The integration test of the bus module's first want shows its node backing it up through the backup seat before that apply and before the one that replaces the server, as every module that keeps data (ADR 0601 §2)

cmd/mesh-host/nodeagent_integration_test.go

@@ -700,6 +700,8 @@ func (r *recordingRuntime) moved(container string) []string {
 // its changed file and records the server as the bus module's, and asks the container runtime to remove, make, stop,
 // rename or restart nothing, since the build does not change the server. A later want whose build changes the server
 // replaces it once, as any module's, once the want carries the yes (§2: the assigner gives it from the release's Approve).
+// ADR 0601 §2: the bus module keeps data (its streams), so before each of those applies the node agent asks its node's
+// backup seat to back it up, as for every module that keeps data, and applies only once the backup says it is done.
 func TestNatsTheBusServerIsTakenOverAndReplacedOnlyWhenItsBuildChangesIt(t *testing.T) {
 	b := newItBed(t)
 	runtime := &recordingRuntime{containers: map[string]string{}}
@@ -749,7 +751,8 @@ func TestNatsTheBusServerIsTakenOverAndReplacedOnlyWhenItsBuildChangesIt(t *test
 	kvOf(link.PackagesBucket)
 	// The bus module at a build, done: its manifest written and the indexer's module naming it at fp.
 	publish := func(fp, env string) {
-		man := `{"module":"nats","version":"1","provides":[{"name":"mesh-bus","scope":"mesh"}],"resources":[` +
+		man := `{"module":"nats","version":"1","provides":[{"name":"mesh-bus","scope":"mesh"}],` +
+			`"data":{"own":[{"id":"jetstream","path":"` + b.dir + `","class":"valuable","why":"the bus's streams"}]},"resources":[` +
 			`{"id":"server-conf","type":"file","path":"` + conf + `","mode":"0644","content":"listen: new\n"},` +
 			`{"id":"server","type":"container","name":"` + server + `","image":"` + image + `"` + env + `}]}`
 		digest := delivery.ManifestDigest([]byte(man))
@@ -790,6 +793,33 @@ func TestNatsTheBusServerIsTakenOverAndReplacedOnlyWhenItsBuildChangesIt(t *test
 			t.Fatal(err)
 		}
 	}
+	// The node's backup seat: its holder says the bus module has something to back up, starts a backup on now, and says
+	// when it ended on its event; each ask of now notes what the bus's file held then, so the order is seen.
+	var backups []string
+	var backupsMu sync.Mutex
+	if _, err := b.conn.Subscribe(link.SeatToolSubject("node-backup", "backed-up", itNode), func(m *nats.Msg) {
+		_ = m.Respond([]byte(`{"result":[{"module":"nats","runs":1}]}`))
+	}); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := b.conn.Subscribe(link.SeatToolSubject("node-backup", "now", itNode), func(m *nats.Msg) {
+		got, _ := os.ReadFile(conf)
+		backupsMu.Lock()
+		backups = append(backups, string(got))
+		backupsMu.Unlock()
+		_ = m.Respond([]byte(`{"result":{"started":true}}`))
+		go func() {
+			time.Sleep(20 * time.Millisecond)
+			_ = b.conn.Publish(backedUpEvent, backupEvent(itNode, "nats", true, time.Now(), ""))
+		}()
+	}); err != nil {
+		t.Fatal(err)
+	}
+	backedUp := func() []string {
+		backupsMu.Lock()
+		defer backupsMu.Unlock()
+		return append([]string(nil), backups...)
+	}
 	first := fpIt("e")
 	publish(first, "")
 	busWant(delivery.Want{FP: first, Self: true})
@@ -809,6 +839,9 @@ func TestNatsTheBusServerIsTakenOverAndReplacedOnlyWhenItsBuildChangesIt(t *test
 	if moved := runtime.moved(server); len(moved) != 0 || runtime.containers[server] != made {
 		t.Fatalf("the first want moved the bus server its build does not change: %v", moved)
 	}
+	if got := backedUp(); len(got) != 1 || got[0] != "listen: old\n" {
+		t.Fatalf("the bus module's first apply was not backed up first: the backups saw %q", got)
+	}
 	state, err := store.Load(b.opts.state)
 	if err != nil {
 		t.Fatal(err)
@@ -835,6 +868,9 @@ func TestNatsTheBusServerIsTakenOverAndReplacedOnlyWhenItsBuildChangesIt(t *test
 			runs++
 		}
 	}
+	if got := backedUp(); len(got) != 2 || got[1] != "listen: new\n" {
+		t.Errorf("the replacing apply of the bus module was not backed up first: the backups saw %q", got)
+	}
 	if runs != 1 || runtime.containers[server] == made {
 		t.Errorf("the build that changes the server made it %d times, its spec still the folded one %v: %v", runs,
 			runtime.containers[server] == made, runtime.moved(server))