Game Server Spawn Flow

In order to spawn a game server right, few steps is done.

1. Creating a room

The Player decides to create a room, or just joins to first room that matches required criteria. In case of second option, if no rooms found that match the criteria, a new one is created.

2. Resolving a Region

Using the IP address of the Player, the closest Region is resolved (the one with closest geo distance to the Player). If no geo information is known upon Player’s IP address, the default Region is chosen instead.

3. Resolving a Host

On the Region from previous step, the least loaded Host it resolved (using Memory / CPU information provided using heartbeats). If a healthy Host cannot be found (either all of them are overloaded or lost connection on heartbeats), and error is returned to the Player.

4. Spawn Request

One the Host is resolved, an HTTP spawn request is sent to that Host. At that point, any result returned is redirected to the Player.

Yet, the Player’s request is not responded until the Game Server instance is spawned and completely initialized. If any error occurs during spawning process, the Player is responded with that error.

5. Spawning Game Server instance

At this point, all communication is happens on the Host the spawning process is happen on.

On the Host machine, depending of which Game Server Configuration is being spawned, a new process is instantiated. That process is completely game-specific. Upon starting a process, multiple command line arguments are passed:

<binary> <zeromq endpoint> <ports> <other> ...

  • <binary> A binary file that is actually being instantiated.
  • <zeromq endpoint> A path to ZeroMQ Endpoint that Controller will communicate with a Game Server upon, depending on the operation system, could have format tcp://<port> (tcp) or ipc://<unix domain socket> (Unix Domain Socket)
  • <ports> A comma separated list of ports us made available for that particular Game Server instance. For example, 32765,32766. Game Server instance may listen on that ports as Player may connect to them.
  • <other> … Additional command line arguments, that may appear as defined in Additional Command Line Arguments section of the Game Server Configuration.


If the Host machine has the firewall enabled, A port’s pool range (by default 38000..40000) must be explicitly enabled.

Alongside with those arguments, depending of Game Server Configuration, a bunch of Environment variables can be defined:

Environment Variable Description
login_access_token A complete and working Access token instance of server-side use.
discovery_services A JSON Object with predefined key/value list of service locations for server-side use. See Discover Services section of the Game Server Configuration.
game_max_players Maximum players the Room can take (on which this Game Server is spawned upon).
room_settings A JSON Object with custom room settings as defined by player.
server_settings A JSON Object with Custom Server Configuration Settings (see according section of the Game Server Configuration).
Environment Variables Those defined in Environment Variables section of the Game Server Configuration.

6. Communication between Game Server and Controller Service

After being spawned, the Game Server instance is required to communicate with Controller Service using JSON-RPC protocol.

In short, a JSON-RPC protocol allows two nodes to send each other requests, end receive responses (in form of JSON objects):

Node A -> { request JSON object } -> Node B
Node A <- { response JSON object } <- Node B

JSON-RPC is a high-level protocol, so the ZeroMQ library is used to proceed transport-level communication:

  • The Game Server instance must create ØMQ Pair socket instance
  • Then, the Game Server instance must bind that socket with Inter-Process Communication transport of the ZeroMQ to listen on that Unix Domain Socket
  • On top of that, each ZeroMQ message should be a complete JSON-RPC object (either request or response).

Examples of opening a ZeroMQ channel for communication:

context = zmq.Context()
socket = context.socket(zmq.PAIR)
socket.connect("<path to zeromq endpoint>")
context = new ZContext();
socket = context.createSocket(ZMQ.PAIR);
socket.connect("<path to zeromq endpoint>")
m_context = std::shared_ptr<zmqpp::context>(new zmqpp::context());
zmqpp::socket_type type = zmqpp::socket_type::pair;
m_socket = std::shared_ptr<zmqpp::socket>(new zmqpp::socket(*m_context, type));
zmqpp::endpoint_t endpoint = "<path to zeromq endpoint>";
m_socket->set(zmqpp::socket_option::linger, 1);

7. Game Server initialization

Once the Game Server instance is completely initialized and ready to receive connections, the inited request should be sent to the Controller.

Example of the JSON-RPC Request object Show/Hide Code

    "jsonrpc": "2.0",
    "method": "inited",
    "params": {
        "settings": {
            "test": 5
    "id": 1
  • If the argument settings passed along the request, the Rooms settings is updated with that argument. For example, if player requested to create a room with {"map": "badone"} and the Game Server instance realized there is no such map, in can choose the other map instead, and pass {"map": "goodone"} as the settings argument to the inited call. That would lead to the Room have correct map setting no matter what setting the Player have passed.
  • The Controller will respond {"status": "OK"} to that request if everything went fine. If the error is returned instead, the Game Server instance should exit the process (and will be forced to at some point).

The Game Server instance has around 30 seconds (as defined in SPAWN_TIMEOUT) to send the inited request to the Controller that the Game Server is completely initialized.


If the Game Server would not manage to initialize within that time, the Game Server instance will be killed, and the error is returned to the Player.

