Varnish 4 cheat sheet

VCL built-in subroutines and their legal returns at the frontend (client) side

subroutine	scope	deliver	fetch	restart	hash	pass	pipe	synth	purge	lookup
vcl_deliver	client	x		x				x
vcl_hash	client									x
vcl_hit	client	x	x	x		x		x
vcl_miss	client		x	x		x		x
vcl_pass	client		x	x				x
vcl_pipe	client						x	x
vcl_purge	client			x				x
vcl_recv	client				x	x	x	x	x
vcl_synth	client	x		x

VCL built-in subroutines and their legal returns at the backend side

subroutine	scope	fetch	deliver	abandon	retry	ok	fail
vcl_backend_fetch	backend	x		x
vcl_backend_response	backend		x	x	x
vcl_backend_error	backend		x		x
vcl_init	vcl.load					x	x
vcl_fini	vcl.discard					x

Variables readable (R) or writable (W) in VCL subroutines

subroutine	req.	bereq.	beresp.	obj.	resp.
vcl_backend_fetch		R/W
vcl_backend_response		R/W	R/W
vcl_backend_error		R/W	R/W
vcl_recv	R/W
vcl_pipe		R/W
vcl_pass	R/W
vcl_hash	R/W
vcl_purge	R/W
vcl_miss	R/W
vcl_hit	R/W			R
vcl_deliver	R/W			R	R/W
vcl_synth	R/W				R/W