Suppose we have two services. they are communicating asynchronously with the help of kafka, will it possible to track with sleuth.
Please check flow diagram for more clarification.
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I am interested in developing an API that is capable of receiving data in real-time and pushing it out to clients connected to an endpoint. I have looked in socket.io and web sockets. However, these depend on events being triggered to send/receive data. This isn't ideal for my use case. What alternatives are there for me to achieve this?
Any help and advice are greatly appreciated.
So If I understand it right, you want to write a streaming service that can push updates on some data in real-time over an endpoint exposed to the clients. I guess webhooks might be a solution looking into your problem statement. I'd recommend you to look into this https://www.youtube.com/watch?v=63grynZmo7c as well. It has got elementary information as to how do you create a webhook and start receiving real-time updates on it
I am reading through nestjs docs and there seems to be no way to use anything else then built in eventbus. Now let's say my cqrs event wants to communicate with microservices with rabbitmq. So 2 questions:
Is it possible to dispatch event that will send queue to external eventbus?
Is it okey from ddd point of view? Where should I dispatch it then? Dispatch from domain layer? And then parties (microservices) that are interested can listen and write to their own db parts of it or whatever.
And if you can please explain it on human level, I am new to ddd with cqrs. And sorry if it's frequently asked question but it's also related to nestjs so I need some clarifications.
Thanks 😊
Integration events should be used to notify external services. A good practice is to keep the external event bus (integration bus) independent from any micro service. And regarding the 2 questions:
It should be possible to send events to external bus with nestjs. Ex: write a domain event handler that create an integration event and dispatch it. You can create a new IntegrationBus interface and implement it in any technology that fits.
It is definitely OK from DDD point of view. Context mapping (how different contexts share data) is part of the strategic patterns of DDD. Usually the integration events are published in the application layer and in some case it can be more convinient to do it in the infrastructure layer.
Here are few articles with more details about domain and integration events:
https://learn.microsoft.com/en-us/dotnet/architecture/microservices/microservice-ddd-cqrs-patterns/domain-events-design-implementation
https://devblogs.microsoft.com/cesardelatorre/domain-events-vs-integration-events-in-domain-driven-design-and-microservices-architectures/
I haven't found an existing post asking this but apologize if I missed it.
I'm trying to get my head round microservices and have come across articles where RabbitMQ is used. I'm confused why RabbitMQ is needed. Is the intention that the services will use a web api to communicate with the outside world and RabbitMQ to communicate with each other?
In Microservices architecture you have two ways to communicate between the microservices:
Synchronous - that is, each service calls directly the other microservice , which results in dependency between the services
Asynchronous - you have some central hub (or message queue) where you place all requests between the microservices and the corresponding service takes the request, process it and return the result to the caller. This is what RabbitMQ (or any other message queue - MSMQ and Apache Kafka are good alternatives) is used for. In this case all microservices know only about the existance of the hub.
microservices.io has some very nice articles about using microservices
A message queue provide an asynchronous communications protocol - You have the option to send a message from one service to another without having to know if another service is able to handle it immediately or not. Messages can wait until the responsible service is ready. A service publishing a message does not need know anything about the inner workings of the services that will process that message. This way of handling messages decouple the producer from the consumer.
A message queue will keep the processes in your application separated and independent of each other; this way of handling messages could create a system that is easy to maintain and easy to scale.
Simply put, two obvious cases can be used as examples of when message queues really shine:
For long-running processes and background jobs
As the middleman in between microservices
For long-running processes and background jobs:
When requests take a significant amount of time, it is the perfect scenario to incorporate a message queue.
Imagine a web service that handles multiple requests per second and cannot under any circumstances lose one. Plus the requests are handled through time-consuming processes, but the system cannot afford to be bogged down. Some real-life examples could include:
Images Scaling
Sending large/many emails (like newsletters)
Search engine indexing
File scanning
Video encoding
Delivering notifications
PDF processing
Calculations
The middleman in between microservices:
For communication and integration within and between applications, i.e. as the middleman between microservices, a message queue is also useful. Think of a system that needs to notify another part of the system to start to work on a task or when there are a lot of requests coming in at the same time, as in the following scenarios:
Order handling (Order placed, update order status, send an order, payment, etc.)
