What does the Random Seed field of JMeter's Open Model Thread Group control?
answer
- Only one step kind consults it
- Zero is the field's default value
- It fixes placement, not volume
- Think about comparing two runs
- Different groups want different values
basics
~20 sIt seeds the generator that decides where arrivals fall inside a random_arrivals span. Zero, the default, means an unseeded generator, so the instants differ on every run; any non-zero value makes the same schedule produce the same arrival instants each time.
solid answer
~40 sThe Open Model Thread Group carries exactly two load properties: the Schedule and `OpenModelThreadGroup.random_seed`. At start the group builds a `java.util.Random` — unseeded when the value is `0`, seeded with the value otherwise — and hands it to the process generator that places arrivals inside each `random_arrivals` span. A fixed seed therefore makes the *timing* of arrivals repeatable from run to run. What it does **not** change is how many arrivals there are: the count of a span is the mean of its start and end rates times its duration, truncated, and that is fixed by the schedule alone. The GUI simply labels the field **Random Seed**; the component reference heads it "Random Seed (change from 0 to random)" and advises that different thread groups in one plan use different seed values.
go deeper
Recall that the field defaults to 0, that 0 means a fresh random pattern each run, and that the setting only affects the random_arrivals steps of the schedule.
Explain that the seed builds the generator that places arrivals inside a span, and that arrival count and total duration are already fixed by the schedule regardless of it.
Show judgement about when repeatability is worth having: pin a seed to compare two builds or reproduce a reported spike, leave it at 0 when a result should not hinge on one arrival pattern.
Own the convention: decide whether your team's plans pin seeds by default, and make the seed part of what a run archives, so any published result can be re-run on demand.
## What the field feeds The Open Model Thread Group has two properties that shape load: `OpenModelThreadGroup.schedule` and `OpenModelThreadGroup.random_seed`. When the group starts, it reads the seed and builds a `java.util.Random`: an unseeded one if the value is `0`, a seeded one otherwise. That single generator is handed to the object that turns the schedule into a list of arrival instants, and it is consulted only by `random_arrivals` spans. So the seed is a knob on **placement**, not on volume and not on shape. ## Seeded and unseeded, side by side | Random Seed | Generator | Effect on two runs of one plan | |---|---|---| | `0` (default) | fresh, unseeded | the same profile, but each arrival falls at a different instant | | any non-zero value | seeded with that value | identical arrival instants in both runs | The GUI labels the field plainly, **Random Seed**; it is the component reference that spells the default out, heading the property "Random Seed (change from 0 to random)" and adding two notes worth remembering: a constant seed makes the thread group generate the same delays on every test start, and different thread groups in the same plan should preferably carry different seed values, so that two groups do not end up firing in lockstep. ## What the seed does not touch This is where people over-claim. Fixing the seed does not make a run reproducible end to end; it fixes one input only. - **Arrival count is already deterministic.** For a span, JMeter takes the mean of the start and end rates, multiplies by the duration and truncates. A flat span of two per second over five seconds schedules ten arrivals whatever the seed is. - **Total duration is already deterministic.** It is the sum of the arrivals and pause spans, and no random number enters it. - **`even_arrivals` never consults the generator.** Its instants are computed from the rates and the duration, so it is unaffected by the seed. - **`pause` never consults it either**, because a pause compiles down to an even-arrivals span held between two zero rates. - **Nothing about the system under test is fixed.** Identical arrival instants do not imply identical response times, and the samples you collect will still differ. ## When to pin it and when not to Pin a seed when you want two runs to differ only in the thing you changed — comparing a build against the previous one, or reproducing a report of a spike at a particular minute. Leave it at `0` when you deliberately want a fresh arrival pattern each time, so that a result does not quietly depend on one lucky or unlucky placement. Worth knowing: JMeter itself normalises the field. When you use the GUI's plan-validation action, the cloned tree sets an Open Model Thread Group's seed string back to `0` and replaces its schedule with a short fixed one — `rate(N / sec) even_arrivals(1 sec) pause(1 hour)`. Note that the determinism there comes from `even_arrivals`, which never consults the generator, not from the seed: `0` is the unseeded value. The point is simply that a validation pass is not at the mercy of the profile you were editing. ## Reading it in a plan The value lives in the `.jmx` under the property name `OpenModelThreadGroup.random_seed`, next to `OpenModelThreadGroup.schedule`. Two habits pay off: 1. Treat the seed as part of the run's identity — record it beside the schedule text in whatever you archive with the results, because a run you cannot reproduce is one you cannot argue about later. 2. If a plan has several open-model groups, give each a distinct seed rather than copying one value across them.
- Does a fixed Random Seed change how many arrivals a random_arrivals span produces?No. The count is the mean of the span's start and end rates times its duration, truncated, and that depends only on the schedule. The seed decides where inside the span those arrivals fall, so two runs with different seeds schedule the same number of arrivals at different instants.
- Why does the manual suggest different thread groups use different seed values?Because two groups seeded identically draw the same sequence, so their arrivals line up instead of interleaving. Giving each group its own value keeps their patterns independent, which is what you normally want when several open-model groups run side by side.
saying these in an interview costs you the question
- Says a fixed seed makes the whole run reproducible
- Thinks the seed changes how many arrivals occur
- Believes zero means no randomness at all
- Expects it to affect even_arrivals or pause
- Copies one seed across every thread group