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ploeh · Jun 6, 2024 · de3f5dd · de3f5dd
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 		<div class="comment-date">2024-06-05 12:42 UTC</div>
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+    <div class="comment" id="2960b65bbaec4db8ade70f551e3f5062">
+        <div class="comment-author"><a href="/">Mark Seemann</a> <a href="#2960b65bbaec4db8ade70f551e3f5062">#</a></div>
+        <div class="comment-content">
+            <p>
+                James, Nicholas, thank you for writing. I realize that there are some unstated assumptions and implied concerns that I should have made more explicit. I certainly have no problem with adding constraints and other rules to model data. For the Danish CPR number, for example, while I wouldn't make it a primary key (for the reasons outlined in the article), I'd definitely put a <code>UNIQUE</code> constraint on it.
+            </p>
+            <p>
+                Another unspoken context that I had in mind is that systems often exist in a wider context where ACID guarantees fall apart. I suppose it's true that if you look at a database in isolation, you may be able to update a foreign key with the help of some cascading changes rippling through the database, but if you've ever shared the old key outside of the database, you now have orphaned data.
+            </p>
+            <p>
+                A simple example could be sending out an email with a link that embeds the old key. If you change the key after sending out the email, but before the user clicks, the link no longer works.
+            </p>
+            <p>
+                That's just a simple and easy-to-explain example. The more integration (particularly system-to-system integration) you have, the worse this kind of problem becomes. I briefly discussed the CPR number example with my doctor wife, and she immediately confirmed that this is a real problem in the Danish health sector, where many independent software systems need to exchange patient data.
+            </p>
+            <p>
+                You can probably work around such problems in various ways, but if you had avoided using natural keys, you wouldn't have had to change the key in the first place.
+            </p>
+        </div>
+        <div class="comment-date">2024-06-06 6:56 UTC</div>
+    </div>
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