[Gamelift] When multiple people join at the same time, server app is not able to create player sessions for each user

The issue you're experiencing with multiple people joining at the same time and the server app not being able to create player sessions for each user (and sometimes crashing) could be due to a few factors:

1. Concurrent requests: When multiple users try to join simultaneously, it can overwhelm the server if not handled properly. This is especially true if you're directly creating game sessions for each request.

2. Resource limitations: The server might not have enough resources to handle multiple simultaneous player session creations, leading to failures or crashes.

3. Race conditions: Concurrent requests might be causing race conditions in your code, leading to unexpected behavior.

To address these issues, consider the following approach:

1. Use Amazon GameLift's matchmaking feature, FlexMatch, in combination with game session queues. Instead of creating game sessions directly for each request, use the StartMatchmaking API call to GameLift FlexMatch from your Lambda function.

2. Implement a queueing system to manage concurrent requests more effectively.

3. Use the CreatePlayerSessions API instead of CreatePlayerSession when adding multiple players at once. This API is designed to handle groups of players joining a game session simultaneously.

4. Ensure your server can handle the maximum number of players per game session, which is 200 in Amazon GameLift (this limit is not adjustable).

For the Unreal Engine server-side code, here's a basic example of how you might implement player session creation:

void AMyGameMode::AcceptPlayerSession(const FString& PlayerSessionId)
{
    auto Outcome = Aws::GameLift::Server::AcceptPlayerSession(TCHAR_TO_ANSI(*PlayerSessionId));
    if (Outcome.IsSuccess())
    {
        UE_LOG(LogTemp, Log, TEXT("Player session created successfully"));
    }
    else
    {
        UE_LOG(LogTemp, Error, TEXT("Failed to create player session: %s"), *FString(Outcome.GetError().GetErrorMessage()));
    }
}

void AMyGameMode::OnPlayerConnected(APlayerController* NewPlayer)
{
    Super::OnPlayerConnected(NewPlayer);

    // Assume you have a way to get the player session ID from the connecting player
    FString PlayerSessionId = GetPlayerSessionIdFromPlayer(NewPlayer);

    AcceptPlayerSession(PlayerSessionId);
}

This code uses the AcceptPlayerSession function from the Amazon GameLift Server SDK to validate and accept incoming player connections. Make sure to properly initialize the SDK and implement error handling and logging as needed.

Remember to test your implementation thoroughly with various concurrent user scenarios to ensure it can handle multiple simultaneous connections without crashing or failing to create player sessions.
Sources
Concurrent create game session requests to join one game session | AWS re:Post
CreatePlayerSessions - Amazon GameLift
Amazon GameLift server SDK for Unreal Engine: Actions - Amazon GameLift