diff --git a/src/allmydata/interfaces.py b/src/allmydata/interfaces.py
index 166aeee..85bcc42 100644
--- a/src/allmydata/interfaces.py
+++ b/src/allmydata/interfaces.py
@@ -630,6 +630,26 @@ class IReadable(Interface):
         """
 
 
+class IWritable(Interface):
+    """
+    I define methods that callers can use to update SDMF and MDMF
+    mutable files on a Tahoe-LAFS grid.
+    """
+    # XXX: For the moment, we have only this. It is possible that we
+    #      want to move overwrite() and modify() in here too.
+    def update(data=IMutableUploadable, offset):
+        """
+        I write the data from my data argument to the MDMF file,
+        starting at offset. I continue writing data until my data
+        argument is exhausted, appending data to the file as necessary.
+        """
+        # to append data: offset=node.get_size_of_best_version()
+        # do we want to support compacting MDMF?
+        # for an MDMF file, this can be done with O(data.get_size())
+        # memory. For an SDMF file, any modification takes
+        # O(node.get_size_of_best_version()).
+
+
 class IMutableFileVersion(IReadable):
     """I provide access to a particular version of a mutable file. The
     access is read/write if I was obtained from a filenode derived from
@@ -798,6 +818,7 @@ class IFilesystemNode(Interface):
     def raise_error():
         """Raise any error associated with this node."""
 
+    # XXX: These may not be appropriate outside the context of an IReadable.
     def get_size():
         """Return the length (in bytes) of the data this node represents. For
         directory nodes, I return the size of the backing store. I return