Add locks to the design document

doc/Design.md

@@ -378,7 +378,43 @@ been returned.
 Locks
 -----
 
+The restic repository structure is designed in a way that allows parallel
+access of multiple instance of restic and even parallel writes. However, there
+are some functions that work more efficient or even require exclusive access of
+the repository. In order to implement these functions, restic processes are
+required to create a lock on the repository before doing anything.
 
+Locks come in two types: Exclusive and non-exclusive locks. At most one
+process can have an exclusive lock on the repository, and during that time
+there mustn't be any other locks (exclusive and non-exclusive). There may be
+multiple non-exclusive locks in parallel.
+
+A lock is a file in the subdir `locks` whose filename is the storage ID of
+the contents. It is encrypted and authenticated the same way as other files
+in the repository and contains the following JSON structure:
+
+    {
+      "time": "2015-06-27T12:18:51.759239612+02:00",
+      "exclusive": false,
+      "hostname": "kasimir",
+      "username": "fd0",
+      "pid": 13607,
+      "uid": 1000,
+      "gid": 100
+    }
+
+The field `exclusive` defines the type of lock. When a new lock is to be
+created, restic checks all locks in the repository. When a lock is found, it
+is tested if the lock is stale, which is the case for locks with timestamps
+older than 30 minutes. If the lock was created on the same machine, even for
+younger locks it is tested whether the process is still alive by sending a
+signal to it. If that fails, restic assumes that the process is dead and
+considers the lock to be stale.
+
+When a new lock is to be created and no other conflicting locks are
+detected, restic creates a new lock, waits, and checks if other locks
+appeared in the repository. Depending on the type of the other locks and the
+lock to be created, restic either continues or fails.
 
 Backups and Deduplication
 =========================