vLLM Mock Engine

vllm-mock-engine is a small engine-side process for frontend stress testing. It joins a frontend-owned startup handshake, reports a large ready response, treats prefill as instant, and emits random decode tokens until each request reaches its max_tokens.

The frontend must own the handshake socket. Start the frontend first, then start the mock engine with the same handshake address.

Start the mock engine

bash

cargo run -p vllm-mock-engine -- \
  --handshake-address tcp://127.0.0.1:29550 \
  --engine-count 1 \
  --output-token-chunk-size 1 \
  --vocab-size 32000 \
  --seed 0 \
  --log-requests

Useful knobs:

--engine-count must match the frontend's expected data-parallel engine count.
--output-token-chunk-size controls how many token IDs appear in one EngineCoreOutput; values greater than 1 are useful for MTP/spec-decode shaped frontend tests.
--vocab-size should stay within the tokenizer vocabulary of the model used by the frontend.

Stop it with Ctrl-C.

Rust Frontend

Terminal 1:

bash

cargo run --bin vllm-rs -- serve Qwen/Qwen3-0.6B \
  --data-parallel-size 1 \
  --data-parallel-size-local 0 \
  --handshake-port 29550

Terminal 2:

bash

cargo run -p vllm-mock-engine -- \
  --handshake-address tcp://127.0.0.1:29550

For multiple mock engines, set both sides to the same count:

bash

cargo run --bin vllm-rs -- serve Qwen/Qwen3-0.6B \
  --data-parallel-size 4 \
  --data-parallel-size-local 0 \
  --handshake-port 29550

cargo run -p vllm-mock-engine -- \
  --handshake-address tcp://127.0.0.1:29550 \
  --engine-count 4

Python Frontend

Use vllm serve with --data-parallel-size-local 0 so the Python process runs as a frontend/API server and waits for external engines on --data-parallel-rpc-port.

Terminal 1:

bash

vllm serve Qwen/Qwen3-0.6B \
  --data-parallel-address 127.0.0.1 \
  --data-parallel-rpc-port 29550 \
  --data-parallel-size 1 \
  --data-parallel-size-local 0

Terminal 2:

bash

cargo run -p vllm-mock-engine -- \
  --handshake-address tcp://127.0.0.1:29550

Smoke Request

After either frontend is ready:

bash

curl http://127.0.0.1:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "Qwen/Qwen3-0.6B",
    "messages": [{"role": "user", "content": "hello"}],
    "max_tokens": 16,
    "stream": true
  }'

Always pass max_tokens; the mock engine stops by length.