upd

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5077 d4fa192b-c00b-0410-8231-f00ffab90ce4

doc/sphinx/phk/backends.rst

+.. _phk_backends:
+
+===============================
+What do you mean by 'backend' ?
+===============================
+
+Given that we are approaching Varnish 3.0, you would think I had this
+question answered conclusively long time ago, but once you try to
+be efficient, things get hairy fast.
+
+One of the features of Varnish we are very fundamental about, is the
+ability to have multiple VCL's loaded at the same time, and to switch
+between them instantly and seamlessly.
+
+So Imagine you have 1000 backends in your VCL, not an unreasonable
+number, each configured with health-polling.
+
+Now you fiddle your vcl_recv{} a bit and load the VCL again, but
+since you are not sure which is the best way to do it, you keep
+both VCL's loaded so you can switch forth and back seamlessly.
+
+To switch seamlessly, the health status of each backend needs to
+be up to date the instant we switch to the other VCL.
+
+This basically means that either all VCLs poll all their backends,
+or they must share, somehow.
+
+We can dismiss the all VCL's poll all their backends scenario,
+because it scales truly horribly, and would pummel backends with
+probes if people forget to vcl.discard their old dusty VCLs.
+
+Share And Enjoy
+===============
+
+In addition to health-status (including the saint-list), we also
+want to share cached open connections and stats counters.
+
+It would be truly stupid to close 100 ready and usable connections to
+a backend, and open 100 other, just because we switch to a different
+VCL that has an identical backend definition.
+
+But what is an identical backend definition in this context ?
+
+It is important to remember that we are not talking physical
+backends:  For instance, there is nothing preventing a VCL for
+having the same physical backend declared as 4 different VCL
+backends.
+
+The most obvious thing to do, is to use the VCL name of the backend
+as identifier, but that is not enough.  We can have two different
+VCL's where backend "b1" points at two different physical machines,
+for instance when we migrate or upgrade the backend.
+
+The identity of the state than can be shared is therefore the triplet:
+	{VCL-name, IPv4+port, IPv6+port} 
+
+No Information without Representation
+=====================================
+
+Since the health-status will be for each of these triplets, we will
+need to find a way to represent them in CLI and statistics contexts.
+
+As long as we just print them out, that is not a big deal, but what
+if you just want the health status for one of your 1000 backends,
+how do you tell which one ?
+
+The syntax-nazi way of doing that, is forcing people to type it all
+every time::
+
+	backend.health b1(127.0.0.1:8080,[::1]:8080)
+
+That will surely not be a hit with people who have just one backend.
+
+I think, but until I implement I will not commit to, that the solution
+is a wildcard-ish scheme, where you can write things like::
+
+	b1				# The one and only backend b1 or error
+
+	b1()				# All backends named b1
+
+	b1(127.0.0.1)			# All b1's on Ipv4 lookback
+
+	b1(:8080)			# All b1's on port 8080, (IPv4 or IPv6)
+
+	b1(192.168.60.1,192.168.60.2)	# All b1's on one of those addresses.
+
+(Input very much welcome)
+
+The final question is if we use shortcut notation for output from
+varnishd, and the answer is no, because we do not want the stats-counters
+to change name because we load another VCL and suddenly need disabiguation.
+
+
+Sharing Health Status
+=====================
+
+To avoid the over-polling, we define that maximum one VCL polls at
+backend at any time, and the active VCL gets preference.  It is not
+important which particular VCL polls the backends not in the active
+VCL, as long as one of them do.
+
+Implementation
+==============
+
+The poll-policy can be implemented by updating a back-pointer to
+the poll-specification for all backends on vcl.use execution.
+
+On vcl.discard, if this vcl was the active poller, it needs to walk
+the list of vcls and substitute another.  If the list is empty
+the backend gets retired anyway.
+
+We should either park a thread on each backend, or have a poller thread
+which throws jobs into the work-pool as the backends needs polled.
+
+The patternmatching is confined to CLI and possibly libvarnishapi
+
+I think this will work,
+
+Until next time,
+
+Poul-Henning, 2010-08-09

doc/sphinx/phk/index.rst

@@ -8,6 +8,7 @@ You may or may not want to know what Poul-Henning think.
 
 .. toctree::
 
+	backends.rst
 	platforms.rst
 	barriers.rst
 	thoughts.rst