Architecture

Modules

Module	Type	Loading phase	Platforms	Content
NsSession	Runtime	Default	All	UNsSessionSubsystem, FNsRoomStatus, the status enums, FNsSteamLobbySearch, and the LogNsSession log category
NsSessionSteam	Runtime	Default	Win64 (PlatformAllowList)	UNsSteamNetConnection

NsSessionSteam does not depend on NsSession. The .uplugin file enables OnlineSubsystem, OnlineSubsystemUtils, and SteamSockets (Win64 only). None of them is optional. OnlineSubsystemSteam is not in the list. The game enables it.

Folders

• Source/NsSession/Public – NsSessionModule.h, NsSessionSubsystem.h, NsSessionTypes.h.
• Source/NsSession/Private – NsSessionModule.cpp, NsSessionSubsystem.cpp, NsSessionTypes.cpp, NsSteamLobbySearch.h, NsSteamLobbySearch.cpp.
• Source/NsSessionSteam/Public – NsSteamNetConnection.h.
• Source/NsSessionSteam/Private – NsSessionSteamModule.cpp, NsSteamNetConnection.cpp.

Main Classes

Type	Responsibility
UNsSessionSubsystem	A UGameInstanceSubsystem with Config = Game. Hosts, joins, and leaves rooms, and reports each status.
FNsRoomStatus	Status, bFailed, RoomCode, Network, Detail. GetDefaultText() gives the English line. GetNetworkName() gives “LAN” or “Steam”.
ENsRoomStatus	The 18 steps and failures of a host or a join.
ENsRoomNetwork	Lan, Steam.
FNsSteamLobbySearch	A private class. Calls Steamworks RequestLobbyList with the worldwide distance filter and returns the Steam ID of the host.
UNsSteamNetConnection	A USteamSocketsNetConnection that overrides CleanUp to keep MappedClientConnections correct when one Steam ID has two connections.
FNsSessionModule	The module class of NsSession. It defines LogNsSession and has no startup code. NsSessionSteam uses FDefaultModuleImpl.

Data Flow

Host:

1. HostRoom returns false when a join search runs or a host is pending. It reports NoMap for an empty path and MapMissing when FPackageName::DoesPackageExist fails.
2. It calls LeaveRoom, makes a code with MakeRoomCode, and picks Steam when CanUseSteam() is true, else LAN.
3. It reports Opening and calls UGameplayStatics::OpenLevel with listen, or listen?bIsLanMatch for LAN.
4. HandlePostLoadMap fires OnRoomEntered for a listen server or client world of this game instance. For a pending host on a listen server, it calls CreateRoomSession.
5. CreateRoomSession creates NAME_GameSession with NumPublicConnections = MaxPlayers, join in progress, and the keys NSCODE and NSGAME. A Steam room also uses lobbies and presence, and sets NSHOST to the Steam ID of the host.
6. When the creation fails, the subsystem writes an error to LogNsSession and clears the code. It sends no status.

Join:

1. JoinRoom normalizes the code. A bad code reports BadCode. Then it returns false when a join search runs or a host is pending.
2. It calls LeaveRoom and starts the LAN search (bIsLanQuery, 64 results). When CanUseSteam() is true, it also starts the Steam search (SEARCH_LOBBIES, one result, filters on NSCODE and NSGAME).
3. When no search starts, it reports SearchFailed. Else it adds a core ticker with a 0.1 s delay and reports Searching.
4. The ticker and each search completion call TryJoinMatch, which checks the results of both searches for the code and the game tag.
5. The first match calls JoinFoundRoom. It stops all searches, reports Found, and calls JoinSession.
6. HandleJoinSessionComplete gets the connect string and calls ClientTravel on the first local player controller. It reports RoomFull, JoinRefused, or NoController on a failure.
7. When both searches end with no match, FinishSearchWithoutMatch starts FNsSteamLobbySearch when Steam can run, and reports SearchingWorldwide. Else it reports NotFound.
8. The worldwide search filters the lobby data keys NSCODE_s and NSGAME_s, and reads NSHOST_s. On a match, the client travels to steam.<host id>:<default port> and the subsystem reports FoundWorldwide.

