Document high-level architecture goals

Create a high-level description of the long-term software architecture
goals.  Closes ticket 32206.

changes/ticket32206

@@ -0,0 +1,3 @@
+  o Documentation:
+    - Create a high-level description of the long-term software
+      architecture goals.  Closes ticket 32206.

src/arch_goals.md

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+@page arch_goals High level code design practices
+
+This page describes the high level design practices for Tor's code.
+This design is a long-term goal of what we want our code to look like,
+rather than a description of how it currently is.
+
+Overall, we want various parts of tor's code to interact with each
+other through a small number of interfaces.
+
+We want to avoid having "god objects" or "god modules".  These are
+objects or modules that know far too much about other parts of the
+code.  God objects/modules are generally recognized to be an
+antipattern of software design.
+
+Historically, there have been modules in tor that have tended toward
+becoming god modules.  These include modules that help more
+specialized code communicate with the outside world: the configuration
+and control modules, for example.  Others are modules that deal with
+global state, initialization, or shutdown.
+
+If a centralized module needs to invoke code in almost every other
+module in the system, it is better if it exports a small, general
+interface that other modules call.  The centralized module should not
+explicitly call out to all the modules that interact with it.
+
+Instead, modules that interact with the centralized module should call
+registration interfaces.  These interfaces allow modules to register
+handlers for things like configuration parsing and control command
+execution.  (The config and control modules are examples of this.)
+Alternatively, registration can happen through statically initialized
+data structures.  (The subsystem mechanism is an example of this.)

src/mainpage.md

@@ -29,6 +29,8 @@ Tor repository.
 
 @subpage intro
 
+@subpage arch_goals
+
 @subpage initialization
 
 @subpage dataflow