Food delivery service (Place an order, prepare an order, deliver food)
Any web service that needs to handle multiple requests
Here is a story explaining how Parkster (a digital parking service) are breaking down their system into multiple microservices by using RabbitMQ.
This guide follow a scenario where a web application allows users to upload information to a web site. The site will handle this information and generate a PDF and email it back to the user. Handling the information, generating the PDF and sending the email will in this example case take several seconds and that is one of the reasons of why a message queue will be used.
Here is a story about how and why CloudAMQP used message queues and RabbitMQ between microservices.
Here is a story about the usage of RabbitMQ in an event-based microservices architecture to support 100 million users a month.
And finally a link to Kontena, about why they chose RabbitMQ for their microservice architecture: "Because we needed a stable, manageable and highly-available solution for messaging.".
Please note that I work for the company behind CloudAMQP (hosting provider of RabbitMQ).
The same question can be why REST is necessary for microservices? Microservice concept is not something new under moon. A long time distribution of workflow was used for backend engineering and asynchronous request processing, Microservice is the same component in a separated jvm which matches with S(single responsibility) in SOLID. What makes it micro SERVICE - is that it is balanced. And that is the all! Particularly (!), it can be REST Service on Spring Cloud/REST base, which is registered by Eureka, has proxy gateway and load balancing over Zuul and Ribbon. But it is not the whole world of microservices!By the way, asynchronous distributed processing is one of tasks which microservices are used for. Long time ago services(components) in separated JVM was integrated over any messaging and the pattern is known as ESB. Microservices are the same subjects the pattern. Due to fashion for Spring Cloud REST seems like it is the only way of microservices. Nope! Message based asynchronous microservice architecture is supported by Vertx https://dzone.com/articles/asynchronous-microservices-with-vertx, for example. Why not to use RabbitMQ as message channel? In this case load balancing can be provided by building RabbitMQ cluster. For example:https://codeburst.io/using-rabbitmq-for-microservices-communication-on-docker-a43840401819. So, world is much wide more.
We have multiple web and windows applications which were deployed to different servers that we are planning to integrate using NservierBus to let all apps can pub/sub message between them, I think we using pub/sub pattern and using MSMQ transport will be good for it. but one thing I am not clear if it is a way to avoid hard code to set sub endpoint to MSMQ QueueName#ServerName which has server name in it directly if pub is on another server. on 6-pre I saw idea to set endpoint name then using routing to delegate to transport-level address, is that a solution to do that? or only gateway is the solution? is a broker a good idea? what is the best practice for this scenario?
When using pub/sub, the subscriber currently needs to know the location of the queue of the publisher. The subscriber then sends a subscription-message to that queue, every single time it starts up. It cannot know if it subscribed already and if it subscribed for all the messages, since you might have added/configured some new ones.
The publisher reads these subscriptions messages and stores the subscription in storage. NServiceBus does this for you, so there's no need to write code for this. The only thing you need is configuration in the subscriber as to where the (queue of the) publisher is.
I wrote a tutorial myself which you can find here : http://dennis.bloggingabout.net/2015/10/28/nservicebus-publish-subscribe-tutorial/
That being said, you should take special care related to issues regarding websites that publish messages. More information on that can be found here : http://docs.particular.net/nservicebus/hosting/publishing-from-web-applications
In a scale out situation with MSMQ, you can also use the distributor : http://docs.particular.net/nservicebus/scalability-and-ha/distributor/
As a final note: It depends on the situation, but I would not worry too much about knowing locations of endpoints (or their queues). I would most likely not use pub/sub just for this 'technical issue'. But again, it completely depends on the situation. I can understand that rich-clients which spawn randomly might want this. But there are other solutions as well, with a more centralized storage and an API that is accessed by all the rich clients.
There are several methods to do IPC in Android - Content Provider, Message, AIDL, Async Task, IntentService ...
Seems like each of them aims to solve particular problem.
How to decide which I should use?
I need a service keep running in background, at the same time other services or activity may acquire data from this service.
Thanks in advance!
If you need a service keeps running in the background, I think you should try to implement a foreground service.
You can read this post for data exchange between activity and service.