Ошибка	Последствие
Менять переменную на клиенте и ждать репликации	Репликация идёт только сервер → клиент; изменение на клиенте локально и будет перезаписано
Забыть `bReplicates = true` на actor	`UPROPERTY(Replicated)` тихо не работает, RPC не маршрутизируются
Забыть `GetLifetimeReplicatedProps` + `DOREPLIFETIME`	Свойство объявлено `Replicated`, но не зарегистрировано — не реплицируется
Вызвать `Multicast` RPC с клиента	Не доходит до других клиентов; multicast вызывается только с сервера
Вызвать `Server` RPC на actor без ownership	RPC игнорируется; нужен owning connection
Использовать `Reliable Multicast` для частых событий	Забивает сетевую очередь; превышение лимита = дисконнект
Класть в RPC большой `TArray` или строку	Превышение `MaxRPCSize` (~64 KB) → дисконнект
Использовать RPC для синхронизации HP	HP — это состояние, делай `Replicated` свойство; RPC для дискретных событий
Проверять `HasAuthority()` в коде, который должен идти на всех	Лишний guard блокирует визуальную логику клиента
Менять `Role`/`RemoteRole` вручную	Это управляется движком; ручная правка ломает репликацию
Тестировать только в PIE с одним клиентом	Many bugs (ownership, ordering) видны только в standalone/dedicated server
Считать, что `OnRep` зовётся тоже на сервере	По умолчанию `OnRep` зовётся только на клиентах; на сервере значение меняется напрямую

advanced

Replication

bReplicates, UPROPERTY(Replicated), RPCs, NetRelevancy, and server authority — Unreal's built-in multiplayer stack.

ue-replication

Sections

Contents

Detailed explanation

Replication — server authoritative, eventual consistency, RPCs for commands

When people say "replication in Unreal", they really mean the built-in multiplayer stack under three strict rules: the server is the single source of truth, state is broadcast through replicated properties (eventual consistency), and discrete commands go through RPCs (point-to-point calls). It is not a symmetric P2P scheme, not "every client broadcasts to every other". It is one-directional: client → server (via Server RPC) → clients (via property replication and Multicast).

The classic mistake is putting gameplay logic on the client: "I clicked, I changed my HP, I told the server". In a server-authoritative model it works the other way around: the client asks via Server_DealDamage, the server validates (validation, anti-cheat), the server mutates the replicated variable, and clients find out about the change through OnRep_Health or simply by reading the new value on the next tick after the packet arrives.

The second recurring confusion is property replication vs RPCs. Different mechanisms with different semantics. A replicated property is state with eventual consistency: a freshly connecting player gets the current value; a dropped packet means the next update overwrites the old value and "things heal themselves". An RPC is a discrete event: "the shot happened", "play this effect"; a lost unreliable RPC is gone forever. The full map lives in the layers below.

Topic map

Replication Basics — server-client model, NetMode, Role vs RemoteRole, HasAuthority().
Server Authority — why the server is the only source of truth, validation, cheat resistance.
Replicated Properties — UPROPERTY(Replicated), ReplicatedUsing, OnRep_*, COND_* conditions.
Property Registration — GetLifetimeReplicatedProps, DOREPLIFETIME, DOREPLIFETIME_CONDITION.
bReplicates Flag — AActor::bReplicates, bAlwaysRelevant, NetUpdateFrequency, NetCullDistance, relevancy.
RPCs — Server/Client/NetMulticast, validation, routing, ownership.
RPC Types — Reliable vs Unreliable, ordering guarantees, payload limits.
Property vs RPC — when to use which, state vs command, late-join.

Common traps

Mistake	Consequence
Mutating a variable on the client and waiting for replication	Replication goes server → client only; client mutation is local and gets overwritten
Forgetting `bReplicates = true` on the actor	`UPROPERTY(Replicated)` silently no-ops, RPCs do not route
Forgetting `GetLifetimeReplicatedProps` + `DOREPLIFETIME`	Property is declared `Replicated` but never registered — nothing replicates
Calling a `Multicast` RPC from a client	Doesn't reach other clients; multicast only fires from the server
Calling a `Server` RPC on an actor without ownership	RPC is dropped; you need an owning connection
Using `Reliable Multicast` for frequent events	Floods the reliable queue; exceeding the limit disconnects the client
Putting a large `TArray` or string into an RPC	Over `MaxRPCSize` (~64 KB) → disconnect
Using RPCs to sync HP	HP is state — use a `Replicated` property; RPCs are for discrete events
Sprinkling `HasAuthority()` checks where the code should run everywhere	Guards block client-side visual logic
Hand-editing `Role` / `RemoteRole`	Driven by the engine; manual writes break replication
Testing only in PIE with one client	Many bugs (ownership, ordering) only appear in standalone / dedicated server
Assuming `OnRep` is also called on the server	By default `OnRep` runs only on clients; on the server the value is set directly

Interview relevance

Replication is a mandatory senior-level topic for any UE5 multiplayer interview. Checks:

That the server is authority and clients are a reflection of its state.
The difference between Role and RemoteRole (on the server my Role is Authority; on the client it is no longer Authority).
How to declare and register a replicated property (two steps: UPROPERTY + DOREPLIFETIME).
When to use ReplicatedUsing=OnRep_X and where OnRep_X runs.
The three RPC kinds and their directions: Server (client→server), Client (server→owning client), NetMulticast (server→everyone).
Reliable vs Unreliable: delivery guarantees, ordering, why you can't spam reliables.
When to replicate a property vs call an RPC: HP is a property, the shot is a multicast RPC, an action attempt is a server RPC.
What Relevancy is and why distant actors do not receive updates.

