Develop Reference

Asp.Net core distributed caching

I am currently using MemoryCache _cache = new MemoryCache(new MemoryCacheOptions()); for caching some data from database that does not change so often, but it does change.
And on create/update/delete of that data I do the refresh of the cache.
This works fine, but the problem is that on production we will have few nodes, so when method for creating of record is called for instance, cache will be refreshed only on that node, not on other nodes, and they will have stale data.
My question is, can I somehow fix this using MemoryCache, or I need to do something else, and if I do, what are the possible solutions?

I think you are looking for is Distributed Caching
Using the IDistributedCache interface you can use either Redis or Sql Server and it supplies basic Get/Set/Remove methods. Changes made on one node will be available to other nodes.
Using Redis is a great way of sharing Session type data between servers in a load balanced environment, Sql Server does not seem to be a great fit given that you seem to be caching to avoid db calls.
It might also be worth considering if you are actually complicating things by caching in the first place. When you have a single application you see the benefit, as keeping them in application memory saves a request over the network, but when you have a load balanced scenario, you have to compare retrieving those records from a distributed cached vs retrieving them from the database.
If the data is just an in memory copy of a relatively small database table, then there is probably not a lot to choose performance wise between the two. If the data is based on a complicated expensive query then the cache is the way to go.
If you are making hundreds of requests a minute for the data, then any network request may be too much, but you can consider what are the consequences of the data being a little stale? For example, if you update a record, and the new record is not available immediately on every server, does your application break? Or does the change just occur in a more phased way? In that case you could keep your in process memory cache, just use a shorter Time To Live.
If you really need every change to propagate to every node straight away then you could consider using a library like Cache Manager in conjunction with Redis which can combine an in memory cache and synchronisation with a remote cache.

Somewhat dated question, but maybe still useful: I agree with what ste-fu said, well explained.
I'll only add that, on top of CacheManager, you may want to take a look at FusionCache ⚡🦥, which I recently released.
On top of supporting an optional distributed 2nd layer transparently managed for you, it also has some other nice features like an optimization that prevents multiple concurrent factory for the same cache key from being executed (less load on the source database), a fail-safe mechanism and advanced timeouts with background factory completion
If you will give it a chance please let me know what you think.
/shameless-plug

Does the "session per request" pattern take advantage of the cache? ("Session per session" or "Session per request")

I build a new application from scratch this days in a web application.
(The technologies are Asp.Net and the ORM I'm using is Entity Framework. if it matters)
I'm uncertain if the widely used pattern session per request is really a good one.
As I see it, the advantage of the pattern is that the cache isn't increases until the database session crash\ being too big and thus inefficient.
But isn't a new session for every request is too much? It means every server call reset the cache, even simple ajax request like auto-complete has a brand new cache, in fact for every key stroke the cache resets.
The chances you will query the same object-entity-row in one request is small.
Isn't Session per session is a better pattern? it has both the advantages meaning
The cache won't grow for ever.
The cache can actually be used...
So... Why is session per request is so widely used and session per session is not?
Clarifications:
When I wrote ORM session it applies both to NHibernate's session and EntityFramework's DbContext.
I do mean to flush-commit-SaveChanges of the session\dbcontext on each request.

Session per request pattern is more natural and robust for using with ORM. It has smaller chances to get dirty entities and has more predictable resource management.
If I got you right and you mean DbContext instance under Session than Session Per Session can be used only in application without data modification, otherwise you would get unexpected data submitting by a request while other request performs data modification. Also I'm not sure Entity Framework context is thread safe - while processing requests is multithread.
I not totally sure but I think Entity Framework doesn't use cache (== identity mapping) as wide as you expect. On selecting entity set it queries database even if all data are in cache - it can only avoid constructing new entities but using existing ones from identity map.
For caching there are other solutions and they are better.

For me, it all about providing consistency by constraining a unit of work to a single request. I'm not sure how a session per session would work when things go wrong.
For example, what would you do if several requests have been handled and then you get an optimistic concurrency exception on the commit? you could have several merge conflicts at that point.
So a session per request just limits your conflict exposure and makes unit of work on the request scope.

Should I use sessions?

