We are just starting to migrate/reimplement an ERP system to Java with Hibernate, targeting a concurrent user count of 50-100 users using the system. We use MS SQL Server as database server, which is good enough for the load.
The old system doesn't use transactions and relies for critical parts (e.g. stock changes) on setting manual locks (using flags) and releasing them. That's something like manual transaction management. But there are sometimes problems with data inconsistency. In the new system, we would like to use transactions to wipe out these problems.
What would be a good/reasonable default transaction isolation level to use for an ERP system, given a usage of about 85% OLTP and 15% OLAP?
Or should I always decide, on a per task basis, which transaction level to use?
The four transaction isolation levels:
99 times out of 100, read committed is the right answer. That ensures that you only see changes that have been committed by the other session (and, thus, results that are consistent, assuming you've designed your transactions correctly). But it doesn't impose the locking overhead (particularly in non-Oracle databases) that repeatable read or serializable impose.
Very occasionally, you may want to run a report where you are willing to sacrifice accuracy for speed and set a read uncommitted isolation level. That's rarely a good idea, but it is occasionally a reasonably acceptable workaround to lock contention issues.
Serializable and repeatable read are occasionally used when you have a process that needs to see a consistent set of data over the entire run regardless of what other transactions are doing at the time. It may be appropriate to set a month-end reconciliation process to serializable, for example, if there is a lot of procedureal code, a possibility that users are going to be making changes while the process is running and a requirement that the process needs to ensure that it is always seeing the data as it existed at the time the reconciliation started.