Thread-Safe In-Memory Load Balancer

Implement an in-memory request load balancer for a Revolut service. It must route requests using either random selection or round-robin selection while safely handling concurrent routing and server membership updates.

Implement load_balancer(initial_servers, algorithm, operations, seed). The function must process operations through a thread-safe LoadBalancer implementation and return the server selected for each route operation.

Formal Specification

• initial_servers is a list of unique non-empty strings.
• algorithm is either "random" or "round_robin".
• operations is a list of dictionaries:
  • {"op": "route"} routes one request.
  • {"op": "add", "server": server} adds a server if absent.
  • {"op": "remove", "server": server} removes a server if present.
  • {"op": "set_algorithm", "algorithm": algorithm} changes the routing algorithm.
• seed is an integer used to initialize the pseudo-random generator.
• Return a list containing one server name, or None, for every route operation.

All operations must be safe when invoked concurrently. A route must observe a consistent server collection and algorithm. Adding the same server twice and removing an unknown server are no-ops. Routing with no available servers returns None.

For round-robin routing, selections continue cyclically across requests. Removing a server must not skip or duplicate the remaining servers.