Serve stale on error

stale_on_error.vcl

#
# Stale-If-Error (v1.0)
#

import std;

sub vcl_recv {
	# First pass, clear SIE headers
	if (!req.restarts) {
		unset req.http.x-sie6-enabled;
		unset req.http.x-sie6-abandon;
	}
	# SIE possible, attempt to serve fresh, skip vcl_recv
	else if (req.restarts == 1 && req.http.x-sie6-enabled) {
		set req.grace = 0s;
		return (hash);
	}
	# SIE serve fresh failed, serve stale, skip vcl_recv
	else if (req.http.x-sie6-abandon) {
		set req.grace = 100y;
		return (hash);
	}
}

sub vcl_hit {
	# Stale object found, attempt SIE
	if (!req.restarts && obj.ttl < 0s && std.healthy(req.backend_hint)) {
		set req.http.x-sie6-enabled = "true";
		return (restart);
	}
}

sub vcl_backend_fetch {
	# Dont do SIE bgfetch on stale delivery
	if (bereq.is_bgfetch && bereq.http.x-sie6-abandon) {
		return (abandon);
	}
}

sub vcl_backend_response {
	# SIE and we got an error, attempt to serve stale
	if (bereq.http.x-sie6-enabled && beresp.status >= 500) {
		return (abandon);
	}
}

sub vcl_backend_error {
	# SIE and we got an error, attempt to serve stale
	if (bereq.http.x-sie6-enabled) {
		return (abandon);
	}
}

sub vcl_synth {
	# Backend SIE error, restart so we can attempt a stale delivery
	if (req.http.x-sie6-enabled && resp.status == 503) {
		unset req.http.x-sie6-enabled;
		set req.http.x-sie6-abandon = "true";
		return (restart);
	}
}

# EOF