What is the Publisher Subscriber Model?

34 thoughts on “What is the Publisher Subscriber Model?

  1. Regarding the first half of the video:
    I like your videos and you're a smart guy, but I think the above was a contrived example. First, you said that the request/response set up is asynchronous but then you said that S1 has to wait. If the request was sent asynchronously, then no S1 doesn't have to wait because the response will come in the handler/callback provided by S1 to the RPC black box. Second, no strong case was presented as to why anybody even needs responses and then you went on to discuss messaging and made everything flow in one direction, i.e. you dropped the responses altogether. Just by switching middleware you suddenly eliminate the need for bidirectional data flow? Third, either you use async req/resp/RPC or PUB/SUB messaging, you have essentially the same set up, except in the RPC case you have a broker that has an internal queue. Before going to S2 or S0, the requests go to the broker, so essentially the same picture.

  2. Hi Gaurav, i am a huge fan of your presentations and appreciate your time and effort on these. QQ- In a publish subscribe model like KAFKA, the publisher S1 or S2 publish the messages to the KAFKA topics and it is upto to the subscriber group to pull the messages isn't it. Listeners need to be configured to the KAFKA topics to pull the messages by the subscribers and the message broker doesn't push them to the subscribers. Please let me know if my understanding is wrong.

  3. Ques: is event-driven DB same as the pub-sub model, why choose one over another? Is the only difference persistent storage or more?

  4. Thanks for explaining this so simply Gaurav. In the financial institution example, How about if we build the debit transaction in S1 and post the message for transfer to MQ? If the transfer fails we can recredit the amount. Dont we see the same thing in current banking system where we get the debit message immediately and the transfer completes after few days. Thanks again.

  5. Very good presentation……the explanation of failure is a bit wrong from POV of design….on can simpy argue just by redesigning so that S2 only sends a message after it has processed, SO takes the commission out, then the design is not going to have the problem stated.
    But kudos to you for explaining this is so much clean detail.

  6. who is this guy trying to send Rs. 1750 even though they have Rs. 1000 in their account. Put them behind bars

  7. If you're looking for a job you got my attention teltech is the company i work for, and co not com is the TLD for our website, mention my name Al Beebe

  8. Thankyou and nice video man. but can you please explain the idempotecncy point in an elaborated manner.

  9. Nice and complete explanation,
    some examples where they are used:
    1) all notifications work this way.

    2)Event driven systems are particularly lightweight, so they are extensively used for Notifier hardware(sensors) with minimal processing power/ power consumption , to broadcast events whenever their is a malfunction/low battery.

    3) they are mainly used for systems which are not inherently in requirement of security, though some kind of crypt might get used to ensure the source of msg to be valid.

    4) not particularly a point but, should have mentioned youtube's own publisher-subcriber system while putting a video on youtube 😉

  10. "You cannot use this for mission critical systems, where either there's a success or a failure. There is no atomicity in this" (9:06 – 9:13)

    Could you please elaborate more on why this is the case? I know atomic transactions are that they either all succeed, or all fail together. Wondering if you could provide more context on this.

    Thanks!

  11. Bro i want to ask something

    suppose i have site called Amazon.com, In this we need to dump lot of code. I think they use micro services like home page served by one machine, log in by another and add to bag and checkout by other server right.

    then those servers are remotely connected right . if we click on add to bag the request goes to add to bag server(i mean the code related to this is present in that server) right ?

    then if one server gets failed the dependency servers are not works right in micro services.

    If we use monolithic that may works but no use of working those pages example with out add to bag page there is no use of checkout page right.

    Is this is the way the system would be.

    thats what i understood, is it fine ?

  12. In your example at 9:00, the business logic of transfer and transfer fee is tied down to each other. so this biz logic cannot be broken down to 2 microservices in first place regardless of event driven service or pub/sub service is used.

  13. When S1 response back in the pub/sub model, it should response with some data, not just stating that the message has been successfully published.

  14. thank you for enlightening me about this system design. i have a comment on the arrows. those arrows supposed to be bi-directional since requests go into one direction and responses go to another direction. looking forward to your videos. thanks again.

  15. Hi Gaurav, your videos are really good, i have a question, as you said publish subscribe model gets the job done for twitter and gaming services, but what is the alternative for financial services, build a monolith app?

  16. I’ve watched a couple of your videos so far. Very good job! I enjoyed them. At 8:56 you mention that you’ll be dealing with ways to avoid the out-of-order problems (consistency, I guess) in a later video. Have you made that yet? If so, where is it?

    I’ve been interested in general message passing systems for a long time because of the loose coupling aspect of it. That’s what attracted me to the Actor model. But in trying to write some small programs with the Actor model (for example I use the C++ Actor Framework) I run into problems with out of order messages similar to what you’ve described here. The nice thing about the model is that each actor is a single thread, making it easier to reason about and avoid deadlocks. But according to the theory, messages are not necessarily in order within the system. So it seems as if it’s really just pushing the complexity of the problem of deadlocks into the realm of message order.

    What I’m looking for is general patterns, techniques, etc. for dealing with out-of-order messages. Or I should say, dealing with cases where messages need to occur in a certain order within a system. If you know of any resources let me know since I haven’t found any so far.

  17. How about this: S1 updates its local DB, then it fails to publish the message to the message broker. Now you have inconsistency in your microservices. Failure to publish can be caused by various reasons and it is very likely to happen, although not frequently.

  18. Awesome work. Very well explained with examples:)
    1. Are there more videos coming out for system design ?
    2. Also I have a question related to system design. I do understand that system design questions are very broad and we need to narrow it down during an interview. But there are so many technologies that need to be learnt and I am quite sure all of them cannot be learnt. So when I am understanding and learning system design problems what are the must know technologies and also do I need to know all of them in depth. Can you cover the must know technologies for system design and maybe make a separated video for the different technolgies

  19. Hi Gaurav , your content is awesome!After watching this video I have a doubt

    Here is the doubt:
    we want to create an app(can be any web,windows or mobile)with client server architecture.
    Following ways we have thought
    1.Normally we would create web api backend and let client consume it through api's endpoint or url.
    but we wanted to try something else this time.
    2.From this video we can adopt messaging queue concept like rabbitmq .Client ll send message to queue and server ll use that and so on.(pub/sub, event driven).
    3.Client and server communication over tcp/socket programming.

    Like I see rabbitmq one as reliable since messages won't be lost and it ll scale better since it is decoupled.
    But which one ll give us high performance(can handle load easily)?
    And are there other ways we can connect client and server? if yes do let us know.Your answer on this would be great help.Thanks

  20. How will adding MQ improve end user response time?

    If S4 is down , S1 still has to wait till S4 process message.

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