Difference between revisions of "Transactions"

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 +
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*After both transactions succeed andrew will be £30 richer.
 +
*One attempts to add £10 the other attempts to add £20
 +
Pink session                  White Session
 +
BEGIN;
 +
                                BEGIN;
 +
UPDATE wealth
 +
    SET amount=amount+10
 +
  WHERE cust = 'andrew';
 +
                                UPDATE wealth
 +
                                  SET amount=amount+20
 +
                                WHERE cust = 'andrew';
 +
                                --This session hangs; its fate rests on the other session
 +
                                --If the other session does rollback andrew will have £120
 +
                                --If the other session does commits  andrew will have £130
 +
COMMIT;
 +
                                SELECT * FROM wealth WHERE cust='andrew';
 +
                                +--------+--------+
 +
                                | cust  | amount |
 +
                                +--------+--------+
 +
                                | andrew |    130 |
 +
                                +--------+--------+
 +
 +
 +
 +
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Revision as of 18:05, 1 November 2016

Transactions allow you to support multiple access safely.

You need to use transactions when the order of processing is important and there are several users (or processes) accessing the data at the same time.

Examples of when you need to do this include customers booking seats on an airplane. You might have:

  • Two customers Alice and Bob both spot there is one seat left on a flight
  • They both attempt to book that seat at (roughly) the same time.
  • Alice's attempt is received first but before it is completed Bob's attempt is started.
  • Alice will get the seat and her credit card will be charged.
  • Bob will not get the seat and his credit card will not be charged.

BEGIN

  • Two sessions, with and without transactions
  • The second session cannot see changes until they have been committed by the first session (do do otherwise would be a "dirty read").
  • We create the bank of wealth - it has two customers, they both have £100
+--------+--------+
| cust   | amount |
+--------+--------+
| andrew |    100 |
| brian  |    100 |
+--------+--------+
DROP TABLE wealth;
CREATE TABLE wealth(
  cust VARCHAR(10) PRIMARY KEY,
  amount INT
);
INSERT INTO wealth VALUES ('andrew',100);
INSERT INTO wealth VALUES ('brian', 100);


REPEATABLE READ

  • When a session starts a transaction it has a consistent copy of the database that is isolated from activity in other sessions.
  • If we are at "REPEATABLE READ" level then even committed transactions (from a different session) are not visible.
Pink session                   White Session
BEGIN;
                               SELECT amount FROM wealth WHERE cust='andrew';
                               --> 100
                               BEGIN;
UPDATE wealth SET amount=200
 WHERE cust = 'andrew';
                               SELECT amount FROM wealth WHERE cust='andrew';
                               --> 100
COMMIT;
                               SELECT amount FROM wealth WHERE cust='andrew';
                               --> 100
                               COMMIT;
                               SELECT amount FROM wealth WHERE cust='andrew';
                               --> 200                               


  • After both transactions succeed andrew will be £30 richer.
  • One attempts to add £10 the other attempts to add £20
Pink session                   White Session
BEGIN;
                               BEGIN;
UPDATE wealth
   SET amount=amount+10
 WHERE cust = 'andrew';
                               UPDATE wealth
                                  SET amount=amount+20
                                WHERE cust = 'andrew';
                               --This session hangs; its fate rests on the other session
                               --If the other session does rollback andrew will have £120
                               --If the other session does commits  andrew will have £130
COMMIT;
                               SELECT * FROM wealth WHERE cust='andrew';
                               +--------+--------+
                               | cust   | amount |
                               +--------+--------+
                               | andrew |    130 |
                               +--------+--------+