diff --git a/LehrerApp.Data.Tests/ChangeHookTests.cs b/LehrerApp.Data.Tests/ChangeHookTests.cs
new file mode 100644
index 0000000..b0f4f6e
--- /dev/null
+++ b/LehrerApp.Data.Tests/ChangeHookTests.cs
@@ -0,0 +1,60 @@
+using LehrerApp.Core.Models;
+using LehrerApp.Data.Repositories;
+using Xunit;
+
+namespace LehrerApp.Data.Tests;
+
+// Vorlage-Test für den OnChange-Hook (Baustein 2), den die übrigen Repositories in den
+// Bausteinen 3/4 auf dieselbe Weise bekommen.
+public sealed class ChangeHookTests
+{
+    private static LiteDbContext NewInMemoryContext() => new(new MemoryStream());
+
+    [Fact]
+    public void StudentRepository_Save_LoestOnChangeMitKorrektenArgumentenAus()
+    {
+        using var db = NewInMemoryContext();
+        var calls = new List<(string EntityType, string EntityId, string Operation, object? Payload)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, id, op, payload));
+        var repo = new StudentRepository(db);
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+
+        repo.Save(student);
+
+        var call = Assert.Single(calls);
+        Assert.Equal("Student", call.EntityType);
+        Assert.Equal(student.Id.ToString(), call.EntityId);
+        Assert.Equal("Save", call.Operation);
+        Assert.Same(student, call.Payload);
+    }
+
+    [Fact]
+    public void StudentRepository_Delete_LoestOnChangeMitNullPayloadAus()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new StudentRepository(db);
+        var student = new Student { FirstName = "Anna", LastName = "Beispiel" };
+        repo.Save(student);
+        var calls = new List<(string EntityType, string EntityId, string Operation, object? Payload)>();
+        db.OnChange = (type, id, op, payload) => calls.Add((type, id, op, payload));
+
+        repo.Delete(student.Id);
+
+        var call = Assert.Single(calls);
+        Assert.Equal("Student", call.EntityType);
+        Assert.Equal(student.Id.ToString(), call.EntityId);
+        Assert.Equal("Delete", call.Operation);
+        Assert.Null(call.Payload);
+    }
+
+    [Fact]
+    public void StudentRepository_Save_OhneGesetztenHook_WirftNicht()
+    {
+        using var db = NewInMemoryContext();
+        var repo = new StudentRepository(db);
+
+        var exception = Record.Exception(() => repo.Save(new Student { FirstName = "Anna", LastName = "Beispiel" }));
+
+        Assert.Null(exception);
+    }
+}
diff --git a/LehrerApp.Data/LiteDbContext.cs b/LehrerApp.Data/LiteDbContext.cs
index 940859d..bd7768b 100644
--- a/LehrerApp.Data/LiteDbContext.cs
+++ b/LehrerApp.Data/LiteDbContext.cs
@@ -3,6 +3,10 @@ using LehrerApp.Core.Models;
 
 namespace LehrerApp.Data;
 
+/// Wird nach jedem Save/Delete einer Entität aufgerufen; payload ist die gespeicherte Entität
+/// bzw. null bei Delete. Sync-agnostisch – siehe <see cref="LiteDbContext.OnChange"/>.
+public delegate void ChangeHandler(string entityType, string entityId, string operation, object? payload);
+
 /// <summary>
 /// Zentrale LiteDB-Verbindung. Singleton – eine Datei = ein Nutzer.
 /// </summary>
@@ -67,6 +71,91 @@ public class LiteDbContext : IDisposable
 
     public int SchemaVersion => ReadSchemaVersion();
 
