From 5b1cad68cbeac4cae8aed1d540522841979ec34d Mon Sep 17 00:00:00 2001
From: danielsdeleo <dan@opscode.com>
Date: Fri, 15 Mar 2013 11:49:33 -0700
Subject: Store design docs in-repo; add one for CHEF-3694

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 design-docs/chef-3694-proposal.md | 196 ++++++++++++++++++++++++++++++++++++++
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 create mode 100644 design-docs/chef-3694-proposal.md

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+# CHEF-3694 Proposal
+
+## Background Reading
+
+* [Jira Ticket](http://tickets.opscode.com/browse/CHEF-3694)
+* [Solution Space Discussion Gist](https://gist.github.com/danielsdeleo/30a8719901eca6545488)
+
+## Issue Summary
+
+The core of the problem is that users can input two resources with the
+same type and name in recipes, for example:
+
+    log "danger" do
+      message "deleting all the backups!"
+    end
+
+    log "danger" do
+      message "restarting a service, you might get paged"
+    end
+
+There is an assumption in Chef's syntax that the name and type together
+form a compound natural key for resource lookup. This is most prominent
+in the syntax for notifications:
+
+    file "/tmp/whatever" do
+      content rand.to_s
+      notifies :write, "log[danger]"
+    end
+
+Chef currently does the following when encountering a resource
+declaration with a type/name pair that already exist in the resource
+collection:
+
+* Create a selective shallow clone of the existing resource. The
+  "selective" shallow clone involves applying a blacklist to ignore some
+  instance variables when cloning. In particular, `not_if` and `only_if`
+  guards are not cloned.
+* Evaluate the block passed to the resource declaration call in the
+  context of the newly cloned resource (as normally happens when
+  creating resources).
+* Add the cloned and modified resource to the resource collection. (As
+  with normal resource creation).
+
+Note that this violates the implicit assumption of type+name uniqueness,
+as there are now two resources with the same type and name in the
+collection.
+
+More info:
+* code: https://github.com/opscode/chef/blob/c7e99e50b1b9bba72b3fd907e1fb3748b901eeee/lib/chef/resource.rb#L283-300
+* Jira ticket: http://tickets.opscode.com/browse/CHEF-26
+
+## Why Do Users Create "Duplicate" Resources?
+
+### Unintentionally
+
+There are several situations in which users create resources with
+duplicate type+name pairs. The most common case we've seen in bug
+reports is unintentional duplication:
+
+    files = %w(one.txt two.txt)
+    files.each do |filename|
+      execute "touch_file" do
+        command "touch /tmp/#{filename}"
+
+        not_if "ls /tmp/#{filename}"
+      end
+    end
+
+The two execute resources here are orthogonal, and the user does not
+expect these to be linked in any way.
+
+The repro case for each of these bugs involves unintentional resource
+duplication:
+
+* http://tickets.opscode.com/browse/CHEF-2812
+* http://tickets.opscode.com/browse/CHEF-894
+* http://tickets.opscode.com/browse/CHEF-2892
+
+### Conflicting Community Cookbook Case
+
+Similar to the above, but more difficult to fix, is a case where two
+cookbooks conflict. Since resources are members of a single global
+collection, it is possible for resources from different cookbooks to use
+the same type, name pair.
+
+I have personally not seen this in the wild, but it is a case users are
+concerned about.
+
+### Insert a Resource Into the Resource Collection at a Different Position
+
+The original use case for the current behavior is to be able to add an
+existing resource to the resource collection with a different action.
+For example, if the user desires to install MySQL from a system package
+but use a non-default filesystem location for the database files, a
+recipe may do this:
+
+    service "mysql" do
+      # Uses runit:
+      start_command "sv mysql start"
+      stop_command "sv mysql start"
+
+      action :stop
+    end
+
+    execute "mv /var/lib/mysql /mnt/mysql"
+
+    service "mysql" do
+      # This will use the start command from above because of resource
+      # cloning:
+      action :start
+    end
+
+## Proposed Behavior Changes
+
+Some of these behavior changes are breaking. They should be implemented
+in an optional way with the default being the present behavior until
+Chef 12.
+
+We will split the responsibilities of the current resource collection
+into a _resource set_ and an _action list_. The resource set will be an
+unordered collection of resources with type,name tuple keys, and no
+duplicate keys. The action list will be an ordered collection of
+resource, action tuples, with no uniqueness constraints. Though not
+strictly required to solve CHEF-3694, this provides flexibility in how
+chef alters contexts for other features and user-defined extensions.
+(See below)
+
+We will add the following to the recipe DSL:
+
+    # TODO: NAME? Signature?
+    trigger(:service, "apache2").to(:start)
+
+    # ALSO: NAME?
+    edit(:service, "apache2") do
+      start_command "sv apache2 start"
+      # etc.
+    end
+
+    # For Completeness:
+    declare_resource(:service, "apache2") do
+    end
+
+The current recipe DSL syntax will just be a shortcut for composing the
+above primitives:
+
+    # This:
+    service "apache2" do
+      start_command "sv apache2 start"
+      action :start
+    end
+
+    # Is a shorter way of doing this:
+    declare_resource(:service, "apache2") do
+      start_command "sv apache2 start"
+    end
+
+    # TODO: make consistent with above
+    trigger(:service, "apache2") do
+      action :start
+    end
+
+These primitives provide enough functionality to 
+
+### Splitting Resource Collection
+
+To be clear: this will be done in a backwards compatible manner, such
+that `run_context.resource_collection` returns an object with the same
+API as currently.
+
+It may be possible to use some ugly workarounds to fix this issue
+without splitting the resource collection's responsibilities; however, I
+think that a look at our two fundamental requirements makes it clear
+that we should use different data structures to implement them:
+
+* Resources should be unique by {type, name}
+* The same resource can have different actions at arbitrary places in
+  the convergence order
+
+This change also makes it possible to fix some inconsistencies in our
+current model:
+
+* Resources with `action :nothing` require a special case to be skipped
+when iterating over the resource collection. If we instead add these
+resources to the resource set, but don't place them in the action list,
+the special casing goes away.
+* There are use cases for LWRPs where the user would like to notify a
+resource outside the LWRP, but otherwise run resources "inline". This
+can be accomplished quite easily if LWRPs run with a separate action
+list, but use the global resource set.
+
+Finally, we can't anticipate every way that people will need to control,
+mangle, or otherwise abuse Chef's reprogrammability to modify execution
+order of resources, but with the changes proposed here, we have a stable
+interface where we can give users total control over execution ordering
+without monkey patching or digging into internals.
+
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