Projections c# api

EventStoreDB
1

Hi,

So far got the server working (run as admin), and got it to save events to a stream, and load events from a stream.

Been meaning to have a play around with it since going to the skills matter launch.

I’m looking for some examples of projections, and how can use the C# API to read from a projection.

Sounds like can use a projection to read from streams and create another stream, is this the way to do it?

Scenario

Say I was recording events on certain actions users were performing.

And I wanted to know how many of each type of these actions a user had done.

How could I do this?

I imagine I would have a stream per user.

And thinking there must be a better way than loading the events in and counting how many of each type there are, but finding a lack of documentation.

And if I wanted to know how many each type of these action a user had done on different days (e.g. multiple events of the same type done on the same day would could as 1)

Should I be doing this outside of Event Store? And recording the last event id that I had processed for a user, and just replay new events?

Or does projections fit in?

Just trying to get an overview of how Event Store fits in for my scenario.

Kind Regards

Ian

2

Sorry for the delay in replying to this - it might be worth taking a look at the latest version on dev (which will be merged to master at some point this week).

You can do this using a projection, either inside the event store, or outside if you host it yourself and subscribe either to individual streams or use the ReadAllEvents* methods.

I’ll post an example of the JS you can use for doing this with a projection shortly.

3

The projection you want would look something like:

fromAll().foreachStream().whenAny(function(state,event) {

//filter out system streams

if(event.streamId.indexOf("$") != 0)

{

var type = event.eventType;

//probably dont want to log types like $stream-created

if(type.indexOf("$") != 0)

{

if(state[type])

{

state[type]++;

} else {

state[type] = 1;

}

}

}

return state;

});

You can then access the stats for a user over http by:

http://localhost:2113/projection/{projection name}/state?partition={your user stream id}