I want to deploy an Windows services in parallel for redundancy and load balancing purposes.
How can i be sure that when the client sends a request to both of these services, that only 1 of them process the actual call?
Example:
When the client or other services sends a message to start a manufacturing process, both of these services will recieve that request. I want to make sure that only one of those services processes this request, so that manufacturing process do not get started twice!
Do they need to able to talk to themself?
Is there a possibility to sync those services?
Which is the most elegant/robust way of handling this problem?
Look into using a mutex to allow both services to only pick up a message once.
Mutex Description C#
Although, you'll need to make sure this can work in the way you want. this can help schedule between application processes and boundaries, but if this is deploy to two different machines, or Cloud services, the Mutex isn't going to work.
for that you'll need to figure out another of communicating across the applications, usually using a database or a MSMQ to create a message queue that you can pop messages off as you need them from each service.
The safest way, and also the best practice, for your example, would be to retrieve (not to peek) messages from a queue leveraging MSMQ. This gives you a clear explanation of the use case: https://learn.microsoft.com/en-us/previous-versions/windows/desktop/msmq/ms706253(v=vs.85)
~Pino
Related
My worker role expose a WCF service and have multiple instances.
I want my client to call this service and make all instances working concurrently.
I'm trying to figure out what is the best way to do this scatter-gather task.
(And I'm trying to avoid service bus and use WCF only)
I can't think of any good way to do this without something like service bus topics. Or using custom functionality that does nearly the same thing. Why are you trying to avoid Service Bus?
There's really no way to make a client-side call to multiple server instances simultaneously, just using Azure's built-in services. Even using Service Bus topics, there's no way to guarantee that multiple subscribers will consume a message at the same time and execute at the same time (even with a message embargo time, you still cannot absolutely guarantee each subscriber will consume + process a message at an exact time).
This will need to be an application-side action. For example: You can queue up your wcf requests. Your queue-reader can then direct-connect to an internal endpoint on each instance, triggering an action to run in parallel. This won't give you exact parallel operation, but it will be pretty close. As another option, you can have several threads available per instance, and you could run the same request on each thread (again, managed by you).
In essence, this is an architectural facet of your app. Azure won't be able to facilitate a parallel-call across instances; you can take advantage of queues, internal services, etc. to accomplish this.
I have three wcf services A,B and C respectively ,since i wanted it to be SOA(Service Oriented Architecture) the way my setup works is when i send a request from client to server.
All the services are self hosted windows services.
Client sends request to service A (client has no clue about the other services B and C);
Service A eventually sends that request to Service B and Service C.
Service B and C sends response back to Service A which would be sent back to the client by service A.
Issue i m facing :If i make any changes in the code of Service B and rebuild and restart the service ,i am having issue getting the response back but when i restart all the remaining services then it works fine.
In other words my client doesn't get the response back unless i restart all the services(A,B and C) even though i just changed the code in only one service and rebuilt it.I know the thing works if i restart all the three services but i want to know is this the problem in my way of designing or it is something i have to deal with self hosted windows services.And all the services(A,B,C) are independent as none depends on each other.
Did some one ever see such things happened in SOA.I would be glad if some one can guide me to appropriate solution ?
Replace WCF between services with any sort of queue (one service publishes something, other can read when they are ready). Can be anything. Can be a simple table where you read from if there is something new. Can be RabbitMQ, NServiceBus, etc, whatever works for you.
Define messages you put into the queue: commands and events. Both are simple classes with properties, no logic there. Commands represent what the system is asked to do (RegisterUser, PlaceOrder, ect), events represent what the system has done (UserRegistered, OrderApproved, PaymentReceived, etc). Be explicit about actions, Don't do something like "I have changed all the properties of a user on the client, now I call SaveUser(user)". Your service supposes to know how to change objects, clients should only command what to do.
Never break your contract. It is easy, easier than it sounds: you can add things to your message contracts, but cannot remove. In other word you just keep your contract backwards compatible.