+    /// Wird von den Repositories nach jedem Save/Delete aufgerufen (payload = die gespeicherte
+    /// Entität bzw. null bei Delete). Bewusst hier statt in LehrerApp.Sync definiert, damit Data
+    /// weiterhin ohne Verweis auf Sync auskommt — die eigentliche Sync-Anbindung setzt diesen
+    /// Hook von außen (siehe AppBootstrapper).
+    public ChangeHandler? OnChange { get; set; }
+
+    /// Führt dieselbe Kaskade wie <c>GroupRepository.Delete</c> aus. Hier auf dem Context statt
+    /// im Repository, damit ein später eingehendes Sync-Ereignis (Baustein 5) dieselbe Kaskade
+    /// nachvollziehen kann, ohne über die Repository-Save/Delete-Methoden (und damit erneut über
+    /// <see cref="OnChange"/>) zu laufen.
+    internal void CascadeDeleteGroup(Guid id)
+    {
+        ExecuteInTransaction(() =>
+        {
+            foreach (var membership in Memberships.Find(e => e.GroupId == id).ToList())
+                Memberships.Delete(membership.Id);
+
+            foreach (var exam in Exams.Find(e => e.GroupId == id).ToList())
+            {
+                foreach (var result in ExamResults.Find(r => r.ExamId == exam.Id).ToList())
+                    ExamResults.Delete(result.Id);
+                Exams.Delete(exam.Id);
+            }
+
+            foreach (var grade in Grades.Find(g => g.GroupId == id).ToList())
+                Grades.Delete(grade.Id);
+
+            foreach (var reportGrade in ReportGrades.Find(g => g.GroupId == id).ToList())
+                ReportGrades.Delete(reportGrade.Id);
+
+            foreach (var scheme in GradingSchemes.Find(s => s.GroupId == id).ToList())
+                GradingSchemes.Delete(scheme.Id);
+
+            foreach (var unit in Units.Find(u => u.GroupId == id).ToList())
+            {
+                foreach (var lesson in Lessons.Find(l => l.UnitId == unit.Id).ToList())
+                    Lessons.Delete(lesson.Id);
+                Units.Delete(unit.Id);
+            }
+
+            foreach (var lesson in Lessons.Find(l => l.GroupId == id).ToList())
+                Lessons.Delete(lesson.Id);
+
+            foreach (var session in ParticipationSessions.Find(s => s.GroupId == id).ToList())
+            {
+                foreach (var entry in ParticipationEntries.Find(e => e.SessionId == session.Id).ToList())
+                    ParticipationEntries.Delete(entry.Id);
+                ParticipationSessions.Delete(session.Id);
+            }
+
+            foreach (var aspect in ParticipationAspects.Find(a => a.GroupId == id).ToList())
+                ParticipationAspects.Delete(aspect.Id);
+
+            foreach (var section in ParticipationSections.Find(s => s.GroupId == id).ToList())
+                ParticipationSections.Delete(section.Id);
+
+            foreach (var plan in SeatingPlans.Find(p => p.GroupId == id).ToList())
+                SeatingPlans.Delete(plan.Id);
+
+            // Dokumentation und Arbeitszeit sind historische Nachweise. Sie bleiben erhalten,
+            // werden aber von der nicht mehr existierenden Lerngruppe entkoppelt.
+            foreach (var documentation in Documentation.Find(d => d.GroupId == id).ToList())
+            {
+                documentation.GroupId = null;
+                documentation.UpdatedAt = DateTime.UtcNow;
+                Documentation.Update(documentation);
+            }
+
+            foreach (var task in Tasks.Find(t => t.GroupId == id).ToList())
+            {
+                task.GroupId = null;
+                task.UpdatedAt = DateTime.UtcNow;
+                Tasks.Update(task);
+            }
+
+            foreach (var timeEntry in TimeEntries.Find(t => t.GroupId == id).ToList())
+            {
+                timeEntry.GroupId = null;
+                TimeEntries.Update(timeEntry);
+            }
+
+            Groups.Delete(id);
+        });
+    }
+
     internal void ExecuteInTransaction(Action action)
     {
         _db.BeginTrans();
diff --git a/LehrerApp.Data/Repositories/AllRepositories.cs b/LehrerApp.Data/Repositories/AllRepositories.cs
index 7fdb50c..7c9af89 100644
--- a/LehrerApp.Data/Repositories/AllRepositories.cs
+++ b/LehrerApp.Data/Repositories/AllRepositories.cs
@@ -35,7 +35,12 @@ public class StudentRepository(LiteDbContext db) : IStudentRepository
         db.ReportGrades.Count(g => g.StudentId == studentId),
         db.ParticipationEntries.Count(e => e.StudentId == studentId),
         db.Documentation.Count(d => d.StudentId == studentId));
-    public void Save(Student s) { s.UpdatedAt = DateTime.UtcNow; db.Students.Upsert(s); }
+    public void Save(Student s)
+    {
+        s.UpdatedAt = DateTime.UtcNow;
+        db.Students.Upsert(s);
+        db.OnChange?.Invoke(nameof(Student), s.Id.ToString(), "Save", s);
+    }
     public void Delete(Guid id)
     {
         var references = GetReferenceSummary(id);
@@ -43,6 +48,7 @@ public class StudentRepository(LiteDbContext db) : IStudentRepository
             throw new InvalidOperationException(
                 "Der Schüler besitzt verknüpfte Daten und kann nur deaktiviert werden.");
         db.Students.Delete(id);
+        db.OnChange?.Invoke(nameof(Student), id.ToString(), "Delete", null);
     }
 }
 
