How to reduce reserved RUs to reduce cost of DocumentDB

Question

We are using DocumentDB on azure. We have a single database with 7 collection, each having maximum 15 records. It does not require much storage.

Only a few developers are using this DB instance. So traffic is also below average.

Still this server is using 67,600 RUs per day. There must be some problem with DocumentDB settings. So, I'm looking for direction to analyse exactly how these RUs are charged and how to reduce it?

Answer 1

There's no problem with DocumentDB settings. You provisioned 7 collections. By default, via the portal, each collection is assigned 1000 RU (which you have at your disposal, regardless whether you use 0 RU or all 1000 RU). The minimum RU setting for a non-partitioned collection is 400.

EDIT - I misread - if you're at 67,000 RU, then you have likely provisioned several partitioned collections (which start at 10,100 RU). For initial dev/test, with only 15 documents, you've grossly over-allocated capacity.

Since you provisioned seven collections (which are likely partitioned, based on your RU sizing), you have a ~70,000 RU deployment. Regardless what you actually consume (you're essentially reserving capacity).

I have no idea what your app needs are, and whether you need 7 collections for some specific reason. But... objectively speaking, there is no rule that says you need to separate different document types into different collections. You can easily store heterogeneous data within a single collection. How you query for specific types is really up to you, but it's trivial to add something like a type property to each document).

Note, since I now believe you're using partitioned collections: You cannot convert these to non-partitioned collections; you'll need to create new non-partitioned collections and move your data from your partitioned collections. (given that you have 15 total documents, this should be trivial).

Note that a single non-partitioned collection may be scaled down to 400 RU. If you then combine your 7 collections into 1 collection, you should be able to reduce your consumption from ~70,000 => 400. (at least during dev/test).

EDIT As of February 2017, the minimum RU for partitioned collections dropped to 2,500 (from the original 10,100 minimum). In December 2017, it dropped again, to 1,000.

Answer 2

It's common for people new to DocumentDB to think of a collection similar to a table in SQL or even what MongoDB calls a "collection". However, DocumentDB is designed differently. It's best to use a single partitioned collection to store all document types and partition on something like geography, tenant, or user. You'll distinguish document types with a type = <MyType> field or I actually prefer to use myType = true approach so I can model inheritance and mixins.

This means, you'll only need to pay for a single partitioned collection. A single partitioned collection may still end up costing you more than table storage, but if you want DocumentDB's near infinite scalability later on, then I highly recommend you start out the way I'm describing.

One more note about David's suggestion to go with non-partitioned collections. That was the only option when DocumentDB first launched but it's now recommended to use partitioned collections. I suspect that non-partitioned collection option may be phased out at some point. You interact with them slightly differently and as David pointed out, there is currently no conversion assistance (especially if you use multiple non-partitioned collections) so transitioning later from non-partitioned collections to a partitioned collection is not hard but it's not as simple as changing your partition type and will cost you development effort. It'll cost you a little more to have a single partitioned collection than a single non-partitioned collection, but it's worth it to save transition costs later, IMHO and it'll cost you less to have a single partitioned collection than it costs to have seven non-partitioned ones.