[Twisted-Python] pb.Cacheable doc question

[Glyph Lefkowitz]

On Oct 8, 2010, at 9:25 AM, exarkun at twistedmatrix.com wrote:

> On 5 Oct, 08:09 pm, stephen.c.waterbury at nasa.gov wrote:
>> First, the "PB Copyable: Passing Complex Types" doc is
>> *great* and the examples are excellent -- my compliments to
>> all who contributed!
>> 
>> My question is about the pb.Cacheable section
>> (http://twistedmatrix.com/documents/current/core/howto/pb- 
>> copyable.html#auto9)
>> -- specifically the first sentence:  'Sometimes the object you
>> want to send to the remote process is big and slow. "big" means
>> it takes a lot of data (storage, network bandwidth, processing)
>> to represent its state.  "slow" means that state doesn't change
>> very frequently.'
>> 
>> I would think that the product of its size and its rate of change
>> is the applicable metric -- i.e.:  the bigger the object is *or*
>> the faster it changes (not the slower), the more applicable
>> Cacheable is, no?
> 
> That seems plausible.  I wonder if the rate comment is motivated by 
> something else, like the chance of the remote cache being out of date 
> when the remote side wants to use some of its data.  This would increase 
> with the rate of change, but I don't know if it really matters.  I 
> haven't ever actually used a Cacheable myself, as far as I can recall.

I think I probably wrote that paragraph, and it was not very well put.  Big objects which are "fast", i.e. change constantly, are perfectly suitable for Cacheables.

The point I believe I was trying to make there was that if a significant proportion of the object's data is changing quickly, Cacheable doesn't make much of a difference over just re-Copyable-ing the whole object, since the delta updates will be the same size as the whole object.

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