How do I "lock" zodb objects?

I want the following process:

See if key is present in root.
[it's not]
Lock key write-access, so that no other client will try to write key.
Compute value for key.
Write value for key.
Unlock key.

How do I achieve this?
Thanks!

  Joseph

-- 
Academic: http://www-etud.iro.umontreal.ca/~turian/
Business: http://www.metaoptimize.com/
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