MySQL Global User Variables UDF 1.0

MySQL Global User Variables UDF is a MySQL extension to store persistent variables.

This shared library adds simple user functions to MySQL in order to keep persistent shared variables in memory. These variables and their values are available to all clients. Any data can be stored into these persistent variables, including BLOBs. Since updates are atomic and way faster than MEMORY tables, this is an easy and efficient way to handle counters and sequences.

Usage:

Storing a value

An unlimited number of user variables can be created, as long as memory is available.
The GLOBAL_STORE(, ) stores a new shared global variable.

Examples:

mysql> DO GLOBAL_STORE("online_users", 42);
mysql> DO GLOBAL_STORE("secret_key", "pajfUyfnd");

The GLOBAL_STORE() function always returns 1 unless an error occurred.

Fetching a value

Reading the value of a variable is the job of the GLOBAL_GET() function.
The value is returned, or NULL is the variable is undefined.

Example:

mysql> SELECT GLOBAL_GET("online_users;);
42

mysql> SELECT id FROM pxs WHERE secret_key = GLOBAL_GET("secret_key");
1

Atomic increments

A single function call can read the previous value, add an integer (that can be negative), and store the new value into the variable.

The function is GLOBAL_ADD(, ) and the return value is the new value of the variable.
Updates are always atomic, if the old value is 18 and you add 1, you will always get back 19.

Example:

mysql> DO GLOBAL_ADD("online_users", 1);
mysql> SELECT GLOBAL_ADD("online_users", -4);
39

If the value of a variable was a string, the new value is the increment:

mysql> SELECT GLOBAL_ADD("secret_key", 12);
12

Adding a value to an undefined variable returns NULL.
A handy variant is GLOBAL_ADDP(, ). GLOBAL_ADDP() is similar to GLOBAL_ADD() but returns the PREVIOUS value of the variable instead of the new one.

Example:

mysql> DO GLOBAL_SET("xxx", 10);
mysql> SELECT GLOBAL_ADDP("xxx", 1);
10
mysql> SELECT GLOBAL_ADDP("xxx", 1);
11

Installation:

On most systems, compiling and installing the library should be as simple as typing (as root):

make install

The shared library is installed as /usr/local/lib/udf_global_user_variables.so

If the base directory of your MySQL installation is not in /usr/local, just type:

make

and then copy udf_global_user_variables.so to the right location for UDFs on your system (maybe /usr/lib/).

The name of a variable is limited to 256 bytes. If that limit is too low for your specific application, just edit the MAX_NAME_LENGTH variable on top of the .c file and reinstall. Variable names can contain binary characters.

Values are limited to 65536 bytes. If that limit is too low for you, edit the MAX_VALUE_LENGTH variable and reinstall.