Memcache: Should we save Model instance or protocol buffer?

[Nickolas Daskalou]

Is it better/safer to store a Model instance into Memcache directly (Method 1 below), or should we convert it to a protocol buffer first, then store it (Method 2 below)?

Method 1:

memcache.set(cache_key, entity)
...
entity = memcache.get(cache_key)


Method 2:

memcache.set(cache_key, db.model_to_protobuf(entity))
...
entity = db.protobuf_to_model(memcache.get(cache_key))


I'm assuming Method 2 results in a smaller Memcache footprint, yes?

Nick

[Sylvain]

I think the answer (and more) is here :
http://blog.notdot.net/2009/9/Efficient-model-memcaching

[Nickolas Daskalou]

Perfect, thanks Sylvain. :)

Nick

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[Andy Freeman]

Note that memcaching a protocol buffer has interesting consequences.
One is that the auto-now datetime properties are updated when the
protocol buffer is created. This update is just on the protocol
buffer - it doesn't affect the datastore copy.

http://code.google.com/p/googleappengine/issues/detail?id=2402

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[Nickolas Daskalou]

Thanks Andy. Do you know what happens to a RefenceProperty that has already had the referenced entity "loaded" on the Model instance? Does the referenced entity also get saved in the ProtocolBuffer, or is only its key saved?

[Andy Freeman]

I'm pretty sure that the protocol buffer saves just the key.

However, I'm virtually certain that pickle will save both the instance
and the key if an reference property has been used. And, if that
instance has used reference properties, their instances are also saved
by pickle. I mention pickle because that's what memcache uses to
store instances.

It's an interesting choice - do you pickle instances as is, which
won't change their datetime now properties, but will save instances if
you've used the a reference property that names them. Or, do you
memcache a protocol buffer, which will update datetime now properties
but won't save instances associated with reference properties.

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