We have an issue where we want to lazily create an entity if it does not
exist. There is some discussion going on about how to do this and I would
like to clarify some things around app engine transactions. I will limit my
query to single entity group transactions.
I am using Go in my examples, but I hope the code is clear enough for
non-Go programmers.
My understanding is that a transaction, on a single entity group, will
succeed only if the entity group is not modified externally during the
transaction. The 'entity group timestamp' indicating when an entity group
was changed is stored in the root entity of the entity group. So during a
transaction the current 'entity group timestamp' is read and the
transaction can only succeed if it hasn't changed by the end of the
transaction.
key := datastore.NewKey(c, "Counter", "mycounter", 0, nil)
count := new(Counter)
err := datastore.RunInTransaction(c, func(c appengine.Context) error {
err := datastore.Get(c, key, count)
if err != nil && err != datastore.ErrNoSuchEntity {
return err
}
count.Count++
_, err = datastore.Put(c, key, count)
return err
}, nil)
In the example above (taken from
https://cloud.google.com/appengine/docs/go/datastore/transactions) there
are two non-error cases, I can see.
1: The Get succeeds and the 'entity group timestamp' on the counter can be
used to ensure no other transactions update the counter during this
transaction.
2: The Get fails with ErrNoSuchEntity and the Put is used to store the
counter for the first time.
In the second case it is possible that another identical transaction is
running. If both transactions' Get return ErrNoSuchEntity how does the
datastore ensure that only one put succeeds? I would expect there to be no
'entity group timestamp' in the datastore to test against?
Does the transaction know that it needs to test for the non-existence of
the counter in order for the Put and the entire transaction to succeed?
Is there a chance in this case for two transactions to succeed and for one
Put to overwrite the other?
If there is documentation, or videos etc, around the mechanism that
controls this I would love to read it.
Thanks in advance.
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