diff --git a/js-assembler/cache.rkt b/js-assembler/cache.rkt
new file mode 100644
index 0000000..94a0f4f
--- /dev/null
+++ b/js-assembler/cache.rkt
@@ -0,0 +1,7 @@
+#lang racket/base
+(define-struct cached-entry (real-path ;; path to a module. 
+                             whalesong-version ;; string
+                             md5   ;; md5 of the original source in real-path
+                             bytes)
+  #:transparent) ;; bytes
+
diff --git a/js-assembler/package.rkt b/js-assembler/package.rkt
index 5b31172..e79d183 100644
--- a/js-assembler/package.rkt
+++ b/js-assembler/package.rkt
@@ -52,6 +52,13 @@
                     (void))))
 
 
+(define-struct cached-entry (real-path ;; path to a module. 
+                             whalesong-version ;; string
+                             md5   ;; md5 of the original source in real-path
+                             bytes)
+  #:transparent) ;; bytes
+
+
 
 
 (define-struct js-impl (name ;; symbol