Add journal file specification

Notice that it's still in WIP status.

journal_file.md

@@ -2,3 +2,76 @@
 
 ## Specification
 
+1. `JRNL` file signature (4 bytes)
+2. The offset to the first transaction (8 bytes)
+3. The offset to the last transaction (8 bytes)
+4. Transactions
+    1. Previous transaction offset (8 bytes)
+    2. Next transaction offset (8 bytes)
+    3. Creation date in Unix timestamp format (8 bytes)
+    4. Transaction flags (1 byte)
+    5. Operations list
+        1. Transaction type (1 byte)
+        2. Transaction data size (4 bytes)
+        3. Transaction data
+        
+## Transactions
+
+### Flags
+
+**TODO**. Now there is the only option -- `JRNL_COMPLETE` on bit 0.
+
+### Consistency
+
+If the last transaction was not written completely, journal rollback is started.
+That means that it will be cleaned since it has not even been written to storage.
+
+If the last transaction is complete in journal but not in storage (flag `JRNL_COMPLETE` is 
+not set), redo is being done. That means the transaction starts again since the journal is 
+consistent. 
+
+### Operations
+
+There are several operation types:
+
+1. Page allocation (`0x01`)
+2. Page modification (`0x02`)
+3. Page removal (`0x03`)
+4. Transaction rollback (`0xFE`)
+5. Transaction completion (`0xFF`)
+
+#### Page allocation
+
+This operation contains this data:
+
+1. Allocated page offset (8 bytes)
+2. Last free page offset before operation (8 bytes)
+3. Last free page offset after operation (8 bytes)
+
+24 bytes in summary.
+
+#### Page modification
+
+This operation contains this data:
+
+1. Modified page offset (8 bytes)
+2. Modified page data offset (2 bytes)
+3. Modified page data size (2 bytes)
+4. 'Before' data image
+5. 'After' data image
+
+12 bytes on header + 2 * (`par. 3`) bytes in summary. 
+Can't be more than 131084.
+
+#### Page removal
+
+This operation contains this data:
+
+1. Removed page offset (8 bytes)
+2. Last free page offset before operation (8 bytes)
+
+16 bytes in summary.
+
+#### Transaction rollback/completion
+
+This operation *may* contain data. By default it does not.