[ZODB-Dev] ReadConflictError while re-indexing big fat index

[Tim Peters]

[Roché Compaan]
...
> Anyway, if I'm not mistaken, a ReadConflict will occur if an object is
> invalidated after it was loaded by the client.

Almost:  change "after" to "before".

    you read object 1
    another transaction commits a change to object 2
    you get an invalidation for object 2 as a result
    you try to read object 2

Now you get a read conflict:  ZODB is protecting you from seeing object 2 in
a state that may be inconsistent with the state you already have for object
1.  If you had read object 2 before it was invalidated, no problem, then the
states you see for objects 1 and 2 are consistent.

MVCC takes a different approach to maintaining consistency:  when you try to
read object 2, under MVCC it doesn't matter that it has been invalidated.
Regardless of whether it was, you'll see the state object 2 had at the time
the transaction began, as if you had read object 2 before it was
invalidated.  This is conceptually straightforward, but required substantial
redesign so that caches could keep track of multiple revisions of an object
correctly.