Sep 14, 2011

URL shortening - make your own URL shortener

If you ever thought about URL shortening, you've probably already figured out, how it works.

Those random letters and numbers which serve as a key (in hashtable data structures) are basically encoded integers, which are AUTO_INCREMENT values in the database.

Here are the encoding and decoding functions:
const ALLOWED_CHARS = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

function id2short($integer, $base = self::ALLOWED_CHARS) {
	$out = '';
	$length = strlen($base);
	while($integer > $length-1) {
		$out = $base[fmod($integer, $length)].$out;
		$integer = floor( $integer / $length);
	}
	return $base[$integer].$out;
}

function short2id($string, $base = self::ALLOWED_CHARS) {
	$length = strlen($base);
	$size = strlen($string)-1;
	$string = str_split($string);
	$out = strpos($base, array_pop($string));
	foreach($string as $i=>$char) {
		$out += strpos($base, $char) * pow($length, $size - $i);
	}
	return $out;
}

Full code here.

Sep 3, 2011

MySQL error 1236: Client requested master to start replication from impossible position

Symptoms:
[1236] Got fatal error 1236 from master when reading data from binary log: 'Client requested master to start replication from impossible position'

110902 16:47:08 [ERROR] Error reading packet from server: Client requested master to start replication from impossible position ( server_errno=1236)
110902 16:47:08 [ERROR] Slave I/O: Got fatal error 1236 from master when reading data from binary log: 'Client requested master to start replication from impossible position', Error_code: 1236
110902 16:47:08 [Note] Slave I/O thread exiting, read up to log 'mysql-bin.000033', position 4621679


on the master
[email protected]:/var/lib/mysql# ls -la mysql-bin.000033
-rw-rw---- 1 mysql mysql 4620018 2011-09-01 13:45 mysql-bin.000033

4620018 is less than 4621679, therefore it's an invalid position.


Causes:
Master server has crashed and the binlog cache has not been flushed to disk. Slave has recieved a new position, did not recieve data, and data gets lost in a crash (however it might have been written to table, but not in binlog).


Solution:
Use this CHANGE MASTER statement on the slave.
CHANGE MASTER TO MASTER_LOG_FILE=[NEXT FILE], MASTER_LOG_POS=4;
SLAVE START;

in my case that would be

CHANGE MASTER TO MASTER_LOG_FILE='mysql-bin.000034', MASTER_LOG_POS=4;
SLAVE START;

I don't know why the master log position needs to be 4 for the new file.

What happens:
When the master server restarts it logs binary changes to a new binlog file, so that we minimize data loss by skipping to the next file (everything from the previous file was written already).

Prevention:
Add this line to my.cnf:

sync_binlog = 1

 With this setting the master server flushes cache in the binlog after every write, so that in case of a crash you can lose one statement at most.