REPEATABLE READ Transactions in Java

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REPEATABLE READ Transactions
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In earlier days REPEATABLE READ was the highest possible isolation level For some DBMSs, it still is REPEATABLE READ won't let through Lost Updates, Dirty Reads, or Non-repeatable Reads; it allows only Phantoms, which are rare With regard to locking, the philosophy of REPEATABLE READ transactions is to keep locks until the transaction ends In this respect they differ from READ COMMITTED transactions, which free locks after finishing with a row In general, it's a bad idea to access the same row twice in a READ COMMITTED transaction, because you're (a) getting a lock, then (b) freeing the lock, and then (c) getting the same lock again When two SQL statements in the same transaction have overlapping WHERE search conditions, or no WHERE clauses at all, there is a danger of a Phantom appearing For example, consider these two SQL statements:
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SELECT * FROM Table1 WHERE column1 = 5 UPDATE Table1 SET column1 = 5 WHERE column1 = 6 /* there might be a Phantom here */
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Even programmers who understand nothing of Phantoms can obey this rule of thumb: Don't write overlapping transactions In the previous example, we should have done the UPDATE first If your WHERE clause is always for a single row on a unique key, and fetch succeeds, Phantoms can't happen with the REPEATABLE READ isolation level If you need the security of REPEATABLE READ, but you would still like locks to get an early release, here are some tricks to force the DBMS to do what you want
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Example #1: (transaction start) SELECT * FROM Table1 SAVEPOINT savepoint1 SELECT * FROM Table2 ROLLBACK TO SAVEPOINT savepoint1 (transaction end)
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In Example #1, the ROLLBACK TO SAVEPOINT statement will eliminate the locks on Table2 only (assuming the DBMS supports this SQL:1999 instruction) ROLLBACK TO SAVEPOINT is also useful if a lock attempt fails and you want to try a different tack
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Example #2: SELECT * FROM Table1; for (n=0; n<#-of-rows; ++n) { FETCH INTO :x[n]; if (NOT_FOUND) break; } ROLLBACK; for (n=0; n<#-of-rows; ++n) { printf("x[n]=%d\n",x[n]); }
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In Example #2, the ROLLBACK statement comes before the display If we'd used a scalar variable for the fetch instead, we would also have had to call printf() from within the fetch loop, thus delaying the transaction end If you can make big arrays, you can end transactions early By the way, we don't bother to close the cursor ROLLBACK handles that REPEATABLE READ is a good isolation level to specify for transactions that contain multiple SQL statements involving data changes Typical REPEATABLE READ situations are transferring money from one bank account to another or producing a customer invoice
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SERIALIZABLE Transactions
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SERIALIZABLE is the default isolation level specified by the SQL Standard and is supported by all the Big Eight except MySQL But all of the Big Eight except Ingres recommend either READ COMMITTED or REPEATABLE READ for the default That tells us that nobody else has found a practical way to implement SERIALIZABLE and look good at the same time You must avoid the SERIALIZABLE level unless it's utterly necessary but, alas, it sometimes is For example, suppose you're coding an application for a bank and want to retrieve and display all bank account balances, followed by a total The total shown must really equal what you'd get by adding up all the individual account lines, so Phantoms are intolerable This means you can't use REPEATABLE READ for your transaction Before DBMSs supported SERIALIZABLE, there was still a way to ensure serialization, namely with a non-standard SQL-extension LOCK statement such as
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LOCK TABLE Accounts IN SHARE MODE
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(or whatever the LOCK statement is for your DBMS) In fact, some DBMSs will internally support SERIALIZABLE through the simple device of huge-grain locks, while others try to be more sophisticated Here is a sampling of what DBMSs do to support this isolation level
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Microsoft keeps locks on a range of index keys, thus blocking transactions that use overlapping WHERE clause search conditions Unlike ordinary index page locks, these range locks are not released until the transaction ends MySQL allows LOCK TABLES UNLOCK TABLES to span statements (statements in MySQL constitute transactions unless you install an optional package)
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SERIALIZABLE is a good isolation level to specify for transactions that contain quickly executed SQL statements that affect only a few rows at a time Typical SERIALIZABLE situations are master/detail, member/aggregate, or account-balance/total-balance reports Bog-standard MySQL won't support SERIALIZABLE transactions (recall that our policy is to ignore non-default installation options; see 1, "Facilis Descensus Averni") The usual solution involves MySQL's non-standard SQL-extension LOCK TABLES statement, but here's another example involving the dread "bank transfer" This transaction should be serializable, and it involves two updates on the same table
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(transaction start) UPDATE Customer1 SET balance = balance - 5 WHERE custid = 1 UPDATE Customer1 SET balance = balance + 5 WHERE custid = 2 (transaction end)
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If you're using MySQL, you could cause this transaction to be SERIALIZABLE by using a single UPDATE statement with CASE, like this:
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UPDATE Customer1 SET balance = CASE WHEN custid = 1 THEN balance - 5 WHEN custid = 2 THEN balance + 5 END WHERE custid IN (1, 2)
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This works because (a) MySQL doesn't have atomic transactions, and (b) MySQL does have atomic statements, therefore (c) transactions must be statements It's not smart, because MySQL handles ORs badly,[2] but might do in a pinch
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