Fix: Construct 3 Multiplayer Photon Host Migration Stuck

A 4-player multiplayer match. The host crashes. Photon promotes player 2 to master. The other three players see frozen NPCs — the spawned enemies lose their controlling client and stop ticking. Restarting the match resolves it; mid-match recovery doesn’t work.

Master Client Authority Model

In Photon’s model, one peer is master client. Authoritative state (NPC AI, world events) typically runs on the master. When the master leaves:

1. Photon detects the disconnect.
2. Promotes another peer to master.
3. Broadcasts On master client changed to all peers.

Owned objects (those tagged with the old master’s ID) need explicit reassignment.

The Fix

On Photon master client changed:
    If Photon.IsMaster:
        // I’m the new master. Take ownership of orphaned objects.
        For each NPC:
            If NPC.OwnerID = Photon.PreviousMasterID:
                NPC Set OwnerID to Photon.LocalID
        Broadcast "AuthorityHandoff" with data {oldMaster: ..., newMaster: ...}

  On Photon message "AuthorityHandoff":
    For each NPC:
        If NPC.OwnerID = message.oldMaster:
            NPC Set OwnerID to message.newMaster

The new master claims orphans; broadcasts the change; clients update their authority bookkeeping. NPCs resume ticking under the new master’s control.

Persistent State via Room Properties

For state that must survive migration (player scores, current wave number):

Photon Set room property "wave_number" to 5
// Property persists in Photon’s room state; new master reads it on promotion

Photon stores these on its server; migration doesn’t lose them. RPCs and ephemeral messages do get lost.

Detecting Migration

On master client changed includes the previous and new master IDs. Use them to identify orphans precisely — you don’t want to reassign objects owned by other still-active clients.

Verifying

Run a 4-player session. Kill the host (Alt+F4 on the master). Verify the other three players continue playing with NPCs ticking under the new master. Score state should persist (room property); in-flight ephemeral events may be lost (acceptable).

Understanding the issue

Multiplayer code has a different correctness model than single-player code. It must tolerate latency, packet loss, and out-of-order delivery while preserving game-state consistency. Each tolerance is engineering work; you choose which network conditions to handle.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

This bug class disproportionately affects late-stage development. The work to surface it is interactive testing in realistic conditions, which only really happens after the gameplay is in place and assets are populated. Catching it early requires deliberate testing of conditions that look unimportant.

At the engine level, the behavior comes from a deliberate design decision in Construct 3. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

Verifying this fix in isolation is straightforward: reproduce the bug, apply the change, confirm the bug no longer reproduces. The harder verification is regression - did this fix introduce a new bug elsewhere? Run your standard regression suite, plus any tests that exercise the same code path with different inputs.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

Related bug classes often share the same root cause. If you find yourself fixing this issue, look for cousins: similar symptoms in adjacent systems, the same data flow but a different value, or the same fix pattern in another module. The catalog of 'we've seen this before' becomes valuable institutional knowledge.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

For shipping titles with a long support window, watch for this issue resurfacing after dependency updates. Engine upgrades, driver updates, OS releases - each one can resurface a bug class you thought you'd fixed because the underlying behavior changed slightly. Regression tests catch the obvious ones; player reports catch the rest.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Construct 3-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

Within Construct 3, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Photon promotes master client automatically. Your game has to take care of authority handover. Listen for the event and reclaim orphans.”

Use room properties for any state that should outlive a single master — treats Photon as durable storage.