I have been working on a project that is basically an e-commerce. It's a multi tenant application in which every client has its own domain and the website adjusts itself based on the clients' configuration.
If the client already has a software that manages his inventory like an ERP, I would need a medium on which, when the e-commerce generates an order, external applications like the ERP can be notified that this has happened to take actions in response. It would be like raising events over different applications.
I thought about storing these events in a database and having the client make requests in a short interval to fetch the data, but something about polling and using a REST Api for this seems hackish.
Then I thought about using Websockets, but if the client is offline for some reason when the event is generated, the delivery cannot be assured.
Then I encountered Message Queues, RabbitMQ to be specific. With a message queue, modeling the problem in a simplistic manner, the e-commerce would produce events on one end and push them to a queue that a clients worker would be processing as events arrive.
I don't know what is the best approach, to be honest, and would love some of you experienced developers give me a hand with this.
I do agree with Steve, using a message queue in your situation is ideal. Message queueing allows web servers to respond to requests quickly, instead of being forced to perform resource-heavy procedures on the spot. You can put your events to the queue and let the consumer/worker handle the request when the consumer has time to handle the request.
I recommend CloudAMQP for RabbitMQ, it's easy to try out and you can get started quickly. CloudAMQP is a hosted RabbitMQ service in the cloud. I also recommend this RabbitMQ guide: https://www.cloudamqp.com/blog/2015-05-18-part1-rabbitmq-for-beginners-what-is-rabbitmq.html
Your idea of using a message queue is a good one, better than database or websockets for the reasons you describe. With the message queue (RabbitMQ, or another server/broker based system such as Apache Qpid) approach you should consider putting a broker in a "DMZ" sort of network location so that your internal ecommerce system can push events out to it, and your external clients can reach into without risking direct access to your core business systems. You could also run a separate broker per client.
Related
I am dealing with communication between microservices.
For example (fictive example, just for the illustration):
Microservice A - Store Users (getUser, etc.)
Microservice B - Store Orders (createOrder, etc.)
Now if I want to add new Order from the Client app, I need to know user address. So the request would be like this:
Client -> Microservice B (createOrder for userId 5) -> Microservice A (getUser with id 5)
The microservice B will create order with details (address) from the User Microservice.
PROBLEM TO SOLVE: How effectively deal with communication between microservice A and microservice B, as we have to wait until the response come back?
OPTIONS:
Use RestAPI,
Use AMQP, like RabbitMQ and deal with this issue via RPC. (https://www.rabbitmq.com/tutorials/tutorial-six-dotnet.html)
I don't know what will be better for the performance. Is call faster via RabbitMQ, or RestAPI? What is the best solution for microservice architecture?
In your case using direct REST calls should be fine.
Option 1 Use Rest API :
When you need synchronous communication. For example, your case. This option is suitable.
Option 2 Use AMQP :
When you need asynchronous communication. For example when your order service creates order you may want to notify product service to reduce the product quantity. Or you may want to nofity user service that order for user is successfully placed.
I highly recommend having a look at http://microservices.io/patterns/index.html
It all depends on your service's communication behaviour to choose between REST APIs and Event-Based design Or Both.
What you do is based on your requirement you can choose REST APIs where you see synchronous behaviour between services
and go with Event based design where you find services needs asynchronous behaviour, there is no harm combining both also.
Ideally for inter-process communication protocol it is better to go with messaging and for client-service REST APIs are best fitted.
Check the Communication style in microservices.io
REST based Architecture
Advantage
Request/Response is easy and best fitted when you need synchronous environments.
Simpler system since there in no intermediate broker
Promotes orchestration i.e Service can take action based on response of other service.
Drawback
Services needs to discover locations of service instances.
One to one Mapping between services.
Rest used HTTP which is general purpose protocol built on top of TCP/IP which adds enormous amount of overhead when using it to pass messages.
Event Driven Architecture
Advantage
Event-driven architectures are appealing to API developers because they function very well in asynchronous environments.
Loose coupling since it decouples services as on a event of once service multiple services can take action based on application requirement. it is easy to plug-in any new consumer to producer.
Improved availability since the message broker buffers messages until the consumer is able to process them.
