Fetch online users's friends for large quantity of users

Have you considered using a relational database instead? They excel at solving this kind of problem.

Either a relational database or a graph database will do the job. DynamoDB is definitely not suitable for modelling relationships.

Thanks but I’m not asking about designing the relation between users.
I’ve already done that and it works really well.

Now to be more specific, my needs are pretty simple, I just need to return a list of friends for a user.
With DynamoDB you can implement a sort of graph using proper primary and sort keys.
It is extremely fast !
I don’t need all the extra features I would get from a graph DB like fetching friends of friends…

This is not the issue I’m trying to solve here.

Whether it’s a graph db or dynamo db, you still have to query the database to fetch the list of friends and you still have to go over them to see which ones of them are online.

I might be missing something but to me this issue has nothing to do with the DB used.
DynamoDb is fine at mapping users’s friends with way better read performance than a SQL DB.

My question is about the strategy to fetch all those data for each user joining.

I could simply load a user’s friends on “demand”, basically when he joins.
Would that scale well when I have 50_000 people joining at the same time ?
May be, may be not but long term I think I need something more efficient to handle all that work.

Or I could take advantage of stuff like genstage/broadway to fetch/update those users’ friends with their current status ?

I might mis-understand your post, but, are you planning for high scale and lots of users being generally there at the same time , or for some reason, your total user number is reasonable (let’s say 80_000) but your domain has a reason for (almost) all of them joining at the same time, making huge bursts of sudden joins ?

I’d say it’s both, lots of users connected at the same time and “huge” bursts of sudden joins sometimes.
Though I’m more preoccupied by the burst of users joining, because my plan is to cache the user friends in ETS table so I would only fetch them once for each user.
Then as mentioned above I would have to filter them out based on whether they are currently available or not, but that step should be straightforward.

DynamoDB is capable of handling more than 10 trillion daily requests, with the ability to support peaks of over 20 million requests per second.
So my question really is to either fetch each user friends separately on demand, or centralize that in a genstage/broadway.

It has everything to do with the database used: the fastest operation is the one that never runs, and with a relational/graph database you can avoid the reads altogether. You ask the database for the friends which are online and get only those in the result, deferring the retrieval of the less-interesting offline friends for later, which may very well be cached end-user-side (e.g. localStorage) on a long-term basis.

The moment that you start doing relational operations over data that you have retrieved from a NoSQL database, you have invented your own half-finished, slow, and bug-ridden relational database.

You will need to adapt the application to make good use of the relational model, but once you have, a sustained “50000 users with 1000 friends each leaving and joining every second” – and likely many of your other problems too – become a non-issue.

Not all your data needs to be in a relational database either, but you may as well put it there: in my experience every clever NoSQL project ends up needing relations everywhere sooner or later anyway.

That expects that the db knows the online status. I guess this topic is about using Phoenix.Presence to track the online status while not having it stored in a database.

Yes: keep it updated, lazily if required. It’ll be less work in aggregate than having to shuffle hundreds or thousands of relation tuples for an application-side join every time a very social user joins.

No this topic is not about using Presence.

Whether you use Presence or not, you still need to get the list of your friends you want to subscribe to.
The topic is about fetching/updating efficiently user’s relationships and their status.

As mentioned above, Presence will be used but at the very last stage, when friends of a users have been fetched. I’m not concerned by that stage as I plan to use Presence sparingly on a subset of the friends returned