[core] Add comments on gera and defaults/vars/userVars mechanism

Author: teo

common/gera/map.go

@@ -26,6 +26,10 @@
 //
 // A gera.Map uses a map[string]interface{} as backing store, and it can wrap other gera.Map instances.
 // Values in child maps override any value provided by a gera.Map that's wrapped in the hierarchy.
+//
+// The name is reminiscent of hiera, as in hierarchical, but it was deemed desirable to avoid future confusion with the
+// Hiera KV store used by Puppet, a different product altogether, so instead of the ancient Greek root, "gera" comes
+// from the Italian root instead where "hi" becomes a soft "g".
 package gera
 
 import (

core/workflow/rolebase.go

@@ -57,6 +57,19 @@ type roleBase struct {
 	status      SafeStatus
 	state       SafeState
 
+	// Defaults, Vars and UserVars are used to store each role's variables.
+	// Defaults are the lowest priority, Vars are the second highest, and UserVars are the highest.
+	//
+	// These use the gera.Map type, which is a wrapper around a map[string]string that provides hierarchical KV store
+	// semantics.
+	// The gera.Map logic allows us to ensure that defaults are overridden by vars, and vars by userVars throughout the
+	// workflow tree, and at the same time that defaults/vars/userVars set in a child role override the relevant values
+	// set in its parent role.
+	// The way we do this is by ensuring parent-child (Wrap) relationships between all the Default members in the
+	// workflow tree, all the Vars members, and all the UserVars members, and then whenever we need to figure out what's
+	// the consolidated KV map seen from the point of view of a given role, we Flatten each of these three, and then
+	// Wrap and re-Flatten between the flattened defaults, vars and userVars (see ConsolidatedVarStack). This results in
+	// a single map, generatable from the POV of any role within the tree.
 	Defaults *gera.WrapMap[string, string] `yaml:"defaults,omitempty"`
 	Vars     *gera.WrapMap[string, string] `yaml:"vars,omitempty"`
 	UserVars *gera.WrapMap[string, string] `yaml:"-"`
@@ -65,6 +78,8 @@ type roleBase struct {
 	Enabled  string                        `yaml:"enabled,omitempty"`
 }
 
+// Needed for the yaml package to correctly unmarshal into gera.Map[string, string] those Defaults and Vars entries from
+// a workflow template, that have a !public tag to include input widget metadata.
 func kvStoreUnmarshalYAMLWithTags(w gera.Map[string, string], unmarshal func(interface{}) error) error {
 	nodes := make(map[string]yaml.Node)
 	err := unmarshal(&nodes)