update TODO.

TODO

@@ -1,5 +1,5 @@
 * streaming - DONE
-* checksums - 1/2 DONE, created on write but not verified on read
+* checksums - DONE
 * cookies - DONE
 * reader concurrency
 * preallocation
@@ -12,3 +12,41 @@
 * Eof tracked in header.
 * start at Eof and skip over all entries to real Eof, write down new Eof if different.
 
+== caching ==
+
+* make metadata files.
+* keep ets of metadata.
+* {Group, Key, Cookie} -> {File, Offset}
+
+== API ==
+
+Lookup by key as location will change on compaction.
+
+=== READ ===
+
+read(Group, Key, Cookie, Fun) -> {ok, Bin} | {error, Reason}
+read(Group, Key, Cookie, Ranges, Fun) -> ok | {error, Reason}
+
+Fun is called with;
+
+{ok, Bin} - a chunk of data.
+{checksum, verified|failed} - checksum validity.
+eof - end of data.
+{error, Reason} - error while reading.
+
+=== WRITE ===
+
+write(Group, Key, Fun) -> {ok, Cookie} | {error, Reason}
+
+Fun is called with one argument, the maximum number of bytes to return in a chunk.
+
+The fun may evaluate to any of the following;
+
+{ok, Bin} - a chunk of data.
+eof - end of data.
+{error, Reason} - error while reading.
+
+=== MISC ===
+
+delete(Group, Key, Cookie) -> ok | {error, Reason}
+compact(Group) -> ok