In ASP.NET, I usually log exceptions at server-side, In windows forms I can either log exceptions server-side or write to a log file on the client. Silverlight seems to fit somewhere in between.
I wanted to know what everyone else is doing to handle their Silverlight exceptions and I was curious if any best practices have emerged for this yet.
For real logging that you could store & track, you will need to do it on the server, since you can't be guaranteed anything on the client will be persisted.
I would suggest exposing a "LogEvent(..)" method on a server side web service (maybe you already have one) which would then do the same kind of logging you do in ASP.net
Here's a video about basic web service calls in Silverlight if you haven't done that yet
http://silverlight.net/learn/learnvideo.aspx?video=66723
I'm not sure about any logging best practices though, my first guess would be to do the best practicies for logging in a web sevice on the server and expose that to the client.
Hope this helps!
I would say that Silverlight fits much better to ASP.NET side of the model. You have server which serves web page. An object (Silverlight app) on the page pings data service to fetch data and display it.
All data access happens on the server side and it does not matter if data is used to create ASP.NET pages on the server or sent raw to the RIA for display. I do log any failures in data service on server side (event log works fine) and do not allow any exception to pass to WCF. When client does not receive expected data (it gets null collection or something similar), it display generic data access error to the user. We may need to extend that soon to pass a bit more information (distinguishing between access denied/missing database/infrastructure failure/internal error/etc), but we do not plan to pass exception error messages to the client.
As for client side, sometimes we may get in situation where async call times out -- it is just another message. For general exceptions from client code (typically, bugs in our code), I just pass exception to the browser to display in same manner as any script exception.
Also take a look at the new Silverlight Integration Pack for Enterprise Library from Microsoft patterns & practices. It provides support for logging exceptions to isolated storage or remote services and is configurable via policies in external config or programmatically. Batch logging and automatic retry (in case of occasionally connected scenarios) are also supported.
Use the Isolated Storage available for Silverlight application. You should store here your log.
Then you can develop a mecanism to send the user log to a webservice like the Windows bug report service.
It very much depends on the type of application that youre developing.
if its an mvc / mvp based architecture then your model, or most of it at least, will be on the server, and this is where most of your exceptions will be thrown i would imagine, so you can log them there and choose to display a message to the user or not.
for exceptions from the client you may want to know the details so just send them back.
Related
What is the correct way of handling errors on the client side of Silverlight applications? I tried building a service endpoint that would receive details about the error and then would write that string to the database. The problem is, the error's text exceeds the maximum byte length, so I can't send the exception message and stacktrace. What would be a better way of handling errors that end up at the client side?
Try handling faults...I used this pattern from MSDN
http://msdn.microsoft.com/en-us/library/dd470096%28VS.96%29.aspx
If you find you message is too long to send to your logging web service then try setting your binding properties such as maxBufferSize and maxStringContentLength to appropriately large values. They default to 16KB, personally i have set mine to 2147483647 (which is int.MaxValue).
Obviously you cannot send the raw exception straight to the logging web service (exceptions are not serializable), what i did was write a function that takes an exception and walks it, translating it into a WCF friendly structure that can then be passed to my logging end point. Of course you need to ensure that if this fails you have a backup plan, like maybe logging it to isolated storage if you are running in browser, or logging it to the user's file system if you are running elevated OOB.
You should not be considering logging of error messages via a service. What if the error that you want to log is related to the service itself? Maybe the server that hosts all dependant services (including the error logging service) is not reachable or down. client errors should be logged on the client side and periodically flushed to the server when connectivity to service is available.
Thats what I would do...
Take a look at the new Silverlight Integration Pack for Enterprise Library from Microsoft patterns & practices. It provides plumbing for both logging (client-side and via a remote service) and exception handling with flexible configuration of policies via config or programmatically.
Ok,
I have developed this WinForms client, which interacts with a server (ASPX Application) by means of WCF calls. I would now like to deploy the server to my shared webhost, but I'm kinda new to WCF and especially the security possibilities behind it.
The goal is to kind of secure the WCF service, so that not everybody that knows or finds out the endpoint address can call it. Rather, only my WinForms client must be able to call the WCF service.
I do not need authentication on a user basis, so no authentication is required from the user of the client. But I want only instances of this WinForms client to be able to interact with the service. The information passed between server and client is not very sensitive, so it's not really required to secure it, but it's a plus if it can easily be done.
Is this possible with a Shared Host (IIS) environment (no HTTPS at disposal) ? What bindings and options should I use ? I suppose wsHttpBinding, but how would I setup the security options ?
Using .NET 4.0
Thanks
From what I understand, you have an internet-facing service which you want to limit to only your client app to be able to call - correct? Or do you envision other clients (like PHP, Ruby etc.) also wanting to call into your service at some point?
To secure your message, you have two options in WCF - message or transport security. Over the internet, with an unknown number of hops between your client and your service, transport security doesn't work - you're left with message security (encrypting the message as it travels across the 'net). For this to work, you typically add a digital certificate to your service (only server-side) that the client can discover and use to encrypt the messages with. Only your service will be able to decrypt them - so you're safe on that end.
