Finish day 2

ntnu/21v/ttk4145/summary/summary.md

@@ -155,4 +155,35 @@ Find the failure modes: What could go wrong?
 **Step 3: Handling with redundancy**
 
 * Timeout 
-* Retransmit message
+* Retransmit message
+
+#### Example with processes and caculations
+
+A calculation is an abstract, so how can we talk generally about the failure modes.
+
+**Step 1: Failure modes**
+
+One failure mode
+
+**Step 2: Detect, simplify, inject errors**
+
+All failed acceptance tests will "PANIC" or "STOP".
+
+**Step 3: Handling with redundancy**
+
+There are three solutions:
+
+1. Checkpoint restart
+    * Do all the work incuding the acceptance test
+    * Wait with the "side effects"
+    * Store a checkpoint
+    * Do the "side effects"
+2. Process pairs
+    * Crash and let an another process take over
+3. Presistent processes
+
+
+## Transactions
+
+A transaction is a design framework for Damage Confinement and Error Recovery.
+