a very simple migration_fallback hook

lib/prefab-cloud-ruby.rb

@@ -54,3 +54,4 @@ module Prefab
 require 'prefab/prefab'
 require 'prefab/murmer3'
 require 'prefab/javascript_stub'
+require 'prefab/migration_fallback'

lib/prefab/config_client.rb

@@ -81,7 +81,7 @@ def get(key, default = NO_DEFAULT_PROVIDED, properties = NO_DEFAULT_PROVIDED)
       if evaluation
         evaluation.report_and_return(@base_client.evaluation_summary_aggregator)
       else
-        handle_default(key, default)
+        handle_default(key, context, default)
       end
     end
 
@@ -95,7 +95,12 @@ def raw(key)
       @config_resolver.raw(key)
     end
 
-    def handle_default(key, default)
+    def handle_default(key, context, default)
+      if @options.migration_fallback
+
+        return @options.migration_fallback.get(key, context, default)
+      end
+
       return default if default != NO_DEFAULT_PROVIDED
 
       raise Prefab::Errors::MissingDefaultError, key if @options.on_no_default == Prefab::Options::ON_NO_DEFAULT::RAISE

lib/prefab/migration_fallback.rb

@@ -0,0 +1,12 @@
+class Prefab::MigrationFallback
+  LOG = Prefab::InternalLogger.new(self)
+
+  def initialize(closure)
+    @closure = closure
+  end
+
+  def get(key, context, default)
+    LOG.debug "Migration fallback for key: #{key}"
+    @closure.call(key, context, default)
+  end
+end

lib/prefab/options.rb

@@ -20,6 +20,7 @@ class Options
     attr_reader :datafile
     attr_reader :global_context
     attr_accessor :is_fork
+    attr_reader :migration_fallback
 
     module ON_INITIALIZATION_FAILURE
       RAISE = :raise
@@ -69,7 +70,8 @@ module DATASOURCES
       datafile: ENV['PREFAB_DATAFILE'],
       x_datafile: nil, # DEPRECATED in favor of `datafile`
       x_use_local_cache: false,
-      global_context: {}
+      global_context: {},
+      migration_fallback: nil
     )
       @api_key = api_key
       @namespace = namespace
@@ -111,6 +113,7 @@ module DATASOURCES
 
       @sse_sources = @sources
       @config_sources = @sources
+      @migration_fallback = Prefab::MigrationFallback.new(migration_fallback)
 
       @telemetry_destination = @sources.select do |source|
         source.start_with?('https://') && (source.include?("belt") || source.include?("suspenders"))

test/test_config_client.rb

@@ -105,4 +105,13 @@ def test_cache_path_respects_xdg
     end
   end
 
+  def test_migration_fallback
+    options = Prefab::Options.new(
+      migration_fallback: ->(key, context, default) { "fallback value" },
+      prefab_datasources: Prefab::Options::DATASOURCES::LOCAL_ONLY,
+    )
+
+    config_client = Prefab::ConfigClient.new(MockBaseClient.new(options), 10)
+    assert_equal "fallback value", config_client.get('anything')
+  end
 end