File downloads (streaming responses)

[bajtos]

To allow LB4 applications to implement file downloads, we need to make improvements in several areas:

1. A route (a controller method) must be able to specify not only the response body, but also additional response headers (e.g. Content-Disposition: attachment; filename="filename.jpg"). This is already covered by the issue Configurable response types/formats #436.
2. It should be possible for routes to provide the response body as a ReadableStream and Buffer too.
3. Nice to have: a helper function similar to res.download provided by Express that will accept a file name and create an LB4 response object with all necessary fields correctly filled (content type, content disposition, body stream, etc.)
4. Documentation: explain how a route can specify the response headers and provide body as a readable stream.

Related issues:

• LB3 implementation: Add support for "file" return args strongloop/strong-remoting#284
• Router: allow methods to return an async-iterator or a readable stream Router: allow methods to return an async-iterator or a readable stream #122
• Equivalent of res.download() (ExpressJS) in the controlller for downloading files Equivalent of res.download() (ExpressJS) in the controlller for downloading files #2037

[jannyHou]

Estimation meeting's discussion:

• We already allow the controller function to return buffer.
• Add some acceptance test case to demo an endpoint for file downloading.

[bajtos]

We already allow the controller function to return buffer.

We can start by supporting only buffers in the first iteration. This is not enough for practical use though. For example, when accessing large data stored in an external storage container (see loopback-component-storage), we don't want to read the entire content into memory, but want to forward the readable stream returned by the outgoing request and write it to the incoming response.

[Bnaya]

Is there any workaround to make stream response atm?

[bajtos]

Is there any workaround to make stream response atm?

You can inject the Response object into your Controller and then write the response directly. See #1760 and loopbackio/loopback4-example-shopping#16, especially the HomeController.