APIS
Client libraries
Custom Functions
This page describes how to write your own custom handler function to make it possible to implement more complex projections.
You can write your own projector code and deploy it as a function wherever you prefer. AWS Lambda, Google Cloud Functions and Azure Functions are all viable alternatives.
Executing custom functions
The external function will be automatically executed by Serialized for every event in the particular feed matching the event type(s) specified in the projection definition.
The structure of a handler using an external function looks like this:
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{
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"eventType": "<YOUR_EVENT_TYPE>",
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"functionUri": "https://example.com/your-function"
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}
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An example of a complete projection definition could look like this:
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{
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"projectionName": "orders-per-customer",
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"feedName": "order",
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"handlers": [
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{
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"eventType": "OrderPlacedEvent",
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"functionUri": "https://example.com/calculate-orders-per-customer"
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}
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]
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}
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Implementing a custom function
Handling the request
To implement an custom function you need to implement a publicly available backend service that can receive a JSON payload that looks like this:
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{
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"metadata": {
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"aggregateId": "a341b64c-b01f-43fb-907c-50c0067df672",
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"timestamp": 1535442699000,
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"createdAt": 1535442699551,
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"updatedAt": 1535442699551
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},
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"currentState": {},
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"event": {
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"eventId": "d710b9b1-063a-4b65-98be-0d46de443bdd",
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"eventType": "UserLoggedInEvent",
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"data": {
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"userId": "618f5a47-9d4c-4f42-9380-ca47c12087a1"
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}
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}
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}
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Producing a response
The response from the custom function should be the new updated state of the projection. The format of the updated state must look like this:
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{
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"updatedState": {
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"loginCount": 1
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}
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}
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Adding a reference to the projection
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Deleting a projection instance
To mark the projection instance for deletion you can set the
delete flag in the response from your custom function.Further calls to get this projection will return a 404, just as if the projection never existed.
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{
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"delete": true
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}
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Securing external function endpoints
The endpoints can be secured with Basic Authentication by including user:password in the
functionUri like this:1
{
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"eventType": "<YOUR_EVENT_TYPE>",
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"functionUri": "https://user:[email protected]/your-function"
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}
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Request signing
All outgoing HTTP request will automatically be HMAC signed using the projection name as the signing key. To provide your own signing secret, include the field
signingSecret in the definition payload.Sample code
For a full example of an implementation of an external custom function using AWS Lambda see our Github repository.
Last modified 10mo ago