Getting your own process ID

Hi All,

I am doing some work where I want to do locking, and prevent scripts from
running in parallel. I see that I could use the semaphore mechanism, but
I'd like for my code to be portable, and that extension is not enabled in
many places.

I need some way for a process to uniquely identify itself. It can then look
at the storage container (flat file, DB, whatever is appropriate in
context), check to see if the requested semaphore is available, and if it
is, acquire it and then mark itself as the owner. It can then check that it
did in fact get ownership (as opposed to another process which attempted to
acquire it at the exact same moment) before proceeding.

However I am stumped at the point where it needs to indicate ownership. How
does the PHP script identify itself? It can't use the script name,
obviously - lots of instances of the script may be running. It can't use
session ID - the user might submit duplicate requests, and they would both
have the same session ID. The best I have been able to think of is to use
the sha1(microtime()) to generate a unique key. But this isn't quite
foolproof, as it is theoretically possible, though unlikely, for two
requests to be at the exact same instant.

The answer that comes to my mind would be to use the process ID. This is
necessarily unique across the entire server, correct? It seems to be
exactly what I need. But I can't seem to figure out how to determine the
current process ID from within PHP. Is this even possible?

Any ideas?

-Josh

p.s. Please forgive me if I have misused the term "semaphore" - I know it
only from the context I have heard it used in, I don't know the textbook
definition.

re: Getting your own process ID

On Thu, 24 Mar 2005 23:31:12 GMT, "Joshua Beall"
<jbeall@donotspam.remove.me.heraldic.us> wrote:
[color=blue]
>The answer that comes to my mind would be to use the process ID. This is
>necessarily unique across the entire server, correct? It seems to be
>exactly what I need. But I can't seem to figure out how to determine the
>current process ID from within PHP. Is this even possible?[/color]

Depends how you're running. If you're running as an Apache module, the current
process ID would be the Apache worker process. It'd only be processing one PHP
script at a time so should be unique enough over the life of the PHP script.

In which case: http://uk.php.net/manual/en/function.posix-getpid.php

Still, this likely wouldn't work under Windows, where Apache (2.0) is
muti-threaded, not multi-process.

--
Andy Hassall / <andy@andyh.co.uk> / <http://www.andyh.co.uk>
<http://www.andyhsoftware.co.uk/space> Space: disk usage analysis tool

re: Getting your own process ID

"Andy Hassall" <andy@andyh.co.uk> wrote in message
news:k4k641989q54ku1j8it88fi25s7k609u6f@4ax.com...[color=blue]
> On Thu, 24 Mar 2005 23:31:12 GMT, "Joshua Beall"
> <jbeall@donotspam.remove.me.heraldic.us> wrote:
>
> Depends how you're running. If you're running as an Apache module, the
> current
> process ID would be the Apache worker process. It'd only be processing one
> PHP
> script at a time so should be unique enough over the life of the PHP
> script.
>
> In which case: http://uk.php.net/manual/en/function.posix-getpid.php
>
> Still, this likely wouldn't work under Windows, where Apache (2.0) is
> muti-threaded, not multi-process.[/color]

Any other ideas? I can't think of anything else that is guaranteed to be
unique per request.

What about remote port number? Is it possible to have multiple connections
from the same remote IP and port? Or would
sha1(microtime().$_SERVER['REMOTE_ ADDR'].$_SERVER['REMOTE_PORT']) generate
a unique ID? We might get repeated requests from the same remote IP and
remote port, but as long as it is not possible for them to be at the same
time, this seems like it would work... ?

-Josh

re: Getting your own process ID

On Thu, 24 Mar 2005 23:59:26 GMT, "Joshua Beall"
<jbeall@donotspam.remove.me.heraldic.us> wrote:
[color=blue]
>"Andy Hassall" <andy@andyh.co.uk> wrote in message
>news:k4k641989q54ku1j8it88fi25s7k609u6f@4ax.com.. .[color=green]
>> On Thu, 24 Mar 2005 23:31:12 GMT, "Joshua Beall"
>> <jbeall@donotspam.remove.me.heraldic.us> wrote:
>>
>> Depends how you're running. If you're running as an Apache module, the
>> current
>> process ID would be the Apache worker process. It'd only be processing one
>> PHP
>> script at a time so should be unique enough over the life of the PHP
>> script.
>>
>> In which case: http://uk.php.net/manual/en/function.posix-getpid.php
>>
>> Still, this likely wouldn't work under Windows, where Apache (2.0) is
>> muti-threaded, not multi-process.[/color]
>
>Any other ideas? I can't think of anything else that is guaranteed to be
>unique per request.
>
>What about remote port number? Is it possible to have multiple connections
>from the same remote IP and port?[/color]

