Implemented hot restart over websocket #2666

jyameo · 2025-08-11T15:29:00Z

implemented hot restart over websocket
fixed refresh race condition bug

related to #2605

nshahan

Do we already have tests configured in this repo that perform a hot restart using the websockets channel? We should have something to make sure we can get an early signal on breakages here.

nshahan · 2025-08-11T17:36:38Z

dwds/lib/src/services/web_socket_proxy_service.dart

+/// Grace period before destroying isolate when no clients are detected.
+/// This handles the race condition during page refresh where the old connection
+/// closes before the new connection is established, preventing premature isolate destruction.
+const _isolateDestructionGracePeriod = Duration(seconds: 2);


Where is the client connection coming from and when does it get sent? Is it sent from the injected client.js running in the browser? I'm concerned about the potential for 2 seconds being too short compared to the 15 second timeout I see in _performWebSocketHotRestart().

The client connection is sent from the injected client running in the browser. This actually doesn’t impact hot restart, but rather a page refresh operation. I found that in my previous implementation, when there’s only a single window open and we trigger a page refresh, the server might not recognize it as a refresh and could start destroying the isolate immediately since there are no connections for a short period. This change just tells the server to wait an extra 2 seconds, then check if any new connections appear before proceeding to destroy the isolate. From my local testing, 2 seconds seems to work fine, but I can increase it if you think it would make more sense.

I increased it to 10 seconds instead

dwds/lib/src/services/web_socket_proxy_service.dart

jyameo · 2025-08-11T18:29:13Z

Do we already have tests configured in this repo that perform a hot restart using the websockets channel? We should have something to make sure we can get an early signal on breakages here.

@nshahan We do not have a test here but I can add a test in a flutter repo where I use a headless chrome browser similar to what I did for hot reload. we can't really test the websocket channel here without establishing the connection between the injected client and DWDS.

nshahan · 2025-08-11T20:59:22Z

We do not have a test here but I can add a test in a flutter repo where I use a headless chrome browser similar to what I did for hot reload. we can't really test the websocket channel here without establishing the connection between the injected client and DWDS.

@jyameo
With the changes @bkonyi made to the PR in the Flutter repo, do we actually still need the headless chrome session involved in the test?

We have these tests already but it is getting harder to understand what scenarios they cover.

@srujzs
I see you added this test a while back, do you recall what is being tested here?
https://github.com/dart-lang/webdev/blame/94c172cc862d0c39c72158c6537f1e20b4432e0e/dwds/test/common/hot_restart_common.dart#L141-L172

I'm getting lost in the details of

hot_restart_common.dart
"and without debugging using WebSockets"
"live reload"

srujzs · 2025-08-11T21:40:30Z

My rough thoughts:

I added that to unify the hot restart tests across the build daemon and frontend server configurations.
Looking at the diff between that test and the test before it, it looks like useSse determines whether to use SSE (true) or WebSockets (false) to connect to DDS.
The live reload I believe refers to the build daemon sending build results to the injected client which then triggers a hard reload.

Also note that that whole group is skipped when using the frontend server as those tests only make sense for the build daemon. It may be less confusing to pull those out into a separate test that's only run with the build daemon.

srujzs

I generally agree that it's better to have tests in this repo as well, but I'm not familiar with how difficult that may be.

dwds/lib/src/services/proxy_service.dart

srujzs · 2025-08-11T22:07:40Z

dwds/lib/src/services/proxy_service.dart

+  /// Default implementation throws UnimplementedError.
+  /// Override in subclasses that support hot restart completion.
+  void completeHotRestart(HotRestartResponse response) {
+    throw UnimplementedError('completeHotRestart not supported');


Why not make this method abstract instead?

Because only WebSocketProxyService implements hot restart completion. The chrome path handles hot restart differently (via dwds_vm_client) therefore the method is an optional, protocol-specific behavior, not core functionality for all proxy services.

dwds/lib/src/services/web_socket_proxy_service.dart

Salakar · 2025-08-11T22:42:03Z

dwds/lib/src/services/web_socket_proxy_service.dart

+/// Grace period before destroying isolate when no clients are detected.
+/// This handles the race condition during page refresh where the old connection
+/// closes before the new connection is established, preventing premature isolate destruction.
+const _isolateDestructionGracePeriod = Duration(seconds: 10);


Not sure I understand this change correctly but wanted to check;

Is the duration here dependant on how long the client takes to load all JS bundles again? In some of our products we have a remote server of flutter run -d web-server.

Loading from a remote server serveral hundred Flutter debug JS bundles unfortunately takes a long time - consistently more than 10s on average, sometimes minutes on a poor connection - so wanted to check if this change affects this in anyway

This is NOT related to how long it takes to load JS bundles. Instead, it handles a specific race condition during page refresh so it should NOT negatively affect your remote server setup

jyameo · 2025-08-12T14:53:35Z

We do not have a test here but I can add a test in a flutter repo where I use a headless chrome browser similar to what I did for hot reload. we can't really test the websocket channel here without establishing the connection between the injected client and DWDS.

@jyameo With the changes @bkonyi made to the PR in the Flutter repo, do we actually still need the headless chrome session involved in the test?

