Scaling with regard to Nested vs Parent/Child Documents

Question

I'm running a proof of concept for us to run nested queries on more "normalised" data in ES.

e.g. with nested

Customer -> - name
- email - events -> - created - type

Now I have a situation where a list of events for a given customer can be moved to another customer. e.g. Customer A has 50 events Customer B has 5000 events

I now want to move all events from customer A into Customer B

At scale with millions of customers and queries are run on this for graphs in a UI is Parent/Child more suitable or should nested be able to handle it?

What are the pros and cons in my situation?

Answer 1

It's hard to give you even rough performance metrics like "Nested is good enough", but I can give you some details about Nested vs Parent/Child that can help. I'd still recommend working up a few benchmark tests to verify performance is acceptable.

Nested

• Nested docs are stored in the same Lucene block as each other, which helps read/query performance. Reading a nested doc is faster than the equivalent parent/child.
• Updating a single field in a nested document (parent or nested children) forces ES to reindex the entire nested document. This can be very expensive for large nested docs
• Changing the "parent" means ES will: delete old doc, reindex old doc with less nested data, delete new doc, reindex new doc with new nested data.

Parent/Child

• Children are stored separately from the parent, but are routed to the same shard. So parent/children are slightly less performance on read/query than nested
• Parent/child mappings have a bit extra memory overhead, since ES maintains a "join" list in memory
• Updating a child doc does not affect the parent or any other children, which can potentially save a lot of indexing on large docs
• Changing the parent means you will delete the old child document and then index an identical doc under the new parent.

It is possible Nested will work fine, but if you think there is the possibility for a lot of "data shuffling", then Parent/Child may be more suitable. Nested is best suited for instances where the nested data is not updated frequently but read often. Parent/Child is better for arrangements where the data moves around more frequently.