proxysql/doc/internal/global_variables.txt

Each module of ProxySQL can have global variables and should be configurable
through the admin interface.
As such, the interface for the management of these global variables should be
either very simple, or managed by a simple API.

Trying to make a standard API, the proposed functions are:
  virtual char **get_variables_list() {return NULL;}
  virtual char *get_variable(char *name) {return NULL;};
  virtual bool set_variable(char *name, char *value) {return false;};
  virtual void commit() {};
  virtual void wrlock() {};
  virtual void wrunlock() {};

In order:
- get_variables_list() returns an array of pointer to variables name, where the
  last element is NULL;
- get_variable() returns the value of a specific variable. If the variable was
  not previously set, the module should return the default value. If the
  variable doesn't exist, NULL should be returned;
- set_variable() set the variable "name" to the value "value" . On success, true
  is returned. If "name" does not exist or if "value" is not valid, false is
  return;
- commit() trigger the module to read the new setting. Its implementation is
  specific to the module;
- wrlock() and wrunlock() are a way to acquire and release mutex or write lock,
  as it is not guaranteed that the any of the previous command is thread-safe.


Current specific implementation.

Standard_ProxySQL_Admin is the current Admin Module.
It allows users to modify such global variables through a table called
global_variables that has 2 columns:
- variable_name (VARCHAR NOT NULL PRIMARY KEY)
- variable_value (VARCHAR NOT NULL)

To distinguish variables from different modules, variable_name has the follow
format: <module>-<name>
For example:
admin-admin_credentials
admin-interfaces
mysql-poll_timeout
mysql-server_version

Current modules that support global variables:
- MySQL_Threads_Handler ("mysql" prefix) : currently implemented in
  Standard_MySQL_Threads_Handler, that then handles Standard_MySQL_Thread;
- ProxySQL_Admin ("admin" prefix) : currently implemented in
  Standard_ProxySQL_Admin . Although ProxySQL_Admin don't interact with other
  modules when processing its own variables, the same interface is used for
  semplicity and to provide threads safety.


Implementation details in Standard_ProxySQL_Admin.
Standard_ProxySQL_Admin uses the follow functions to handle global variables: 
- flush_<module>_variables___database_to_runtime
- flush_<module>_variables___runtime_to_database

flush_<module>_variables___database_to_runtime :
a) selects all the variables that have name like "<module>-%" and strips the
   prefix
b) for each of them try to set it with set_variable()
c) if #b fails, it calls get_variable() with the same name
d) if #c succeed, update the value in the table; if #c fails, delete the row
   from the table