README.md

@@ -278,6 +278,9 @@ exist yet, upaas has Docker create it as an empty directory, owned by root, when
 the app's container starts; there is no need to create it first. An existing
 host path is left as it is. This needs Docker Engine 23.0 or later.
 
+A deploy or rollback replaces the app's container with a new one, which keeps
+only the files the app wrote to its volume mounts.
+
 ## License
 
 WTFPL

TODO.md

@@ -20,6 +20,11 @@ regress.
 
 # Completed Steps
 
+- 2026-10-01: An app with no volume mounts says on its page, and in the log of
+  each deploy, that a deploy or rollback loses the files it writes and a restart
+  keeps them; the README's Volume mounts section says a deploy or rollback keeps
+  only the files the app wrote to its volume mounts (#248).
+
 - 2026-10-01: Builds attach a BuildKit session, as the docker command line does,
   so a base image that is not on the host is pulled instead of the build failing
   with "no active sessions" on Docker Engine 27. Container logs, and so the

internal/handlers/app.go

@@ -21,6 +21,7 @@ import (
 	"sneak.berlin/go/upaas/internal/database"
 	"sneak.berlin/go/upaas/internal/models"
 	"sneak.berlin/go/upaas/internal/service/app"
+	"sneak.berlin/go/upaas/internal/service/deploy"
 	"sneak.berlin/go/upaas/templates"
 )
 
@@ -194,6 +195,7 @@ func (h *Handlers) HandleAppDetail() http.HandlerFunc {
 			"EnvVars":          envVars,
 			"Labels":           labels,
 			"Volumes":          volumes,
+			"NoVolumesWarning": deploy.NoVolumesWarning,
 			"Ports":            ports,
 			"Deployments":      deployments,
 			"LatestDeployment": latestDeployment,

internal/handlers/app_no_volumes_test.go

@@ -0,0 +1,45 @@
+package handlers_test
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"sneak.berlin/go/upaas/internal/models"
+	"sneak.berlin/go/upaas/internal/service/deploy"
+)
+
+// TestAppPageSaysFilesAreLostOnlyWhenAppHasNoVolumes checks that the app page
+// shows deploy.NoVolumesWarning until the app gets a volume mount.
+func TestAppPageSaysFilesAreLostOnlyWhenAppHasNoVolumes(t *testing.T) {
+	t.Parallel()
+
+	testCtx := setupTestHandlers(t)
+	createdApp := createTestApp(t, testCtx, "no-volumes-app")
+
+	renderAppPage := func() string {
+		request := httptest.NewRequestWithContext(
+			t.Context(), http.MethodGet, "/apps/"+createdApp.ID, nil,
+		)
+		request = addChiURLParams(request, map[string]string{"id": createdApp.ID})
+		recorder := httptest.NewRecorder()
+
+		testCtx.handlers.HandleAppDetail().ServeHTTP(recorder, request)
+		require.Equal(t, http.StatusOK, recorder.Code)
+
+		return recorder.Body.String()
+	}
+
+	assert.Contains(t, renderAppPage(), deploy.NoVolumesWarning)
+
+	volume := models.NewVolume(testCtx.database)
+	volume.AppID = createdApp.ID
+	volume.HostPath = "/srv/no-volumes-app"
+	volume.ContainerPath = "/data"
+	require.NoError(t, volume.Save(t.Context()))
+
+	assert.NotContains(t, renderAppPage(), deploy.NoVolumesWarning)
+}

internal/service/deploy/deploy.go

@@ -56,6 +56,12 @@ var (
 	ErrNoPreviousImage = errors.New("no previous image available for rollback")
 )
 
+// NoVolumesWarning is shown on the page of an app with no volume mounts and
+// written into each of its deploy logs.
+const NoVolumesWarning = "This app has no volume mounts, so the files it writes " +
+	"are lost whenever a deploy or rollback replaces its container; " +
+	"a restart of the container keeps them."
+
 // logFlushInterval is how often to flush buffered logs to the database.
 const logFlushInterval = time.Second
 