Now you have a much better design: services communicate only through messages in queues, messages are backward compatible. This means that you can stop any of the services at any time without impacting others: they will continue sending messages into queues, and when the stopped service comes back again it will catch up processing all the stuff from the queue.
Then, if you want, you can use the same approach with client interactions: if instead of calling WCF clients would only put their commands in some sort of a queue then service upgrades or other downtime would not impact user experience.
Example: if I use WCF to place an order or to put an item into a shopping card then if there is a problem or a service is down for maintenance I will not be able to do it. I would click a button and have a nasty error. More importantly my order will not make into the system.
In contrast, if there is a queue in the middle, I only put my command into the queue. Now even if my service is down at the moment, or experience a high load (and therefore slow) then my user experience is still the same and does not degrade. It is just my command will be processed a bit later, but as a client I don't really care. And my order will not be lost in this scenario. The system became fault-tolerate and self-balanced.
There are all sorts of fantastic tricks you can do if you simply put a queue in the middle instead of experiencing problems with spatial and temporal coupling that comes with WCF :)
And what I described is just the beginning... :)
You may want to consider using a service bus such as NServiceBus to help you accomplish your functionality.
The first issue it will help you address is the decoupling of your services via publish/subscribe messaging pattern. Rather than invoking web services in one or the other service, publish events that notify the respective services when something has occurred. In your case this would look something like this:
Client invokes web service in Service A.
Service A publishes a message "Client Command Received" which Service B and C subscribe to.
Service B and C handle this event and then publish events of their own.
Service A subscribes to both events and replies to the client.
The first and immediate benefit of using something NServiceBus is reliability. On top of that you are able to easily version your message without affecting your client or your respective services. NServiceBus has full WCF integration so your client can continue to send messages to your service as before.
One of the things that makes your scenario interesting is that you can't guarantee when Service B and C send their responses back to you. Do you keep the connection to the client open until Service has received their responses? Do you need both responses before you can send a the client its response? What happens if either or one of the service crash? What if there is a time limit to how long you can wait before a response is received by Service A? All of these questions and more can be answered with a feature in NServiceBus called Sagas. Check it out.
If using NServiceBus is not possible then things become more difficult. WCF doesn't support publish/subscribe out of the box so you will have to bake your own. At a minimum I would recommend using this to decouple your services. How you manage state and temporal coupling in your services is another matter. Save yourself the trouble.
There are other frameworks out there but if you want a developer centric, cost effective way to create a .NET based solution then recommend using NServiceBus.
So basically I am thinking about attempting load testing on my asp.net application using various features all at once. There is a lot of dependencies and ajax requests being performed in this application so it seems like a simple replay of captured http requests will not suffice and due to other features like picking out random operations, performing then verifying results across several machines, simple load testing software will not suffice.
Also there is no budget to this project for spending, so commercial implementations can not be used. I'm debating on trying to use MSMQ (never used before) to handle communication between clients, but if that is really complicated to set up then I would either use a database table as a queue or a simple TCP server with each test machine as its clients.
Features I want are: immediate failure (one client crashes, then all clients should stop), each test run should start with a brand new scenario with no prior messages, and ability to publish a start and stop event. Also it would be nice if I don't have to worry about state management (leaning towards TCP server for this over database) or concurrency.
It doesn't sound like MSMQ is what you need. It is a message-passing asynchronous communication method, akin to email. You can send a message to another queue that no one is even listening to (i.e. the application isn't running). It seems to me you want a more "online" communication model.
How about creating agents (client applications that sit on many machines and create the load) that expose a WCF service where a controller program can connect to all of them and instruct the agents what to do? It can be a duplex contract, so that the agents can send the controller a notifications. When one of them send a error notification, the controller can instruct all the other agents to shut down. Also I'd go for a Net.TCP binding rather than HTTP binding.