*scrunches forehead* I _think_ this is guaranteed unique at any given time...
[color=blue]
>Or would sha1(microtime().$_SERVER['REMOTE_ ADDR'].$_SERVER['REMOTE_PORT']) generate
>a unique ID?[/color]

Well, there you're throwing in a hashing function - SHA1 has a pretty large
range, but it does not guarantee uniqueness, just that it is Very Hard to
deliberately produce two different plain-data values that result in the same
SHA1 hash.

Drop the sha1() and you might actually have something more likely to be
unique.

--
Andy Hassall / <andy@andyh.co.uk> / <http://www.andyh.co.uk>
<http://www.andyhsoftware.co.uk/space> Space: disk usage analysis tool

re: Getting your own process ID

"Joshua Beall" <jbeall@donotspam.remove.me.heraldic.us> wrote in message
news:45I0e.20505$b_6.11577@trnddc01...[color=blue]
> Hi All,
>
> I am doing some work where I want to do locking, and prevent scripts from
> running in parallel. I see that I could use the semaphore mechanism, but
> I'd like for my code to be portable, and that extension is not enabled in
> many places.
>
> I need some way for a process to uniquely identify itself. It can then
> look at the storage container (flat file, DB, whatever is appropriate in
> context), check to see if the requested semaphore is available, and if it
> is, acquire it and then mark itself as the owner. It can then check that
> it did in fact get ownership (as opposed to another process which
> attempted to acquire it at the exact same moment) before proceeding.
>
> However I am stumped at the point where it needs to indicate ownership.
> How does the PHP script identify itself? It can't use the script name,
> obviously - lots of instances of the script may be running. It can't use
> session ID - the user might submit duplicate requests, and they would both
> have the same session ID. The best I have been able to think of is to use
> the sha1(microtime()) to generate a unique key. But this isn't quite
> foolproof, as it is theoretically possible, though unlikely, for two
> requests to be at the exact same instant.
>
> The answer that comes to my mind would be to use the process ID. This is
> necessarily unique across the entire server, correct? It seems to be
> exactly what I need. But I can't seem to figure out how to determine the
> current process ID from within PHP. Is this even possible?
>
> Any ideas?
>
> -Josh
>
> p.s. Please forgive me if I have misused the term "semaphore" - I know it
> only from the context I have heard it used in, I don't know the textbook
> definition.
>[/color]

Process ID would not be unique on a multi threaded server and hence not
portable if that is an requirement.

PHP has a function to generate a unique ID if that is what you are after:

http://se.php.net/manual/en/function.uniqid.php

Neither that function guarantees the ID to be unique, if generated on
several hosts at the same microsecond... probably won't happen in a trillion
years though...

You could use a transactional database to generate unique ids or you could
use file locking.

I serialize session requests per session ID by making a lock on the sesssion
ID in MySQL:

"SELECT GET_LOCK('$sessionID', 10)" and "DO RELEASE_LOCK('$sessionID')"

If you wish to serialize requests per script, I would use a lock file and
use file locking. That is how most applications do it.

If you want everything you do to be bullet proof, you will end up spending
most of your time on just that, and not on your application.

Best Regards,

Peter Albertsson

re: Getting your own process ID

Carved in mystic runes upon the very living rock, the last words of
Joshua Beall of comp.lang.php make plain:
[color=blue]
> However I am stumped at the point where it needs to indicate
> ownership. How does the PHP script identify itself?[/color]

