docs(database): add transaction side-effect recipes

- explain why external side effects should be deferred until commit
- add afterCommit and afterRollback examples
- clarify Model callbacks are not commit-aware by themselves

Signed-off-by: memleakd <121398829+memleakd@users.noreply.github.com>

Author: memleakd

user_guide_src/source/database/transactions.rst

@@ -180,6 +180,31 @@ This is useful for side effects that should only happen after committed data is
 visible, such as dispatching a queued job or sending a notification, and for
 cleanup that should only happen after a real rollback.
 
+Deferring Side Effects
+----------------------
+
+Code that changes external state should usually not run in the middle of a
+transaction. If the transaction rolls back after the side effect has already
+run, the application may send a notification for data that was never saved,
+invalidate a cache for a write that did not persist, or start background work
+that cannot find the committed row it expects.
+
+Register those side effects with ``afterCommit()`` instead:
+
+.. literalinclude:: transactions/013.php
+
+Use ``afterRollback()`` for cleanup that should happen only when the transaction
+does not commit, such as removing a temporary file created before the database
+write:
+
+.. literalinclude:: transactions/014.php
+
+Model callbacks such as ``afterInsert`` and ``afterUpdate`` run after the Model
+query has executed, but not necessarily after the surrounding transaction has
+committed. If a Model callback needs to run a side effect only after commit, it
+should register that work with the database connection's ``afterCommit()``
+method while an active transaction is already open.
+
 Callbacks run after the database transaction has already committed or rolled
 back. If a callback throws an exception, that exception bubbles to the caller,
 but the transaction outcome is not changed.

user_guide_src/source/database/transactions/013.php

@@ -0,0 +1,18 @@
+<?php
+
+use CodeIgniter\Events\Events;
+
+$orderId = $this->db->transaction(static function ($db) use ($order): int {
+    $db->table('orders')->insert($order);
+    $orderId = $db->insertID();
+
+    $db->afterCommit(static function () use ($orderId): void {
+        service('cache')->delete('orders_list');
+        Events::trigger('order_created', $orderId);
+
+        // Dispatch a queued job or send a notification here.
+        // The new order is committed and visible to other database connections.
+    });
+
+    return $orderId;
+});

user_guide_src/source/database/transactions/014.php

@@ -0,0 +1,11 @@
+<?php
+
+$this->db->transaction(static function ($db) use ($temporaryPath, $record): void {
+    $db->afterRollback(static function () use ($temporaryPath): void {
+        if (is_file($temporaryPath)) {
+            unlink($temporaryPath);
+        }
+    });
+
+    $db->table('documents')->insert($record);
+});