Common wrong answer: "Replication is syncing variables between clients — slap Replicated on a property and it broadcasts itself." Reality — the model is server-authoritative, the mutation happens on the server; without DOREPLIFETIME the Replicated flag does nothing; the direction is one-way; commands from client to server need a Server RPC, not an assignment.

Why it matters

Replication is **not "syncing values across clients"**. It is a **server-authoritative** model with eventual consistency for state and RPCs for discrete commands. People who miss this put gameplay on the client, use `Multicast` where they need `Server`, and watch the game fall apart under packet loss.

Ошибка	Последствие
Менять переменную на клиенте и ждать репликации	Репликация идёт только сервер → клиент; изменение на клиенте локально и будет перезаписано
Забыть `bReplicates = true` на actor	`UPROPERTY(Replicated)` тихо не работает, RPC не маршрутизируются
Забыть `GetLifetimeReplicatedProps` + `DOREPLIFETIME`	Свойство объявлено `Replicated`, но не зарегистрировано — не реплицируется
Вызвать `Multicast` RPC с клиента	Не доходит до других клиентов; multicast вызывается только с сервера
Вызвать `Server` RPC на actor без ownership	RPC игнорируется; нужен owning connection
Использовать `Reliable Multicast` для частых событий	Забивает сетевую очередь; превышение лимита = дисконнект
Класть в RPC большой `TArray` или строку	Превышение `MaxRPCSize` (~64 KB) → дисконнект
Использовать RPC для синхронизации HP	HP — это состояние, делай `Replicated` свойство; RPC для дискретных событий
Проверять `HasAuthority()` в коде, который должен идти на всех	Лишний guard блокирует визуальную логику клиента
Менять `Role`/`RemoteRole` вручную	Это управляется движком; ручная правка ломает репликацию
Тестировать только в PIE с одним клиентом	Many bugs (ownership, ordering) видны только в standalone/dedicated server
Считать, что `OnRep` зовётся тоже на сервере	По умолчанию `OnRep` зовётся только на клиентах; на сервере значение меняется напрямую

advanced

Replication

bReplicates, UPROPERTY(Replicated), RPCs, NetRelevancy, and server authority — Unreal's built-in multiplayer stack.

ue-replication

Practice: 21 questions →

Sections

Contents

Detailed explanation

Replication — server authoritative, eventual consistency, RPCs for commands

Topic map

Replication Basics — server-client model, NetMode, Role vs RemoteRole, HasAuthority().
Server Authority — why the server is the only source of truth, validation, cheat resistance.
Replicated Properties — UPROPERTY(Replicated), ReplicatedUsing, OnRep_*, COND_* conditions.
Property Registration — GetLifetimeReplicatedProps, DOREPLIFETIME, DOREPLIFETIME_CONDITION.
bReplicates Flag — AActor::bReplicates, bAlwaysRelevant, NetUpdateFrequency, NetCullDistance, relevancy.
RPCs — Server/Client/NetMulticast, validation, routing, ownership.
RPC Types — Reliable vs Unreliable, ordering guarantees, payload limits.
Property vs RPC — when to use which, state vs command, late-join.

Common traps

Mistake	Consequence
Mutating a variable on the client and waiting for replication	Replication goes server → client only; client mutation is local and gets overwritten
Forgetting `bReplicates = true` on the actor	`UPROPERTY(Replicated)` silently no-ops, RPCs do not route
Forgetting `GetLifetimeReplicatedProps` + `DOREPLIFETIME`	Property is declared `Replicated` but never registered — nothing replicates
Calling a `Multicast` RPC from a client	Doesn't reach other clients; multicast only fires from the server
Calling a `Server` RPC on an actor without ownership	RPC is dropped; you need an owning connection
Using `Reliable Multicast` for frequent events	Floods the reliable queue; exceeding the limit disconnects the client
Putting a large `TArray` or string into an RPC	Over `MaxRPCSize` (~64 KB) → disconnect
Using RPCs to sync HP	HP is state — use a `Replicated` property; RPCs are for discrete events
Sprinkling `HasAuthority()` checks where the code should run everywhere	Guards block client-side visual logic
Hand-editing `Role` / `RemoteRole`	Driven by the engine; manual writes break replication
Testing only in PIE with one client	Many bugs (ownership, ordering) only appear in standalone / dedicated server
Assuming `OnRep` is also called on the server	By default `OnRep` runs only on clients; on the server the value is set directly

Interview relevance

Replication is a mandatory senior-level topic for any UE5 multiplayer interview. Checks:

That the server is authority and clients are a reflection of its state.
The difference between Role and RemoteRole (on the server my Role is Authority; on the client it is no longer Authority).
How to declare and register a replicated property (two steps: UPROPERTY + DOREPLIFETIME).
When to use ReplicatedUsing=OnRep_X and where OnRep_X runs.
The three RPC kinds and their directions: Server (client→server), Client (server→owning client), NetMulticast (server→everyone).
Reliable vs Unreliable: delivery guarantees, ordering, why you can't spam reliables.
When to replicate a property vs call an RPC: HP is a property, the shot is a multicast RPC, an action attempt is a server RPC.
What Relevancy is and why distant actors do not receive updates.

Why it matters

Репликация

Репликация — server authoritative, eventual consistency, RPC для команд

Карта темы

Частые ошибки и ловушки

Значение для собеседований

Replication

Replication — server authoritative, eventual consistency, RPCs for commands

Topic map

Common traps

Interview relevance

Репликация

Репликация — server authoritative, eventual consistency, RPC для команд

Карта темы

Частые ошибки и ловушки

Значение для собеседований

Replication

Replication — server authoritative, eventual consistency, RPCs for commands

Topic map

Common traps

Interview relevance