How to store large arrays in DynamoDB

Question

I am new to DynamoDB and I am curious what is the best way to store potentially large arrays.

I have a user object that looks like this:

UserId: String
Watching: Card[]
Listings: Card[]

I am aware there is a limit to the size of objects in Dynamo - I think 1MB? Therefore I think if a user had many listings it might go past this limit. What would be the best practice to store potential large arrays like this? Would it be to maybe store an array of CardIds and then do a second query to get cards from this?

Answer 1

One option is to use the Single Table Design approach where all users, watchings, and listings are in the same table.

User items would have a primary key of user#uid and sk of user#uid (the same as pk), each watching item would have a pk of user#uid and sk of watching#wid, and each listing would have a pk of user#uid and sk of listing#lid.

For example:

pk	sk	other attributes
user#1	user#1	yes
user#2	user#2	yes
user#42	user#42	yes
user#42	watching#12	optional
user#42	watching#29	optional
user#42	listing#901	optional
user#42	listing#472	optional

This approach has no real limit on the number of watching or listing relationships.

You can then query all items for a given user simply by issuing a query for pk=user#42 and that will yield the user and all associated watching and listing items (pagination notwithstanding). You can query all listings for a given user with pk=user#42 and sk begins_with("listing#").

Note that this will increase the table size because of the additional "user", "watching", and "listing" prefixes on attribute values so you may want to consider abbreviating those.

To quote Alex DeBrie:

The main reason for using a single table in DynamoDB is to retrieve multiple, heterogenous item types using a single request.