Understanding How a Projection Partitions State per Entity

EventStoreDB
1

I am completely new to ES, so apologise in advance for the stupid question. I’m trying to use projections in JS to process different event types from a single stream to build up state per policy_id and write to a new stream when a certain event is encountered (insurance_policy.bound).

The source stream has the following event types per policy_id:

  • insurance_policy.created
  • Many insurance_policy.rate_added
  • insurance_policy.bound

My code is below and I am wondering about the following:

  • How does the projection know to partition the state per policy_id
  • I am wondering do I need to do “Reset state after flushing to new stream” part? If I remove s.rates = [] I get rates being mixed up in the emitted event from different policy rate_added events.
fromStream('policies')
.when({
    $init:function(){
        return {
            transaction_type: null,
            policy_id: null,
            agency_data: {},
            rates: []
        }  
    },
    "insurance_policy.created": function(s, ev){
        s.policy_id = ev.data.insurance_policy_id
    },
    "insurance_policy.rate_added": function(s, ev) {
        s.rates.push({
            policy_rate_id : ev.data.insurance_policy_rate_id,
            key : ev.data.key
        })
    },
    "insurance_policy.bound": function(s, ev) {
        s.transaction_type = 'bound'
        emit(
            'analytics.policy_transactions',
            'policy_bound',
            s
        )

        // Reset state after flushing to new stream
        s.transaction_type = null
        s.policy_id = null
        s.agency_data = {}
        s.rates = []        
    }
})

Thank you for your help.
Jag

2

Jag,

If you want to partitions per Policy, I think you would need to do this:

fromCategory(policies)
.foreachStream()

That would (concurrently) process each of your policies.
This means you would have state per policy (assuming that’s what you need)

You can then emit you new event.

3

Currently the single stream I am reading from has events for all policies in it. Is there a way keep this and still have state per policy ID in the projection, or do I need to split the source stream into different ones like so?
policies-1
policies-2
policies-3

Disclaimer: I maybe thinking about this the wrong way :slight_smile:

Thanks!

4

Jag,

You could create a new projection which would manage building the polciy streams.
So something like:

fromStreams(‘policies’)
.when({
$any: function (s, e) {

        if (e === null || e.data === null || e.data.IsJson === false)
            return;

	var streamname = "policy-" + e.data.policyId.replace(/-/gi, "");
	
        //linkTo or emit to using e and streamname
    }
});

You could then use another proejction (using the foreach example) for partitions.

Just a side note though, if you are putting all policies into one stream, is there not a danger this stream will become too big?
Do you have specific domain rule which is forcing you to have 1 stream?

Without knowing anything about your domain, I would have thought 1 stream per policy, and if u need to check across all policies for a domain rule, then handle that in a different manner.

5

That worked Steven! Thanks!

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