I have a Blazor Server App that is running its frontend as a Scoped process and that uses a memory cache to handle objects across requests. Within the scoped process, it is obviously easy enough to use a ReleaseWriterLockSlim object to prevent multi-threading issues, however I need to do so across numerous instances.
I have read through several posts focused on this issue, but none appear to work as I require and I wondered if I could just create a class, expose a ReleaseWriterLockSlim as a public readonly object and then register that class as a singleton.
Shouldn't this provide the same effect no matter where it is called? I feel that I must be missing something that will cause massive issues.
Thanks!
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I am new to WCF & Service development and have a following question.
I want to write a service which relies on some data (from database for example) in order to process client requests and reply back.
I do not want to look in database for every single call. My question is, is there any technique or way so that I can load such data either upfront or just once, so that it need not go to fetch this data for every request?
I read that having InstanceContextMode to Single can be a bad idea (not exactly sure why). Can somebody explain what is the best way to deal with such situation.
Thanks
The BCL has a Lazy class that is made for this purpose. Unfortunately, in case of a transient exception (network issue, timeout, ...) it stores the exception forever. This means that your service is down forever if that happens. That's unacceptable. The Lazy class is therefore unusable. Microsoft has declared that they are unwilling to fix this.
The best way to deal with this is to write your own lazy or use something equivalent.
You also can use LazyInitializer. See the documentation.
I don't know how instance mode Single behaves in case of an exception. In any case it is architecturally unwise to put lazy resources into the service class. If you want to share those resources with multiple services that's a problem. It's also not the responsibility of the service class to do that.
It all depends on amount of data to load and the pattern of data usage.
Assuming that your service calls are independent and may require different portions of data, then you may implement some caching (using Lazy<T> or similar techniques). But this solution has one important caveat: once data is loaded into the cache it will be there forever unless you define some expiration strategy (time-based or flush on write or something else). If you do not have cache entry expiration strategy your service will consume more and more memory over time.
This may not be too important problem, though, if amount of data you load from the database is small or majority of calls access same data again and again.
Another approach is to use WCF sessions (set InstanceContextMode to PerSession). This will ensure that you have service object created for lifetime of a session (which will be alive while particular WCF client is connected) - and all calls from that client will be dispatched to the same service object. It may or may not be appropriate from business domain point of view. And if this is appropriate, then you can load your data from the database on a first call and then subsequent calls within same session will be able to reuse the data. New session (another client or same client after reconnect) will have to load data again.
I'm currently investigating some code which has a cache layer which at the bottom level uses the MemoryCache class. This is a c# Windows Service app so not web/IIS. There is a section of the code which spawns off a number of threads in which it creates and executes some code in a POC to perform some calculations. These are then stored in the above mentioned cache layer. What is been seen is it looks like cached values seem to be getting stored per thread and not at apication level. I thought that MemoryCache was a Singleton that would sit out side of the individual threads.
Can anybody confirm this behaviour would be expected?
Many thanks for any comments.
A MemoryCache is thread-safe but there is no reason to assume it's a singleton. If you want different threads to access the same MemoryCache instance you need to give them all a reference to the same instance (either as a singleton (really bad) static (still bad) or through argument passing as dependency injection (good)).
The simple way to do it (which does use global state) is to access the default memory cache:
var cache = MemoryCache.Default; // not really a good idea and harder to test, works
You can find the specific docs here. Make sure to configure it in your app/web.config file.
I know several topics on the subject have been discussed, because I have been reading a lot to try to resolve my issue, but somehow they happen to not fulfill my needs (maybe for the lack of detail). Anyway, if you think some specific 'topic' might be useful, please link it.
I'm developing a desktop application with WPF (and MVVM) and I'm using NHibernate. After researching about possible ways to manage my session, I have decided to use the session-per-form approach. By this way, I think I can fully use the features of NHibernate like lazy-loading, cache and so on.
As I'm working with a database, I don't want to freeze my UI while I'm loading or saving my entities, so I thought I should use a dedicated thread (in each form, which I think simplifies the development) to handle the database interaction. The problem, though, is how I should 'reuse' the thread (supposing I have a session associated with that thread) to make my 'database calls'.
I think I couldn't use TPL because I'm not guaranteed that the two tasks would run in the same thread (it's not even guaranteed that they will be run in different threads than the invoker)
I would prefer to use session-per-form, as I have seen similar discussions that end by using session-per-conversation or something like that. But anyway, if you find that session-per-conversation would be better, please tell me (and hopefully explain why)
Threads don't provide a way to directly run more than one method, so I think I would have to 'listen' for requests, but I'm still unsure if I really have to do this and how I would 'use' the session (and save it) only inside the thread.
EDIT:
Maybe I'm having this problem because I'm confusing thread-safety with something else.
When the NHibernate documentation says that ISession instances are not thread-safe, does it means that I will (or could) get into trouble if two threads attempt to use it at the same time, right? In my case, if I use TPL, different threads could use the same session, but I wouldn't perform more than one operation in the same session at the same time. So, would I get into trouble in that situation?
If I may make a suggestion, desktop applications are poorly suited to interact with the database directly. The communication is not encrypted and it's really easy for someone with even the slightest amount of know-how to grab the database password and begin messing with records using a SQL connection and corrupt your database.
It would be better to create a web service with authentication that stands between the desktop application and the database as you could create credentials for each person and every transaction would be forcibly subjected to your various business rules.
This would also take care of your threading issue as you would be able to create HTTP connections on another thread with little to no trouble concerning session management. A cookie value is likely all that would be required and RestSharp makes this fairly trivial.
I have an application that use a static class too large and complex for this reason can not use the standard Asp.net Session. More telho problems with the stability of my application because when the pool closed by an error in a estarna dlls, all static variables are discharged.
I wonder if there is a setting for each "client" open a pool. So if a User does not fall knocks others.
If you have a static class there is only one of that class for the application pool. If this class has something different for each user it shouldn't be static. If the class only contains general information not pertaining to a specific session and you don't want to make it an instance class then make sure there are no exceptions being thrown in the static class' constructor.
In addition to YetAnotherSoftwareDeveloper's answer, application pooling is used to provide a mechanism that can be used to isolate applications for stability and security reasons, not to isolate individual client sessions.
If you have an application that is unstable, you can keep it from having a detrimental effect on other applications by isolating it in its own application pool. This will not provide any stabilizing effect on the application having problems, but will keep it from crashing other applications on the same server.
Context: I have a set of View/Presenters and I've noticed that for complex views I get some performance issues at the time of the InitializeComponent() call
Is there any way to instruct the spring container to pre-instantiate objects scoped as prototype? Something similar to a queue with the objects ready when the application requests them?
We had exactly the same problem. We also found that this performance overhead occured only the first time we requested a form from the container. We didn't find a clean solution, so we decided to write an initialization routine that runs in the background and requests all objects of type Form from the container. When this routine is finished, all forms open quickly.
Looking forward to a better sution, but this worked for us. Main disadvantage of this workaround is, that during the initialization routine, users might still experience some slow loading forms.