http://www.php.net/manual/en/function.uniqid.php

--
Alan Little
Phorm PHP Form Processor
http://www.phorm.com/

re: Getting your own process ID

"Joshua Beall" <jbeall@donotspam.remove.me.heraldic.us> kirjoitti
viestissä:45I0e.20505$b_6.11577@trnddc01...[color=blue]
> Hi All,
>
> I am doing some work where I want to do locking, and prevent scripts from
> running in parallel. I see that I could use the semaphore mechanism, but
> I'd like for my code to be portable, and that extension is not enabled in
> many places.
>
> I need some way for a process to uniquely identify itself. It can then
> look at the storage container (flat file, DB, whatever is appropriate in
> context), check to see if the requested semaphore is available, and if it
> is, acquire it and then mark itself as the owner. It can then check that
> it did in fact get ownership (as opposed to another process which
> attempted to acquire it at the exact same moment) before proceeding.
>
> However I am stumped at the point where it needs to indicate ownership.
> How does the PHP script identify itself? It can't use the script name,
> obviously - lots of instances of the script may be running. It can't use
> session ID - the user might submit duplicate requests, and they would both
> have the same session ID. The best I have been able to think of is to use
> the sha1(microtime()) to generate a unique key. But this isn't quite
> foolproof, as it is theoretically possible, though unlikely, for two
> requests to be at the exact same instant.
>
> The answer that comes to my mind would be to use the process ID. This is
> necessarily unique across the entire server, correct? It seems to be
> exactly what I need. But I can't seem to figure out how to determine the
> current process ID from within PHP. Is this even possible?
>
> Any ideas?
>[/color]

here's something of a suggestion

use a file, only one file handle can be opened at one time. This is the
first thing. Next is to use timestamps. The last is to build a system
that'll try and retry to open the file untill successfull. Last thing is to
ensure you don't get duplicate id's. Here's how:

<?php

for($i=0;$i<10;$i++){
// If for some reason the opening of the file never succeeds,
// attempt at most 10 times
$fp = fopen("timestamp.log","r+");
// attempt tp open file
if($fp){ // file opened
// read the previous timestamp from file
$prev_timestamp = fread($fp, 1024);
// create own timesampt from cpu time
$my_timestamp = microtime();
// make sure they don't match
if($my_timestamp!=$prev_timestamp){
// place filepointer in the beginning
rewind($fp);
// write your own timestamp as
//"previous" for the next instance
fwrite($fp, $my_timestamp, 1024);
// close, so the next instance can open
fclose($fp);
// exit for-loop
break;
}
}
// unsuccesful? wait 1 second and retry...
sleep(1);
}
?>

I'm not sure at all of this works or not and I haven't tested it, but it's
an idea for you to experiment with...

re: Getting your own process ID

"Joshua Beall" <jbeall@donotspam.remove.me.heraldic.us> writes:
[color=blue]
> Hi All,
>
> I am doing some work where I want to do locking, and prevent scripts from
> running in parallel. I see that I could use the semaphore mechanism, but
> I'd like for my code to be portable, and that extension is not enabled in
> many places.
>
> I need some way for a process to uniquely identify itself. It can then look
> at the storage container (flat file, DB, whatever is appropriate in
> context), check to see if the requested semaphore is available, and if it
> is, acquire it and then mark itself as the owner. It can then check that it
> did in fact get ownership (as opposed to another process which attempted to
> acquire it at the exact same moment) before proceeding.
>
> However I am stumped at the point where it needs to indicate ownership. How
> does the PHP script identify itself? It can't use the script name,
> obviously - lots of instances of the script may be running. It can't use
> session ID - the user might submit duplicate requests, and they would both
> have the same session ID. The best I have been able to think of is to use
> the sha1(microtime()) to generate a unique key. But this isn't quite
> foolproof, as it is theoretically possible, though unlikely, for two
> requests to be at the exact same instant.
>
> The answer that comes to my mind would be to use the process ID. This is
> necessarily unique across the entire server, correct? It seems to be
> exactly what I need. But I can't seem to figure out how to determine the
> current process ID from within PHP. Is this even possible?
>
> Any ideas?
>
> -Josh
>
> p.s. Please forgive me if I have misused the term "semaphore" - I know it
> only from the context I have heard it used in, I don't know the textbook
> definition.
>
>[/color]
Check out the following function calls:
getmypid()
uniqid()

Complete documentation at http://www.php.net/

--
John
__________________________________________________ _________________
John Murtari Software Workshop Inc.
jmurtari@following domain 315.635-1968(x-211) "TheBook.Com" (TM)
http://thebook.com/