I am designing an online time tracking software to be used internally. I am fairly new to c# and .NET though I have extensive PHP experience.
I am using Windows Forms Authentication, and once the user logs in using that, I create a Timesheet object (my own custom class).
As part of this class, I have a constructor that checks the SQL DB for information (recent entries by this user, user preferences, etc.)
Should I be storing this information in a session? And then checking the session object in the constructor first? That seems the obvious approach, but most examples I've looked at don't make much use of sessions. Is there something I don't know that others do (specifically related to .NET sessions of course)?
EDIT:
I forgot to mention two things. 1. My SQL DB is on another server (though I believe they are both on the same network, so not much of an issue)2. There are certain constants that the user will not be able to change (only the admin can modify them) such as project tasks. These are used on every page, but loaded the first time from the DB. Should I be storing these in a session? If not, where else? The only other way I can think of is a local flat file that updates each time the table of projects is updated, but that seems like a hack solution. Am I trying too hard to minimize calls to the DB?

There is a good overview on ASP.NET Session here: ASP.NET Session State.
If you don't have thousands of clients, but need "some state" stored server-side, this is very easy to use and works well. It can also be stored in the database in multi server scenarios, without changing a line in your code, just by configuration.
My advise would be not to store "big", or full object hierarchies in there, as storing in a session (if the session is shared among servers in a web farm in a database for example) can be somewhat costy. If you plan to have only one server, this is not really a problem, but you have to know that you won't be able to easily move to a multiple server mode easily.
The worst thing to do is follow the guys who just say "session is bad, whooooo!", don't use it, and eventually rewrite your own system. If you need it, use it :-)

I would shy away from session objects. And actually I would say look into .net MVC as well.
The reason I don't use the session is because I feel it can be a crutch for some developers.
I would save all of the information that you would have put into a session into a db. This will allow for better metrics tracking, support for Azure (off topic but worth mentioning) and is cleaner imo.

ASP developers know session state as a great feature, but one that is somewhat limited. These limitations include:
ASP session state exists in the process that hosts ASP; thus the actions that affect the process also affect session state. When the process is recycled or fails, session state is lost.
Server farm limitations. As users move from server to server in a Web server farm, their session state does not follow them. ASP session state is machine specific. Each ASP server provides its own session state, and unless the user returns to the same server, the session state is inaccessible. http://msdn.microsoft.com/en-us/library/ms972429.aspx

One of the main problems with Session is, that by default, it is stored in memory. If you have many concurrent users that store data in the session this could easily lead to performance problems.
Another thing is that application recycle will empty your in memory session which could lead to errors.
Off course you can move your session to SqlServer or a StateServer but then you will lose on performance.

Look into the HttpContext.User (IPrincipal) property. this is where user information is stored in the request.

Most people avoid session state simply because people like to avoid state in general. If you can find an algorithm or process which works all the time regardless of the previous state of an object, that process tends to be more fool proof against future maintenance and more easily testable.
I would say for this particular case, store your values in the database and read them from there any time you need that information. Once you have that working, take a look at the performance of the site. If it's performing fine then leave it alone (as this is the simplest case to program). If performance is an issue, look at using the IIS Cache (instead of session) or implementing a system like CQRS.

Session State Disadvantage
Session-state variables stay in memory until they are either removed or replaced, and therefore can degrade server performance. Session-state variables that contain blocks of information, such as large datasets, can adversely affect Web-server performance as server load increases. Think what will happen if you significant amount of users simultaneously online.
NOTE :- I haven't mentioned the advantages because they are straightforward which are : Simple implementation, Session-specific events, Data persistence, Cookieless support etc.

The core problem with sessions are scaleability. If you have a small application, with a small number of users, that will only ever be on one server, then it may be a good route for you to save small amounts of data - maybe just the user id - to allow quick access to the preferences etc.
If you MAY want multiple web servers, or the application MAY grow, then don't use session. And only use it for small pieces of information.

