Add an Architecture section with diagrams.

Closes issue #92.

docs/source/architecture.rst

+.. _architecture:
+
+Architecture
+============
+
+This section provides details of the overall BuildGrid architecture.
+
+.. toctree::
+   :maxdepth: 3
+
+   architecture_overview.rst
\ No newline at end of file

docs/source/architecture_overview.rst

+.. _architecture-overview:
+
+Remote execution overview
+=========================
+
+Remote execution aims to speed up a build process and to rely on two separate
+but related concepts that are remote caching and remote execution itself. Remote
+caching allows users to share build outputs while remote execution allows the running
+of operations on a remote cluster of machines which may be more powerful than what
+the user has access to locally.
+
+The `Remote Execution API`_ (REAPI) describes a `gRPC`_ + `protocol-buffers`_
+interface that has three main services for remote caching and execution:
+
+- A ``ContentAddressableStorage`` (CAS) service: a remote storage end-point
+  where content is addressed by digests, a digest being a pair of the hash and
+  size of the data stored or retrieved.
+- An ``ActionCache`` (AC) service: a mapping between build actions already
+  performed and their corresponding resulting artifact.
+- An ``Execution`` service: the main end-point allowing one to request build
+  job to be perform against the build farm.
+
+The `Remote Worker API`_ (RWAPI) describes another `gRPC`_ + `protocol-buffers`_
+interface that allows a central ``BotsService`` to manage a farm of pluggable workers.
+
+BuildGrid is combining these two interfaces in order to provide a complete
+remote caching and execution service. The high level architecture can be
+represented like this:
+
+.. graphviz::
+   :align: center
+
+    digraph remote_execution_overview {
+	node [shape = record,
+	      width=2,
+	      height=1];
+
+	ranksep = 2
+	compound=true
+	edge[arrowtail="vee"];
+	edge[arrowhead="vee"];
+
+	client [label = "Client",
+	color="#0342af",
+	fillcolor="#37c1e8",
+	style=filled,
+	shape=box]
+
+	subgraph cluster_controller{
+	    label = "Controller";
+	    labeljust = "c";
+	    fillcolor="#42edae";
+	    style=filled;
+	    controller [label = "{ExecutionService|BotsInterface\n}",
+			fillcolor="#17e86a",
+			style=filled];
+
+	}
+
+	subgraph cluster_worker0 {
+	    label = "Worker 1";
+	    labeljust = "c";
+	    color="#8e7747";
+	    fillcolor="#ffda8e";
+	    style=filled;
+	    bot0 [label = "{Bot|Host-tools}"
+		  fillcolor="#ffb214",
+		  style=filled];
+	}
+
+	subgraph cluster_worker1 {
+	    label = "Worker 2";
+	    labeljust = "c";
+	    color="#8e7747";
+	    fillcolor="#ffda8e";
+	    style=filled;
+	    bot1 [label = "{Bot|BuildBox}",
+		  fillcolor="#ffb214",
+		  style=filled];
+	}
+
+	client -> controller [
+	    dir = "both",
+	    headlabel = "REAPI",
+	    labelangle = 20.0,
+	    labeldistance = 9,
+	    labelfontsize = 15.0,
+	    lhead=cluster_controller];
+
+	controller -> bot0 [
+	    dir = "both",
+	    labelangle= 340.0,
+		labeldistance = 7.5,
+		labelfontsize = 15.0,
+	    taillabel = "RWAPI     ",
+	    lhead=cluster_worker0,
+	    ltail=cluster_controller];
+
+	controller -> bot1 [
+	    dir = "both",
+	    labelangle= 20.0,
+	    labeldistance = 7.5,
+	    labelfontsize = 15.0,
+		taillabel = "     RWAPI",
+	    lhead=cluster_worker1,
+	    ltail=cluster_controller];
+
+    }
+
+BuildGrid can be split up into separate endpoints. It is possible to have
+a separate ``ActionCache`` and ``CAS`` from the ``Controller``. The
+following diagram shows a typical setup.
+
+.. graphviz::
+   :align: center
+
+    digraph remote_execution_overview {
+
+	node [shape=record,
+	      width=2,
+	      height=1];
+
+	compound=true
+	graph [nodesep=1,
+	       ranksep=2]
+
+	edge[arrowtail="vee"];
+	edge[arrowhead="vee"];
+
+	client [label="Client",
+		color="#0342af",
+		fillcolor="#37c1e8",
+		style=filled,
+		shape=box]
+
+	cas [label="CAS",
+	     color="#840202",
+	     fillcolor="#c1034c",
+	     style=filled,
+	     shape=box]
+
+	subgraph cluster_controller{
+	    label="Controller";
+	    labeljust="c";
+	    fillcolor="#42edae";
+	    style=filled;
+	    controller [label="{ExecutionService|BotsInterface\n}",
+			fillcolor="#17e86a",
+			style=filled];
+
+	}
+
+	actioncache [label="ActionCache",
+		     color="#133f42",
+		     fillcolor="#219399",
+		     style=filled,
+		     shape=box]
+
+	subgraph cluster_worker0 {
+	    label="Worker";
+	    labeljust="c";
+	    color="#8e7747";
+	    fillcolor="#ffda8e";
+	    style=filled;
+	    bot0 [label="{Bot}"
+		  fillcolor="#ffb214",
+		  style=filled];
+	}
+
+	client -> controller [
+	    dir="both"];
+
+	client -> cas [
+	    dir="both",
+	    lhead=cluster_controller];
+
+	controller -> bot0 [
+	    dir="both",
+	    lhead=cluster_worker0];
+	    //ltail=cluster_controller];
+
+	cas -> bot0 [
+	    dir="both",
+	    lhead=cluster_worker0];
+
+	actioncache -> controller [
+	    dir="both"];
+
+	client -> actioncache [
+	    dir="both",
+	    constraint=false,
+    ];
+
+
+    }
+
+.. _Remote Execution API: https://github.com/bazelbuild/remote-apis/blob/master/build/bazel/remote/execution/v2
+.. _gRPC: https://grpc.io
+.. _protocol-buffers: https://developers.google.com/protocol-buffers
+.. _Remote Worker API: https://github.com/googleapis/googleapis/tree/master/google/devtools/remoteworkers/v1test2

docs/source/index.rst

@@ -19,6 +19,7 @@ Remote execution service implementing Google's REAPI and RWAPI.
    using.rst
    reference.rst
    contributing.rst
+   architecture.rst
    resources.rst