We have these tests already but it is getting harder to understand what scenarios they cover.

https://github.com/dart-lang/webdev/blob/main/dwds/test/common/hot_restart_common.dart

https://github.com/dart-lang/webdev/blob/main/dwds/test/common/hot_restart_correctness_common.dart

@srujzs I see you added this test a while back, do you recall what is being tested here? https://github.com/dart-lang/webdev/blame/94c172cc862d0c39c72158c6537f1e20b4432e0e/dwds/test/common/hot_restart_common.dart#L141-L172

I'm getting lost in the details of

hot_restart_common.dart

"and without debugging using WebSockets"

"live reload"

@nshahan Will land this PR for now and look into adding a test in a follow up PR

auto-submit · 2025-08-12T15:45:00Z

autosubmit label was removed for dart-lang/webdev/2666, because - The status or check suite unit_test; windows; Dart dev; PKG: dwds; `dart test --total-shards 3 --shard-index 0 --exclude-ta... has failed. Please fix the issues identified (or deflake) before re-applying this label.

Revisions updated by `dart tools/rev_sdk_deps.dart`. ai (https://github.com/dart-lang/ai/compare/6b4b2bc..ee5b2b2): ee5b2b2 2025-08-13 Greg Spencer Stop reporting errors for non-zero exits (dart-lang/ai#262) protobuf (https://github.com/dart-lang/protobuf/compare/0b73b0d..6e9c9f4): 6e9c9f4 2025-08-12 Ömer Sinan Ağacan Improve clone and deepCopy tests (google/protobuf.dart#1039) 086dfab 2025-08-12 Ömer Sinan Ağacan Make proto3_json.dart aware of well known types (google/protobuf.dart#1037) 277b45a 2025-08-12 Ömer Sinan Ağacan Release protobuf 4.2.0 (google/protobuf.dart#1038) shelf (https://github.com/dart-lang/shelf/compare/2a46b4f..400fc39): 400fc39 2025-08-11 Kevin Moore [shelf_router_generator] latest deps, prepare release (dart-lang/shelf#480) web (https://github.com/dart-lang/web/compare/72cdd84..4310354): 4310354 2025-08-12 Nikechukwu [interop] Add Support for JSDoc Documentation (dart-lang/web#435) webdev (https://github.com/dart-lang/webdev/compare/94c172c..c0492f1): c0492f1b 2025-08-13 Srujan Gaddam [dwds] Wait for scripts to be parsed on a hot restart and publish DWDS 25.0.0 (dart-lang/webdev#2667) 595f8768 2025-08-13 Nicholas Shahan [dwds] Cleanup unused null safety related fields (dart-lang/webdev#2660) b43030e2 2025-08-12 Jessy Yameogo Implemented hot restart over websocket (dart-lang/webdev#2666) Change-Id: I1cb2a0c2ed693bb740ab4c40ea70678b03eec596 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/445340 Auto-Submit: Devon Carew <devoncarew@google.com> Commit-Queue: Konstantin Shcheglov <scheglov@google.com> Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

implemented hot restart over websocket

2bd06e4

github-actions bot added the package:dwds label Aug 11, 2025

remove service callback for fullReload

c992a36

jyameo requested review from biggs0125, srujzs and nshahan August 11, 2025 15:43

nshahan reviewed Aug 11, 2025

View reviewed changes

addressed comments

fbeacb6

jyameo requested a review from nshahan August 11, 2025 18:29

srujzs approved these changes Aug 11, 2025

View reviewed changes

Salakar reviewed Aug 11, 2025

View reviewed changes

addressed comments

f2f1023

jyameo added the autosubmit label Aug 12, 2025

auto-submit bot removed the autosubmit label Aug 12, 2025

jyameo merged commit b43030e into dart-lang:main Aug 12, 2025
51 of 52 checks passed

This was referenced Aug 15, 2025

Hot restart is broken when running DWDS without Chrome Debug Port #2642

Closed

Issue with page refreshes (from VSCODE) #2644

Closed

Implemented hot restart over websocket #2666

Implemented hot restart over websocket #2666

Uh oh!

Conversation

jyameo commented Aug 11, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

nshahan left a comment

Choose a reason for hiding this comment

Uh oh!

nshahan Aug 11, 2025

Choose a reason for hiding this comment

Uh oh!

jyameo Aug 11, 2025

Choose a reason for hiding this comment

Uh oh!

jyameo Aug 11, 2025

Choose a reason for hiding this comment

Uh oh!

Uh oh!

jyameo commented Aug 11, 2025

Uh oh!

nshahan commented Aug 11, 2025

Uh oh!

srujzs commented Aug 11, 2025

Uh oh!

srujzs left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

srujzs Aug 11, 2025

Choose a reason for hiding this comment

Uh oh!

jyameo Aug 12, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Salakar Aug 11, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

jyameo Aug 12, 2025

Choose a reason for hiding this comment

Uh oh!

jyameo commented Aug 12, 2025

Uh oh!

auto-submit bot commented Aug 12, 2025

Uh oh!

Uh oh!

Uh oh!

jyameo commented Aug 11, 2025 •

edited

Loading

jyameo Aug 12, 2025 •

edited

Loading

Salakar Aug 11, 2025 •

edited

Loading