WASP - How to Choose Between RPS and VUs

When writing a test, you need to decide whether to use RPS or VUs as your load type.

RPS (Requests Per Second)

You should use RPS when your goal is to measure throughput. This load type implements an open model, where:

• The number of requests is fixed.
• The load is adjusted to meet the target RPS.
• The system's response time has no impact on the rate of load generation.

In an RPS-based test, new requests are sent continuously, regardless of how long previous requests take.

Example RPS Test Diagram

---
title: RPS test
---
sequenceDiagram
    participant Profile(Test)
    participant Scheduler
    participant Generator(Gun)
    participant Promtail
    participant Loki
    participant Grafana
    loop Test Execution
        Profile(Test) ->> Generator(Gun): Start with (API, TestData)
        loop Schedule
            Scheduler ->> Scheduler: Process schedule segment
            Scheduler ->> Generator(Gun): Set new RPS target
            loop RPS load
                Generator(Gun) ->> Generator(Gun): Execute Call() in parallel
                Generator(Gun) ->> Promtail: Save CallResult
            end
            Promtail ->> Loki: Send batch<br/>when ready or timeout
        end
        Scheduler ->> Scheduler: All segments done<br/>wait all responses<br/>test ends
        Profile(Test) ->> Grafana: Check alert groups<br/>FAIL or PASS the test
    end

VirtualUser (VUs)

Use VUs when working with stateful protocols or workflows. With VirtualUser, the requests per second rate cannot be guaranteed because:

• Each iteration begins only after the previous one finishes.
• The load follows a closed model, where the system's response time affects the RPS.

If the system takes longer to respond, iterations take longer, and the RPS decreases.

Example VUs Test Diagram

---
title: VUs test
---
sequenceDiagram
    participant Profile(Test)
    participant Scheduler
    participant Generator(VUs)
    participant VU1
    participant VU2
    participant Promtail
    participant Loki
    participant Grafana
    loop Test Execution
        Profile(Test) ->> Generator(VUs): Start with (API, TestData)
        loop Schedule
            Scheduler ->> Scheduler: Process schedule segment
            Scheduler ->> Generator(VUs): Set new VUs target
            loop VUs load
                Generator(VUs) ->> Generator(VUs): Add/remove VUs
                Generator(VUs) ->> VU1: Start/end
                Generator(VUs) ->> VU2: Start/end
                VU1 ->> VU1: Run loop, execute multiple calls
                VU1 ->> Promtail: Save []CallResult
                VU2 ->> VU2: Run loop, execute multiple calls
                VU2 ->> Promtail: Save []CallResult
                Promtail ->> Loki: Send batch<br/>when ready or timeout
            end
        end
        Scheduler ->> Scheduler: All segments done<br/>wait all responses<br/>test ends
        Profile(Test) ->> Grafana: Check alert groups<br/>FAIL or PASS the test
    end

In this model, VU2 starts only after VU1 has finished its iteration.

Summary

• Use RPS for:

  • Stateless protocols.
  • Measuring throughput.
  • Scenarios where the load generation rate must remain constant.

• Use VUs for:

  • Stateful protocols.
  • Workflows that involve multiple steps.
  • Scenarios where response time impacts the load generation rate.