README + prevent data inconsistency on db/add.

README.md

@@ -6,6 +6,12 @@ When storing simple files directly on the file-system is enough.
 
 This project is a work in progress.
 
+# Constraints (WIP)
+
+TODO:
+- parallelism: run dodb on several threads (solved by a locking system)
+- fs data integrity: in case of a crash, dodb won't corrupt data on disk (solved by a temp file on the same filesystem, then a simple mv)
+
 # Crystal version
 
 Follow [this link for the Crystal version of DODB][dodbcr].

src/dodb.cl

@@ -62,8 +62,6 @@
                  (setf current-index id))
                (setf (gethash id data) value)))
 
-    (format t "Max current index: ~A~&" current-index)
-
     (make-db
       :struct-name struct-name
       :path path
@@ -172,7 +170,7 @@
     (db-data database)))
 
 ; Example: database -> "000000000000018".
-(defun db/add/new-data-filename (database)
+(defun db/add/new-data-basename (database)
   (number->filename (db-current-index database)))
 
 ; Example: {db-path}/data/000000000000018
@@ -181,20 +179,23 @@
 
 (defun db/add (database object)
   (incf (db-current-index database))
-  (let ((file-name (db/add/new-data-filename database)))
+  (let* ((file-basename     (db/add/new-data-basename database))
+         (tmp-file-basename (concatenate 'string file-basename "_tmp"))
+         (tmp-file-path     (get-filepath database tmp-file-basename)))
 
-    ; write object to file
-    (util:write-object-to-file
-      object
-      (get-filepath database file-name))
+    ; write object to temporary file
+    (util:write-object-to-file object tmp-file-path)
+
+    ; rename the temporary file
+    (rename-file tmp-file-path file-basename)
 
     ; handle indexes
     (loop for index in (db-indexes database)
-       do (db/index/new database index object file-name))
+       do (db/index/new database index object file-basename))
 
     ; handle partitions
     (loop for partition in (db-partitions database)
-       do (db/partition/new database partition object file-name)))
+       do (db/partition/new database partition object file-basename)))
 
   ; store new in-memory data
   ; database.data[database.current-index] = object
@@ -249,6 +250,11 @@
 (alias db/get-by-index     db/search/fs/get-by-index)
 (alias db/get-by-partition db/search/fs/get-by-partition)
 
+;(defun db/update (database object-index)
+;  (let ((object (gethash object-index (db-data database))))
+;    ; TODO: write the object on the FS
+;  ))
+
 ;
 ; TEST SCENARIO
 ;
@@ -257,11 +263,11 @@
 
 (defparameter cars (db/new "vehicle" "./storage/cars/"))
 
-;(db/add cars (make-vehicle :name "Corvet"       :color "Red"))
-;(db/add cars (make-vehicle :name "Ferrari"      :color "Red"))
-;(db/add cars (make-vehicle :name "Deudeuch"     :color "Beige"))
-;(db/add cars (make-vehicle :name "BMW"          :color "Blue"))
-;(db/add cars (make-vehicle :name "Suzuki Wagon" :color "Blue"))
+(db/add cars (make-vehicle :name "Corvet"       :color "Red"))
+(db/add cars (make-vehicle :name "Ferrari"      :color "Red"))
+(db/add cars (make-vehicle :name "Deudeuch"     :color "Beige"))
+(db/add cars (make-vehicle :name "BMW"          :color "Blue"))
+(db/add cars (make-vehicle :name "Suzuki Wagon" :color "Blue"))
 
 (db/new-index cars "name")
 (db/new-partition cars "color")