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[Sean Dague <sean@xxxxxxxxx>]

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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