I am using cloud endpoints with objectify and Firestore in Datastore mode. Although it says in the documentation that all queries are strongly consistent, I have found that they are not in the following examples:
Example 1
I made an endpoint that queries for an entity by a property, adds +1 to a count property on it, and saves it back to the datastore. I then have 50 different clients all execute that method at the same time. I would expect the count property to be 50, however, it usually ends up being somewhere between 25-30.
Example 2
I have an endpoint that queries for an entity by a property. If the entity does not exist, I create the entity and save it to the datastore. If it exists, I just return it. Again, I hit this endpoint with 50 different clients at the same time. I would expect there to only be one entity in the Datastore. However, I will have maybe 5-10 of the same entity.
It seems to me this is not strongly consistent. If I take my code in the above endpoints and put them in a transaction with retries, all works as intended. I looked around in objectify to see if there is a ReadOptions set somewhere, but from what I can see, there is not, so it should be using the default of read_consistency=STRONG
For example 1, you need to use transactions to ensure that writes do not stomp on each other.
For example 2, again you need to use a transaction to get consistency across clients.
Strong consistency means that if a client writes a value, it can read or query it back after the write succeeds. Not that if a client reads a value, another reads the same value, they each do a transformation, and try to write that the blinds writes for each client will merge together.