Perhaps I'm going about this the wrong way, but here's my current "setup".
I have a silverlight client that uses Caliburn.Micro and a MEF container with a "LoadCatalog" class, to keep everything loosely coupled in an MVVM way.
I have a "common" dll where all the interfaces are kept.
All my views and viewmodels are separate projects, that only have a reference to the common dll.
The viewmodels use WCF (regular) to communicate to the backend. The frontend itself has a duplex connection to the backend.
Now here's where the question comes to mind. Whenever the backend thinks it's time to have a new screen appear at the frontend, it uses the callback channel to tell the frontend to load the next screen.
Does this seem like a good pattern to use? Or should I leave the management of what screen to load when to the frontend? I think it's nice to have this in the backend, but perhaps this is some kind of anti-pattern I'm not aware of, hence the question.
Now for argument sake, lets say I want to keep this in the backend.
What would be the best way to go about managing the collection of callback channels on the backend? If I enable SessionMode.Required on all the regular WCF endpoints, as well as the duplex channel, does this persist state together over multiple endpoints (regular+duplex)? Or will this persist state only within a single endpoint?
My guess (from the tests I have been able to do so far) is that I need to add some logic, like for example provide the frontend with a guid as soon as the callback connection is made. And then use that guid in the regular endpoint connections so the backend knows which "client" it is.
And would I "ever" be able to reliably collect all the channels and detect current state if I made a collection of the callback channels that I receive? I can intercept the callback channel now (just 1 instance atm, no collections made or anything, so single user) and use that to tell the frontend what to do. But sometimes when the client stops abrubtly (in other words when an error occurs) and I start the client again, it seems like the previous (faulted?) connection is still "re-used" or something, without luck so the communication flow stops after connection the duplex endpoint.
Does this make any sense?
Hope there's someone who has some experience in the matter that can shed some light on it for me. I'm no total newbie, but with regards to multiple connection and keeping them separated, I might need some pointers in the right direction.
Thanks!
Huron.
I managed to get this up and running.
When I create the duplex connection from the frontend to the backend I return a unique Guid. From now on I use this guid for all communication I do with the backend.
This makes the backend "recognize" the client.
In the backend I have a list of connections (grabbed the callback channel and stored it together with the Guid).
Just had to make sure to lock the list object whenever I iterate it or when I add/remove items from it, since it will be used from multiple threads by design.
The pattern to take control from the backend seems to work great so far.
Related
I am implementing a micro-service architecture for the first time.
Some of my services (.NET Core Web APIs) need to communicate with each other through HTTP requests. For that purpose, I am injecting a wrapper around HttpClient.
But I suspect that I am reinventing the wheel. Among micro-service practitioners, is there a pattern or even a third-party library to solve this problem?
In a micro-service architecture, the most important thing is a clear separation of concerns and application boundaries. Imagine a simple setup, with Product and Price micro services
An important concept is each service is master of data, and owns its own database. In this example,
a client of the 'Product' service will make an HTTP call to the Product API.
the product API will make a call to the Price API to get prices for the products
the product API therefore depends on the Price API to create a response
These are the synchronous parts of the process, generally achieved through HTTP calls across boundaries. You'll also have asynchronous parts of your solution, in this example,
the Price API publishes an event to a bus whenever a price is changed
the product API publishes an event whenever a product is created
There may be one or more subscribers to these events, that will respond and probably call an API to retrieve the changed data.
The critical parts of this are clearly defining your API and message contracts, understanding if things will be async or sync, having the right level of telemetry across the entire architecture to track and understand distributed system behaviour, and keeping everything as independently buildable/testable/deployable components.
First and foremost, if you're not using containers, start, along with orchestration (both natively supported in Visual Studio, assuming you have Docker, etc. actually installed). Among the many benefits, you can reference your services via hostname without having to worry about ports and different locations for different environments.
As far as actual communication goes. There's not really a magic solution here. HttpClient is what you use, of course, and generally, yes, you want to have a wrapper around that to abstract away the low-level HTTP communication stuff, so the rest of your code can simply call simple methods on that wrapper.
If you aren't using IHttpClientFactory, start. If you already have a wrapper class, you're halfway there, and with that, not only do you get efficient management of HttpMessageHandlers so you don't exhaust your server's connection pool, but you can also use the Polly integration to handle transient HTTP errors and even do retry policies, circuit breakers, etc. for your microservice connections.
Finally, there is the Refit library which can make things a tad more straight-forward. I find it to have more use with huge third-party APIs like Facebook, Google, etc., though. Since microservices should by design be simple, you're probably not saving much code over just having your own wrapper class. Regardless, the way it works is that you define an interface that represents the API, and then Refit uses that to actually make appropriate requests. It's kind of like a wrapper class for free, but you still need to create the interface.
I am experimenting with using Reactive Extensions to create a Windows Service.
