Difference between revisions of "Transactions"

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(Update Madness)
(Update Madness)
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This is the Linux bash script to run the command 100 times:
 
This is the Linux bash script to run the command 100 times:
 
<source lang=bash>
 
<source lang=bash>
for i in `seq 1 100`; do php trnt.php andrew brian 1; done
+
for i in `seq 100`; do php trnt.php andrew brian 1; done
 
</source>
 
</source>
 
You will find that if you run these two commands in different putty sessions at the same time then serious errors can occur:
 
You will find that if you run these two commands in different putty sessions at the same time then serious errors can occur:
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<tr><td>
 
<tr><td>
 
<source lang=bash>
 
<source lang=bash>
for i in `seq 1 100`; do php trnt.php andrew brian 1; done
+
for i in `seq 100`; do php trnt.php andrew brian 1; done
 
</source>
 
</source>
 
</td><td>
 
</td><td>
 
<source lang=bash>
 
<source lang=bash>
for i in `seq 1 100`; do php trnt.php andrew brian -1; done
+
for i in `seq 100`; do php trnt.php andrew brian -1; done
 
</source>
 
</source>
 
</td></tr></table>
 
</td></tr></table>

Revision as of 19:49, 2 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                               


Transaction left hanging

  • Two transactions attempt to increase andrew's wealth
  • One attempts to add £10 the other attempts to add £20
  • After both transactions succeed andrew will be £30 richer.
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 |
                               +--------+--------+


One transaction must die

  • We try the same pair of transactions as above but this time we do a read first.
Pink session                   White Session
BEGIN;
                               BEGIN;
                               SELECT * FROM wealth WHERE cust='andrew';
                               +--------+--------+
                               | cust   | amount |
                               +--------+--------+
                               | andrew |    130 |
                               +--------+--------+
UPDATE wealth
   SET amount=amount+10
 WHERE cust = 'andrew';
--Hangs
                               UPDATE wealth
                                  SET amount=amount+20
                                WHERE cust = 'andrew';
--Fails                        --Succeeds
                               COMMIT;
                               SELECT * FROM wealth WHERE cust='andrew';
                               +--------+--------+
                               | cust   | amount |
                               +--------+--------+
                               | andrew |    150 |
                               +--------+--------+


Update Madness

We will try a naive update to see how badly things can go wrong. We will then try to fix it so that the errors are more manageable.

We create the wealth table and add two rows:

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);

We create a php program trnt.php to transfer money from andrew to brian or the other way around.

<?php
$payer = $argv[1];
$payee = $argv[2];
$value = $argv[3];
$dbh = new PDO('mysql:host=localhost;dbname=scott','scott','tiger');
$dbh->setAttribute(PDO::ATTR_EMULATE_PREPARES,false);
$sth = $dbh->prepare("SELECT amount FROM wealth WHERE cust=?");
$sth->execute(array($payer));
$a = $sth->fetchAll()[0][0];
$sth->execute(array($payee));
$b = $sth->fetchAll()[0][0];
$sth = $dbh->prepare("UPDATE wealth SET amount=? WHERE cust=?");
$sth->execute(array($a-$value,$payer));
$sth->execute(array($b+$value,$payee));
echo "$payer has ".($a-$value).", $payee has ".($b+$value)."\n";

This can be run from the command line - you give three parameters - payer, payee and amount.

This Linux bash script command gives one pound from andrew to brian:

php trnt.php andrew brian 1

This is the Linux bash script to run the command 100 times:

for i in `seq 100`; do php trnt.php andrew brian 1; done

You will find that if you run these two commands in different putty sessions at the same time then serious errors can occur:

for i in `seq 100`; do php trnt.php andrew brian 1; done
for i in `seq 100`; do php trnt.php andrew brian -1; done

Update Consistency - detecting failure

This version of the program includes transactions.

  • Before running any SQL read or write statements we BEGIN a transaction.
  • After all of the SQL statements have been run we issue a COMMIT
  • Every SQL statement that runs is checked - if a failure is detected we show an error message and abandon the program.

The php program trwt.php to transfer money now includes transactions and failure detection.

<?php
$payer = $argv[1];
$payee = $argv[2];
$value = $argv[3];
$dbh = new PDO('mysql:host=localhost;dbname=scott','scott','tiger');
$dbh->setAttribute(PDO::ATTR_EMULATE_PREPARES,false);
$dbh->query("SET TRANSACTION ISOLATION LEVEL SERIALIZABLE");
$dbh->beginTransaction();
$sth = $dbh->prepare("SELECT amount FROM wealth WHERE cust=?");
$comm = "php ".join(' ',$argv)."\n";
if (!$sth->execute(array($payer))){
  die("#SELECT failed\n$comm");
}
$a = $sth->fetchAll()[0][0];
if (!$sth->execute(array($payee))){
  die("#SELECT failed\n$comm");
}
$b = $sth->fetchAll()[0][0];
$sth = $dbh->prepare("UPDATE wealth SET amount=? WHERE cust=?");
if (!$sth->execute(array($a-$value,$payer))){
  $dbh->rollback();
  die("#UPDATE failed\n$comm");
}
if (!$sth->execute(array($b+$value,$payee))){
  $dbh->rollback();
  die("#UPDATE failed\n$comm");
}
$dbh->commit();
echo "#$payer has ".($a-$value).", $payee has ".($b+$value)."\n";

It is still possible for a transaction to fail - but this time the transaction either completes or fails entirely. It is no longer possible for money to go missing from the system altogether.

When a transaction does fail we are notified and we can take steps to cope with that - typically we can try again.

Update Consistency - retry on failure

The final version of the program includes a retry on failure

  • As before we enclose all the SQL execute attempts with a transaction; BEGIN at the top COMMIT at the end
  • If any of the four execute statements fails then we simply reissue the command - we try again.
  • So long as failures are rare this is a safe thing to do.
    • Our previous test showed one or two failures per hundred transactions
    • This is a terrifyingly high number of failures - in a practical application we would hope that failures were much more unusual.
    • Even with a 2% failure rate the retry will not hugely increase the total run time.

The php program trwr.php to transfer money now includes transactions and failure detection.

<?php
$payer = $argv[1];
$payee = $argv[2];
$value = $argv[3];
$dbh = new PDO('mysql:host=localhost;dbname=scott','scott','tiger');
$dbh->setAttribute(PDO::ATTR_EMULATE_PREPARES,false);
$dbh->query("SET TRANSACTION ISOLATION LEVEL SERIALIZABLE");
$dbh->beginTransaction();
$sth = $dbh->prepare("SELECT amount FROM wealth WHERE cust=?");
$comm = "php ".join(' ',$argv)."\n";
if (!$sth->execute(array($payer))){
  $dbh->rollback();
  `$comm`;
  die("#SELECT failed\n");
}
$a = $sth->fetchAll()[0][0];
if (!$sth->execute(array($payee))){
  $dbh->rollback();
  `$comm`;
  die("#SELECT failed\n");
}
$b = $sth->fetchAll()[0][0];
$sth = $dbh->prepare("UPDATE wealth SET amount=? WHERE cust=?");
if (!$sth->execute(array($a-$value,$payer))){
  $dbh->rollback();
  `$comm`;
  die("#UPDATE failed\n");
}
if (!$sth->execute(array($b+$value,$payee))){
  $dbh->rollback();
  `$comm`;
  die("#UPDATE failed\n");
}
$dbh->commit();
echo "#$payer has ".($a-$value).", $payee has ".($b+$value)."\n";

This time when a failure occurs we simply try again - nothing can possibly go wrong!