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Writing Backend Code

Overview

Backend function code executes in a serverless environment associated with your 8th Wall account. All backend functions must export a top level async method called handler, which is the entry point into the backend function.

Example entry file code:

const handler = async (event: any) => {
// Custom backend code goes here

return {
body: JSON.stringify({
myResponse,
}),
}
}

export {
handler,
}

Client Method

When you create a backend function, a client method is automatically created for you. This client method is a wrapper around fetch, meaning you can pass the same arguments to this function as you would with a normal fetch call. See Fetch API for more details.

This client method is how you send requests from module client code to the backend function.

FetchWrapper

Function Event Parameters

The handler method is invoked with an event object each time the client method is called. event has the following properties:

PropertyTypeDescription
pathstringThe URL path passed to the client method ('/getUser/foo' , '/checkAnswer', etc).
bodystringCall JSON.parse(event.body) to get transform the body into an object.
httpMethodstringThe HTTP method used to call the backend function. One of 'GET', 'PUT', 'POST', 'PATCH', 'DELETE'.
queryStringParametersRecord<string, string>Key/value pairs containg the query string parameters from the request.
headersRecord<string, string>Key/value pairs containing request headers.

Return Object

All properties are optional.

PropertyTypeDescription
statusCodenumberThe status code of the response. Default is 200.
headersRecord<string, string>The headers associated with the response.
bodystringThe JSON.stringify()'d body object associated with the response.

Error Handling

If the backend function throws an uncaught exception, the function will return statusCode: 500 with an error object in the response body.

If you own the module and are in development mode, the error object will contain name, message and stack:

{error: {name: string, message: string, stack: string}}

Example:

{
"error": {
"name": "TypeError",
"message": "Cannot read properties of undefined (reading 'foo')",
"stack": "TypeError: Cannot read properties of undefined (reading 'foo')\n at call (webpack:///src/index.ts:8:24)\n ...
}
}

For non-development mode, the error object will not contain a name or stack property and the message will be a generic "Internal Server Error".

Pinning Targets

Please refer to https://www.8thwall.com/docs/guides/modules/pinning-targets/ for full details on module pinning targets.

When pinning to a Version, Allowed Updates must be set to None

BFNVersionPinning

When pinning to a Commit, select a specific commit. Latest is not supported.

BFNCommitPinning