To create useful projections we need to merge the event data with the projection data in different ways. We provide templating support built on JsonPath to support this out-of-the-box.

If the templating support we provide is too restricted for your use-case we encourage you to write your own projector, deployed either as an external function (hosted on eg. AWS) or external service using the Feed API and a storage of your choice. Read more about customized projectors in the next chapter.

The structure of a handler using the provided JsonPath templating looks like this:

{
  "eventType": "<YOUR_EVENT_TYPE>",
  "functions": [
    {
      "function": "merge",
      "targetSelector": "$.projection",
      "eventSelector": "$.event"
    }]
}

This example will merge the content of the incoming event by using the JsonPath selector $.event with the current state of the projection by targeting the full projection state using the JsonPath selector $.projection.

In a handler function you always have access to $.event (the current event being handled) and $.projection (the current projection state).

Besides that, there is also a $.metadata struct containing the fields aggregateId, timestamp, createdAt and updatedAt.

The above example can also be simplified to:

{ "function": "merge" }

since the default values for targetSelector and eventSelector are $.event and $.projection, respectively.

Providing raw data

If you have a static value that you want to use as the data for the projection instead of some value that is provided in the event, you can use the rawData argument instead of the eventSelector. You can then provide any JSON value which will be used as-is.

Basic object modification

merge

Merges event data with existing projected data.

{
  "eventType": "OrderStatusChangedEvent",
  "functions": [
    {
      "function": "merge"
    }
  ]
}

set

Sets/replaces the value of an existing key.

{
  "eventType": "EmailUpdatedEvent",
  "functions": [
    {
      "function": "set",
      "targetSelector": "$.projection.user.email",
      "eventSelector": "$.event.newEmail"
    }
  ]
}

unset

Removes an existing key.

{
  "eventType": "EmailRemovedEvent",
  "functions": [
    {
      "function": "unset",
      "targetSelector": "$.projection.user.email"
    }
  ]
}

clear

Clear the entire projection or a specific field, i.e reset it to a default value.

{
  "eventType": "ListClearedEvent",
  "functions": [
    {
      "function": "clear",
      "targetSelector": "$.projection.todoList"
    }
  ]
}

delete

Delete a projection instance entirely. Future requests will result in a 404 Not Found.

{
  "eventType": "UserDeletedEvent",
  "functions": [
    {
      "function": "delete"
    }
  ]
}

Working with arrays/lists

append

Adds anything to the end of an array.

{
  "eventType": "RunnerFinishedRaceEvent",
  "functions": [
    {
      "function": "append",
      "targetSelector": "$.projection.finishers",
      "eventSelector": "$.event['runnerName','finishTime']"
    }
  ]
}

prepend

Adds anything to the beginning of an array.

{
  "eventType": "TodoAddedEvent",
  "functions": [
    {
      "function": "prepend",
      "targetSelector": "$.projection.todos",
      "eventSelector": "$.event['todoId','todoText']"
    }
  ]
}

remove

Removes an existing key or an array element matching filter expression.

{
  "eventType": "TodoRemovedEvent",
  "functions": [
    {
      "function": "remove",
      "targetSelector": "$.projection.todos[?]",
      "targetFilter": "[?(@.todoId == $.event.todoId)]"
    }
  ]
}

Performing arithmetic

add

Sums two numbers together.

{
  "eventType": "OrderPlacedEvent",
  "functions": [
    {
      "function": "add",
      "targetSelector": "$.projection.totalAmount",
      "eventSelector": "$.event['orderAmount']"
    }
  ]
}

subtract

Subtracts two numbers.

{
  "eventType": "OrderRefundedEvent",
  "functions": [
    {
      "function": "subtract",
      "targetSelector": "$.projection.totalAmount",
      "eventSelector": "$.event['orderAmount']"
    }
  ]
}

inc

Increases a number by one.

{
  "eventType": "TicketReservedEvent",
  "functions": [
    {
      "function": "inc",
      "targetSelector": "$.projection.ticketCount"
    }
  ]
}

dec

Decreases a number by one.

{
  "eventType": "TicketReleasedEvent",
  "functions": [
    {
      "function": "dec",
      "targetSelector": "$.projection.ticketCount"
    }
  ]
}

Adding lookup via references

You can make your projections

setref

Marks a projection field as a reference which makes it filterable in the API.

{
  "eventType": "TicketReleasedEvent",
  "functions": [
    {
      "function": "setref",
      "targetSelector": "$.projection.releaseDate"
    }
  ]
}

clearref

Clears the reference for the projection, removing the ability to filter.

{
  "eventType": "TicketReleasedEvent",
  "functions": [
    {
      "function": "clearref"
    }
  ]
}

Filters

Functions can also provide two filters: targetFilter and eventFilter.

To add a filter to a selector you provide a [?] in the selector text, as described in the JsonPath documentation. The filter for the selector is then applied for the given function. This is useful for matching on ids in nested lists or to apply conditional logic for when/how to process events.

All Filter Operators that are described here are supported.