Explain the lifecycle of a single HTTP sampler request through JMeter's processor chain?

Before the request is actually sent, any Pre-Processors attached to (or scoped over) the sampler run first, in the order they appear in the tree - this is where a script might modify request parameters, set a header dynamically, or perform setup logic that needs to happen right before this specific request goes out.

JMeter then constructs and sends the actual HTTP request, incorporating whatever Config Elements are in scope (HTTP Request Defaults for shared connection details, the Cookie Manager for any stored cookies, the Cache Manager for cache-related headers or a simulated cache hit) alongside the sampler's own configured parameters.

Once a response is received, or the request fails to complete, JMeter records the raw sample result - response code, response time, response data, and success/failure status based on the protocol-level outcome (like a valid HTTP response versus a connection timeout).

Post-Processors attached to the sampler then run, in tree order, typically extracting data from that response into variables for later use; after that, Assertions attached to the sampler evaluate against the response, and any assertion failure overrides the sample's status to failed even if the underlying protocol-level result had been technically successful.

The fully processed sample - including its final pass/fail status after assertions - is then what gets sent to any active listeners and written to the result file, which is why a sample can show an HTTP 200 status code in the raw response data while still being correctly reported as a failure overall, if an assertion attached to it determined the actual content wasn't what was expected.

flowchart LR
  A[Pre-Processors run] --> B[Request constructed: Config Elements + sampler params applied]
  B --> C[HTTP request sent]
  C --> D[Raw sample recorded: code, time, data]
  D --> E[Post-Processors run: extract data to variables]
  E --> F[Assertions evaluate: may override pass/fail]
  F --> G[Final sample sent to listeners and result file]