Question

I am trying to wrap my head around Entity Groups in Google AppEngine. I understand them in general, but since it sounds like you can not change the relationships once the object is created AND I have a big data migration to do, I want to try to get it right the first time.

I am making an Art site where members can sign up as regular a regular Member or as one of a handful of non-polymorphic Entity "types" (Artist, Venue, Organization, ArtistRepresentative, etc). Artists, for example can have Artwork, which can in turn have other Relationships (Gallery, Media, etc). All these things are connected via References and I understand that you don t need Entity Groups to merely do References. However, some of the References NEED to exist, which is why I am looking at Entity Groups.

From the docs: "A good rule of thumb for entity groups is that they should be about the size of a single user s worth of data or smaller."

That said, I have a couple hopefully yes/no questions.

Question 0: I gather you don t need Entity Groups just to do transactions. However, since Entity Groups are stored in the same region of Big Table, this helps cut down on consistency issues and race conditions. Is this a fair look at Entity Groups and Transactions together?

Question 1: When a child Entity is saved, do any parent objects get implicitly accessed/saved? i.e. If I set up an Entity Group with path Member/Artist/Artwork, if I save an Artwork object, do the Member and Artist objects get updated/accessed? I would think not, but I am just making sure.

Question 2: If the answer to Question 1 is yes, does the accessing/updating only travel up the path and not affect other children. i.e. If I update Artwork, no other Artwork child of Member is updated.

Question 3: Assuming it is very important that the Member and its associated account type entity exist when a user signs up and that only the user will be updating its Member and associated account type Entity, does it make sense to put these in Entity Groups together?

i.e. Member/Artist, Member/Organization, Member/Venue.

Similarly, assuming only the user will be able to update the Artwork entities, does it make sense to include those as well? Note: Media/Gallery/etc which are references to Artwork may be related to lots of Artwork, not just those owned by the user (i.e. many to many relations).

It makes sense to have all the user s bits in an entity group if it works the way I suspect (i.e. Q1/Q2 are "no"), since they will all be in the same region of BigTable. However, adding the Artwork to the entity group seems like it might violate the "keep it small" principal and honestly, may not need to be in Transactions aside from saving bandwidth/retrys when users are uploading artwork images.

Any thoughts? Am I approaching Entity Groups wrong?

Answer 1

0: You do need entity groups for transactions among multiple entities
1: Modifying/accessing children does not modify/access a parent
2: N/A
3: Sounds reasonable. My feeling is, entity groups should not be used unless you need transactions among them.

It is not necessary to have the the Artwork as a child for permission purposes. But if you need transactional modification to them (including e.g. creation and deletion) it might be better. For example: if you delete an account, you delete the user entity but before you delete the child, you get DeadlineExceeded or the server crashes. Now you have an orphaned Artwork. If you have more than 1,000 Artworks for an Artist, you must delete in batches.

Good luck!

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