Replication: Entity state bags - #277
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WalkthroughThe change adds typed, scoped state bags to network entities. It synchronizes state changes through a bounded RPC, seeds construction data, exposes state bags to scripts, handles ownership changes, and adds unit tests. ChangesState Bag Replication
Priority: ➖ Normal Estimated code review effort: 4 (Complex) | ~60 minutes Change: Feature Sequence Diagram(s)sequenceDiagram
participant Script
participant StateBag
participant ReplicationManager
participant StateBagSync
Script->>StateBag: set(key, value, scope)
StateBag->>ReplicationManager: MarkStateBagDirty(entity)
ReplicationManager->>StateBagSync: Send state changes
StateBagSync->>StateBag: Apply(key, value, removed)
Merge Risk: 🟡 Moderate · up to Previous owners may retain stale owner-only state, so the revocation path should be corrected before merge. Ownerless subscriptions also require cleanup handling. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches 💡 1📝 Generate docstrings 💡
🧪 Generate unit tests (beta)
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Every NetworkEntity gains an arbitrary key/value bag that replicates to the connections which can see it, so a game can hang its own state off an entity without a typed field being added here for each one. Bags do not ride SerializeFields. That path runs through VariableDeltaSerializer, which identifies variables by position in a fixed per-tick sequence, so keys that come and go shift every later variable's slot and corrupt the delta. They travel as their own RPC instead, on Channel::Events -- which shares an ordering channel with construction, so a change naming an entity arrives after the seed that created it, and no sequence number is needed. ReplicationManager flushes once per tick from NetworkPeer::Update, next to RebuildInterest, and sends each change only to connections that have the entity constructed. Bags therefore inherit interest scoping, virtual worlds and interest budgets rather than restating them, and no state reaches a client that cannot see the entity. A dirty list keyed by NetworkID keeps the flush proportional to what changed rather than to how many entities exist. Three scopes decide who a key reaches: broadcast, the owning connection only, or the server alone. The construction seed is written per destination, so an owner's copy carries its owner-scoped keys and nobody else's; SetOwner re-sends them when authority moves, which is the one path construction cannot cover. Writes are the server's. Key, value and bag-size limits are enforced at the write: every write crosses the network to every viewer of the entity, so an unbounded bag is a denial of service one careless resource away.
Adds a StateBag builtin and reaches it from `entity.state`, so every handle a mod derives from Entity carries its bag without registering anything of its own. Reads are on both sides: get, has, keys and toObject. Writes -- set and remove -- are registered only for server scripting, the same split Entity already draws around setVirtualWorld. Scalars cross as themselves and anything structured is serialized to JSON, so the wire format never needs to know a game's shapes. Reads are per key rather than per bag: get is a lookup, and toObject is the explicit call for a whole copy. A write that breaches a limit throws; a call on a handle whose entity is gone is a silent no-op, per the conventions next to the builtins.
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Inline comments:
In `@code/framework/src/networking/replication/replication_manager.cpp`:
- Around line 142-144: Update the ownership-change flow in the replication
manager to capture the previous owner before reassignment, then remove its
owner-scoped keys before seeding the new owner. Use the previous owner’s replica
and OwnerKeys data to send removal changes, including when that replica is
retained; keep this separate from normal state-scope transitions.
In `@code/framework/src/networking/replication/state_bag.cpp`:
- Around line 100-102: Update StateBag::Set and FlushStateBags to retain each
key’s previous scope when its scope changes, send the current value only to
recipients in the new audience, and send removed=true to constructed recipients
excluded from the previous audience. Ensure transitions to StateScope::Server
are marked dirty when removal is needed, and add regression coverage for
narrowing transitions both within the same tick and after delivery.
In `@code/framework/src/networking/rpc/state_bag_sync.h`:
- Line 49: Validate the deserialized count in the state-bag sync flow before
allocation: if count exceeds kMaxChanges, reject or return using the existing
invalid-input path, then call changes.assign only for accepted counts. Preserve
the uint16_t deserialization behavior and anchor the change to the count check
immediately before changes.assign.
