vault backup: 2024-02-06 13:38:14

.obsidian/workspace.json

@@ -155,7 +155,7 @@
       }
     ],
     "direction": "horizontal",
-    "width": 300
+    "width": 200
   },
   "left-ribbon": {
     "hiddenItems": {

Semester 2/Database Systems/Week 4/Week 4 Database Systems.md

@@ -1 +1,29 @@
-# 
+# Concurrency Control
+
+Process of managing simultaneous operations on the database without interference.
+Prevents interference when >2 users access and update the db
+
+# Lost Update Problem
+
+A successful update is overwritten by another user - a read operation of a tuple happens at the same time as a value update of the tuple. This causes the next value update to access the incorrect value, as the read value is incorrect too.
+This would be avoided if any reads are prevented until all updates are completed.
+
+# Uncommitted Dependency Problem
+
+A transaction reads the value of a tuple before another transaction is completed / committed. If the 2nd transaction aborts / rolls back, the value read by the 1st is inconsistent.
+This would be avoided by preventing the 1st transaction from reading until the 2nd has completed ( commit or abort ).
+
+# Inconsistent Analysis Problem
+
+A transaction reads several values, a second transaction updates some of them during the execution of the first. This creates inconsistent values across the effected tuples.
+This would be avoided by preventing reads of targeted tuples until the first transaction has committed or aborted updates.
+
+# Conflicting Actions - General Rules
+
+- Two transactions do not conflict if they:
+	- Are only reading data items
+	- Operate on completely different data items
+- Two operations conflict if all of these are true:
+	- Belong to different transactions
+	- Access same data item
+	- At least on writes the item