diff --git a/lib/active_file/base.rb b/lib/active_file/base.rb
index a73e0ae2..a074cfe1 100644
--- a/lib/active_file/base.rb
+++ b/lib/active_file/base.rb
@@ -2,6 +2,7 @@ module ActiveFile
 
   class Base < ActiveHash::Base
     extend ActiveFile::MultipleFiles
+    @@instance_lock = Mutex.new
 
     class_attribute :filename, :root_path, :data_loaded, instance_reader: false, instance_writer: false
 
@@ -13,10 +14,12 @@ def delete_all
       end
 
       def reload(force = false)
-        return if !self.dirty && !force && self.data_loaded
-        self.data_loaded = true
-        self.data = load_file
-        mark_clean
+        @@instance_lock.synchronize do
+          return if !self.dirty && !force && self.data_loaded
+          self.data = load_file
+          mark_clean
+          self.data_loaded = true
+        end
       end
 
       def set_filename(name)
@@ -53,6 +56,10 @@ def actual_root_path
         end
       end
 
+      def all_in_process
+        return super if data_loaded
+        @records || []
+      end
     end
   end
 
diff --git a/lib/active_hash/base.rb b/lib/active_hash/base.rb
index b326fd1c..7ba1a4e4 100644
--- a/lib/active_hash/base.rb
+++ b/lib/active_hash/base.rb
@@ -145,7 +145,7 @@ def insert(record)
       end
 
       def next_id
-        max_record = all.max { |a, b| a.id <=> b.id }
+        max_record = all_in_process.max { |a, b| a.id <=> b.id }
         if max_record.nil?
           1
         elsif max_record.id.is_a?(Numeric)
@@ -153,6 +153,11 @@ def next_id
         end
       end
 
+      def all_in_process
+        all
+      end
+      private :all_in_process
+
       def record_index
         @record_index ||= {}
       end
diff --git a/spec/active_yaml/base_spec.rb b/spec/active_yaml/base_spec.rb
index a28e14b5..b94b629e 100644
--- a/spec/active_yaml/base_spec.rb
+++ b/spec/active_yaml/base_spec.rb
@@ -32,6 +32,12 @@ class Empty        < ActiveYaml::Base ; end # Empty YAML
     it 'can load empty yaml' do
       expect(Empty.first).to be_nil
     end
+
+    it 'is thread-safe' do
+      (1..5).map do
+        Thread.new { expect(City.count).to eq(2) }
+      end.each(&:join)
+    end
   end
 
   describe ".all" do