Just a technology update, now that .NET 4.0 is out.
I write an application that communicates to the server through what is basically a message bus (instead of method calls). This is based on the internal architecture of the application (which is multi threaded, passing the messages around).
There are a limited number of messages to go from the client to the server, quite a lot more from the server to the client. Most of those can be handled via a separate specialized mechanism, but at the end we talk of possibly 10-100 small messages per second going from the server to the client.
The client is supposed to operate under "internet conditions". THis means possibly home end users behind standard NAT devices (i.e. typical DSL routers) - a firewalled secure and thus "open" network can not be assumed.
I want to have as little latency and as little overhad for the communication as possible.
What is the technologally best way to handle the message bus callback? I Have no problem regularly calling to the server for message delivery if something needs to be sent...
...but what are my options to handle the messagtes from the server to the client?
WsDualHttp does work how? Especially under a NAT scenario?
Just as a note: polling is most likely out - the main problem here is that I would have a significant overhead OR a significant delay, both aren ot really wanted. Technically I would love some sort of streaming appraoch, where the server can write messags to a stream while he generates them and they get sent to the client as they come. Not esure this is doable with WCF, though (if not, I may acutally decide to handle the whole message part outside of WCF and just do control / login / setup / destruction via WCF).
For bidirectional communications, your best bet is NetTcpBinding, rather than the http bindings, if they're available.
This has the advantage of only requiring that the client can initiate a connection with the server.
I would go with Windows Azure Service Bus. See my answer in the following question:
WCF, 4.0, Bidirectional
Take a look at Windows AppFabric, good place to start is Here. It fundamentally wraps up WCF and WF into an application server, with WCF activation supported through WAS. Its where I would host this type of app. It offerd full duplex connection orientated, p2p or sessions between client and server. Don't confuse the Windows appfabric with Azure appfabric, (formely called Azure Service Bus).
As regards bindings above, both NetTcpBinding and WsDualHttp offer callbacks, but the ws binding you get a lot for your cash, especially if it's a mixed programming environment and you have to flatten the wsdl to make interop work. I also think that WsDual is easier on routers traversal, although I understand talking to friends, that Windows AppFabric mitigates this, with new Relay Services, (which i've not seen, and I think have now been renamed).
Hope that helps.
Related
I have a windows phone 8 application, which communicates with WCF service using basicHttpBinding. The service is hosted on IIS7 (and not using windows azure)
As the service may go down for any reason, I am exploring the use of message queues to increase the reliability of the system.
I have looked at NetMsmqBinding provided in WCF - but it looks like this binding is not supported by WP8 client.
I am also looking at using RabbitMQ, but cannot find any working example with WP8 client using WCF.
Please can anyone suggest what is the best way forward? Any sample code (or links) will be much appreciated.
Thanks
First off, netMsmqBinding cannot be used across the internet. This is because it uses MSMQ which is not exposed over http.
When you're making calls to a resource across the internet, unreliability is something you need to factor into your application. Because of the number of possible problems you can encounter, it's generally not a case of if, but when, there is a failure and it's how your application deals with this which is important.
Even so, there are things you can do to minimize the reliability issues you experience, one of which does involve queuing.
Where queuing can be useful is taking large, complex, and long running processes offline. Because calls to such processes implemented synchronously often time out, you can gain a lot of reliability by making the actual processing call asynchronous.
As an example, it would be fairly common to have the web server invoke some offline process via message queuing and return to the client that their request is being processed. Because doing this is inexpensive calls are far less likely to fail. Your problem then becomes one of how to return the response to the client once the offline processing has been done.
I am sure that this is answered dozens of times, but I am at a loss as to what keywords to search for and thus I hope that someone can at least tell me where I should be looking given an explanation of my scenario.
I need two services (one can be just a client if that is easier) to talk to each other, but the client will be on a private network whereas the server will be on the internet. I want to be able to push jobs to the client, but the server obviously does not have an IP to hit the client. I'd rather not poll from the client every X seconds. I have read various topics all circling this issue and so I am going to throw out a few terms that I think are relevant, but I am not sure which to use or exactly how.
Comet, SignalR, WebSockets, XSockets, Publisher/Subscriber Pattern...
