Simulation Lifecycle

Complete Execution Flow

This document provides a detailed walkthrough of what happens when you run a Gatling simulation from start to finish.

mvn gatling:test -Dgatling.simulationClass=io.learn.gatling.simulations.http.Sim01_BasicHttp
        │
        ├─→ [PHASE 1: INITIALIZATION]
        ├─→ [PHASE 2: WARM-UP]
        ├─→ [PHASE 3: INJECTION]
        ├─→ [PHASE 4: EXECUTION]
        ├─→ [PHASE 5: COOL-DOWN]
        ├─→ [PHASE 6: REPORTING]
        └─→ [Done: target/gatling/.../ report generated]

Phase-by-Phase Breakdown

Phase 1: Initialization (< 1 second)

  • Load Simulation class
  • Parse Protocol, Scenario, setUp()
  • Validate syntax and configuration
  • Initialize metrics collection system
  • Prepare HTTP client infrastructure

Phase 2: Warm-up (1-5 seconds)

  • Establish initial TCP connections
  • Perform TLS handshakes
  • JVM JIT compiler starts warming up
  • Connection pools initialized
  • Ready for test traffic

Phase 3: Injection & Ramp-up

  • Users are created according to injection profile
  • Each user starts executing scenario independently
  • Requests begin being sent
  • Metrics start being collected

Phase 4: Steady State / Execution

  • Users continue executing scenarios
  • Responses recorded
  • Latencies measured
  • Errors tracked
  • Live console statistics updated

Phase 5: Cool-down

  • Injection stops (no new users created)
  • Existing users finish their current scenarios
  • In-flight requests complete
  • Graceful shutdown

Phase 6: Reporting

  • Aggregate all metrics
  • Calculate percentiles (p50, p95, p99)
  • Generate HTML report with charts
  • Evaluate assertions
  • Print summary to console

Per-User Execution Flow

Each virtual user executes the scenario independently:

User 1 Timeline:
├─ [0s]   User created, scenario starts
├─ [1s]   GET /posts request sent
├─ [2.5s] Response received, latency = 1500ms
├─ [2.5s] Response validated (check: status = 200)
├─ [2.5s] pause(1) begins
├─ [3.5s] pause complete
├─ [3.5s] GET /posts/1 request sent
├─ [4.2s] Response received, latency = 700ms
├─ [4.2s] Response validated
├─ [4.2s] pause(1) begins
├─ [5.2s] pause complete
├─ [5.2s] GET /comments?postId=1 request sent
├─ [6.0s] Response received, latency = 800ms
├─ [6.0s] Response validated
├─ [6.0s] Scenario complete
└─ [6.0s] User finishes (or loops back to start if configured)

Total: 6 seconds per user, per scenario iteration

See Also

Next Steps

→ Read next: HTTP vs Kafka