diff --git a/lib/unleash/backup_file_writer.rb b/lib/unleash/backup_file_writer.rb
new file mode 100644
index 0000000..459e901
--- /dev/null
+++ b/lib/unleash/backup_file_writer.rb
@@ -0,0 +1,21 @@
+require 'unleash/configuration'
+
+module Unleash
+  class BackupFileWriter
+    def self.save!(toggle_data)
+      Unleash.logger.debug "Will save toggles to disk now"
+
+      backup_file = Unleash.configuration.backup_file
+      backup_file_tmp = "#{backup_file}.tmp"
+
+      File.open(backup_file_tmp, "w") do |file|
+        file.write(toggle_data)
+      end
+      File.rename(backup_file_tmp, backup_file)
+    rescue StandardError => e
+      # This is not really the end of the world. Swallowing the exception.
+      Unleash.logger.error "Unable to save backup file. Exception thrown #{e.class}:'#{e}'"
+      Unleash.logger.error "stacktrace: #{e.backtrace}"
+    end
+  end
+end
diff --git a/lib/unleash/streaming_event_processor.rb b/lib/unleash/streaming_event_processor.rb
index e07c2c1..3825cc4 100644
--- a/lib/unleash/streaming_event_processor.rb
+++ b/lib/unleash/streaming_event_processor.rb
@@ -1,4 +1,5 @@
 require 'json'
+require 'unleash/backup_file_writer'
 
 module Unleash
   class StreamingEventProcessor
@@ -41,7 +42,8 @@ def handle_connected_event(event)
     def handle_updated_event(event)
       handle_delta_event(event.data)
 
-      # TODO: update backup file
+      full_state = @toggle_engine.get_state
+      Unleash::BackupFileWriter.save!(full_state)
     rescue JSON::ParserError => e
       Unleash.logger.error "Unable to parse JSON from streaming event data. Exception thrown #{e.class}: '#{e}'"
       Unleash.logger.debug "stacktrace: #{e.backtrace}"
diff --git a/lib/unleash/toggle_fetcher.rb b/lib/unleash/toggle_fetcher.rb
index 1c881df..ebcb3be 100755
--- a/lib/unleash/toggle_fetcher.rb
+++ b/lib/unleash/toggle_fetcher.rb
@@ -1,5 +1,6 @@
 require 'unleash/configuration'
 require 'unleash/bootstrap/handler'
+require 'unleash/backup_file_writer'
 require 'net/http'
 require 'json'
 require 'yggdrasil_engine'
@@ -54,25 +55,7 @@ def fetch
       # always synchronize with the local cache when fetching:
       update_engine_state!(response.body)
 
-      save! response.body
-    end
-
-    def save!(toggle_data)
-      Unleash.logger.debug "Will save toggles to disk now"
-
-      backup_file = Unleash.configuration.backup_file
-      backup_file_tmp = "#{backup_file}.tmp"
-
-      self.toggle_lock.synchronize do
-        File.open(backup_file_tmp, "w") do |file|
-          file.write(toggle_data)
-        end
-        File.rename(backup_file_tmp, backup_file)
-      end
-    rescue StandardError => e
-      # This is not really the end of the world. Swallowing the exception.
-      Unleash.logger.error "Unable to save backup file. Exception thrown #{e.class}:'#{e}'"
-      Unleash.logger.error "stacktrace: #{e.backtrace}"
+      Unleash::BackupFileWriter.save! response.body
     end
 
     private
diff --git a/spec/unleash/streaming_event_processor_spec.rb b/spec/unleash/streaming_event_processor_spec.rb
new file mode 100644
index 0000000..7c406be
--- /dev/null
+++ b/spec/unleash/streaming_event_processor_spec.rb
@@ -0,0 +1,81 @@
+RSpec.describe Unleash::StreamingEventProcessor do
+  let(:engine) { YggdrasilEngine.new }
+  let(:processor) { Unleash::StreamingEventProcessor.new(engine) }
+  let(:backup_file) { Unleash.configuration.backup_file }
+
+  before do
+    Unleash.configure do |config|
+      config.url = 'http://test-url/'
+      config.app_name = 'test-app'
+    end
+    Unleash.logger = Unleash.configuration.logger
+  end
+
+  after do
+    File.delete(backup_file) if File.exist?(backup_file)
+  end
+
+  class TestEvent
+    attr_reader :type, :data
+
+    def initialize(type, data)
+      @type = type
+      @data = data
+    end
+  end
+
+  def feature_event(name, enabled = true)
+    {
+      "events": [{
+        "type": "feature-updated",
+        "eventId": 1,
+        "feature": {
+          "name": name,
+          "enabled": enabled,
+          "strategies": [{ "name": "default" }]
+        }
+      }]
+    }.to_json
+  end
+
+  def backup_contains_feature?(name)
+    return false unless File.exist?(backup_file)
+
+    parsed = JSON.parse(File.read(backup_file))
+    feature_names = parsed['features'].map { |f| f['name'] }
+    feature_names.include?(name)
+  end
+
+  describe '#process_event' do
+    it 'processes valid events and saves full engine state' do
+      event = TestEvent.new('unleash-updated', feature_event('test-feature'))
+      processor.process_event(event)
+
+      expect(engine.enabled?('test-feature', {})).to eq(true)
+      expect(backup_contains_feature?('test-feature')).to eq(true)
+    end
+
+    it 'ignores unknown event types' do
+      event = TestEvent.new('unknown-event', feature_event('test-feature'))
+      processor.process_event(event)
+
+      expect(File.exist?(backup_file)).to eq(false)
+      expect(engine.enabled?('test-feature', {})).to be_falsy
+    end
+
+    it 'saves full engine state, not partial event data' do
+      processor.process_event(TestEvent.new('unleash-updated', feature_event('first-feature', true)))
+      processor.process_event(TestEvent.new('unleash-updated', feature_event('second-feature', false)))
+
+      expect(backup_contains_feature?('first-feature')).to eq(true)
+      expect(backup_contains_feature?('second-feature')).to eq(true)
+    end
+
+    it 'handles invalid JSON gracefully without creating backup' do
+      event = TestEvent.new('unleash-updated', 'invalid json')
+
+      expect { processor.process_event(event) }.not_to raise_error
+      expect(File.exist?(backup_file)).to eq(false)
+    end
+  end
+end