@@ -369,6 +375,11 @@ func (svc *Service) Deploy(
 
 	svc.logWebhookPayload(bgCtx, deployment, webhookEvent)
 
+	volumes, err := app.GetVolumes(bgCtx)
+	if err == nil && len(volumes) == 0 {
+		_ = deployment.AppendLog(bgCtx, NoVolumesWarning)
+	}
+
 	err = svc.updateAppStatusBuilding(bgCtx, app)
 	if err != nil {
 		return err

internal/service/deploy/deploy_volumes_test.go

@@ -0,0 +1,79 @@
+package deploy_test
+
+import (
+	"context"
+	"log/slog"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"sneak.berlin/go/upaas/internal/config"
+	"sneak.berlin/go/upaas/internal/database"
+	"sneak.berlin/go/upaas/internal/docker"
+	"sneak.berlin/go/upaas/internal/logger"
+	"sneak.berlin/go/upaas/internal/models"
+	"sneak.berlin/go/upaas/internal/service/deploy"
+	"sneak.berlin/go/upaas/internal/service/notify"
+)
+
+// deployLog deploys a new app, with one volume mount when withVolume is set,
+// and returns the deploy's log. Docker is not connected, so the deploy fails
+// at the git clone, after the start of its log is written.
+func deployLog(t *testing.T, withVolume bool) string {
+	t.Helper()
+
+	log := logger.NewForTest(slog.New(slog.NewTextHandler(os.Stderr, nil)))
+	dataDir := t.TempDir()
+	cfg := &config.Config{DataDir: dataDir, HostDataDir: dataDir}
+	db := database.NewTestDatabase(t)
+
+	dockerClient, err := docker.New(nil, docker.Params{Logger: log, Config: cfg})
+	require.NoError(t, err)
+
+	notifySvc, err := notify.New(nil, notify.ServiceParams{Logger: log})
+	require.NoError(t, err)
+
+	svc, err := deploy.New(nil, deploy.ServiceParams{
+		Logger: log, Config: cfg, Database: db,
+		Docker: dockerClient, Notify: notifySvc,
+	})
+	require.NoError(t, err)
+
+	ctx := context.Background()
+
+	app := models.NewApp(db)
+	app.ID = "volumesapp-id"
+	app.Name = "volumesapp"
+	app.Branch = "main"
+	require.NoError(t, app.Save(ctx))
+
+	if withVolume {
+		volume := models.NewVolume(db)
+		volume.AppID = app.ID
+		volume.HostPath = "/srv/volumesapp"
+		volume.ContainerPath = "/data"
+		require.NoError(t, volume.Save(ctx))
+	}
+
+	err = svc.Deploy(ctx, app, nil, false)
+	require.ErrorIs(t, err, docker.ErrNotConnected)
+
+	deployments, err := app.GetDeployments(ctx, 1)
+	require.NoError(t, err)
+	require.Len(t, deployments, 1)
+
+	return deployments[0].Logs.String
+}
+
+func TestDeployLogSaysFilesAreLostOnlyWhenAppHasNoVolumes(t *testing.T) {
+	t.Parallel()
+
+	logWithoutVolumes := deployLog(t, false)
+	assert.Equal(t, 1, strings.Count(logWithoutVolumes, deploy.NoVolumesWarning),
+		logWithoutVolumes)
+
+	assert.NotContains(t, deployLog(t, true), deploy.NoVolumesWarning)
+}

templates/app_detail.html

@@ -318,6 +318,8 @@
                 </tbody>
             </table>
         </div>
+        {{else}}
+        <p class="alert-warning">{{.NoVolumesWarning}}</p>
         {{end}}
         <form method="POST" action="/apps/{{.App.ID}}/volumes" class="flex flex-col sm:flex-row gap-2 items-end">
                 {{ .CSRFField }}