In `@code/framework/src/scripting/builtins/state_bag.cpp`:
- Line 269: Update the “key” parameter metadata in the state-bag binding to
document a maximum of 64 bytes rather than 64 characters, matching the server
validation limit.
- Line 38: Update the options.scope lookup in StateBag.set so it returns false
immediately when Get(...).ToLocal(&scope) fails; only assign the default
Broadcast scope when the lookup succeeds and returns null or undefined.
- Line 218: Update the state-key assignment in ReadKey to define each key as an
own data property rather than using Object::Set, preventing __proto__ from
invoking the inherited setter while preserving the existing converted value.
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code/framework/CMakeLists.txtcode/framework/src/networking/network_peer.cppcode/framework/src/networking/replication/network_entity.cppcode/framework/src/networking/replication/network_entity.hcode/framework/src/networking/replication/replication_manager.cppcode/framework/src/networking/replication/replication_manager.hcode/framework/src/networking/replication/state_bag.cppcode/framework/src/networking/replication/state_bag.hcode/framework/src/networking/rpc/state_bag_sync.hcode/framework/src/scripting/builtins/builtins.cppcode/framework/src/scripting/builtins/entity.cppcode/framework/src/scripting/builtins/state_bag.cppcode/framework/src/scripting/builtins/state_bag.hcode/tests/framework_ut.cppcode/tests/modules/state_bag_ut.h
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A change callback used to be one per manager, so every listener woke for every key of every entity and paid to have its arguments built. A busy server changes bags many times a tick; a script watching one field of one body should not be paying for the rest of the world. ReplicationManager now keeps a registry of subscriptions, each with an optional entity and key filter, and consults only the ones a change can match: the bucket that named its key, plus those that named none. Handles rather than iterators, and each looked up again at call time, so a handler may cancel itself or another mid-dispatch; one added during a dispatch starts from the next change. `entity.state.onChange(key, handler)` exposes it, with a null key meaning every key of that entity. It returns an unsubscribe function, matching what Events.on hands back, and the registering resource owns the subscription: CallResourceStop drops it, as it already does for event and message handlers, so a stopped resource stops being called and its retained function stops holding its objects alive. The handler and the unsubscribe function are typed Function rather than by signature. The declaration generator rejects a function type anywhere in its grammar, so the shape is spelled in the doc text.
Review found two ways a peer kept a value it was no longer meant to have. Both come from the same gap: a client stores what it is sent, under broadcast scope whatever scope it arrived on, so withholding later updates leaves the old value sitting there. Nothing took one back. A key's audience can shrink. A dirty key now records the scope it has now and the scope it had before, and the flush sends the value to the new audience and a removal to every constructed connection that was in the old one and is not in the new. A write to Server scope dirties when the key had previously reached the wire, which it did not before -- so a key narrowed from Broadcast to Server stayed on every client holding it. Several writes in one tick keep the audience from before the first of them, because that is who holds the stale value. The flush also took its audience from the dirty record, which after a Broadcast write followed by a Server write in the same tick still said Broadcast: the server-only value went to everyone. It now uses the scope the key actually ended on. SetOwner is the same problem with the audience rather than the scope moving: the peer losing authority keeps every owner-scoped value it was sent. It is now sent a removal for each before the new owner is seeded. Also from review: StateBagSync read a uint16 count without bounding it, so a malformed sender could name 65535 changes and have the reader allocate for them. The comment claimed a bound the code did not apply.
All three from review. A throwing getter or proxy on the options object failed the scope read, and taking that for "no scope given" would have written and broadcast the value with an exception already pending. The read now fails the call. toObject built its result with Object::Set, so a stored key of __proto__ ran the inherited setter and reassigned the result's prototype instead of becoming a property on it. It now creates an own data property. The key limit is measured in bytes but documented as characters, which a non-ASCII key can satisfy and still be refused.
