CodSpeed Performance Report

Merging #5969 will not alter performance

_{Comparing masenf/rehydrate-on-reconnect (e842f4d) with main (81a1b2f)}

Summary

✅ 8 untouched

Greptile Overview

Greptile Summary

This PR implements a frontend-driven reconnection hydration mechanism to fix issue #5963 where stale state would linger on the client after socket reconnection when backend state expired. The solution introduces a rehydrate flag that triggers a special "reconnect" hydrate event on socket reconnection, which fetches the latest complete state without resetting client storage or re-running on_load events.

Key Changes:

• Frontend now sets a rehydrate flag when manually reconnecting and sends a hydrate event with is_reconnect: true upon connection
• Backend hydrate middleware skips client storage reset and is_hydrated flag manipulation when processing reconnect hydrates
• Changed link_token_to_sid to use state_manager.modify_state instead of app.modify_state to avoid emitting unnecessary deltas (full state is sent via reconnect hydrate)
• Removed the previous backend-driven reload check that was added in commit 8388d74

Architecture:
This replaces the previous backend-driven approach (checking if router_data exists on connect) with a cleaner frontend-driven approach where the client explicitly requests a reconnect hydrate. This gives the frontend more control and reduces backend complexity.

Confidence Score: 4/5

• This PR is safe to merge with one minor assumption that should be verified through testing
• The implementation is well-structured and addresses the stated issue effectively. The changes are minimal and focused, replacing a backend-driven solution with a frontend-driven one. However, the score is 4 instead of 5 due to one assumption: the code directly accesses initialEvents()[0] and mutates its payload without defensive checks. If initialEvents() returns an empty array or the hydrate event structure changes, this could cause a runtime error. The fix is straightforward (add a null check), but the likelihood of this being an issue in practice is low since initialEvents() is generated code that should always include the hydrate event as the first element.
• Pay attention to reflex/.templates/web/utils/state.js - verify that initialEvents()[0] always returns the expected hydrate event object in all scenarios

Important Files Changed

File Analysis

Sequence Diagram

sequenceDiagram
    participant Client as Frontend (state.js)
    participant Socket as WebSocket
    participant Backend as EventNamespace
    participant Middleware as HydrateMiddleware
    participant State as State Manager

    Note over Client,State: Normal Reconnection Flow
    
    Client->>Client: Detect disconnection
    Client->>Client: Call socket.reconnect()
    Client->>Client: Set rehydrate flag = true
    Client->>Socket: Connect with token
    
    Socket->>Backend: on_connect(sid, token)
    Backend->>Backend: link_token_to_sid(sid, token)
    Backend->>State: state_manager.modify_state()
    Note right of Backend: No delta emitted (changed from app.modify_state)
    Backend->>State: Update router_data[SESSION_ID] = sid
    
    Socket-->>Client: "connect" event
    
    Client->>Client: Check rehydrate flag = true
    Client->>Client: Get initialEvents()[0]
    Client->>Client: Set payload.is_reconnect = true
    Client->>Client: Reset rehydrate flag = false
    Client->>Socket: Queue hydrate event with is_reconnect=true
    
    Socket->>Backend: Process hydrate event
    Backend->>Middleware: preprocess(event)
    
    alt is_reconnect = true
        Note over Middleware: Skip _reset_client_storage()
        Note over Middleware: Skip setting is_hydrated = False
        Note over Middleware: On_load events will NOT run
    else is_reconnect = false (normal hydrate)
        Middleware->>State: _reset_client_storage()
        Middleware->>State: Set is_hydrated = False
        Note over Middleware: On_load events WILL run
    end
    
    Middleware->>State: Get full state dict via _resolve_delta()
    Middleware->>State: _clean()
    Middleware-->>Backend: Return StateUpdate with full delta
    
    Backend->>Socket: Emit state update
    Socket-->>Client: Receive full state
    Client->>Client: Apply state update to frontend