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From Caolan McMahon <caolan.mcma...@gmail.com>
Subject Re: CommonJS module updates
Date Tue, 22 Feb 2011 17:20:19 GMT
I can see the case for not wanting to store state on modules between
requests, as this could break caching rules for those resources. Even
though there is a performance hit. How would you know if the results
for a show or list function had changed?

It also sounds like this would be unreliable anyway since there are
multiple JS processes. However, I think we must have caching between
require()'s within a single request, as otherwise some modules will
not work and we can't handle circular dependencies.

The question is, should CouchDB enforce the fact that applications
should not store state between requests (except for caching) and take
the performance hit, or should this be left to commonjs module
developers?

I assume freezing modules would cancel out the benefits of increased
module compatability...


On 22 February 2011 16:57, Paul Davis <paul.joseph.davis@gmail.com> wrote:
> On Tue, Feb 22, 2011 at 11:47 AM, Caolan McMahon
> <caolan.mcmahon@gmail.com> wrote:
>> As some of you may know, I've been working on a couchapp framework
>> which makes heavy use of commonjs modules (http://kansojs.org). While
>> developing this I've run into a number of issues which prevent the use
>> of some modules, and makes writing my own more difficult:
>>
>> 1. Modules are not cached - eval'ing a complex application, consisting
>> of many modules on each request would have an impact on performance.
>> It also means you can't use modules which use the module object to
>> store state. This is commonly used by template libraries to store
>> loaded templates in a cache, or 'memoize' expensive functions.
>>
>> 2. Circular dependencies blow the stack - Its not possible to require
>> module A from module B, if module B also requires module A. This
>> happens more often than you might think, and is handled by other
>> require() implementations by setting the cached module to an empty
>> object before eval'ing it. The fix for this requires a module cache to
>> be in place.
>>
>>
>> Since these are really hindering progress, I've forked on github and
>> committed my proposed patches with associated tests:
>> https://github.com/caolan/couchdb/compare/7f553e82ef...6c66675a23
>>
>> Please, if you have time, review the code and provide me with your feedback.
>>
>>
>> Thanks,
>>
>> Caolan
>>
>
> The circular dependency section looks good.
>
> The bit on caching and testing that things are cached is not good on
> the other hand. The way that JS processes are used you can never be
> sure if it'll be the same os process handling the request. In the test
> suite, its more than likely to be the same OS process because of how
> the server gets restarted often and there's a single serialized
> client.
>
> I'm a bit iffy on whether we should cache modules because that'd be a
> pretty easy place to break view updates and could lead to other weird
> bits in the show/list stuff. Though I also understand the concern for
> avoiding all the recompilation. I wonder, is it possible to freeze the
> module maybe?
>

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