Add 'Understanding Eventsourcing' chapters 22-23

public/data/blogs/blogs.json

@@ -327,7 +327,7 @@
             "title": "Understanding Event Sourcing - a review",
             "teaser": "Understand the events, use the source",
             "dir": "",
-            "date": "2024-12-09",
+            "date": "2024-12-10",
 			"categories": ["review"]            
         }
     ]

public/data/blogs/understanding-eventsourcing/entry.md

@@ -446,3 +446,31 @@ Axon also makes it easy to test the slices. It is interesting to see how aggrega
 are initialized. That can sometimes be a puzzle to figure that out.
 
 The rule with maximum three items in the card is implemented by counting _ItemAdded_-events.
+
+### 22. Implementing state view slices using Live-Projections
+
+A _Live report_ or _Live projection_ is a query which is calculated from the events
+every time it is requested. This is rarely too slow. If it is you can chose a
+stored projection.
+
+The code in the is highly dependent on the Axon framework but the principles used
+are very simple.
+
+Axon does not provide any help for testing queries, so the has an example of a more
+manual approach using Test Containers. It gets the job done.
+
+### 23. Implementing Remove-Item and Clear-Cart
+
+In here it is stated very clearly that the command handler is not responsible
+for changing the state and attributes of the aggregate. The job of the command
+handler is _to enforce invariants, validate business rules and apply the correct
+events in case all rules apply_.
+
+_The sole purpose of the command handler is to make a decision whether a command can
+be processed by validating the existing state of the system._
+
+_The only purpose of the event-sourcing handler is to evolve the state of the aggregate._
+
+_An aggregate's internals only changes because of events that occured, never because of an action._
+
+Just to make that clear!

public/data/quotes.json

@@ -515,6 +515,22 @@
         "author": "Ryan Singer",
         "languages": ["dk", "en"],
         "source": "Shape Up"
+    },{
+        "quote": "The sole purpose of the command handler is to make a decision whether a command can be processed by validating the existing state of the system",
+        "author": "Martin Dilger",
+        "languages": ["dk", "en"],
+        "source": "Understanding Eventsourcing"
+    },{
+        "quote": "The only purpose of the event-sourcing handler is to evolve the state of the aggregate",
+        "author": "Martin Dilger",
+        "languages": ["dk", "en"],
+        "source": "Understanding Eventsourcing"
+    },{
+        "quote": "An aggregate's internals only changes because of events that occured, never because of an action",
+        "author": "Martin Dilger",
+        "languages": ["dk", "en"],
+        "source": "Understanding Eventsourcing"
     }
+
 ]
 }