Failures:

• HandleTravelFailure calls LeaveRoom and reports TravelFailed.
• HandleNetworkFailure calls LeaveRoom and reports HostLeft for ConnectionLost and ConnectionTimeout, else ConnectionFailed.
• ReportStatus writes the line to LogNsSession (Warning for a failure, Display for a step), stores LastStatus, stores LastFailure for HostLeft and ConnectionFailed, and fires OnRoomStatus.

Threading: Steam calls FImpl::OnLobbyList on its callback thread. The function moves the result to the game thread with AsyncTask. A cancelled search drops the result. All other code runs on the game thread.

Lifetime

1. Initialize binds FCoreUObjectDelegates::PostLoadMapWithWorld, GEngine->OnTravelFailure(), and GEngine->OnNetworkFailure().
2. Deinitialize stops the searches and removes the three bindings.
3. The subsystem lives as long as the game instance. The code, the last status, and the last failure stay across map travel.
4. LeaveRoom stops all searches, clears the pending host and the code, and destroys NAME_GameSession on the Null and the Steam session interfaces. It does not travel.
5. GetSessionInterface uses Online::GetSubsystem(GetWorld(), ...), so each Play In Editor world gets its own online subsystem instance.

Extension Points

• OnRoomStatus (FOnNsRoomStatus, const FNsRoomStatus& Status) – BlueprintAssignable.
• OnRoomEntered (FOnNsRoomEntered, no parameters) – BlueprintAssignable.
• static UNsSessionSubsystem* Get(const UObject* WorldContext) – C++ access.
• static FString NormalizeRoomCode(const FString& Code) and static FString MakeRoomCode() – C++ only.
• FNsRoomStatus::GetDefaultText() – Use it, or map Status to your own text.
• GameTag and MaxPlayers – UPROPERTY(Config) in DefaultGame.ini, section [/Script/NsSession.NsSessionSubsystem]. See the User Guide.
• UNsSteamNetConnection – Config = Engine. Set it as NetConnectionClassName of SteamSocketsNetDriver.

The subsystem has no virtual functions of its own. Its private handlers are not for override.

Dependencies

Dependency	Kind	Why
Core, CoreUObject, Engine	Public module (both modules)	Base types, the subsystem, travel, and the net driver
CoreOnline, OnlineSubsystem, OnlineSubsystemUtils	Public module of NsSession	Session interfaces, identity, and Online::GetSubsystem
Steamworks	Private third-party library of NsSession, Win64 only	The worldwide lobby search. Sets NSSESSION_WITH_STEAM=1 on Win64, else 0
SteamSockets	Public module of NsSessionSteam	USteamSocketsNetConnection
OnlineSubsystem, OnlineSubsystemUtils	Plugin, not optional	Enabled by the .uplugin file
SteamSockets	Plugin, not optional, Win64	Enabled by the .uplugin file

Performance

• The subsystem does not tick. The search ticker runs only while a LAN or Steam join search runs, with a 0.1 s delay.
• TryJoinMatch walks up to 64 LAN results and one Steam result on each ticker call.
• Nothing runs while the player is in a room, except the engine delegates for map load, travel failure, and network failure.

Networking

• The plugin has no replicated properties and no RPCs. It uses the online session interfaces and engine travel.
• Each machine keeps its own room code. The host makes it, and a client keeps the code that it typed.
• Session keys: NSCODE (the code), NSGAME (the game tag), and NSHOST (the Steam ID of the host, Steam only). All use ViaOnlineServiceAndPing. Steam stores a string key with the suffix _s in the lobby data.
• CanUseSteam() is false in Play In Editor, with -lanroom, and when the Steam identity is not logged in for user 0.
• UNsSteamNetConnection::CleanUp points the address key in MappedClientConnections at itself before the engine removes it. Then it gives the key to another open connection from the same address. It logs that case to LogNsSessionSteam.

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