Build ESI onerror and alt support into 404 handler?

[trieloff]

Give that we post-process most resources that could contain ESI anyway (in helix-pipeline or helix-static), it might be practical to implement support for the alt and onerror through rewritten ESI and modified VCL.

1. <esi:include src="foo.html" alt="bar.html" /> becomes <esi:include src="foo.html?alt=bar.html"/>

When the 404 handler detects that this is an ESI request with an alt query string parameter, it will change the URL and restart the request.

2. <esi:include src="foo.html" onerror="continue" /> becomes <esi include src="foo.html?onerror=continue"/>

When the 404 handler detects that this is an ESI request with the onerror request parameter, it handle the request depending on the value of the onerror request parameter:

• continue: create an empty synthetic response
• static: create a synthetic response that contains the current URL (slightly rewritten, so recreate the required fallback for static)

In all other cases, run the standard 404 behavior.

[tripodsan]

what are the chances that fastly implements this in their ESI processing out-of-the-box?

I don't like that the control logic is done via request params. since fastly knows the esi include depth, it must keep a state. so it might be possible to use this somehow?

[trieloff]

what are the chances that fastly implements this in their ESI processing out-of-the-box?

I'd say slim, but I can check.

[kptdobe]

@trieloff any update on this ?

[kptdobe]

FYI, I followed up on the Fastly slack channel. Waiting now for ideas from @drwilco ;)