EQUALS formats its operands through a printf-style variadic, so a std::string passed to it is undefined behaviour. Clang rejects it outright -- the macOS job failed to compile -- while MSVC and GCC let it through, which is why this was green locally and on two of the three runners. Every string assertion now uses STREQUALS against a c_str(), as the rest of the suite does. The one remaining double operand is wrapped in a comparison for the same reason: %lld would not have formatted it.
No behaviour change. The subscription dispatch was a sixty-line lambda nested inside the lambda that registered it, three levels deep in GetClass. It is now a named Dispatch, and the registration reads as one line. Resource attribution was a copy of the three-tier resolution in events.cpp: the handler's script origin, then the loading context, then the call stack. ResourceManager owns all three tiers, so it now owns the policy as ResolveResourceContext and both callers ask it. Two copies of that order would have drifted the first time one moved. Also drops a duplicated include block and trims comments that restated their code.
The event was left for each mod to raise, which made every mod carry the same forty lines: subscribe unfiltered, enter the isolate, build the four arguments, emit. Only one line of that is a game's own opinion -- which handle its scripts should see for an entity -- and the framework already has that seam for players as WrapScriptPlayer. Both integration instances now install the subscription once scripting is up and release it on shutdown, and the emit itself lives in one header they share. A mod gets the event by existing. The new WrapScriptEntity virtual is the part a game overrides, defaulting to the base Entity builtin, so a mod with no handles of its own overrides nothing at all. Four copies of the undefined-versus-null rule for a removed key would have been four chances to disagree about it.
Raising the event from the framework left the two instances carrying a verbatim copy of the release block and of the default entity wrapper, which is the duplication the change was meant to remove, one layer down. Both now live beside the install in the shared header, so each instance is a call. Building the handler's arguments is lifted out of the subscription lambda into StateChangeArgs. It is the part of the emit with a decision in it -- undefined for a removed key and for one that held nothing, so a stored null stays readable as null -- and out on its own it needs an isolate rather than a whole session to test. Adds a state_bag_scripting module: both value conversions over every type, a cyclic value refused rather than half-stored, a malformed JSON value read back as text without leaving an exception pending, and the four argument shapes. The emit itself stays untested and the file says why -- it wants a rig, not a test.
A catalog merge skipped any symbol the project also defined, whole. That is right for a class -- both sides declare Player with different members and blending them is not expressible in TypeScript -- but EventMap is not a redefinition, it is one map both sides put their own events into. Skipping it meant the framework could not document an event it raises itself, and every mod had to restate the set by hand: a copy that goes stale the moment a tuple here changes, and silently, because the export still writes. A data type defined on both sides is now blended property by property, with the project's own declaration winning on a name they share. A class is still taken whole from the project, and the kinds are checked on both sides because adding a data type over a constructor throws inside the registry. RegisterEventMetadata then declares what the framework raises: resourceStart, resourceStop and entityStateChange. Only events the framework is the sole raiser of, and only with the tuple it actually emits -- an entry that overstates the surface is worse than a missing one, because a script written against it compiles and then does not work. The rest of the framework's events are still undocumented and want the same treatment once their shapes are confirmed.
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Actionable comments posted: 2
🤖 Prompt for all review comments with AI agents
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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.
Inline comments:
In `@code/framework/src/networking/replication/replication_manager.cpp`:
- Line 138: Update the owner-revocation logic around OwnerKeys() to also include
dirty keys whose previous scope was Owner and whose final scope has no client
audience, covering removed or Server-reassigned owner keys before SetOwner.
Merge these keys with the existing OwnerKeys() results and deduplicate before
revoking tombstones from the previous owner.
In `@code/framework/src/scripting/builtins/state_bag.cpp`:
- Around line 346-356: Update the onChange subscription flow around
ResolveResourceContext to reject the handler when no resource name is resolved,
before creating or storing the Subscription and its v8::Global callback. Match
the existing Events.on behavior by throwing in this case, while preserving
normal registration for handlers associated with a valid resource.
