Aggregating events

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Aggregating events

Projections can also be configured to merge events from different aggregates which makes it easy to build things such as reporting features or similar.

Creating your first aggregated projection
To illustrate aggregated projections we will create an aggregated projection that counts the amount of paid orders and their total amount. We use a POST command to create a new projection definition:
cURL
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curl -i https://api.serialized.io/projections/definitions \
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  --header "Content-Type: application/json" \
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  --header "Serialized-Access-Key: <YOUR_ACCESS_KEY>" \
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  --header "Serialized-Secret-Access-Key: <YOUR_SECRET_ACCESS_KEY>" \
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  --data '
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  {
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    "projectionName": "order-totals",
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    "feedName": "order",
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    "aggregated": true,
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    "handlers": [
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      {
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        "eventType": "OrderPlacedEvent",
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        "functions": [
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          {
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            "function": "add",
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            "targetSelector": "$.projection.orderAmount",
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            "eventSelector": "$.event.orderAmount"
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          },
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          {
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            "function": "inc",
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            "targetSelector": "$.projection.orderCount"
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          }
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        ]
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      }
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    ]
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  }
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  '
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Query an aggregated projection
Since aggregated projections are combinations of multiple aggregates they don’t have any id’s. We query them directly by their name:
cURL
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curl -i \
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  --header "Serialized-Access-Key: <YOUR_ACCESS_KEY>" \
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  --header "Serialized-Secret-Access-Key: <YOUR_SECRET_ACCESS_KEY>" \
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  https://api.serialized.io/projections/aggregated/order-totals
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Response
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HTTP/1.1 200 OK
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Content-Type: application/json
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Date: Wed, 20 Sep 2017 11:45:27 GMT
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ETag: "7b2b05dab4a33088d9aee48b90c4a786"
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Last-Modified: Tue, 19 Sep 2017 19:53:08 GMT
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Vary: Accept-Encoding
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Content-Length: 91
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Connection: keep-alive
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​
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{
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  "projectionId": "order-totals",
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  "updatedAt": 1505850788368,
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  "data": {
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    "orderAmount":"1000",
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    "orderCount":"2"
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  }
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}
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Given that we have two OrderPlacedEvent that together sum up to 1000 we should get the response above.

Projecting events

Reacting to events

Last modified 3yr ago

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Contents

Creating your first aggregated projection

Query an aggregated projection