Re: fleeing sql

About multi threading in a client application, my opinion is similar : 
you have to lock something for writing. I don't know however the effect 
of a write operation while another thread is reading... maybe memory 
mapped file would save you, but I wouldn't bet on it.

On the other hand, in a web environment, where multiple processes would 
access the same metakit file, my opinion is that the "file lock" method 
as you explained cannot be reliable.

If I had to use metakit in an HTTP server-side application, and if I 
needed a write access, I would build a single process on the server that 
does all metakit access for all processes, and I would talk to that 
single process with sockets. Of course you loose the easy of metakit 
API, but your server is safe.

As I mentioned, I'm not sure of the effect of a write operation while 
another thread/process is reading. So I would also include read 
operations on this single process.

Maybe I'm wrong, please let me know.

David McNab wrote:
> On Wed, 2007-08-08 at 12:34 -0700, abused by speech recognition wrote:
>> If I remember correctly, metakit is multi-reader single writer right?
>> since I need multi-writer, guess I'm best solving that with embedding
>> metakit in a server using internal access locks?  does metakit work in
>> a python multi threaded environment either as multi thread access or a
>> single thread fed by multiple request threads/
>>
>> thanks for any feedback
> 
> In a python single-process multi-threading environment, I'm sure that
> metakit could be made to work well, even if you have to surround the
> access statements in each thread with .acquire() and .release() of a
> threading.Lock object.
> 
> I'm also interested in a multi-process scenario, such as CGI-based web
> server. This could be made to work well by tuning the http
> request/response cycle:
> 
>  - create a metakit.storage() object at start of the http response
>    cycle, with the database open in read mode
> 
>  - for a very small part of the http response cycle:
>      - acquire a file lock on the metakit database file
>      - create another metakit.storage() object on the file, this one
>        with the database open in read/write mode
>      - perform all needed write operations
>      - commit() the the read-write storage object
>      - del the read-write storage object
>      - release the file lock
> 
>  - continue with the rest of the http response cycle, using the
>    read-only instance of the storage object
> 
> The smaller the 'write window', the less the performance impact in a
> busy server environment.
> 
> My questions on this scenario:
>  - would it work?
>  - would there be any problems with having two different metakit.storage
>    objects open at the same time in the same process, one with readonly
>    access and one with read/write access?
>  - how would it compare, performance-wise, to a DB built for concurrency
>    such as MySQL?
> 
> Cautions with this approach:
>  - all View and row objects acquired from the read-write storage object
>    would have to be freed (ie, not stored in any object instances or
>    globals or closure-locals spaces) prior to doing the commit
> 
> Metakit can offer significant speed and simplification over *SQL
> databases. Mk4Py allows a friendly object interface to views and rows,
> where to get the same for *SQL databases, one needs to load in something
> like SQLObject, which is hugely expensive.
> 
> I would be interested in people's thoughts on all this.
> 
> Cheers
> David
> 
> 
> 
> 
> > 
> 

-- 
Très cordialement,

Riccardo Cohen
ligne directe : 02-47-49-63-24
-------------------------------------------
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