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code/framework/src/integrations/client/instance.cppcode/framework/src/integrations/client/instance.hcode/framework/src/integrations/client/scripting/module.cppcode/framework/src/integrations/server/instance.cppcode/framework/src/integrations/server/instance.hcode/framework/src/integrations/server/scripting/module.cppcode/framework/src/integrations/shared/scripting/state_bag_events.hcode/framework/src/networking/replication/replication_manager.cppcode/framework/src/networking/replication/replication_manager.hcode/framework/src/networking/replication/state_bag.cppcode/framework/src/networking/replication/state_bag.hcode/framework/src/networking/rpc/state_bag_sync.hcode/framework/src/scripting/builtins/events.cppcode/framework/src/scripting/builtins/state_bag.cppcode/framework/src/scripting/builtins/state_bag.hcode/framework/src/scripting/event_metadata.hcode/framework/src/scripting/resource/resource_manager.cppcode/framework/src/scripting/resource/resource_manager.hcode/framework/src/scripting/scripting_catalog.hcode/tests/framework_ut.cppcode/tests/modules/scripting_catalog_ut.hcode/tests/modules/state_bag_scripting_ut.hcode/tests/modules/state_bag_ut.h
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Two more from review. An owner-scoped key removed or narrowed to Server earlier in the same tick is gone from OwnerKeys, so the handover did not revoke it -- and the flush that would have is too late, because by then the key's audience is evaluated against the owner that just took over. The peer losing authority kept the value. OwnerRevokeKeys is the set that is actually at risk: the live owner-scoped entries plus this tick's tombstones that no longer reach any client. A key that widened to Broadcast is not one of them; it is still going out to everyone, the outgoing owner included. StateBag.onChange stored an empty resource name when attribution failed, and CleanupResource matches on the name -- so that subscription and its retained function survived every resource stop, which is the opposite of what owning it was for. It now refuses the call, with the message Events.on gives for the same case.
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Adds arbitrary key/value state to any
NetworkEntity, replicated to the connections that can see it.Every replicated value currently has to be a typed field on an entity class. A game that wants "this player is a blacksmith" visible to everyone either hand-rolls an RPC with its own reconciliation, or the framework grows another field. Neither scales with the number of things a gamemode invents, and the second makes this repo the bottleneck on every mod's design. A bag moves that decision back to the mod.
How it replicates
Not through
SerializeFields. That path runs throughVariableDeltaSerializer, which identifies variables by their position in a fixed per-tick sequence, so keys that come and go shift every later variable's slot and silently corrupt the delta. A dynamic key set cannot be a sequence of VDS variables.Bags travel as their own RPC on
Channel::Events, which shares an ordering channel withChannel::Construction. A change naming an entity therefore arrives after the construction that created and seeded it, so bags need no sequence number of their own.ReplicationManager::FlushStateBagsruns once per tick fromNetworkPeer::Update, right afterRebuildInterest, and sends each change only to connections that have the entity constructed (Connection_RM3::HasReplicaConstructed). That is deliberately not a broadcast: interest scoping, virtual-world filtering and per-type interest budgets all already decide construction, so asking that one question makes bags inherit every one of those rules with no bag-side logic that can drift out of sync with the real one. A broadcast would also hand both the value and the entity's existence to clients outside the interest set.Cost is O(dirty entities × connections) per tick, with one packet per connection, chunked at 256 changes. A dirty list keyed by
NetworkID— not by pointer, since an entity can be destroyed between dirtying and the flush — keeps that proportional to what changed rather than to how many entities exist.The construction seed is written in
SerializeConstruction, thefinalone, rather than the mod-overridableOnSerializeConstructionwhere a forgotten base call would silently drop every key.Scopes and authority
Three scopes decide who a key reaches:
BroadcastOwnerServerBecause construction is written per destination, an owner's own snapshot carries its owner-scoped keys and nobody else's, so a fresh owner needs no separate seeding pass.
