What methods can be used to ensure that only one user is connected to the API at the same time?

To ensure that only one user can connect to the API at the same time, several approaches can be used:

1. Sessions and tokens: Issue a unique session token when the user logs in. When a new connection is made, invalidate the old token, thereby disconnecting the previous session.

2. Server-level locking: Store the user's connection state (for example, in a database or cache). If the user tries to reconnect, check for an active session and reject the new request.

3. WebSocket or long polling with control: If a persistent connection is used, the server can monitor active connections and close old ones when new connections appear.

Example in PHP using sessions and Redis to store active sessions:

session_start();
$userId = $_SESSION['user_id'];
$redis = new Redis();
$redis->connect('127.0.0.1', 6379);

$activeSession = $redis->get("user_session_{$userId}");
$currentSessionId = session_id();

if ($activeSession && $activeSession !== $currentSessionId) {
    // Reject connection or terminate the old session
    die('An active connection already exists');
} else {
    $redis->set("user_session_{$userId}", $currentSessionId);
    // Continue operation
}

This approach allows controlling simultaneous connections and limiting them to one per user.