• Poul-Henning Kamp's avatar
    First part of major backend overhaul. · 1cb4d736
    Poul-Henning Kamp authored
    *** Please do not use -trunk in production until I say so again ***
    
    I have not entirely decided in the precise terminology, so the following
    may sound a lot more complicated than it really is:
    
    In VCL we can now have "backends" and "directors" both of which we
    treat as a "backend".
    
    When we define backends and directors in VCL, they refer to "backend
    hosts" which is just another way to say "hostname+portname" but later
    these will grow other parameters (max connections etc).
    
    A director is a piece of code that selects a "backend host" somehow,
    "random" and "round-robin" are the first algorithms.  A backend
    can still be specified directly of course, that's the "simple director"
    that always return the same "backend host".
    
    This is probably where an example is in order:
    
    
    	/* A backend as we know it */
    	backend b1 {
    		.host = "fs";
    		.port = "80";
    	}
    
    	/* A director */
    	director b2 random {
    		{
    			/* We can refer to named backends */
    			.backend        = b1;
    			.weight         = 7;
    		}
    		{
    			/* Or define them inline */
    			.backend        = {
    				.host = "fs2";
    			}
    			.weight         = 3;
    		}
    	}
    
    	sub vcl_recv {
    		if (req.url ~ "\[[a-z]]") {
    			set req.backend = b2;
    		} else {
    			set req.backend = b1;
    		}
    	}
    
    This results in quite a lot of changes in the C code, VRT API and
    VCL compiler, the major thrust being:
    
    Directors like "simple" and "random" will not have to think about
    the actual connections to the backends, but just concentrate on
    selecting which backend should be used.
    
    When a new VCL is loaded, it will instantiate all directors, but
    try to reuse any preexisting "backend hosts" (which we still
    call "backend" in the C code).
    
    This is simple for a backend like "b1" in the example above, but
    sligthly more complex for the backend inlined in b2.  The VCL
    compiler solves this, by qualifying the ident string for the inlined
    backend host with the prefix "b2 random :: 2 :: ", so that a reload
    of the same director with the same (unchanged) inline backend host
    will match, but none other will.
    
    One implication of instantiating all directors for every VCL load,
    is that private statistics cannot be reused, but stats on the
    backend host can.  This is likely a very fine point of no consequence.
    
    Once the backend is selected by the director, the generic code in
    cache_backend.c will cope with reusing the connection pool,
    establishing connections and all that, moving most of the nastyness
    out of directors, leaving cache_dir_simple.c with only 96 lines of
    code, of which the license is a large fraction.
    
    Until now, we have done automatic DNS re-lookups, but they seem to
    cause more grief than advantage (I suspect some of the DNS lookups
    to be resposible for long timeouts), so that will be dropped, and
    instead we might add an explicit CLI command for this later.
    
    The code as here committed can handle a couple of simple requests,
    but there are a large number of INCOMPL()'s that need to be resolved
    before this is ready for prime time again.
    
    
    
    
    git-svn-id: http://www.varnish-cache.org/svn/trunk@2437 d4fa192b-c00b-0410-8231-f00ffab90ce4
    1cb4d736
vcl.h 1.02 KB