ASP.NET Session - Use or not use and best practices for an e-commerce app

I have used ASP.NET in mostly intranet scenarios and pretty familiar with it but for something such as shopping cart or similar session data there are various possibilities. To name a few:
1) State-Server session
2) SQL Server session
3) Custom database session
4) Cookie
What have you used and what our your success or lessons learnt stories and what would you recommend? This would obviously make a difference in a large-scale public website so please comment on your experiences.
I have not mentioned in-proc since in a large-scale app this has no place.
Many thanks
Ali

The biggest lesson I learned was one I already knew in theory, but got to see in practice.
Removing all use of sessions entirely from an application (does not necessarily mean all of the site) is something we all know should bring a big improvement to scalability.
What I learnt was just how much of an improvement it could be. By removing the use of sessions, and adding some code to handle what had been handled by them before (which at each individual point was a performance lose, as each individual point was now doing more work than it had before) the performance gain was massive to the point of making actions one would measure in many seconds or even a couple of minutes become sub-second, CPU usage became a fraction of what it had been, and the number of machines and amount of RAM went from clearly not enough to cope, to be a rather over-indulgent amount of hardware.
If sessions cannot be removed entirely (people don't like the way browsers use HTTP authentication, alas), moving much of it into a few well-defined spots, ideally in a separate application on the server, can have a bigger effect that which session-storage method is used.

In-proc certainly can have a place in a large-scale application; it just requires sticky sessions at the load balancing level. In fact, the reduced maintenance cost and infrastructure overhead by using in-proc sessions can be considerable. Any enterprise-grade content switch you'd be using in front of your farm would certainly offer such functionality, and it's hard to argue for the cash and manpower of purchasing/configuring/integrating state servers versus just flipping a switch. I am using this in quite large scaled ASP.NET systems with no issues to speak of. RAM is far too cheap to ignore this as an option.

In-proc session (at least when using IIS6) can recycle at any time and is therefore not very reliable because the sessions will end when the server decides, not when the session actually times out. The sessions will also expire when you deploy a new version of the web site, which is not true of server-based session providers. This can potentially give your users a bad experience if you update in the middle of their session.
Using a Sql Server is the best option because it is possible to have sessions that never expire. However, the cost of the server, disk space, its maintenance, and peformance all have to be considered. I was using one on my E-commerce app for several years until we changed providers to one with very little database space. It was a shame that it had to go.
We have been using the state service for about 3 years now and haven't had any issues. That said, we now have the session timeout set at an hour an in E-commerce that is probably costing us some business vs the never expire model.
When I worked for a large company, we used a clustered SQL Server in another application that was more critical to remain online. We had multiple redundency on every part of the system including the network cards. Keep in mind that adding a state server or service is adding a potential single point of failure for the application unless you go the clustered route, which is more expensive to maintain.
There was also an issue when we first switched to the SQL based approach where binary objects couldn't be serialized into session state. I only had a few and modified the code so it wouldn't need the binary serialization so I could get the site online. However, when I went back to fix the serialization issue a few weeks later, it suddenly didn't exist anymore. I am guessing it was fixed in a Windows Update.

If you are concerned about security, state server is a no-no. State server performs absolutely no access checks, anybody who is granted access to the tcp port state server uses can access or modify any session state.
In proc is unreliable (and you mentioned that) so that's not to consider.
Cookies isn't really a session state replacement since you can't store much data there
I vote for a database based storage (if needed at all) of some kind, it has the best possibility to scale.

What is the most cost-effective way to break up a centralised database?

