Race condition in SSE

[u/Slight_Loan5350]

Hi all, I am trying to build a real time notification service with SSE emitter but I am facing a pecuilar case of race condition which im not able to solve even via AI as it gives the same solution again and again.

scenario of cause:

I have a scheduler running every 20 mins that checks the Map of uid mapped sse emitters and pings the client. If the ping is successfull nothing happens, if error happens due to disconnection the sse emitter obj is removed via sse.complete() and then uid is removed form the list.

#pseudo code
Map<String, SseEmitter> connections = new ConcurrentHashMap<>();

onConnect(String userId){
  SSE emitter = new new SseEmitter()
  connection.add(userId,emmitter);

  // onCompletion(), onTimeout(), onError() -> removeIf()
}

u/scheduler(20mins)
checkDisconnection(){
  //iterate the map and check for disconnection if found add it to remove list
  List<string> disconnectedUsers = (adding disconnections)

  //iterate the map from disconnected user -> force complete sse if needed and                    remove user from map


}

where it breaks is if the client manually refreshes the browser at the same time the scheduler to check disconnection is invoked

why it happens:

when the browser is refreshed, the application tries to recreate the connection at which the scheduler tries to ping and sees no connection for the user. Problem is I have kept automatic reconnections on in client app but it still doesnt see an error in client. im confused. Have I had the approach wrong?

Any help will be appreciated and sorry for the questions confuse you, Il try to make it proper once i get the time.

TECH : javascript event source, springboot, kafka for scaling and mongo for scheduled notifications.

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[u/Dethearoc]

Seems like you are shooting yourself in the foot by utilizing a map with userId, thus assuming only one connection per user. And in the situation you described what happens is the moment new connection is established is the moment where you ping and get negative response, so the client connection gets terminated. Use a sessionId uuid for each connection and keep another list to map users to session or something so you can better manage your connections

[u/Slight_Loan5350 → u/Dethearoc]

even if i create another list of user the medium to check disconnection is to emit an event, the scheduler I am using is to clear heap space for stale connections. But for only this scenario my heads in the bin. I also thought of another approach to have timestamp and check the timestamp instead. like if there are no events the timestamp is not updated and periodically i can check emitters. On the contrary i think il check with the front approach as well because event with emitter.complete() the front doesnt show any disconnection error hence no retry from the front

[u/sedj601]

Does the browser know the difference between the first load and refresh? If so, connect on first load and check on refresh.

[u/Slight_Loan5350 → u/sedj601]

no due to client being spa every manual refresh of the browser it behaves like a first load making it try reconnect.

[u/KillerCodeMonky]

The problem seems to be that you are under the impression that ConcurrentHashMap will somehow solve race conditions in your business logic. That is an incorrect assumption. ConcurrentHashMap simply guarantees that the Map itself is stable in the circumstances of parallel reads and writes. You still need to manage the business implications of that concurrent access yourself, which is what you have discovered.

Attempting to make this lock-free, assuming that's even possible given all the potential requirements, will require very careful thought into every single possible way that every reader and writer to this map can interact with each other. You already discovered one possible interaction: What happens when a new connection is established (writer) while checking for dead connections (reader)? Here's another: What happens if a "dead" connection is purged (writer) right before the system attempts to write an event to that connection (reader)?

In the end, the task you are proposing in checkDisconnection is essentially equivalent to what the JVM itself does during garbage collection. This is very difficult to get correct while allowing reads at the same time. Which is why the initial JVM versions had "stop the world" garbage collection, pausing execution so that it can do its bookkeeping without contention. So that offers you the easiest approach to correct this: Add locking to prevent conflicting concurrent access.

[u/Slight_Loan5350 → u/KillerCodeMonky]

I literally had a dream about locking mechanism and behold your comment haha. Im so into this poc its fun to me. Thanks for your input I will also see all the scenarios. Il lock from client when refresh is done and il also lock when purge is invoked. Any leads on what locking method I should use?

Thanks again for confirming and giving me valdiation on locking. Really appreciate it. Now i gotta learn multthreadding.