Basics of Multiplayer with EOS

The Epic Online Services (EOS) SDK provides two different services that you can use for multiplayer matchmaking:

Lobbies : Lobbies provide a persistent connection between users for the purpose of sharing game and user state with real-time updates. Typically, users can create or join lobbies to form teams, select pre-game options, and wait for additional players to join in before playing together. Using the Lobby Interface, your users can create, join, leave, and manage lobbies.
Sessions : Epic Online Services (EOS) gives players the ability to host, find, and interact with online gaming sessions through the Sessions Interface. A session can be short, like filling a certain number of player slots before starting a game, then disbanding after the game ends, or it could be longer, like keeping track of a game that cycles through matches on multiple maps or levels. The Sessions Interface also manages game-specific data that supports the back-end service searching and matchmaking functionality.

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If your game uses dedicated servers, you must use sessions. Dedicated servers cannot use lobbies.

Difference between Lobbies and Sessions

Players have a persistent connection to the lobby with WebSocket, and hence don't need to send an HTTP request each time they query or update information.

Players do not have a persistent connection. Each call your game makes, such as when your game registers a new player to the session, requires a new request.
Your game must push session updates to the EOS session service manually and keep each player's remote view of the session up to date.

Automatically registers players when they join the lobby and unregisters players when they leave the lobby.

Your game must register players when they join the session and unregister players when they leave the session.
This keeps the player count accurate for the session and prevents other players from trying to join the session if it is already at the maximum player capacity.

Voice chat integration.
Host migration: the owner can transfer ownership when they leave or if their connection drops.
The owner can remove (kick) players from the lobby.

Fixed attributes and user attributes that you can populate to best suit your game's needs.
Players can search for a lobby or session by the fixed attributes and user attributes that you've marked as public.
Players already in the lobby or session can invite other players to join them.
Players can join the presence lobby or presence session that a friend is in.
Players can join multiple lobbies and sessions at the same time.