问题
I'm new to NHibernate and am having difficulties setting it up for my current website. This website will run on multiple webservers with one database server, and this leaves me facing some concurrency issues. The website will have an estimated 50.000 users or so registered, and each user will have a profile page. On this page, other users can 'like' another user, much like Facebook. This is were the concurrency problem kicks in.
I was thinking of using second-level cache, most likely using the MemChached provider since I'll have multiple webservers. What is the best way to implement such a 'Like' feature using NHibernate? I was thinking of three options:
- Use a simple Count() query. There will be a table 'User_Likes' where each row would represent a like from one user to another. To display the number the number of likes, I would simply ask the number of Likes for a user, which would be translated to the database as a simple
SELECT COUNT(*) FROM USER_LIKES WHERE ID = xor something. However, I gather this would be come with a great performance penalty as everytime a user would visit a profile page and like another user, the number of likes would have to be recalculated, second-level cache or not. - Use an additional
NumberOfLikescolumn in the User table and increment / decrement this value when a user likes or dislikes another user. This however gives me concurrency issues. Using a simple for-loop, I tested it by liking a user 1000 times on two servers and the result in the db was around 1100 likes total. That's a difference of 900. Whether a realistic test or not, this is of course not an option. Now, I looked at optimistic and pessimistic locking as a solution (is it?) but my current Repository pattern is, at the moment, not suited to use this I'm afraid, so before I fix that, I'd like to know if this is the right way to go. - Like 2, but using custom HQL and write the update statement myself, something along the lines of
UPDATE User SET NumberOfLikes = NumberOfLikes + 1 WHERE id = x. This won't give me any concurrency issues in the database right? However, I'm not sure if I'll have any datamismatch on my multiple servers due to the second level caching.
So... I really need some advice here. Is there another option? This feels like a common situation and surely NHibernate must support this in an elegant manner. I'm new to NHIbernate so a clear, detailed reply is both necessary and appreciated :-) Thanks!
回答1:
I suspect you will see this issue in more locations. You could solve this specific issue with 3., but that leaves other locations where you're going to encounter concurrency issues.
What I would advise is to implement pessimistic locking. The usual way to do this is to just apply a transaction to the entire HTTP request. With the BeginRequest in your Global.asax, you start a session and transaction. Then, in the EndRequest you commit it. With the Error event, you go the alternative path of doing a rollback and discarding the session.
This is quite an accepted manner of applying NHibernate. See for example http://dotnetslackers.com/articles/aspnet/Configuring-NHibernate-with-ASP-NET.aspx.
回答2:
I'd go with 3. I believe this in this kind of application it's not so critical if some pages show a slightly outdated value for a while.
IIRC, HQL updates do not invalidate the entity cache entry, so you might have to do it manually.
来源：https://stackoverflow.com/questions/4246051/nhibernate-counters-with-concurrency-and-second-level-caching