Intro

Hermes is a TCP proxy written in Erlang that simulates network conditions between servers. It introduces configurable latency by delaying data forwarding on each send operation, where each operation transmits up to one buffer's worth of data. Hermes delivers better performance than kffl/speedbump. It also supports runtime delay injection via an HTTP API, a feature not available in Speedbump.

Benchmarks

redis-benchmark

Benchmarking script is available at benchmarks/redis_test.sh. The following benchmarking results are based on 1M requests across 100 concurrent clients with buffer size 65536 (64 KB) across different latencies.

Baseline (without any proxy) vs Speedbump (0ms upstream latency) vs Hermes (0ms upstream latency)

Baseline (without any proxy) vs Speedbump (5ms upstream latency) vs Hermes (5ms upstream latency)

Docker compose setup (recommended)

services:
  hermes:
    image: xillar/hermes:latest
    container_name: hermes
    ports:
      - "6389:6389"
      - "8000:8000"
    environment:
      LISTEN_HOST: "0.0.0.0"
      LISTEN_PORT: "6389"
      FORWARD_HOST: "localhost"
      FORWARD_PORT: "6379"
      LATENCY_MSECS: "5"
      BUFFER_SIZE: "65536"
      LOG_LEVEL: "INFO"
      API_HOST: "0.0.0.0"
      API_PORT: "8000"

Docker public image: xillar/hermes:latest

Configuration (Environment Variables)

The proxy is configured using environment variables. Below are the available options:

Variable	Default	Description
LISTEN_HOST	127.0.0.1	Proxy bind address
LISTEN_PORT	6380	Proxy listen port
FORWARD_HOST	127.0.0.1	Target server address
FORWARD_PORT	6379	Target server port
BUFFER_SIZE	65536	Read buffer size (bytes)
API_HOST	127.0.0.1	Latency API host
API_PORT	8000	Latency API port
LATENCY_MSECS	0	Artificial delay (ms) per flush

Latency API

GET /latency — read current latency

curl http://localhost:8000/latency
# {"latency": 5}

POST /latency — update latency

curl -X POST http://localhost:8000/latency \
     -H 'Content-Type: application/json' \
     -d '{"latency": 20}'
# {"latency": 20}