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<chapter id="tutorial-advanced">
<sect1 id="tutorial-transactions">
<title>Transactions</title>
- <comment>This section needs to be written.</comment>
+ <indexterm zone="tutorial-transactions">
+ <primary>transactions</primary>
+ </indexterm>
+
+ <para>
+ <firstterm>Transactions</> are a fundamental concept of all database
+ systems. The essential point of a transaction is that it bundles
+ multiple steps into a single, all-or-nothing operation. The intermediate
+ states between the steps are not visible to other concurrent transactions,
+ and if some failure occurs that prevents the transaction from completing,
+ then none of the steps affect the database at all.
+ </para>
+
+ <para>
+ For example, consider a bank database that contains balances for various
+ customer accounts, as well as total deposit balances for branches.
+ Suppose that we want to record a payment of $100.00 from Alice's account
+ to Bob's account. Simplifying outrageously, the SQL commands for this
+ might look like
+<programlisting>
+UPDATE accounts SET balance = balance - 100.00
+ WHERE name = 'Alice';
+UPDATE branches SET balance = balance - 100.00
+ WHERE name = (SELECT branch_name FROM accounts WHERE name = 'Alice');
+UPDATE accounts SET balance = balance + 100.00
+ WHERE name = 'Bob';
+UPDATE branches SET balance = balance + 100.00
+ WHERE name = (SELECT branch_name FROM accounts WHERE name = 'Bob');
+</programlisting>
+ The details of these commands are not important here; the important
+ point is that there are several separate updates involved to accomplish
+ this rather simple operation. Our bank's officers will want to be
+ assured that either all these updates happen, or none of them happen.
+ It would certainly not do for a system failure to result in Bob
+ receiving $100.00 that was not debited from Alice. Nor would Alice long
+ remain a happy customer if she was debited without Bob being credited.
+ We need a guarantee that if something goes wrong partway through the
+ operation, none of the steps executed so far will take effect. Grouping
+ the updates into a <firstterm>transaction</> gives us this guarantee.
+ A transaction is said to be <firstterm>atomic</>: from the point of
+ view of other transactions, it either happens completely or not at all.
+ </para>
+
+ <para>
+ We also want a
+ guarantee that once a transaction is completed and acknowledged by
+ the database system, it has indeed been permanently recorded
+ and won't be lost even if a crash ensues shortly thereafter.
+ For example, if we are recording a cash withdrawal by Bob,
+ we do not want any chance that the debit to his account will
+ disappear in a crash just as he walks out the bank door.
+ A transactional database guarantees that all the updates made by
+ a transaction are logged in permanent storage (i.e., on disk) before
+ the transaction is reported complete.
+ </para>
+
+ <para>
+ Another important property of transactional databases is closely
+ related to the notion of atomic updates: when multiple transactions
+ are running concurrently, each one should not be able to see the
+ incomplete changes made by others. For example, if one transaction
+ is busy totalling all the branch balances, it would not do for it
+ to include the debit from Alice's branch but not the credit to
+ Bob's branch, nor vice versa. So transactions must be all-or-nothing
+ not only in terms of their permanent effect on the database, but
+ also in terms of their visibility as they happen. The updates made
+ so far by an open transaction are invisible to other transactions
+ until the transaction completes, whereupon all the updates become
+ visible simultaneously.
+ </para>
<para>
+ In <productname>Postgres</>, a transaction is set up by surrounding
+ the SQL commands of the transaction with
+ <command>BEGIN</> and <command>COMMIT</> commands. So our banking
+ transaction would actually look like
+<programlisting>
+BEGIN;
+UPDATE accounts SET balance = balance - 100.00
+ WHERE name = 'Alice';
+-- etc etc
+COMMIT;
+</programlisting>
+ If, partway through the transaction, we decide we don't want to
+ commit (perhaps we just noticed that Alice's balance went negative),
+ we can issue the command <command>ROLLBACK</> instead of
+ <command>COMMIT</>, and all our updates so far will be canceled.
+ </para>
+ <para>
+ <productname>Postgres</> actually treats every SQL statement as being
+ executed within a transaction. If you don't issue a <command>BEGIN</>
+ command,
+ then each individual statement has an implicit <command>BEGIN</> and
+ (if successful) <command>COMMIT</> wrapped around it. A group of
+ statements surrounded by <command>BEGIN</> and <command>COMMIT</>
+ is sometimes called a <firstterm>transaction block</>.
</para>
+
+ <note>
+ <para>
+ Some client libraries issue <command>BEGIN</> and <command>COMMIT</>
+ commands automatically, so that you may get the effect of transaction
+ blocks without asking. Check the documentation for the interface
+ you are using.
+ </para>
+ </note>
</sect1>
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