@@ -77,77 +83,7 @@ public class GroupRepository(LiteDbContext db) : IGroupRepository
     public void Delete(Guid id)
     {
         ArchivedGroupWriteGuard.EnsureActive(db, id);
-        db.ExecuteInTransaction(() =>
-        {
-            foreach (var membership in db.Memberships.Find(e => e.GroupId == id).ToList())
-                db.Memberships.Delete(membership.Id);
-
-            foreach (var exam in db.Exams.Find(e => e.GroupId == id).ToList())
-            {
-                foreach (var result in db.ExamResults.Find(r => r.ExamId == exam.Id).ToList())
-                    db.ExamResults.Delete(result.Id);
-                db.Exams.Delete(exam.Id);
-            }
-
-            foreach (var grade in db.Grades.Find(g => g.GroupId == id).ToList())
-                db.Grades.Delete(grade.Id);
-
-            foreach (var reportGrade in db.ReportGrades.Find(g => g.GroupId == id).ToList())
-                db.ReportGrades.Delete(reportGrade.Id);
-
-            foreach (var scheme in db.GradingSchemes.Find(s => s.GroupId == id).ToList())
-                db.GradingSchemes.Delete(scheme.Id);
-
-            foreach (var unit in db.Units.Find(u => u.GroupId == id).ToList())
-            {
-                foreach (var lesson in db.Lessons.Find(l => l.UnitId == unit.Id).ToList())
-                    db.Lessons.Delete(lesson.Id);
-                db.Units.Delete(unit.Id);
-            }
-
-            foreach (var lesson in db.Lessons.Find(l => l.GroupId == id).ToList())
-                db.Lessons.Delete(lesson.Id);
-
-            foreach (var session in db.ParticipationSessions.Find(s => s.GroupId == id).ToList())
-            {
-                foreach (var entry in db.ParticipationEntries.Find(e => e.SessionId == session.Id).ToList())
-                    db.ParticipationEntries.Delete(entry.Id);
-                db.ParticipationSessions.Delete(session.Id);
-            }
-
-            foreach (var aspect in db.ParticipationAspects.Find(a => a.GroupId == id).ToList())
-                db.ParticipationAspects.Delete(aspect.Id);
-
-            foreach (var section in db.ParticipationSections.Find(s => s.GroupId == id).ToList())
-                db.ParticipationSections.Delete(section.Id);
-
-            foreach (var plan in db.SeatingPlans.Find(p => p.GroupId == id).ToList())
-                db.SeatingPlans.Delete(plan.Id);
-
-            // Dokumentation und Arbeitszeit sind historische Nachweise. Sie bleiben erhalten,
-            // werden aber von der nicht mehr existierenden Lerngruppe entkoppelt.
-            foreach (var documentation in db.Documentation.Find(d => d.GroupId == id).ToList())
-            {
-                documentation.GroupId = null;
-                documentation.UpdatedAt = DateTime.UtcNow;
-                db.Documentation.Update(documentation);
-            }
-
-            foreach (var task in db.Tasks.Find(t => t.GroupId == id).ToList())
-            {
-                task.GroupId = null;
-                task.UpdatedAt = DateTime.UtcNow;
-                db.Tasks.Update(task);
-            }
-
-            foreach (var timeEntry in db.TimeEntries.Find(t => t.GroupId == id).ToList())
-            {
-                timeEntry.GroupId = null;
-                db.TimeEntries.Update(timeEntry);
-            }
-
-            db.Groups.Delete(id);
-        });
+        db.CascadeDeleteGroup(id);
     }
 }