Are projections good for this?

EventStoreDB
#1

Hello Community! :slight_smile:

Do the projections have enough power to solve this?

I have two event types:

A: {	
	Message,
	CorrelationId
}

and

B: {	
	ParentCorrelationId,
	CorrelationId
}

I would like for each event A emit new event C:

C : {
	Message,
	ContextPath
}

where ContextPath is CorrelationId1/CorrelationId2/CorrelationId3/… computed from B (join) events.

For example, for sequence:

A: {Message: "something", CorrelationId: 1}
A: {Message: "something2", CorrelationId: 1}
B: {ParentCorrelationId: 1, CorrelationId: 2}
A: {Message: "something3", CorrelationId: 2}
A: {Message: "something4", CorrelationId: 1}
B: {ParentCorrelationId: 2, CorrelationId: 3}
A: {Message: "something5", CorrelationId: 3}
B: {ParentCorrelationId: 1, CorrelationId: 4}
A: {Message: "something6", CorrelationId: 4}

We should produce:

C: {Message: "something",  ContextPath: "1"}
C: {Message: "something2", ContextPath: "1"}
C: {Message: "something3", ContextPath: "1/2"}
C: {Message: "something4", ContextPath: "1"}
C: {Message: "something5", ContextPath: "1/2/3"}
C: {Message: "something6", ContextPath: "1/4"}

Is it possible with projections?

#2

I think so, but are Events A and B on the same stream?
If your proejction montiros the stream(s) that A and B are written to.
Assuming correlationId would be you partition id?

#3

Well, if you think it is possible, I will try to write a projection or a group of projections that do this.

Although I think it will not be easy to construct CorrelationId 1 / CorrelationId 2 / … path by sequential observation of events A and B with projection state that has constant size and not contain array of previous events.

As for your questions. Yes, I planned to have events A and B on the same stream, but if that helps, they can be on different streams.

“Assuming correlationId would be you partition id?” - are you suggesting using partitionBy() function?

#4

Yeah, using partitions could solve this.
You only need to actually use partitionBy if you want to change the partition name.

For example, I have a projection which processes all my sales:

fromCategory("SalesTransaction")
    .foreachStream()
    .when({
            $init: function(s, e) {
                return initialiseState();
            },
            $any: function(s, e) {
				
            }
        });

This will create the partition for me (say I had a called SalesTransaction- d547edae2cd94d5a8eb4371045c79567)
You would only need to call byPartition() if you wanted to change the name of that.

Does that help?

#5

Thank you for your response.
I’ve tried to solve mentioned problem using projections but I encountered big difficulties.
So I gave up and decided to use C# and separate program for this.
Projections are not for everything.
And there are strange restrictions, for example: why can’t two projections write to the same stream?

#6

why can’t two projections write to the same stream?

It would mess up the checkpoint on the projection stream and give you non deterministic results in case of a database crash. Longer answer here: "Multiple projections emitting to the same stream detected" after recreating a projection