Caching models like this probably really should see a spec just to think
through the access patterns, especially as people keep proposing to make
MD writable from the guest, which impacts this.
** Changed in: nova
Status: New => Opinion
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https://bugs.launchpad.net/bugs/1549814
Title:
Metadata caching is fundamentally broken
Status in OpenStack Compute (nova):
Opinion
Bug description:
The caching backend for memcached/dogpile stores whole Python objects
by creating a deep copy. Metadata is cached by storing the metadata
Python object, and therefore the whole object tree underneath,
including the "instance" attribute. Whenever the metadata is fetched,
it also fetches the old version of instance with it, and works on it.
This has several implications, like for example lazy fetched fields of
the "instance" object are never cached, if they are not pre-fetched
before caching. Also it might implicate security issues.
It would be probably a better approach to actually cache the DB query
results only, and not the whole populated "living" object.
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