CODEWITHSUNDEEP
google-app-enginegoogle-cloud-datastore
aloo
aloo

Reputation: 5389

Does eventual consistency apply to the set of results of a query? Or the entities themselves that are returned?

I'm using the HRD on Appengine.

Say I have a query that cuts across entity groups (i.e. not an ancestor query). I understand that the set of results returned by this query may not be consistent:

For example, the query may return 4 entities {A, B, C, D} even though and 5th entity E, matches the query. This makes sense.

However, in the inconsistent query above, is it ALSO the case that any of the results in the set may themselves not be consisitent (i.e. their fields are not the freshest)? That is, if A has a property called foo, is foo consistent?

My question boils down to, which part of the query is inconsistent - the set of results, the properties of the returned results, or both?

Upvotes: 2

Views: 287

Answers (2)

lukelazarovic
lukelazarovic

Reputation: 1500

I think the answer is that inconsistency can occur in both the set of results and properties of the returned results. Because incosistency occurs when you query a replica (or data center as in Google docs) that doesn't know yet about some write you made before. And the write can be anything, creating new entity or updating existing one.

So if you have for example the entity A with property x and you:

  1. update x on A to 50 (previously it was 40)
  2. query for entities with x >= 30

Then you certainly get this entity in the resut set but it can has an old value of x (40), in case that the replica you queried didn't yet know about your update.

Upvotes: 0

Nick Johnson
Nick Johnson

Reputation: 101139

Eventual consistency applies to both the entities themselves and the indexes. This means that if you modify an entity, then query with a filter that matches only the modified one (not the value before modification), you could get no records. It also means that potentially you could get entities back from a query whose current versions do not match the index criteria they were fetched for.

You can ensure you have the latest copy of an entity by doing a consistent get (though outside a transaction, this is fairly meaningless, since it could have changed the moment you do the get), but there's no equivalent way to do a consistent index lookup.

Upvotes: 3

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