SetOwnerre-sends them when authority moves — the one path construction cannot cover, and the only special case in the design.Writes are the server's alone, matching the rule the entity layer already enforces in
Deserialize. Client writes are deliberately out of scope here; see below.Limits
Key ≤ 64 bytes, value ≤ 4 KiB, 128 keys per bag, enforced at the write and surfaced to script as a thrown
Error.These are not optional. Every write crosses the network to every viewer of the entity, so without them a single careless resource — or one hostile one, once clients can write — is a session-wide denial of service at a few hundred keys of a megabyte each. Raising a limit later is safe; lowering one breaks live gamemodes, so they start strict.
Scripting
entity.stateon the existingEntityhandle, so every type a mod derives from it gets a bag without registering anything of its own.get,has,keysandtoObjectare on both sides;setandremoveare registered only for server scripting, the same splitEntityalready draws aroundsetVirtualWorld. Scalars cross as themselves and anything structured is serialized to JSON, so the wire format never needs to know a game's shapes. Reads are per key —getis a hash lookup, andtoObjectis the explicit call when a whole copy is wanted.entity.state.onChange(key, handler)watches one entity, with a null key meaning every key of it. It returns an unsubscribe function, matching whatEvents.onhands back.The filter is applied before the handler runs, which is the point of it. A busy server changes bags many times a tick, and a listener watching one field of one body should not be paying to have arguments built for every other change in the world.
ReplicationManagerkeeps subscriptions in buckets by key and consults only those a change can match — the bucket that named its key, plus those that named none — then checks the entity filter on each. Subscriptions are held by handle and looked up again at call time, so a handler may cancel itself or another mid-dispatch, and one added during a dispatch starts from the next change.The registering resource owns its subscriptions:
CallResourceStopdrops them, as it already does for event and message handlers, so a stopped resource stops being called and its retained function stops holding its objects alive.Both integration instances also raise an
entityStateChange(entity, key, value, previous)event on the global bus for every change, installed once scripting is up and released on shutdown. A mod gets it by existing; the only part a game has an opinion about is which handle its scripts should see, which is the newWrapScriptEntityvirtual, defaulting to the baseEntitybuiltin alongside the existingWrapScriptPlayer.ReplicationManager::AddStateChangeHandleris the same mechanism from C++, and an empty filter sees everything.Tests
code/tests/modules/state_bag_ut.h, 34 cases: storage and removal, every limit boundary including the exact-limit key and a full bag still accepting an overwrite, unchanged-write suppression, the scope routing decisions, the seed for owner versus non-owner, a round trip of all five value types, bag replacement on re-seed, apply-does-not-dirty, the RPC payload, and an unknown value tag degrading to null rather than indexing the enum. Subscriptions get their own: each filter alone and both together, an empty filter seeing everything, delivery stopping after removal, a double removal being harmless, a handler that cancels itself mid-dispatch, and one added mid-dispatch starting from the next change. The full suite passes at 374.One thing is not covered and the file says so: the per-connection routing in
FlushStateBags.HasReplicaConstructedis only meaningful after a real construction handshake over a bound socket, so a unit test here could only assert against a stub of the very thing under test. What feeds that routing — which keys are dirty, the scope each carries, what a seed writes for an owner versus everyone else — is pinned instead.The handler and the unsubscribe function are typed
Functionin the generated declarations rather than by signature: the declaration generator rejects a function type anywhere in its grammar, so the shape is spelled in the doc text instead. Worth fixing in the generator rather than here.Deliberately not in this change
clientWritableflag per key is the natural next step, but it is the whole new attack surface and wants a rate limiter alongside the size caps, not just the caps.Verification
FrameworkTests,KCDCServerandKCDCClientall build and the suite passes at 374. Both commits were built in isolation, not just the tip. The wire path has not yet run between two live peers — storage and serialization are covered by tests, the send path is not.Summary by CodeRabbit
entity.state.entityStateChangeevents with current and previous values.