Following on from this question...
What to do when you’ve really screwed up the design of a distributed system?
... the client has reluctantly asked me to quote for option 3 (the expensive one), so they can compare prices to a company in India.
So, they want me to quote (hmm). In order for me to get this as accurate as possible, I will need to decide how I'm actually going to do it. Here's 3 scenarios...
Scenarios
Split the database
My original idea (perhaps the most tricky) will yield the best speed on both the website and the desktop application. However, it may require some synchronising between the two databases as the two "systems" so heavily connected. If not done properly and not tested thouroughly, I've learnt that synchronisation can be hell on earth.
Implement caching on the smallest system
To side-step the sync option (which I'm not fond of), I figured it may be more productive (and cheaper) to move the entire central database and web service to their office (i.e. in-house), and have the website (still on the hosted server) download data from the central office and store it in a small database (acting as a cache)...
Set up a new server in the customer's office (in-house).
Move the central database and web service to the new in-house server.
Keep the web site on the hosted server, but alter the web service URL so that it points to the office server.
Implement a simple cache system for images and most frequently accessed data (such as product information).
... the down-side is that when the end-user in the office updates something, their customers will effectively be downloading the data from a 60KB/s upload connection (albeit once, as it will be cached).
Also, not all data can be cached, for example when a customer updates their order. Also, connection redundancy becomes a huge factor here; what if the office connection is offline? Nothing to do but show an error message to the customers, which is nasty, but a necessary evil.
Mystery option number 3
Suggestions welcome!
SQL replication
I had considered MSSQL replication. But I have no experience with it, so I'm worried about how conflicts are handled, etc. Is this an option? Considering there are physical files involved, and so on. Also, I believe we'd need to upgrade from SQL express to SQL non-free, and buy two licenses.
Technical
Components
ASP.Net website
ASP.net web service
.Net desktop application
MSSQL 2008 express database
Connections
Office connection: 8 mbit down and 1 mbit up contended line (50:1)
Hosted virtual server: Windows 2008 with 10 megabit line

Having just read for the first time your original question related to this I'd say that you may have laid the foundation for resolving the problem simply because you are communicating with the database by a web service.
This web service may well be the saving grace as it allows you to split the communications without affecting the client.
A good while back I was involved in designing just such a system.
The first thing that we identified was that data which rarely changes - and immediately locked all of this out of consideration for distribution. A manual process for administering using the web server was the only way to change this data.
The second thing we identified was that data that should be owned locally. By this I mean data that only one person or location at a time would need to update; but that may need to be viewed at other locations. We fixed all of the keys on the related tables to ensure that duplication could never occur and that no auto-incrementing fields were used.
The third item was the tables that were truly shared - and although we worried a lot about these during stages 1 & 2 - in our case this part was straight-forwards.
When I'm talking about a server here I mean a DB Server with a set of web services that communicate between themselves.
As designed our architecture had 1 designated 'master' server. This was the definitive for resolving conflicts.
The rest of the servers were in the first instance a large cache of anything covered by item1. In fact it wasn't a large cache but a database duplication but you get the idea.
The second function of the each non-master server was to coordinate changes with the master. This involved a very simplistic process of actually passing through most of the work transparently to the master server.
We spent a lot of time designing and optimising all of the above - to finally discover that the single best performance improvement came from simply compressing the web service requests to reduce bandwidth (but it was over a single channel ISDN, which probably made the most difference).
The fact is that if you do have a web service then this will give you greater flexibility about how you implement this.
I'd probably start by investigating the feasability of implementing one of the SQL server replication methods
Usual disclaimers apply:

Splitting the database will not help a lot but it'll add a lot of nightmare. IMO, you should first try to optimize the database, update some indexes or may be add several more, optimize some queries and so on. For database performance tuning I recommend to read some articles from simple-talk.com.
Also in order to save bandwidth you can add bulk processing to your windows client and also add zipping (archiving) to your web service.
And probably you should upgrade to MS SQL 2008 Express, it's also free.
It's hard to recommend a good solution for your problem using the information I have. It's not clear where is the bottleneck. I strongly recommend you to profile your application to find exact place of the bottleneck (e.g. is it in the database or in fully used up channel and so on) and add a description of it to the question.
EDIT 01/03:
When the bottleneck is an up connection then you can do only the following:
1. Add archiving of messages to service and client
2. Implement bulk operations and use them
3. Try to reduce operations count per user case for the most frequent cases
4. Add a local database for windows clients and perform all operations using it and synchronize the local db and the main one on some timer.
And sql replication will not help you a lot in this case. The most fastest and cheapest solution is to increase up connection because all other ways (except the first one) will take a lot of time.
If you choose to rewrite the service to support bulking I recommend you to have a look at Agatha Project

Actually hearing how many they have on that one connection it may be time to up the bandwidth at the office (not at all my normal response) If you factor out the CRM system what else is a top user of the bandwidth? It maybe the they have reached the point of needing more bandwidth period.
But I am still curious to see how much information you are passing that is getting used. Make sure you are transferring efferently any chance you could add some easy quick measures to see how much people are actually consuming when looking at the data.