The game built in the YouTube series Building an MMO: The Engineering Series: a C++ server, a C++ client, and everything around them. Each episode's code is a git tag (ep001/end, ep002/end, ...).

Build, test and run

You need CMake, Ninja and a C++20 compiler. The client draws with raylib, on top of the system's OpenGL:

macOS and Windows: nothing more to install.
Ubuntu: sudo apt install xorg-dev libgl1-mesa-dev, the X11 and OpenGL headers.

CMake fetches and builds the libraries: GoogleTest, raylib, Dear ImGui, rlImGui, abseil, protobuf, Asio and prometheus-cpp. The first build compiles protobuf, so it takes a few minutes.

cmake -S . -B build -G Ninja
cmake --build build
ctest --test-dir build
./build/bin/mmo_server

Then, in another terminal or two:

./build/bin/mmo_client

The server listens on TCP port 7777, and the client connects to localhost, or to the host you give it (./build/bin/mmo_client 192.168.1.20). Each player is a cube on a board of tiles. W, A, S and D, or the arrow keys, move yours, which has a white marker over it. View, Settings changes the background.

protocol/mmo.proto is the contract between the client and the server. The build runs protoc on it, and both sides use the C++ it generates.

Or with only Docker installed:

# Build and run the tests on Ubuntu.
docker build --target test .
# Check the formatting.
docker build --target format .
# The server image, if the tests pass.
docker build --target server -t mmo-server .
# Run the server, on port 7777, with its metrics on 9464.
docker run --rm -p 7777:7777 -p 9464:9464 mmo-server
# API docs in site/.
docker build --target docs --output site .

In Docker, the client's tests open their window on a virtual display from Xvfb, and render with llvmpipe, Mesa's OpenGL driver that runs on the CPU. So they don't need a screen or a GPU.

Monitoring and load

The server serves its metrics for Prometheus at http://localhost:9464/metrics: the players online, how long each tick takes, the bytes it sends and the messages it receives.

compose.yaml starts the server together with Prometheus, which scrapes it every two seconds, and Grafana, with a dashboard of those metrics. The dashboard and Grafana's data source are files in monitoring/, so they're set up the same way on any machine.

docker compose up -d --build

Grafana: http://localhost:3000, which opens on the MMO server dashboard.
Prometheus: http://localhost:9090.

mmo_bot connects a swarm of bots that wander around like players at their keys. It takes the host, the port, how many bots, and how many seconds to spread their arrival over:

./build/bin/mmo_bot localhost 7777 100 10

Or in Docker, next to the server: 300 bots, arriving over two minutes.

docker compose up -d bots

A few hundred bots need more open files than macOS allows a shell by default, so on the Mac, run ulimit -n 4096 first. Stop everything with docker compose --profile load down.