In Kafka I know that I can subscribe my consumer to multiple topics doing:
passing directly the list of topics I want to subscribe to.
Or passing a pattern
In my case I cannot pass directly a list of topics or a pattern because the list of topics is not known in advance.
For example a user will specify a topic in the morning by using a UI (for example), and a second topic in the evening, so the consumer will finish subscribed to these two topics.
Is it possible to do this?
You can make use of the subscribe and unsubscribe API of the KafkaConsumer to dynamically change the subscriptions of your consumer.
If you sent a message to one of the subscribed topics with key=subscribe/unsubscribe and value=topic1/topic2/... you could implement a logic in your consumer that changes its subscriptions based on those messages.
Related
I'm making a social media app like Facebook using Flutter and Firebase.
I'm using Firebase Cloud Messaging to make notification service.
I want to make users who joined community or groups to subscribe their group's topic. Therefore, I can send notification to them by using method which is "subscribeToTopic()". However, I don't know how to make all users in community or groups to subscribe to certain topics.
If you know how to make all users to subscribe to certain topics, please let me know. Thank you!
There is no API to get the current subscribers to a topic, nor is there an API that verbatim subscribes all subscribers to one topic to another topic.
If you already track group membership yourself, you can either let each client subscriber themselves to the additional topic, or you can determine the list of tokens for the group members and then subscribe them to the topic on the server.
Below is a typical GCP pubsub model:
My question: is it possible for one subscriber(application or job) to subscribe to multiple subscriptions? Like this:
I mean we can filter at the subscription level that one subscription takes one event type (A or B). I know it will be easier if we have two topics (Topic A and B) and create two subscriptions, but again, it will boil down to the same question, is it possible for one subscriber to subscribe to multiple subscriptions?
Or the only alternative way I can imagine is that at the subscriber level, I can classify the event type, A or B, but that requires the publisher to pass the attribute to the topic level.
I have the control of publisher and I just wanna do one Subscriber instead of multiple subscribers.
An application can subscribe to multiple subscriptions, yes. You would need to instantiate multiple instances of the subscriber client, one for each subscription for which you want to receive messages.
If you want the subscriber to be able to receive messages without knowing the names of all of the subscriptions, then you could use push subscriptions and set the endpoint to different subscriptions to the same URL. Then, the subscriber behind that URL would receive messages from the different subscriptions.
Let's suppose a simplified scenario like this:
There are two Kafka topics, users and orders and three microservices user-service, order-service and shipping-service.
When an order is placed through the order service, an OrderCreated event is added to the orders topic and listened by the shipping service. This service needs to get the user information to send the order. According to my requirements I can't make a REST call to user-service but use a stateful approach. That is to say, the shipping service is a Kafka Streams application that listens to the users topic, having a KTable backed by a local store with the full user table information. Thus, when processing the order it already has the user information available locally.
However, one concern of this approach is the consistency of the local user information in the shipping service, e.g:
A user updates its shipping address in the user-service, it updates its local SQL database and publishes an event in the user topic with this change.
The user places an order, so order-service publishes it in the order topic.
For whatever reason shipping service could process the OrderCreated event from order topic before reading the UserUpdated information from the user topic so it would use an address which is not valid anymore.
How could I guarantee that the shipping service always has an updated user information in this event-carried state transfer scenario?
If you need ordering guarantees, you would need to write both the user information update as well as the order into the same topic (and in particular into the same partition) because Kafka only guarantees order within a single partition.
You could call this topic "user_action" with a unique user-id as key (both an user information update as well as an user order is an user action). In your case, all three services would consume the "user_action" topic. While the user service only considers user updates and the order service only considers orders, the shipping service considers both.
This blog post might help, too: https://www.confluent.io/blog/put-several-event-types-kafka-topic/
I am developing an application where I have several Consumers (an project which will consume a message and process it and after processing it send back to the topic where it comes from)
An Getway project which has two REST method Post method, this Post method will produce a messaage to one of the consumer and that particular consumer will process and send back to the Getway
now, I have a Get method that will consume the message which is sent back by the consumer.
but I want to remove this Get call, and I want all this done in single Post call, where I can produce a message to consumer and once the consumer processed I want to read it
how to achieve this? should I use Poll?
If so then where should I write Poll and is there a way to handle this on event basis like once message send by Getway it reads the response on some event.
In my project we used spring kafka template.
Setup:
I have setup a pubsub service wherein the publishers publish geolocation data at regular intervals.
The subscribers receive the location data of the publishers.
The subscribers are not presence subscribed, in the sense, the subscribers are not in the publishers rosters.
Problem:
The subscribers need to know the presence status of publishers.
Is there a way for the subscribers to know the presence status of publishers?
No, since there is no direct relationship between subscribers and publishers, which is typical of any pubsub design. To accomplish this the subscribers would need to know who the publishers are, which is not a good generic pubsub design.
It sounds like what you actually want is PEP (Personal Eventing Protocol), which is a subset of pubsub. In this case, the subscribers are subscribing to nodes belonging to the actual user they are interested in. If they are subscribed to the users presence, they automatically have access to the users nodes.
NOTE: I have recently found out that the newer version of the spec does in fact support an attribute that identifies the publisher. Thus making it feasible to get their presence, but you would still have to subscribe or query for it.