Drawback
Additional complexity of message broker, which must be highly available
Debugging an event request is not that easy.
Personally I am not a fan of using a message broker for RPC. It adds unnecessary complexity and overhead.
How do you host your long-lived RabbitMQ consumer in your Users web service? If you make it some static singleton, in your web service how do you deal with scaling and concurrency? Or do you make it a stand-alone daemon process? Now you have two User applications instead of one. What happens if your Users consumer slows down, by the time it consumes the request message the Orders service context might have timed-out and sent another message or given up.
For RPC I would suggest simple HTTP.
There is a pattern involving a message broker that can avoid the need for a synchronous network call. The pattern is for services to consume events from other services and store that data locally in their own database. Then when the time comes when the Orders service needs a user record it can access it from its own database.
In your case, your Users app doesn't need to know anything about orders, but your Orders app needs to know some details about your users. So every time a user is added, modified, removed etc, the Users service emits an event (UserCreated, UserModified, UserRemoved). The Orders service can subscribe to those events and store only the data it needs, such as the user address.
The benefit is that is that at request time, your Orders service has one less synchronous dependency on another service. Testing the service is easier as you have fewer request time dependencies. There are also drawbacks however such as some latency between user record changes occuring and being received by the Orders app. Something to consider.
UPDATE
If you do go with RabbitMQ for RPC then remember to make use of the message TTL feature. If the client will timeout, then set the message expiration to that period. This will help avoid wasted work on the part of the consumer and avoid a queue getting backed up under load. One issue with RPC over a message broker is that once a queue fills up it can add long latencies that take a while to recover from. Setting your message expiration to your client timeout helps avoid that.
Regarding RabbitMQ for RPC. Normally we use a message broker for decoupling and durability. Seeing as RPC is a synchronous communication, that is, we are waiting for a response, then durability is not a consideration. That leaves us decoupling. The question is does that decoupling buy you anything over the decoupling you can do with HTTP via a gateway or Docker service names?
We have developed modular web app with very powerful API and now we need queuing tool for delayed|time consuming jobs. We are looking at RabbitMQ or AWS SQS. But these two just store messages, and you have to manually get messages from them or I misunderstood it?
We would like to channel all messages through our API, so when message is published to Queue in should be POST-ed (after some delay) to to our Interface.
So my question:
Is there any tool for queuing that support http post (with oauth2)?
If not, is this approach somehow valid:
Create worker that poll messages from queue
and POST them to API with some client?
(we have to maintain cli tool, and we want to avoid that).
Are there any alternatives?
When using SQS polling is the only way out.
To make things easier you can write this polling logic in AWS Lambda because lambda functions do not have the overhead of maintaining infrastructure and servers
I am designing a whatsapp like messenger application for the desktop using WPF and .Net. Now, when a user creates a group I want other members of the group to receive a notification that they were added to a group. My frontend is built in C#.Net, which is connected to a RESTful Webservice (Ruby on Rails). I am using Postgres for the database. I also have a Redis layer to cache my rails models.
I am considering the following options.
1) Use Postgres's inbuilt NOTIFY/LISTEN mechanism which the clients can subscribe to directly. I foresee two issues here
i) Postgres might not be able to handle 10000's of clients subscribed directly.
ii) There is no guarantee of delivery if the client is disconnected
2) Use Redis' Pub/Sub mechanism to which the clients can subscribe. I am still concerned with no guarantee of delivery here.
3) Use a messaging queue like RabbitMQ. The producer of this queue will be postgres which will push in messages through triggers. The consumer of-course will be the .Net clients.
So far, I am inclined to use the 3rd option.
Does anyone have any suggestions how to design this?
In an application like WhatsApp itself, the client running in your phone is an integral part of a large and complex event-based, distributed system.
Without more context, it would be impossible to point in the right direction. That said:
For option 1: You seem to imply that each client, as in a WhatsApp client, would directly (or through some web service) communicate with Postgres as an event bus, which is not sound and would not scale because you can only have ONE Postgres instance.
For option 2: You have the same problem that in option 1 with worse failure modes.
For option 3: RabbitMQ seems like a reasonable ally here. It is distributed in nature and scales well. As a matter of fact, it runs on erlang just as most of WhatsApp does. Using triggers inside Postgres to publish messages however does not make a lot of sense.
