In a JMeter HTTP Request's Body Data field, how do you generate a unique order id per request?

## The body field is interpolated like any other An HTTP Request's **Body Data** box is an ordinary JMeter text field. A reference in it is compiled once when the plan is prepared and re-evaluated every time that sampler runs: ``` {"orderId":"ORD-${__UUID}","sku":"${SKU}","qty":1} ``` Nothing else is required — no pre-processor, no script element. The only real question is which function gives you the uniqueness you actually need. ## What each candidate guarantees | Call | New value on | Unique across | | --- | --- | --- | | `${__UUID}` | every evaluation | threads, Thread Groups and JVMs | | `${__counter(TRUE,)}` | every evaluation, per thread | that one thread | | `${__counter(FALSE,)}` | every evaluation, shared | threads in one JVM | | `${__threadNum}` | never — fixed for the thread's life | its own Thread Group only | | `${__RandomString(12,0123456789ABCDEF)}` | every evaluation | nothing; collisions are possible | | `${__timeShift(yyyyMMddHHmmssSSS,,,,)}` | every evaluation | nothing; threads share a millisecond | `${__UUID}` is the honest default. It takes no arguments, returns a type-4 UUID, and needs no coordination, so it keeps working when you add threads, add a second Thread Group, or run the same plan from more than one machine. ## The counter traps 1. **Every occurrence is its own counter.** A call is compiled into one function instance per occurrence in the plan, so two fields containing `${__counter(TRUE,)}` count independently. To share one sequence, produce it once with the second argument — `${__counter(TRUE,ORDER_SEQ)}` — and reference `${ORDER_SEQ}` everywhere else. 2. **`FALSE` is not global to the test.** It is global to that one function instance inside that one JVM. 3. **It is backed by an int**, so the sequence stops meaning anything past 2,147,483,647. 4. **Each server in a distributed run evaluates its own copy of the plan**, so counter-based and thread-number-based ids repeat once per server. `${__UUID}` does not. ## __threadNum is Thread Group-local JMeter names its worker threads `<group name> <group number>-<thread number>`, and `__threadNum` returns the text after the last hyphen — the thread's 1-based index inside its own Thread Group. Add a second Thread Group and there are two threads numbered 1. If you want the number to identify a virtual user across the whole plan, prefix it with `${__threadGroupName}`, which also takes no arguments. Two further limits are easy to trip over: `__threadNum` does not work in a config element such as User Defined Variables, because config elements are processed from a separate thread, and it means nothing on the Test Plan element. ## Readable ids versus safe ids Two shapes are worth knowing, and they trade against each other: - `ORD-${__UUID}` — unique with no reasoning required, but it tells you nothing when you find it in a server log. - `ORD-${__threadGroupName}-${__threadNum}-${__counter(TRUE,)}` — reads as *group, user, iteration*, so a failed request traces straight back to the virtual user and iteration that sent it; unique only within one JVM. A common compromise carries both: a UUID as the id the system under test stores, and the readable composite in a custom header or the sample label so the run itself stays navigable. How realistic the rest of the payload has to be is a test-data question rather than a JMeter-syntax one, and is decided elsewhere. ## Use the same id twice, on purpose If the id must appear in the body *and* in a header, or in a later request, do not write `${__UUID}` twice — the second call returns a different UUID. `__UUID` has no store-to-variable argument, so capture it once (a User Parameters pre-processor is the usual place) and reference the variable afterwards. `__counter` and `__RandomString` do take a variable name as their last argument, which is the cheaper route when either of those fits. Finally, check the substitution once. An unresolved reference is passed through as literal text, so a mistyped `${__UUId}` puts the characters `${__UUId}` into the order id rather than failing anything. Behaviour described is Apache JMeter 6.0.0.