I am working on one Asp.Net application and need to send mails periodically based on some event. First I thought of creating a thread in global.asax and start thread in application_start. But that becomes a bit of problem when application pool crashes or something. So I implemented a windows service and started thread in that and log any errors in windows event log. This works fine. But I need to know whether I am implementing it correctly or is there a better way of doing it?
I think you are moving (or moved already) to the right direction.
We have similar architectures as well, in some cases we used MSMQ to queue outgoing notifications from the ASP.NET application then the Windows Service, usually called Messaging Manager, can grab asynchronously the incoming messages and send the emails or alerts out.
this proves to be effective and robust, if anything crashes after the message has been queued, nothing will be lost because the windows service will always process the messages in the queue, so you can have ASP.NET recycling or the machine with the windows service being rebooted, nothing is lost ever. And in fact in normal production mode, messages are sent out instantly, the decoupling or loose of sync is mostly hidden when everything is working smooth and servers are not overloaded or suffering anything.
In a later project we are now implementing something similar using TIBCO technologies, EMS for the queues and Business Works for queue subscribers.
Using a Windows Service for this kind of tasks is the preferred way instead of doing it in the ASP.NET application. You may also take a look at Quartz.NET which could simply your code for scheduling the task execution and dealing with threads. But if you don't want to write Windows Services probably the simplest would be to have a console application that will do the job of sending emails and then simply use the Windows Task Scheduler to run it at regular intervals.
Another option is a message-based approach. You could have a Windows Service/Console Application reading messages of a message queue (like msmq) and send email when a message is recieved. You can then have your ASP.NET application publish messages to this queue.
Minibuss is a lightweight client for msmq which is very easy to work with. Another options is NServiceBus.
I'm creating an application that I want to put into the cloud. This application has one main function.
It hosts socket CLIENT sessions on behalf of other users (think of Beejive IM for the iPhone, where it hosts IM sessions for clients to maintain state on those IM networks, allowing the client to connect/disconnect at will, without breaking the IM network connection).
Now, the way I've planned it now, is that one 'worker instance' can likely only handle a finite number of client sessions (let's say 50,000 for argument sake). Those sessions will be very long lived worker tasks.
The issue I'm trying to get my head around is that I will sometimes need to perform tasks to specific client sessions (eg: If I need to disconnect a client session). With Azure, would I be able to queue up a smaller task that only the instance hosting that specific client session would be able to dequeue?
Right now I'm contemplating GoGrid as my provider, and I solve this issue by using Apache's Active Messaging Queue software. My web app enqueues 'disconnect' tasks that are assigned to a specific instance Id. Each client session is therefore assigned to a specific instance id. The instance then only dequeues 'disconnect' tasks that are assigned to it.
I'm wondering if it's feasible to do something similar on Azure, and how I would generally do it. I like the idea of not having to setup many different VM's to scale, but instead just deploying a single package. Also, it would be nice to make use of Azure's Queues instead of integrating a third party product such as Apache ActiveMQ, or even MSMQ.
I'd be very concerned about building a production application on Azure until the feature set, pricing, and licensing terms are finalized. For starters, you can't even do a cost comparison between it and e.g. GoGrid or EC2 or Mosso. So I don't see how it could possibly end up a front-runner. Also, we know that all of these systems will have glitches as they mature. Amazon's services are in much wider use than any of the others, and have been publicly available for much years. IMHO choosing Azure is a recipe for pain as they stabilize.
Have you considered Amazon's Simple Queue Service for queueing?
I think you can absolutely use Windows Azure for this. My recommendation would be to create a queue for each session you're tracking. Then enqueue the disconnect message (for example) on the queue for that session. The worker instance that's handling that connection should be the only one polling that queue, so it should handle performing the task on that connection.
Regarding the application hosting socket connections for clients to connect to, I'd double-check on what's allowed as I think only HTTP and HTTPS connections are allowed to be made with Azure.