The next point is: who can call your service? If you want to be totally open to anyone, then yes, you need wsHttpBinding (or the RESTful variant - webHttpBinding). If you want to allow non-.NET clients, you're typically limited to no authentication (anyone can call), or username/password schemes which you will validate on the server side against a database of valid users.
If you only want to allow your own .NET client in, then you can do several things:
disable metadata on your service; with this, you would "hide" your endpoints and the services they provide - someone using a "metadata scanner" (if that exists) wouldn't be able to just stumble across your service and find out what methods it provides etc. This however also makes it impossible for another developer outside your organization to do an Add Service Reference to your service.
you could define and use a custom binary http binding - only other clients with this setup could even call your service. The binary http binding would bring some speed improvements, too. See this blog post on how to do this.
you need to somehow identify those callers that are allowed in - one possible method would be to put an extra header into your WCF messages that you then check for on the server side. This would simply make sure that a casual hacker who discovers your service and figures out the binary http binding would still be rejected (at least for some time). See this blog post here on how to implement such a message inspector extension for WCF.
the ultimate step would be to install a digital certificate on the client machine along with your service. You would then set up your client side proxy to authenticate with the service using that certificate. Only client machine that have that certificate could then call into your service.
It really depends on how far you want to go - WCF gives you a lot of options, but you need to decide how much effort you want to put into that .
The first thing you need to ask your self is: "What can someone do to your WCF service if they connected their own customized client?" Look at all of the functionality that is being exposed via WCF and assume that it could be accessed at will. You have absolutely no control over the client, and you will never have this ability.
HTTPS is beautiful, its a damn shame that your forced to be vulnerable to OWASP A9: Insufficient Transport Layer Protection. If it where up to me, I would move to a different host that cared about security. If you are throwing usernames and passwords over the network, then your putting your users in danger.
One of the biggest problems I have seen with a WCF service is that they had a "executeQuery()" function that was exposed. The developer allowing the client to build queries to be executed by the server. This approach is fundamentally flawed as you are just handing your database over to an attacker. This type of vulnerability isn't SQL Injection, it falls under CWE-602: Client-Side Enforcement of Server-Side Security.
Along the same lines as CWE-602 is OWASP A4: Insecure Direct Object References. Could an attacker fool your WCF service into thinking its another user by providing a different user id? Are you trusting the client to tell the truth?
The next classification of vulnerabilities that you must take into consideration is OWASP A1: Injection, other wise known as "Taint and Sink". For instance if you are exposing a function where one of its parameters is being used in a CreateProcess() which is invoking cmd.exe. This parameter could be controlled by the attacker, and there for this variable is "tainted", the call to CreateProcess() is a "sink". There are many types of vulnerabilities along these lines, including but not limited to; SQL Injection, LDAP Injection, XPATH Injection. These types of vulnerabilities affect all web applications.
The scenario:
I have a PRISM application developed in Silverlight (4), and I'm using a ASP.NET server side application to host several web-services (which, in turn, accesses WCF-services, but that's not really important here). The Silverlight application must be able to call the web services cross-domain (meaning that the web services isn't necessarily on the same server hosting the silverlight application).
The Silverlight application consists of several modules, each accessing the ASP.NET web-services.
I do not have much experience with Silverlight and PRISM, but as far as I can see, this is not a very unusual scenario...
The problem:
My challange is, that when 2 different modules access the web-services, I get 2 new sessions on the web-server. I would have thought that since both modules live on the same HTML-page (and then also in the same browser session), they would get the same session on the web-server...?
I have tried to make the web-service Proxy-client globally available in the container (using Unity), by registering an instance (using Container.RegisterInstance), and then getting this instance whenever a module needs to make a web-service call (using Container.Resolve), but this doesn't seem to help.
However, any calls made within the same module always gets the same session on the server.
Can anyone see what I'm missing here...?
Thanks!
Jon
Looks like I found my own answer.
The problem was that my application was firing multiple web-service calls upon startup (the different PRISM-modules working independently). And when several calls were made before any responses was given from the web-server, no session (and hence no "ASP.NET_SessionId" cookie was provided) back to the client before subsequent calls were made.
My solution was to make sure I make one call (async as always), for example to a simple Ping-like web-service, then hold all other calls to the web-server until this response is back. Then, all subsequent calls are given the same Session on the server (because now they all contain the "ASP.NET_SessionId" cookie in the header).
In pratice, this call is made by the PRISM-shell, and no modules are beeing loaded before I receive the reponse. Then I'm absolutely sure that none of the other modules get trigger-happy before I have a session-state on hand.
Still, if anybody sees any other problems with this solution, I'm more than happy to hear from you.
Thanks!
Jon
I have one very weird question.
There are 2 Silverlight Client
1. Admin
2. User
Now, I want a scenario wherein the Admin Silverlight can initiate a function call on the User Silverlight.
Pretty much a newbie with SL so wonder if that would be possible.
I'd appreciate any help.
Thanks
I suppose the applications are not in the same browser / machine, and when you describe the usage pattern as admin and user, I take that there are probably more users than admins.