8. The Game Server instance details

Once the inited request is called, the Master Service will return the Game Server instance details to the player (as described in step 4):

  • The host location of the Game Server instance
  • The ports made available for that particular Game Server instance
  • the Room Registration Key
  • the Room Settings (original or as Game Server instance modified them)

That information is need to be used by Player to perform a connection to the Game Server Instance.

9. The Game Server instance status

After complete initialization, Game Controller service with periodically check (or heartbeat) the Game Server instance status using status request.

Please note that this request comes from the Game Controller side, to the Game Server instance:

Controller Service -> { request 'status' } -> Game Server instance

The Game Server instance is required to respond to that request with {"status": "ok"} object. If other response is received, or no response received in certain time, the Game Server instance will be shot down as “hang”.

Join Room Flow

The Player is required to be joined into the Room in order to connect to the Game Server. The join process ensures that no extra player can join the Game Server due to concurrency issues (as hundreds of Players are constantly join to different Game Servers).

The flow goes like this:

  1. The Player successfully “joins” into the Room, gets room location and key in return.
  2. Using the location information, the Player connects to the Game Server using any protocol, that’s up to the game
  3. The Player sends the key to the Game Server. The Game Server checks the key, registering the Player in the Room at the same time. If the key has been rejected, the Player gets disconnected.

Also, the join process makes the Access token of the Player to be available on the Game Server, yet with no Access token being sent directly to the Game Server (for server-side use) as Access token is a sensitive piece of information and communication between the Game Server instance and the Player if often unencrypted.

1. Room Registration

After the join call, no matter if the Game Server instance have just spawned, or it’s an old room, a registration process on that room is performed. Registration process ensures that:

  • Player has a valid access token for a join
  • Player has not exceeded the join rate limits
  • There is enough space for that Player in the Room
  • Player has not been banned from Matchmaking

Due to concurrency, multiple Players can perform a join request on the same room at the same time, yet it may has only one free slot left. Is that case, only the first one will succeed.

As a response to a successful registration the Master Service will respond to the Player with some information:

  • The host location of the Game Server instance for that room
  • The ports made available for that particular Game Server instance
  • the Room registration Key
  • the Room Settings (original or as Game Server instance modified them)

the Room registration Key is important and acts as a proof that the Player has the right to join that room.


At that point, the registration is temporary and will be released automatically within 30 seconds (as described in AUTO_REMOVE_TIME). To ensure the registration is permanent, the Player need to do the next steps.

2. Connecting

Then, the Player connects to the Game Sever instance, using the information in the previous step (such as a host location, or ports). The connection protocol (either UDP or TCP or even both) is completely up to the game.

After the successful connection, the Player sends the Room registration Key to the Game Server instance (again, the way it is sent is completely up to the game). If no registration Key is sent within some time, the Game Server instance must drop that connection.

Then, the Game Server instance should try to exchange the registration Key using a JSON-RPC request joined.

Arguments for that command are:

Argument Description
key The registration Key
extend_token, extend_scopes (Optional) See step 2a for more information.

Example of the JSON-RPC Request object

    "jsonrpc": "2.0",
    "method": "joined",
    "params": {
        "key": "<Player's registration key>",
        "extend_token": "<see step 2a>",
        "extend_scopes": "<see step 2a>"
    "id": 2

If the request is successful, the Controller will respond:

    "access_token": "<Player's access token>",
    "account": "<Player's account id>",
    "info": { ... custom player's info },
    "scopes": ["<A list of Player's access token scopes>"]

That token then should be used by the Game Server Instance to communicate with any service in behalf ot the Player (for example, update the Player’s profile, or post a score to a leaderboard etc). The scopes field may be used to give the Player certain admin rights inside the game.

Also, a successful request will make room registration permanent (until the Player leaves the server).

2a. Token Extension

If both extend_token and extend_scopes are passed diring the joined request, the Access token of the player will be extended (see Extend Access Token) using extend_token as master token and extend_scopes as a list of scopes the Player’s Access token should be extended with.

Token extention is used to do strict actions server side in behalf of the Player while the Player itself cannot. For example,

  1. User Authenticates asking for profile scope. This scope allows only to read user profile, but not to write;
  2. The Game Server instance Authenticates itself with profile_write scope access (allows to modify the profile);
  3. The Game Server extends this token to the more powerful one, so server can write the profile in behalf of the Player;
  4. At the same time, user still have perfectly working access token, without such possibility;
  5. So player can only read Player’s profile, but the Game Server can also write it.

3. Disconnecting

Once player left the Game Server instance (intentionally or due to connection error), the Controller needs to be notified about it using the left request.

Arguments for that command are:

Argument Description
key The registration Key

Example of the JSON-RPC Request object

    "jsonrpc": "2.0",
    "method": "left",
    "params": {
        "key": "<Player's registration key>"
    "id": 3

After a successful response, a slot it room is freed for future joins.