Description:
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I'm not a PHP extension developer, nor am I familiar with 
APC's design, but from what I can tell by reading the source 
in the SVN, apc_store/add/delete are not atomic.

IMHO this destroys the main advantage of being able to pass 
multiple key/value pairs into apc_store/add/delete().

For example, my application caches data for a single large 
tree. Each node is stored as a separate cache item. Child 
nodes are referenced in parent nodes and vice versa. If the 
tree is modified (e.g. nodes added, removed, etc.) and the 
parent/child cache items are updated individually, the tree 
may be read in a broken state (nodes can either be read from 
child->parent or parent->child). If apc_store/add/delete 
worked in an atomic fashion, the tree would never be 
readable in a partially-updated (broken) state.