NHibernate Transactions on Reads

Question

I have read the documentation and explanation on why it is highly recommended to use transactions on read operations in NH. However, I still haven't totally "bought" into it yet. Can someone take a stab at explaining it without just telling me to RTFM, which I have already done? ;)

Answer 1

This post from one of the authors might have your answer:

Even if we are only reading data, we want to use a transaction, because using a transaction ensure that we get a consistent result from the database. NHibernate assume that all access to the database is done under a transaction, and strongly discourage any use of the session without a transaction.

Leaving aside the safety issue of working with transactions, the assumption that transactions are costly and we need to optimize them is a false one. As already mentioned, databases are always running in transaction. And databases have been heavily optimized to work with transactions. The question is whatever this is per statement or per batch. There is some amount of work that need to be done to create and dispose a transaction, and having to do it per statement is actually more costly than doing it per batch.

Answer 2

What the others have said is true, but they've not pointed out that the problem with not controlling transactions yourself is that if you perform several NHibernate operations without an explicit transaction, each of these operations will take place in separate transactions.

So you can easily get inconsistency between the operations. By explicitly starting an NHibernate transaction then performing the operations in it, you're guaranteed consistency across these operations.

This is, of course, true for ANY data access layer that implicitly starts transactions for you if you do not. It's not limited to NHibernate.