Essentially what I want is for the Observer to sit on the server, the clients able to create observables and have them pushed to the server, the server informing the client of the progress of the job (not sure how to do this or what mechanism to use to do it), and then when it's done, having the server send the client the return code and output of the program it called. Can this be done? Is it the best way to do what I'm trying to do? If you need any more information, what would you need to know to help me?
This seems back to front. Generally clients know about servers (how to find then and connect). In contrast the Observer pattern (and therefore Rx) is about allowing something to callback to another observer that is does not know about.
In your case I think you simply want to have clients call methods on a server. Potentially these are bound to a single connection/session. The client however maybe an observer of the progress from the server and the final result.
See the Reactive Trader project by the team at Adaptive to see a .NET client server app using Rx.
does someone know if it possible to use one WCF Data Service as data source of another WCF Data Service? If so, how?
So the short answer is yes. Actually I have consumed one WCF service in another (HttpBinding coming to a service on computer, then that service had a NamedPipesBinding service to communicate with multiple desktop apps, but it did some data transformation in the middle). That would not be an issue at all, you would set up a proxy/client just like you would in a desktop client, and handle everything in your new service as if it was just passing information along, you could even create a shared library for the DataContracts and such.
HOWEVER I would not suggest the leapfrog method in your implementation. Depending on how many customers you are potentially opening the door too, you may be introducing a bottlekneck, if you have a singleton service, or overload your existing service in the case of many connections from the new one. Since you have a SQL server, why would you not have a WCF service on your web/app server (public) that connected to it and provided the data you need? I'm only thinking this because your situation can become exponentially complicated when you start trying to pass credentials for authentication and authorization between the two, depending on your security settings. Another thing to consider is the complexity in debugging this new service and the old one, and a client at the same time, as if it wasn't a pain just to do server and client, since you are opening it to a public facing port, there are different things to set up, and debugging everything on the same machine is not the same as a public facing application server.
Sorry if this goes against what you were hoping to hear. I'm just saying that it is possible, but not suggested (at least by me) in your particular case.
I have got two applications that need to communicate via WCF:
Called A and B.
A suppose to push values to B for storage/update
B suppose to push a list of values stored in it to A
the senior programmer in my team wants to open a WCF server at A and another WCF server at B.
I claim that one should be the server and the other should be the client and use server call back In order to avoid splitting the interface into two, avoid circular dependency, and duplication of code settings. he doesn't understand it. can anyone help me explain him why his solution is bad code?
It depends on your criteria. Let's assume a client/server model where A is the client and B is the server. You state that B should "push" data to A.
If you truly need push then you should make B into a duplex server. This does put some strain on your bandwith, so if you have a bandwith restriction, this might not be the right choice.
If you can incur some delay at A than you might want to opt for a polling mechanism of your own (maybe based on timing, or some other logic).
If both are not an option, you can try and swap roles. So then make B the client and A the server. It's les intuitive, but it might fit your scenario. If you can incur a delay on storing data, make B poll A for changes in the data and save at an interval.
If there can be no delay in both and bandwith is limited, you do end up with two WCF services. Altough it may look silly at first glance, keep in mind they are services and not servers. It does make things a bit more complex, so I would keep it as a last resort.
A service should encapsulate a set of functionality that other applications can consume. All it does is wait for and respond to requests from other components, it doesn't initiate actions by itself.
If Application B is storing data, then it can of course be provided to Application A as a service. It provides the "service" of storing data without application A having to worry about how or where, and returns successfully stored data. This is exactly the kind of thing that WCF Services are meant to handle.
I am assuming that application A is the one initiating the requests (unless you have an unmentioned 3rd application, one of them must be the initiator). If Application A is initiating actions (for example, it has a UI, or is triggered to do some batch processing etc.) then it should not be modeled as a "service".
I hope that helps :)
I have found myself responsible for carrying on the development of a system which I did not originally design and can't ask the original designers why certain design decisions were taken, as they are no longer here. I am a junior developer on design issues so didn't really know what to ask when I started on the project which was my first SOA / WCF project.
The system has 7 WCF services, will grow to 9, each self-hosted in a seperate console app/windows service. All of them are single instance and single threaded. All services have the same OperationContract: they expose a Register() and Send() method. When client services want to connect to another service, they first call Register(), then if successful they do all the rest of their communication with Send(). We have a DataContract that has an enum MessageType and a Content propety which can contain other DataContract "payloads." What the service does with the message is determined by the enum MessageType...everything comes through the Send() method and then gets routed to a switch statement...I suspect this is unusual
Register() and Send() are actually OneWay and Async...ALL results from services are returned to client services by a WCF CallbackContract. I believe that the reson for using CallbackContracts is to facilitate the Publish-Subscribe model we are using. The problem is not all of our communication fits publish-subscribe and using CallbackContracts means we have to include source details in returned result messages so clients can work out what the returned results were originally for...again clients have a switch statements to work out what to do with messages arriving from services based on the MessageType (and other embedded details).
