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Test race condition? (unit test?)

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  Test race condition? (unit test?) Simon <bad@example.com> - 2011-10-10 10:16 +0200
    Re: Test race condition? (unit test?) Jerry Stuckle <jstucklex@attglobal.net> - 2011-10-10 08:36 -0400
      Re: Test race condition? (unit test?) Simon <bad@example.com> - 2011-10-10 15:48 +0200
        Re: Test race condition? (unit test?) The Natural Philosopher <tnp@invalid.invalid> - 2011-10-10 15:43 +0100
        Re: Test race condition? (unit test?) Jerry Stuckle <jstucklex@attglobal.net> - 2011-10-10 13:42 -0400

#3358 — Test race condition? (unit test?)

Hi,

I have written a very simple file based lock/unlock system.
the way it works is as follows.

process A looks for File 'X', if file 'X' does not exists it tries to 
creates an empty file, 'Xa', and locks it, it then creates file 'X' and 
deletes file 'Xa'.

process B is also looking for File 'X', if file 'X' does not exists it 
tries to create 'Xa', on failure it waits for a seconds and tries to 
obtain the lock again. Once/if the lock is obtained, it looks for file 
'X' again and either uses it or creates it.

process C looks for file 'X', if file 'X' exists it will use it.

I try to write as much unit tests as possible and, as this is an 
important process, I would like to test the process above.

1- Can you think of any way I could test the above? As far as I know you 
cannot start multiple threads in php, (even in a dev environment).

2- Is there a better way of getting a lock like I am doing above?

Many thanks

Simon

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#3363

On 10/10/2011 4:16 AM, Simon wrote:
> Hi,
>
> I have written a very simple file based lock/unlock system.
> the way it works is as follows.
>
> process A looks for File 'X', if file 'X' does not exists it tries to
> creates an empty file, 'Xa', and locks it, it then creates file 'X' and
> deletes file 'Xa'.
>
> process B is also looking for File 'X', if file 'X' does not exists it
> tries to create 'Xa', on failure it waits for a seconds and tries to
> obtain the lock again. Once/if the lock is obtained, it looks for file
> 'X' again and either uses it or creates it.
>
> process C looks for file 'X', if file 'X' exists it will use it.
>
> I try to write as much unit tests as possible and, as this is an
> important process, I would like to test the process above.
>
> 1- Can you think of any way I could test the above? As far as I know you
> cannot start multiple threads in php, (even in a dev environment).
>
> 2- Is there a better way of getting a lock like I am doing above?
>
> Many thanks
>
> Simon

Race conditions are always difficult to duplicate and troubleshoot when 
they occur.  And although with some third party tools you could process 
php scripts in different threads, you probably don't want to (locking 
between threads is different than locking between processes).  The way 
to typically troubleshoot such problems is to run multiple copies of the 
script/program with waits for input at critical places.  Run the script 
concurrently from multiple windows to emulate the system.

The real question here is - what is the problem you're trying to 
resolve.  Without knowing that, it's difficult to recommend if there is 
a better way.

-- 
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
==================

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#3364

>
> The real question here is - what is the problem you're trying to
> resolve. Without knowing that, it's difficult to recommend if there is a
> better way.
>

Thanks for the reply. I am not trying to solve a problem per say.
I am 'just' trying to prevent duplicate work.

When a request for a certain page first comes, it make a few calls to 
the db and then caches the page.
Once the page is cached, only one call needs to be made.

But I sometime get hundreds of calls for a certain page, all at the same 
time.
So it would make no sense for all of them to create the very same cache 
page.

It makes more sense for the first request to cache the page wile others 
wait a few milliseconds for it.
By locking the page, the first requests is instructing others that the 
work is already been done and they should wait a few milliseconds.

Every so often, (for various reasons), the cache is marked as 'dirty' 
and a new cache will then be recreated.
If one request 'locks' the page, and a cache is available, then the 
other requests will continue to serve the dirty page.

I just want to prevent the same cache been created by many requests at 
the same time.

I hope this makes sense.

Simon

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#3365

Simon wrote:
>>
>> The real question here is - what is the problem you're trying to
>> resolve. Without knowing that, it's difficult to recommend if there is a
>> better way.
>>
> 
> Thanks for the reply. I am not trying to solve a problem per say.
> I am 'just' trying to prevent duplicate work.
> 
> When a request for a certain page first comes, it make a few calls to 
> the db and then caches the page.
> Once the page is cached, only one call needs to be made.
> 
> But I sometime get hundreds of calls for a certain page, all at the same 
> time.
> So it would make no sense for all of them to create the very same cache 
> page.
> 
> It makes more sense for the first request to cache the page wile others 
> wait a few milliseconds for it.
> By locking the page, the first requests is instructing others that the 
> work is already been done and they should wait a few milliseconds.
> 
> Every so often, (for various reasons), the cache is marked as 'dirty' 
> and a new cache will then be recreated.
> If one request 'locks' the page, and a cache is available, then the 
> other requests will continue to serve the dirty page.
> 
> I just want to prevent the same cache been created by many requests at 
> the same time.
> 

well if you are using a database, it will already have cached the data 
comprising the page.

Unless your manipulation of that page is excessively memory or CPU 
intensive, there is little point in caching the result too.

And that is probably something a proxy server does better, anyway.



> I hope this makes sense.
> 
> Simon

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#3368

On 10/10/2011 9:48 AM, Simon wrote:
>>
>> The real question here is - what is the problem you're trying to
>> resolve. Without knowing that, it's difficult to recommend if there is a
>> better way.
>>
>
> Thanks for the reply. I am not trying to solve a problem per say.
> I am 'just' trying to prevent duplicate work.
>
> When a request for a certain page first comes, it make a few calls to
> the db and then caches the page.
> Once the page is cached, only one call needs to be made.
>
> But I sometime get hundreds of calls for a certain page, all at the same
> time.
> So it would make no sense for all of them to create the very same cache
> page.
>
> It makes more sense for the first request to cache the page wile others
> wait a few milliseconds for it.
> By locking the page, the first requests is instructing others that the
> work is already been done and they should wait a few milliseconds.
>
> Every so often, (for various reasons), the cache is marked as 'dirty'
> and a new cache will then be recreated.
> If one request 'locks' the page, and a cache is available, then the
> other requests will continue to serve the dirty page.
>
> I just want to prevent the same cache been created by many requests at
> the same time.
>
> I hope this makes sense.
>
> Simon

Are you sure you need this?  Databases will cache the results and future 
db requests for the same information are generally quite fast.

Are you having performance problems?  And if so, have you looked at 
where your performance problems are?  And if so, have you tried to 
optimize your code?

Depending on what's going on, trying to cache pages can actually add 
overhead to the server instead of relieving it.

-- 
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
==================

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