Question

In an app, Users and Cases have a many-to-many relationship. Users pull their list of Cases often, Users can update a single case at a time (a 1-10 second operation, requiring more than one UPDATE). Under READCOMMITTED, any in-use Case would block all associated Users from pulling their list of Cases. Also, the most recent data is a hotspot for both reads and writes to the Cases table.

I think I want to employ dirty reads to keep the experience snappy. READPAST on Cases won’t work for this purpose. NOLOCK will work, but I’d like to be able to show which records are dirty when they are listed.

I don’t know of any native way to show which records are dirty, so I’m thinking that for each update or insert to Cases, an INUSE flag will be set. This flag must be cleared by the end of the updating transaction such that under READCOMMITTED, this flag will never appear to be set. Note that this is NOT to replace concurrency management, only to show which records are potentially dirty to the User.

My question is whether this is reliable – if we UPDATE two or more fields (INUSE plus the other fields) in a single statement, is it possible that a concurrent NOLOCK query would read some of the new values but not others? If so, is it possible to guarantee that INUSE be set first?

And if I’m thinking about this all wrong, please enlighten me. My ideal situation would be to, in a manageable way, be able to show the values as they were PRIOR to any related transaction so the data is immediately available and always consistent (but partially out-dated). But I don’t think this is available – especially in the more complex actual database.

Thanks!

Answer 1

Have you considered using SNAPSHOT isolation level? When used for a query, it requires no locks whatsoever, and it gives precisely the semantics that you’re asking for:

show the values as they were PRIOR to any related transaction so the data is immediately available and always consistent (but partially out-dated)