You need a message bus because you would have lots of updates to do in the background, not to directly connect your users to each other. As you said, clients can be offline.
Architecture is more about deferring decisions than taking them.
I suggest that you start simple. Build a small, monolithic, synchronous system first, pushing updates as persisted data to all the involved users. For example; In a group of n users, just write n records to a table. It is already complicated to reliably keep track of who has received and read what.
This heavy "group" updates can then be moved to long-running processes using RabbitMQ or the like, but a system with several thousand users can very well work without such thing, especially because a simple message from user A to user B would not need many writes.
I heard Amazon uses HTTP for its microservice based architecture. An alternative is to use a messaging system like RabbitMQ or Solace systems. I personally have experience with Solace based microservice architecture, but never with REST.
Any idea what do various big league implementations like Amazon, Netflix, UK Gov etc use?
Other aspect is, in microservices, following things are required (besides others):
* Pattern matching
* Async messaging.. receiving system may be down
* Publish subscribe
* Cache load event.. i.e. on start up, a service may need to load all data from a couple of other services, and should be notified when data is completely loaded, so that it can 'know' that it is now ready to service requests
These aspects are naturally done with messaging rather than REST. Why should anyone use REST (except for public API). Thanks.
A standard that I've followed in the past is to use web services when the key requirement is speed (and data loss isn't critical) and messaging when the key requirement is reliability. Like you've said, if the receiving system is down, a message will sit on a queue until the system comes back up to process it. If it's a REST endpoint and it's down, requests will simply fail.
REST API presumes use of HTTP only. it is quite stone age technology and does not accept async. messaging. To plugin messaging there, I would consider WebSockets Gateways
-sorry for eventually dummy statements
I have an Azure hosted web application which works alongside a number of instances of a worker role. Currently the web app passes work to these workers by placing messages in an Azure queue for the workers to pick up. The workers pass status and progress messages back by placing messages into a 'feedback' queue. At the moment, in order to inform my browser clients as to progress, I make ajax based periodic polling calls in the browser to an MVC controller method which in turn reads the Azure 'feedback' queue and returns these messages as json back to the browser.
Obviously, SignalR looks like a very attractive alternative to this clumsy polling / queing approach, but I have found very little guidance on how to go about doing this when we are talking about multiple worker roles (as opposed to the web role) needing to send status to individual or all clients .
The SignalR.WindowsAzureServiceBus by Clemens vasters looks superb but leaves one a bit high and dry at the end i.e. a good example solution is lacking.
Added commentary: From my reading so far it seems that no direct communication from worker role (as opposed to web role) to browser client via the SignalR approach is possible. It seems that workers have to communicate with the web role using queues. This in turn forces a polling approach ie the queues must be polled for messages from the worker roles - this polling has to originate (be driven from) from the browser it appears (how can a polling loop be set up in a web role?)
In summary, SignalR, even with the SignalR.WindowsAzureServiceBus scale out approach of Clemens Vasters, cannot handle direct comunication from worker role to browser.
Any comments from the experts would be appreciated.
You can use your worker roles as SignalR clients, so they will send messages to the web role (which is SignalR server) and the web role in turn will forward messages to clients.
We use Azure Service Bus Queues to send data to our SignalR web roles which then forward on to the clients.
The CAT pages have very good examples of how to set up asynchronous loops and sending.
Keep in mind my knowledge of this two technologies is very basic, I'm just starting. And I might have misunderstood your question, but it seems pretty obvious to me:
Web roles are capable of subscribing to a queue server where the worker role deposits the message?. If so there would be no client "pulling", the queue service would provide the web server side code with a new message, and through SignalR you would push changes to the client without client requests involved. The communication between web and worker would remain the same (which in my opinion, it's the proper way to do it).
If you are using the one of the SignalR scaleout backplanes you can get workers talking to connected clients via your web application.
How to publish messages using the SignalR SqlMessageBus explains how to do this.
It also links to a fully worked example which demonstrates one way of doing this.
Alternative message bus products such as NServiceBus could be worth investigating. NServiceBus has the ability to deliver messages asynchronously across process boundaries without the need for polling.