Internals of a custom application how-custom-application-works

Use the following illustration to understand the end-to-end workflow when a digital asset is processed using a custom application by a client.

Custom application workflow

Figure: Steps involved when processing an asset using Adobe Asset Compute Service.

Registration registration

The client must call /register once before the first request to /process so it can set up and retrieve the journal URL for receiving Adobe I/O Events Events for Adobe Asset Compute.

curl -X POST \ \
  -H "x-ims-org-id: $ORG_ID" \
  -H "x-gw-ims-org-id: $ORG_ID" \
  -H "Authorization: Bearer $JWT_TOKEN" \
  -H "x-api-key: $API_KEY"

The @adobe/asset-compute-client JavaScript library can be used in NodeJS applications to handle all the necessary steps from registration, processing to asynchronous event handling. For more information on the required headers, see Authentication and Authorization.

Processing processing

The client sends a processing request.

curl -X POST \ \
  -H "x-ims-org-id: $ORG_ID" \
  -H "x-gw-ims-org-id: $ORG_ID" \
  -H "Authorization: Bearer $JWT_TOKEN" \
  -H "x-api-key: $API_KEY" \

The client is responsible for correctly formatting the renditions with pre-signed URLs. The @adobe/node-cloud-blobstore-wrapper JavaScript library can be used in NodeJS applications to pre-sign URLs. Currently the library only supports Azure Blob Storage and AWS S3 Containers.

The processing request returns a requestId that can be used for polling Adobe I/O Events.

A sample custom application processing request is below.

    "source": "",
    "renditions" : [
            "worker": "",
            "name": "rendition1.jpg",
            "target": "https://some-presigned-put-url-for-rendition1.jpg",
    "userData": {
        "my-asset-id": "1234567890"

The Asset Compute Service sends the custom application rendition requests to the custom application. It uses an HTTP POST to the provided application URL, which is the secured web action URL from App Builder. All requests use the HTTPS protocol to maximize data security.

The Asset Compute SDK used by a custom application handles the HTTP POST request. It also handles downloading of the source, uploading renditions, sending Adobe I/O Events and error handling.

Application code application-code

Custom code only needs to provide a callback that takes the locally available source file (source.path). The rendition.path is the location to place the final result of an asset processing request. The custom application uses the callback to turn the locally available source files into a rendition file using the name passed in (rendition.path). A custom application must write to rendition.path to create a rendition:

const { worker } = require('@adobe/asset-compute-sdk');
const fs = require('fs').promises;

// worker() is the entry point in the SDK "framework".
// The asynchronous function defined is the rendition callback.
exports.main = worker(async (source, rendition) => {

    // Tip: custom worker parameters are available in rendition.instructions.
    console.log(; // should print out `rendition.jpg`.

    // Simplest example: copy the source file to the rendition file destination so as to transfer the asset as is without processing.
    await fs.copyFile(source.path, rendition.path);

Download source files download-source

A custom application only deals with local files. The Asset Compute SDK handles the downloading of the source file.

Rendition creation rendition-creation

The SDK calls an asynchronous rendition callback function for each rendition.

The callback function has access to the source and rendition objects. The source.path already exists and is the path to local copy of source file. The rendition.path is the path where the processed rendition must be stored. Unless the disableSourceDownload flag is set, the application must use exactly the rendition.path. Otherwise, the SDK cannot locate or identify the rendition file and fails.

The over simplification of the example is done to illustrate and focus on the anatomy of a custom application. The application just copies the source file to the rendition destination.

For more information about the rendition callback parameters, see Asset Compute SDK API.

Upload renditions upload-rendition

After each rendition is created and stored in a file with the path provided by rendition.path, the Asset Compute SDK uploads each rendition to a cloud storage (either AWS or Azure). A custom application gets multiple renditions at the same time if, and only if, the incoming request has multiple renditions pointing to the same application URL. The upload to cloud storage is done after each rendition and before running the callback for the next rendition.

The batchWorker() has a different behavior. It processes all renditions, and only after all have been processed, it uploads them.

Adobe I/O Events aio-events

The SDK sends Adobe I/O Events for each rendition. These events are either type rendition_created or rendition_failed depending on the outcome. For more information, see Asset Compute asynchronous events.

Receive Adobe I/O Events receive-aio-events

The client polls the Adobe I/O Events journal according to its consumption logic. The initial journal URL is the one provided in the /register API response. Events can be identified using the requestId that is present in the events and is the same as returned in /process. Every rendition has a separate event that gets sent as soon as the rendition has been uploaded (or failed). When it receives a matching event, the client can display or otherwise handle the resulting renditions.

The JavaScript library asset-compute-client makes the journal polling simple using the waitActivation() method to get all the events.

const events = await assetCompute.waitActivation(requestId);
await Promise.all( => {
    if (event.type === "rendition_created") {
        // get rendition from cloud storage location
    else if (event.type === "rendition_failed") {
        // failed to process
    else {
        // other event types
        // (could be added in the future)

For details on how to get journal events, see Adobe I/O Events API.