Posted by Andy Hassall on 01/26/06 23:38

On Fri, 27 Jan 2006 10:06:58 +1300, Chris Hope <blackhole@electrictoolbox.com>
wrote:

>> mysql_insert_id() is connection specific, and MySQL connections
>> aren't shared
>> between concurrent PHP requests, so this avoids the condition you
>> describe.
>
>What about if persistant connections are used? I'm a little unclear of
>this myself. Does each PHP thread use a different persistant
>connection, or can the share them? I would assume the former.

In multiprocess webservers, e.g. Apache 1.3, it's simple - there's a
completely separate instance of PHP per Apache worker process, each with its
own persistent connection pool, which will have zero or one connection for a
given database and credentials. So, there's nothing else that can use the
connection, since this instance of PHP can only run one script at a time.

The persistent connection list is stored in the "persistent_list" member of a
struct called "executor_globals". From looking through the PHP source code,
there's a section in Zend/zend_globals_macros.h, where if you have "ZTS"
defined (which means you're building a thread-safe PHP) then there's this:

/* Executor */
#ifdef ZTS
# define EG(v) TSRMG(executor_globals_id, zend_executor_globals *, v)
#else
# define EG(v) (executor_globals.v)
extern ZEND_API zend_executor_globals executor_globals;
#endif

... i.e. it defines EG() as accessing a single global copy of this struct if
not building threaded, but if threaded then it uses the Thread Safe Resource
Manager (TSRM), so presumably if you trace this further through the code then
you end up with a copy of executor_globals per-thread.

Going over to php_mysql.c, it uses this "EG" macro to access persistent_list
in php_mysql_do_connect.

--
Andy Hassall :: andy@andyh.co.uk :: http://www.andyh.co.uk
http://www.andyhsoftware.co.uk/space :: disk and FTP usage analysis tool

[Back to original message]