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Berkeley DB Reference Guide:
  Transaction Protected Applications

# Why transactions?

Perhaps the first question to answer is "Why transactions?" There are a number of reasons for including transactional support in your applications. The most common ones are:

Recoverability
  Applications often need to ensure that, no matter how the system or application fails, previously saved data is available the next time the application runs.

Deadlock avoidance
  When multiple threads of control change the database at the same time, there is usually the possibility of deadlock, that is, where each of the threads of control owns a resource another thread wants, and so no thread is able to make forward progress, all waiting for a resource. Deadlocks are resolved by having one of the operations involved release the resources it controls so the other operations can proceed. (The operation releasing its resources usually just tries again later.) Transactions are necessary so that any changes that were already made to the database can be undone as part of releasing the held resources.

Atomicity
  Applications often need to make multiple changes to one or more databases, but want to ensure that either all of the changes happen, or none of them happen. Transactions guarantee that a group of changes are atomic, that is, if the application or system fails, either all of the changes to the databases will appear when the application next runs, or none of them.

Repeatable reads
  Applications sometimes need to ensure that, while doing a group of operations on a database, the value returned as a result of a database retrieval doesn't change, that is, if you retrieve the same key more than once, the data item will be the same each time. Transactions guarantee this behavior.

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