I have looked at each of these and I am not sure which is the right way to go. The client can certainly "subscribe" to the server on startup, so that should not be an issue. But the client should be either a console app, windows service, or WCF service. It seems Comet and SignalR are more for ASP.NET apps, where the client is JS in a browser. I just need "server(client)" to server connection where the client is behind firewalls.
Which of these terms (or none of them) is a good way to handle server -> client push notifications?
Pub/Sub architecture pattern with something like Azure Service Bus should help you create the solution you desire. This does require that service and the client are aware of the bus. For the plumbing of the client and the services use the WCF which has built in bindings to facilitate the use of this pattern.
Azure: How to Use Service Bus Topics/Subscriptions.
Azure SB has a counter part that works on-premises as well. There are other popular message bus tech (NServiceBus, MasTransit, etc.)
You can have a look at node.js together with socket.io.
This will give everything you need.
socket.io uses web sockets, and if the browser does not support web sockets, it gracefully falls back to other communication mechanism like xhr, flash, polling, aso.
After developing mini project with WCF duplex (Chat Service | Sms Service), I got a Point that maybe not be correct!!
I believed Duplex theory is good and useful but there is a lot problem about using Wcf Duplex. (like reliable session, Time-out exceptions, Client address-Management on server side, proxy management on Client Side)
am I think wrong ? am I miss something?
For more Information I Used wsDualHttpBinding not tcpBinding.
If you need bidirectional communication and you want to use WCF, duplex channels are the way to go. You just need to design your application correctly and correctly handle all problems you have described. If you feel that these problems are overhead and make things even worse you can always use network programming directly (sockets) or handle bidirectional communication by yourselves exposing separate service on server and another on client (where first call from client inform server about clients address) - this scenario will suffer from the same communication problems as WsDualHttpBinding.
WsDualHttpBinding itself is special kind of duplex communication. I personally don't like it because people very often misuse it. The problem is that this binding uses two separate connections - one from client to server and second from server to client. That is big difference to net.tcp where only connection initiated from client to server is used. Obviously using WsDualHttpBinding over internet (= you don't have control over client machines) becomes much more complicated because each client must configure its firewall (in computer, on home internet gateway, etc.) to allow connection on some port. Also if you want to run more then one instance of application on the same client machine, each instance must use its own port.
We're planning a system running on Windows/.Net 3.5 that has a number of "services" that need to run in the background. Some will be active all of the time, but some will only be called occassionally and can be stood-up on demand.
As far as I can see, my options are:
Windows Services - always running(?)
IIS hosted something - called on demand
COM+/ .Net Enterprise Sevices - most complex option, but most powerful?
Distributed transactions is not a requirement, these are mainly computation engines, rather than transaction processors.
Does anyone have any experience of working with all of these and what further pros & cons can be claimed for each technology?
EDIT
Is suppose there are multiple ways of hosting code in IIS, web services, WCF (as pointed out below), any others? Relative pros/cons?
WCF feels like the right way to go. There are still many choices to make. WCF provides a number of communication mechanisms and hosting environments:
WCF combines the following technologies under one set of APIs-
ASMX;
WSE;
Remoting;
COM+;
MSMQ.
So for instance you can use persistent messages from MSMQ for occassionaly connected clients or standard XML encoding SOAP messages over an HTTP transport layer. You can also use new features in 3.5 like binary encoding of XML or JSON encoding over HTTP.
Hosting environments include:
Console applications
Windows services
WCF services inside IIS 7.0
and on Windows Vista or Windows Server 2008 you can use WAS (Windows Activation Services) to host WCF services.
Different hosting environments have pros and cons. I suggest you look at MSDN for more details (e.g. http://msdn.microsoft.com/en-us/library/bb332338.aspx).
Because WCF encompasses a lot of functionality it is more difficult to learn than any one of the technologies it replaces. I still think it pays for itself in the long run.
It depends on what the software will do, and how (and if) users or systems need to interact with it. Depending on those things, there may be one more, often overlooked, option: set it up as a scheduled task. This is often a very good alternative to a windows service, if the software is of the kind that will act on certain time intervals (check for a change in a database, act on the changed data and send it somewhere, for instance).
If you will have other systems talking directly to your software, I would imagine that a WCF application hosted in IIS would be a rather straighforward way. We use both those approaches in my current assignment; WCF services for looking up and storing data, and scheduled tasks for data calculations that run on a regular basis.
The scheduled task has one upside compared to the others in one specific field; it uses system resources only when running.
