clock that compensates for drift, prevents overlapping async ticks, bounds catch-up work, and reports dropped retained lag instead of replaying forever.

## Explain it like I’m five

FixedStepService is a metronome for game logic. Even when the computer hesitates, it advances the simulation in measured beats without starting two beats at once.

## When should I use it?

Use it for authoritative simulations or periodic work that needs stable step sizes, bounded catch-up, and explicit handling of excessive lag.

## Follow the example

This API pattern illustrates the named surface; it may require application-owned classes, credentials, or assets. Start from a complete recipe for a runnable application.

```js
const { FixedStepService } = require('redweb')

class Simulation extends FixedStepService {
  constructor() { super('simulation', 50, 3) }
  async onTick(stepMs, tick) {
    await game.update(stepMs, tick)
  }
}
```

1. The service schedules ticks using the configured fixed interval.
2. A slow asynchronous tick finishes before another begins.
3. Limited catch-up reduces drift, while old excess lag is reported and discarded instead of causing an endless spiral.

## Methods and members

### constructor(name, tickRateMs, maxCatchUpTicks?, maxRetainedLagMs?)

Creates a fixed-step route service with finite catch-up and retained-lag limits.

### onTick(stepMs, tick)

Implement one simulation step. Async work never overlaps the next pulse.

### onLagDropped(milliseconds)

Optional observability hook called when retained lag is deliberately discarded.

## What should I watch for?

A fixed step does not make expensive work free. Measure onTick duration, set conservative catch-up limits, and keep network I/O outside the critical simulation path.