Should I use one projection per entity or category?

Question

I am coding a new application usign CQRS+ES architecture with Event Store DB. In my app, I have the following streams:

• user-1
• user-2
• user-3
• ...

Each stream contains all events regarding a given user.

I am now creating a projection called user-account, which consists in basic data regarding my user's account (like first name, email, and others)

What is the optimal way to design that projection?

I should have a single projection for each user, creating projections called:

• user-account-1
• user-account-2
• user-account-3
• ...

Or a single projection for all user-accounts? Being it a key-value pair record (that may store millions of keys in the future)

Answer 1

You can go with one stream per user. Projections are like dimensions. A user can exist in different "dimensions" (CDC naming) and have a different shape in each.

Read https://www.eventstore.com/blog/the-cost-of-creating-a-stream

Answer 2

First, subscribing to individual streams (aggregate or entity streams) won't ever work. You will end up with thousands of subscriptions, which are sitting there doing nothing (how often the user details change?).

The category stream is one way to go, you will project all the events for all the users. Not only you need just one subscription for all your users, but you'll also have more interesting possibilities like "users pending activation" or "blocked users" projections.

I prefer subscribing to $all and apply server-side filtering if necessary. It might have a bit of overhead as you receive more events than you need, but you get so much more power by combining events from different aggregates.

I wrote a little about it in Eventuous documentation.