In Gatling, which builders declare a whole stepped load profile in one expression, and what does each call in that chain set?

Gatling calls a stepped load profile a **staircase**, and it ships one builder per workload model so that a capacity profile is a single chained expression rather than a hand-written run of alternating ramps and holds. ## The two entry points | builder | model | what the argument means | type | |---|---|---|---| | `incrementUsersPerSec(rateIncrement)` | open | the difference in **arrivals per second** between consecutive levels | `Double` | | `incrementConcurrentUsers(usersIncrement)` | closed | the difference in **concurrent users** between consecutive levels | `Int` | The commonest misreading is treating that first number as the load itself. It is not. Gatling's own API documentation calls the parameter *"the difference of users per second rate between levels of the stairs"*. So `incrementUsersPerSec(20)` does not ask for twenty arrivals a second — it asks for levels **twenty apart**. The builder you pick fixes which model you are declaring, and every step inside one injection call must be of the **same** model kind, so an `incrementUsersPerSec` staircase cannot sit beside a `constantConcurrentUsers` hold in the same call. (What an open or a closed workload model *is*, and which one your system deserves, is performance-testing theory that this page does not settle.) ## The chain is a type ladder, not a bag of options Each call returns a different type, and that is what enforces the order at compile time: 1. `incrementUsersPerSec(20)` yields a `Stairs` whose **only** method is `times`. 2. `.times(8)` yields a stage whose **only** method is `eachLevelLasting`. 3. `.eachLevelLasting(d)` yields a `Composite` — and this is the first point at which you hold a usable injection step. 4. `.startingFrom(x)` and `.separatedByRampsLasting(d)` are declared on that `Composite`, return another `Composite`, and may be written in either order. Two consequences follow. You cannot skip `times` or `eachLevelLasting` — there is no overload that lets you — and you cannot call `.startingFrom(...)` before `.eachLevelLasting(...)`, because the earlier stages do not declare it. ## What the four calls set * **`times(levels)`** — the number of levels in the staircase. * **`eachLevelLasting(duration)`** — how long every level holds. This is the same for all levels; the builder has no per-level duration. * **`separatedByRampsLasting(duration)`** — *optional*. Inserts a linear ramp of that length between levels. Omit it and the profile jumps from one level straight to the next. * **`startingFrom(x)`** — *optional*. The level the staircase begins at. Omit it and it begins at zero, which is almost never what a capacity run wants. The duration arguments follow the SDK's usual spelling: a bare number means **seconds** everywhere, Java and Kotlin also take a `java.time.Duration`, Scala takes a `scala.concurrent.duration` literal such as `2.minutes`, and the JavaScript/TypeScript SDK takes an object literal such as `{ amount: 2, unit: "minutes" }`. ## A worked profile A capacity staircase of eight levels twenty arrivals per second apart, each level held for two minutes: ```java setUp( scn.injectOpen( incrementUsersPerSec(20) .times(8) .eachLevelLasting(Duration.ofMinutes(2)) .startingFrom(20) ) ).protocols(httpProtocol); ``` That declares levels of 20, 40, 60, 80, 100, 120, 140 and 160 arrivals per second, sixteen minutes of run time, and roughly 86,400 injected users — one expression in place of eight hand-written steps. With no ramps declared, the staircase expands to exactly one `constantUsersPerSec(rate).during(2 min)` block per level, so eight levels means eight blocks. (Adding `separatedByRampsLasting` to this `startingFrom(20)` profile would make it fifteen: eight levels and seven ramps.) ## What the staircase expands into The helper is shorthand. Gatling turns the finished chain into an ordinary list of injection blocks — but which list depends on which branch the expansion takes, and there are two of them. **With no ramps, or with ramps over a non-zero starting level**, it is one constant-rate block per level plus one ramp block per gap (`levels - 1` ramps, and none at all without `separatedByRampsLasting`), and the rate of the level at index `k` is simply: ```text levelRate = k * increment + startingRate // k = 0 .. levels-1 ``` **With ramps and a starting level of zero** — which is what you get when `startingFrom` is omitted — Gatling takes the other branch. It emits `levels` ramps rather than `levels - 1`, because there is a leading ramp out of zero that sits in no gap, and the rates are: ```text levelRate = (k + 1) * increment // k = 0 .. levels-1 ``` so that profile peaks at `levels x increment`, one full increment above what the first formula would predict. Carrying the first formula into the ramped-from-zero case is the commonest arithmetic slip on this builder. Two things follow from the expansion. Everything true of `constantUsersPerSec` and `rampUsersPerSec` is true of the blocks a staircase produces, because they *are* those blocks. And the total run length is derived rather than declared: `levels x levelDuration` with no ramps, plus the ramp time when ramps are present. Nothing in the chain states a total duration, so a reviewer computes it from the level count and the level duration. ## Where it sits in the simulation The finished `Composite` **is** an injection step, so it goes wherever any other step goes: alone inside `injectOpen(...)`, or chained beside other open steps such as a trailing `constantUsersPerSec(...).during(...)`. In Java, Kotlin, JavaScript and TypeScript you pass it to `scn.injectOpen(...)` or `scn.injectClosed(...)`; in Scala you pass it to the single `inject(...)`, which resolves open versus closed from the step type rather than from the method name. One limit worth knowing up front: there is no `.randomized()` on a staircase. That modifier exists on `constantUsersPerSec` and `rampUsersPerSec` only, so the levels of a staircase always inject at regular intervals.