Linking to multiple streams


If I have 10 streams called:





And I have a projection that processes an event of a certain type, can I issue a linkTo call that puts the event in all streams of a certain category?


If you looped yourself yes but this would be an expensive projection (each event of type x would write n events) so the write amplification in this case would be 10-1. What are you trying to accomplish here? What are the message loads?

The message loads are small right now but they won’t be for long. Sounds like it won’t work.

The scenario is partitioning messages for different users. A users join the system, a stream is set up for all types of event that the user might be interested in. This particular type of event would be considered as a system wide message that all users are interested in, therefore the event should be placed on all user streams.

Make sense? Is there a better way to achieve this or does it start to bend GES in a way it was not designed?


How many event types are there?

No more than 10 currently. And there is only 1 event type that should it be sent to GES would need to be linked to all user category streams.

Why not just have the users read a stream with those events in it?

I want the order in which all these events were raised to be used. So a system wide message could appear in the middle of a list of other even types/messages. So the order of the stream is important and ideally I would like to read from a single stream that represents a particular user.

You could do the equivalent of fromstreams() and join two streams.

How many events/sec are being discussed?

It will be really low for now. Would that merge taking into account the date each event was raised to create an ordered stream?

How might the projection(s) look?

I mean to do the equivalent from the outside (we have considered exposing fromstreams to the outside). Basically fromstreams just does a join of n streams.

If your message volume is low writing a link is no problem but it does not scale well imagine 1000 users where you have each event written 1000 times)

From the outside it would be perfect. I’d have a stream per user and then I could join on the system wide stream too. Messages in the system wide stream would be minimal.

Is there scope to expose this sort of thing?

You can also do it yourself. Listen to two streams with a slight delay to allow reordering. You can reorder the vents into correct order.

Tis true :slight_smile:

Thanks, food for thought.

I’ve a slightly more complex variant of this now that would make polling multiple streams externally a bit of none goer. Unless there is another way of looking at the problem…

Given a set of users, they can post messages to a central location, call it a wall for now. In posting a message they effectively join that wall and are interested in seeing further updates from it. If I look at the stream for the wall I should be able to see all interaction i.e. the messages from different users. This should work fine.

As a user I can view a separate stream of my own activity (again, this should be fine), plus I can see another view that aggregates all my interactions and other people’s interaction across the walls I am interested in. This is where it falls down.

So on receiving message events into the system I can linkto new streams of:



But then I need to produce other streams that represent a view for each user, including messages from other users, which we made on walls the base user is interested in.

I’m thinking of these things as a stack of items, which is wrong. It feels like there should be a solution but I may be looking to bend GES in a direction it was not designed for. Please say so if this is the case!