You might want to take a look at duplex bindings for WCF services - this is a web service binding that allows pushing notifications to clients from the server. When all clients establish such a channel, you can implement hub-and-spoke communication between clients.
This blog post gives a good receipt for getting started:
http://silverlightforbusiness.net/2009/06/23/pushing-data-from-the-server-to-silverlight-3-using-a-duplex-wcf-service/
If they are both in the same frame/browser, you could call JavaScript in the first using the HtmlPage API, which could interact with the second.
So:
Silverlight control -> injects JS into HtmlPage -> JS interacts with Silverlight control 2 (assuming this is possible, please correct me if wrong) -> Silverlight control responds.
If they are in separate windows or running "out of browser", I would expect it wouldn't work.
If the 2 instances are seperated (i.e., the admin is on one machine and the user is on another) there's no direct way to do it. However, you can rig it up with a publisher/subscriber style system.
Assumption: You have some sort of shared data store between the two, maybe a database or something.
Idea: You have the admin client write a request to this shared data store; an entry in a table, or a new file in a network share, or something. You have the user client app regularly scan this table/share for new entries, say every .5 seconds or so. When it sees the entry, it executes the requested operation, storing any return values back to the shared store. When the admin sees the return value, he knows the operation has been successfully executed.
There are a couple of options that I can think of.
You could implement some sort of remote procedure call via web services whereby one Silverlight app posts a request to call the method, and the other Silverlight regularly checks for method call requests.
If hosted on the same HTML page in a browser, you could use javascript to allow the two controls to interact.
However, direct communication between two Silverlight instances isn't supported, and while the suggestions may help to achieve something close to what you want, they don't provide a complete solution that will work in all scenarios.
I'm working on building a Silverlight application whereas we want to be able to have a client hit a url like:
http://{client}.domain.com/
and login, where the {client} part is their business name. so for example, google's would be:
http://google.domain.com/
What I was wondering was if anyone has been able, in silverlight, to be able to use this subdomain model to make decisions on the call to the web server so that you can switch to a specific database to run a query? Unfortunately, it's something that is quite necessary for the project, as we are trying to make it easy for their employees to get their company specific information for our software.
Wouldn't it work to put the service on a specific subdomain itself, such as wcf.example.com, and then setup a cross domain policy file on the service to allow it to access it?
As long as this would work you could just load the silverlight in the proper subdomain and then pass that subdomain to your service and let it do its thing.
Some examples of this below:
Silverlight Cross Domain Services
Silverlight Cross Domain Policy Helpers
On the server side you can check the HTTP 1.1 Host header to see how the user came to your server and do the necessary customization based on that.
I think you cannot do this with Silverlight alone, I know you cannot do this without problems with Javascript, Ajax etc. . That is because a sub domain is - for security reasons - treated otherwise than a sub-page by the browsers.
What about the following idea: Insert a rewrite rule to your web server software. So if http://google.domain.com is called, the web server itself rewrites the URL to something like http://www.domain.com/google/ (or better: http://www.domain.com/customers/google/). Would that help?
Georgi:
That would help if it would be static, but alas, it's going to all be dynamic. My hope was to have 1x deployment for the application, and to use the http://google.domain.com/ idea to switch to the correct database for the user. I recall doing this once when we built an asp.net website, using the domain context to figure out what skin to use, etc.
Ates: Can you explain more about what you are saying... sounds like you are close to what I am trying to come up with. Have you seen such a tutorial for this?
The only other way I have come up with to make this work is to have a metabase that when the user logs in, it will switch them to the appropriate database as required... was just thinking as well that telling Client x to hit:
http://ClientX.domain.com/ would have been sweeter than saying to hit http://www.domain.com/ and login. It seemed as if they were to hit their name, and to show it personalized for them right from the login screen would have been much more appealing for the client base.
#Richard B: No, I can't think of any such tutorial that I've seen before. I'll try to be more verbose.
The server-side approach in more detail:
Direct *.example.com to the same IP in your DNS settings.
The backend app that handles login checks the Host HTTP header (e.g. the "HTTP_HOST" server variable in some platforms). That would contain the exact subdomain.example.com that the client used for reaching your server. Extract the subdomain part and continue...
There can also be a client-side-only approach. I don't know much about Silverlight but I'm assuming that you should be able to interface Silverlight with JavaScript. You could read document.location with JavaScript and pass it to your Silverlight applet, whereon further data fetching etc. logic would rely on the subdomain that was passed in by JavaScript.
#Ates:
That is what we did when we wrote the ASP.Net system... we pushed a slew of *.example.com hosts against the web server, and handled using the HTTP headers. The hold-up comes when dealing with WCF pushing the info between the client and the server... it can only exist in one domain...
So, for example, when you have {client}.example.com and {sandbox}.example.com, the WCF service can't be registered to both. It also cannot be registered to just *.example.com or example.com, so that's where the catch 22 is coming in at. everything else I have the prior knowledge of handling.
I recall a method by which an application can "spoof" another domain name in certain instances. I take it in this case, I would need to do such a configuration? Much to research yet I believe.