Nested transactions
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## Nested transactions

Berkeley DB provides support for nested transactions. Nested transactions allow an application to decompose a large or long-running transaction into smaller units that may be independently aborted.

Normally, when beginning a transaction, the application will pass a NULL value for the parent argument to DB_ENV->txn_begin(). If, however, the parent argument is a TXN handle, the newly created transaction will be treated as a nested transaction within the parent. Transactions may nest arbitrarily deeply. For the purposes of this discussion, transactions created with a parent identifier will be called child transactions.

Once a transaction becomes a parent, as long as any of its child transactions are unresolved (that is, they have neither committed nor aborted), the parent may not issue any Berkeley DB calls except to begin more child transactions, or to commit or abort. For example, it may not issue any access method or cursor calls. After all of a parent's children have committed or aborted, the parent may again request operations on its own behalf.

The semantics of nested transactions are as follows. When a child transaction is begun, it inherits all the locks of its parent. This means that the child will never block waiting on a lock held by its parent. Further, locks held by two children of the same parent will also conflict. To make this concrete, consider the following set of transactions and lock acquisitions.

Transaction T1 is the parent transaction. It acquires a write lock on item A and then begins two child transactions: C1 and C2. C1 also wants to acquire a write lock on A; this succeeds. If C2 attempts to acquire a write lock on A, it will block until C1 releases the lock, at which point it will succeed. Now, let's say that C1 acquires a write lock on B. If C2 now attempts to obtain a lock on B, it will block. However, let's now assume that C1 commits. Its locks are anti-inherited, which means they are given to T1, so T1 will now hold a lock on B. At this point, C2 would be unblocked and would then acquire a lock on B.

Child transactions are entirely subservient to their parent transaction. They may abort, undoing their operations regardless of the eventual fate of the parent. However, even if a child transaction commits, if its parent transaction is eventually aborted, the child's changes are undone and the child's transaction is effectively aborted. Any child transactions that are not yet resolved when the parent commits or aborts are resolved based on the parent's resolution -- committing if the parent commits and aborting if the parent aborts. Any child transactions that are not yet resolved when the parent prepares are also prepared.

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