You mentioned starting up a process "on demand". WAS - Windows Activation Service, or sometimes called Windows Process Activation Servvice, though it is never abbreviated "WPAS" - is the thing inside Windows that provides on-demand process activation. The way it works - when a message arrives, WAS can start a worker process to handle the message. WAS was, prior to IIS7, fairly tightly integrated into IIS. It was used primarily to activate processes that did web work - like an ASP.NET worker process. With IIS7, WAS is generalized so that it can activate worker processes based on non-HTTP as well as HTTP messages. If you write your app to receive messages through WCF, you can get activation essentially "for free". That applies if it is HTTP, TCP, MSMQ; SOAP or otherwise.
The key thing with this on-demand startup though, is that it is tied to the communication. In fact the process lifecycle model for WAS is tied to communication as well. By default if there are no incoming messages after a while, the process will be shut down by WAS. That may or may not be what you want.
As for process hosting - COM+ offers a hosting environment but it is primarily intended for use as a host for processes that communicate. This may not be the perfect fit for you.
If you have compute engines, you may just want to run a Windows Service. A service like that can be started and stopped either administratively or programmatically. In the latter case, you could imagine a WAS-activated worker process programmatically starting a windows service.
You could also imagine writing a simple Windows Service that watches a location (filesystem, message queue, etc) for a message, and when that file or message arrives, the Windows Service starts up a compute engine process, which itself is NOT a Windows Service, but is just a process.
Speaking of MSMQ - That is basically the same model as MSMQ triggers. You can configure MSMQ to start a process when a message arrives on a particular queue.
There are lots of options.
I am looking at using MSMQ as a solution to do asynchronous execution in my upcoming project. I want to know the differences between using WCF and frameworks like MassTransit or even hand written MSMQ client to place/read task off MSMQ.
Basically the application will be several websites (internal through LAN or external through the Internet) reading/writing data through a service layer (be it WCF or normal web service). Then this service layer will do one of two things: 1. write data to database 2. and/or trigger the background process by placing a message in the queue. 3. obviously it can also retrieve data from database. The little agent (a windows service) on the other side of the queue will monitor the queue and execute based on the task command.
This architecture will be quite easy to scale (add more queues and agents) and easy to implement compared to RPC or distributed execution or whatever. And the agent processing doesn’t need to be real time. And the agent and service layer are separate applications except they share the common domain objects and Repositories etc.
What do you think? Architecture suggestions for the above requirements are welcomed. Thank you!
WCF adds an abstraction over MSMQ. In fact, once you define compatible contracts (operations must be OneWay), you can switch out MSMQ in the config, transparently. (For instance, you could switch to normal HttpWS or a NetTcp binding.)
You should evaluate the other WCF benefits, like security and so on, to see how those fit in with your needs. Again, they should be reasonably transparent of the fact you're using MSMQ underneath. For instance, adding SOAP security and so on should "just work", independent of using MSMQ.
(Although, IIRC, you still need to login to the desktop on each machine that uses MSMQ, with the service account that will use MSMQ, to generate the certificate in the machines local profile. And then, it doesn't work very well from IIS6, since user profiles aren't loaded. A real pain in general, but nothing to do with WCF specifically.)
Apart from that:
Have you looked at SQL Server Service Broker? After using MSMQ + WCF and SSSB, I think that SSSB is vastly easier to configure and manage. SSSB works with T-SQL commands over any SQL client (I use it from Mono, on Linux, with transactions). It'll also give you transactional send/receive, even remotely (I think MSMQ 4 now allows this). It really takes a lot of the pain away from message queuing, and if you're using SQL Server already...
SSSB is often overlooked since the SQL Management Studio doesn't have GUI designers for it all, but it isn't hard and is a great option. The one downside is that if you want local send capability (i.e., queue message when network is down), you'll need to run a local SQL Express instance.
Your architecture seems sound and reasonable. However you should consider using the WCF net MSMQ transport over hand coded MSMQ classes. WCF wraps this common functionality into a nice programming model. Also I believe there is some improvements in the protocol used by wcf compared to basic System.Messaging
Have a look at the value-add over plain MSMQ:
http://readthedocs.org/docs/masstransit/en/latest/overview/valueadd.html
In summary, you get a lot of messaging concepts clearly presented in the API with MassTransit; to an extent you wouldn't have if you hand-coded it or used WCF.