Distributed Master / Worker

Overview

LoadDensity exposes Locust’s distributed runner via a runner_mode parameter on start_test / prepare_env. Three modes are supported:

local — single process (default).
master — coordinates a cluster of workers, optionally serves the Locust Web UI.
worker — joins a master and runs the requested user count.

Master

from je_load_density import start_test

start_test(
    user_detail_dict={"user": "fast_http_user"},
    runner_mode="master",
    master_bind_host="0.0.0.0",
    master_bind_port=5557,
    expected_workers=4,                # wait for 4 workers
    web_ui_dict={"host": "0.0.0.0", "port": 8089},
    user_count=400,
    spawn_rate=40,
    test_time=600,
    tasks=[...],
)

The master waits up to 60 s for expected_workers workers to join before starting the load ramp. If only N workers (N < expected) join, it logs a warning and starts anyway.

Worker

Run on each load-generating node:

start_test(
    user_detail_dict={"user": "fast_http_user"},
    runner_mode="worker",
    master_host="10.0.0.10",
    master_port=5557,
    tasks=[...],
)

Workers do not start a Web UI and skip the local stats greenlets — the master collects and publishes aggregate stats.

Tips

Open the master master_bind_port in your firewall. Default Locust port is 5557.
Use master_bind_host="0.0.0.0" only when the master is reachable by the workers; bind to a private interface IP otherwise.
Match the user template (http_user / fast_http_user / …) on master and workers — the master broadcasts the user class name.
If you parameterise tasks with ${csv.X.col}, register the same CSV files on every worker (they don’t share state).