In terms of topology: the services form "nodes" in a graph. Each service has hardcoded a list of other services it must connect to when it starts, and wont allow client services to "Register" with it until is has made all of the connections it needs. As an example, we have a LoggingService and a DataAccessService. The DataAccessSevice is a client of the LoggingService and so the DataAccess service will attempt to Register with the LoggingService when it starts. Until it can successfully Register the DataAccess service will not allow any clients to Register with it. The result is that when the system is fired up as a whole the services start up in a cascadeing manner. I don't see this as an issue, but is this unusual?
To make matters more complex, one of the systems requirements is that services or "nodes" do not need to be directly registered with one another in order to send messages to one another, but can communicate via indirect links. For example, say we have 3 services A, B and C connected in a chain, A can send a message to C via B...using 2 hops.
I was actually tasked with this and wrote the routing system, it was fun, but the lead left before I could ask why it was really needed. As far as I can see, there is no reason why services cannot just connect direct to the other services they need. Whats more I had to write a reliability system on top of everything as the requirement was to have reliable messaging across nodes in the system, wheras with simple point-to-point links WCF reliabily does the job.
Prior to this project I had only worked on winforms desktop apps for 3 years, do didn't know any better. My suspicions are things are overcomplicated with this project: I guess to summarise, my questions are:
1) Is this idea of a graph topology with messages hopping over indirect links unusual? Why not just connect services directly to the services that they need to access (which in reality is what we do anyway...I dont think we have any messages hopping)?
2) Is exposing just 2 methods in the OperationContract and using the a MessageType enum to determine what the message is for/what to do with it unusual? Shouldnt a WCF service expose lots of methods with specific purposes instead and the client chooses what methods it wants to call?
3) Is doing all communication back to a client via CallbackContracts unusual. Surely sync or asyc request-response is simpler.
4) Is the idea of a service not allowing client services to connect to it (Register) until it has connected to all of its services (to which it is a client) a sound design? I think this is the only design aspect I agree with, I mean the DataAccessService should not accept clients until it has a connection with the logging service.
I have so many WCF questions, more will come in later threads. Thanks in advance.
Well, the whole things seems a bit odd, agreed.
All of them are single instance and
single threaded.
That's definitely going to come back and cause massive performance headaches - guaranteed. I don't understand why anyone would want to write a singleton WCF service to begin with (except for a few edge cases, where it does make sense), and if you do have a singleton WCF service, to get any decent performance, it must be multi-threaded (which is tricky programming, and is why I almost always advise against it).
All services have the same
OperationContract: they expose a
Register() and Send() method.
That's rather odd, too. So anyone calling will first .Register(), and then call .Send() with different parameters several times?? Funny design, really.... The SOA assumption is that you design your services to be the model of a set of functionality you want to expose to the outside world, e.g. your CustomerService might have methods like GetCustomerByID, GetAllCustomersByCountry, etc. methods - depending on what you need.
Having just a single Send() method with parameters which define what is being done seems a bit.... unusual and not very intuitive / clear.
Is this idea of a graph topology with
messages hopping over indirect links
unusual?
Not necessarily. It can make sense to expose just a single interface to the outside world, and then use some internal backend services to do the actual work. .NET 4 will actually introduce a RoutingService in WCF which makes these kind of scenarios easier. I don't think this is a big no-no.
Is doing all communication back to a
client via CallbackContracts unusual.
Yes, unusual, fragile, messy - if you can ever do without it - go for it. If you have mostly simple calls, like GetCustomerByID - make those a standard Request/Response call - the client requests something (by supplying a Customer ID) and gets back a Customer object as a return value. Much much simpler!
If you do have long-running service calls, that might take minutes or more to complete - then you might consider One-Way calls which just deposit a request into a queue, and that request gets handled later on. Typically, here, you can either deposit the answer into a response queue which the client then checks, or you can have two additional service methods which give you the status of a request (is it done yet?) and a second method to retrieve the result(s) of that request.
Hope that helps to get you started !
All services have the same OperationContract: they expose a Register() and Send() method.
Your design seems unusual at some parts specially exposing only two operations. I haven't worked with WCF, we use Java. But based on my understanding the whole purpose of Web Services is to expose Operations that your partners can utilise.
Having only two Operations looks like odd design to me. You generally expose your API using WSDL. In this case the WSDL would add nothing of value to the partners, unless you have lot of documentation. Generally the operation name should be self-explanatory. Right now your system cannot be used by partners without having internal knowledge.
Is doing all communication back to a client via CallbackContracts unusual. Surely sync or asyc request-response is simpler.
Agree with you. Async should only be used for long running processes. Async adds the overhead of correlation.