Iris

Iris is UE 5.x's new replication system, designed to replace the original UNetDriver plumbing for high-scale multiplayer. Optional, must be opted into per project.

Status (UE 5.7)

• Production: Yes, Epic uses it for Fortnite
• Default for new projects: No — old system still ships
• API stability: Improving, still some moving pieces

What it changes

Feature	Old	Iris
Property replication path	Per-property check each tick	Push Model first-class
Subobject replication	Manual	First-class via IrisObjectReplicationBridge
Relevancy	Per-actor IsRelevant	Replication Graph + Iris filter chains
Dirty tracking	Per-property compare	Dirty-bit driven
Bandwidth	OK	Significantly lower
Authoring	UPROPERTY + DOREPLIFETIME	UPROPERTY + Push Model

Enable

DefaultEngine.ini:

[/Script/IrisCore.ReplicationSystem]
bIsEnabled=true

Then per-actor:

AMyActor::AMyActor()
{
    bReplicates = true;
    bReplicateUsingRegisteredSubObjectList = true;
}

When to use

• Yes: 10k+ replicated actors, MMO-scale, large-world multiplayer
• Maybe: 100–10k actors, want best bandwidth
• No: Small multiplayer, simple twitch games — original system is fine

Migration considerations

• Auto-detected for most projects, but custom NetDriver subclasses need updates.
• Replication Graph integration is the recommended scaling layer on top of Iris.

See also

• Replication
• UE — Iris