Understanding Unreal Engine’s DORM_Initial dormancy setting

This article is a collaboration with WizardCell and on it, you’ll learn how DORM_Initial works and why you should be careful with it.

Introduction

If we read the description of the DORM_Initial dormancy setting, we see what follows:

This actor is initially dormant for all connection if it was placed in map.

Which made me understand that this setting would work like DORM_DormantAll just with a couple extra optimizations. But it isn’t the case, let’s see what are the implications of using this setting.

DORM_DormantAll versus DORM_Initial

When an Actor becomes relevant to a connection, all the properties of said Actor are sent to the connection, no matter whether the Actor is DORM_DormantAll or DORM_Awake. That means that the state the client receives from the Actor will be consistent of what the server sees about it.

However, that is not the case of DORM_Initial Actors, as they will not send any C++ replicated property if a connection becomes relevant:

Watch out! Blueprint replicated properties ignore Actor dormancy settings.

How to use DORM_Initial

So shall I simply not use DORM_Initial? No, you should! DORM_Initial Actors won’t be considered for replication at all even under relevancy, and sometimes that can be a huge optimization.

To ensure incoming connections get the new values over DORM_Initial Actors, you should call FlushNetDormancy on them after updating their properties. This will make the Actor to become DORM_DormantAll but only when strictly needed.

What about calling ForceNetUpdate to also flush the dormancy?

Be very careful with this, as ForceNetUpdate also calls FlushNetDormancy but its guarded with a NetDriver check:

void AActor::ForceNetUpdate()
{
	...

	if (GetLocalRole() == ROLE_Authority)
	{
		// ForceNetUpdate on the game net driver only if we are the authority...
		if (NetDriver && NetDriver->GetNetMode() < ENetMode::NM_Client) // ... and not a client
		{
			NetDriver->ForceNetUpdate(this);

			if (NetDormancy > DORM_Awake)
			{
				FlushNetDormancy();
			}
		}
	}

	...
}

So, if the NetDriver is Null at the moment you call ForceNetUpdate*, the Actor will never call FlushNetDormancy, causing it to stay in a DORM_Initial state. For that reason it is always recommended to explicitly call FlushNetDormancy when dealing with DORM_Initial Actors.

*Worlds don’t get a NetDriver until UWorld::Listen is called.

Conclusion

Thanks for reading!

Let’s hope everything is clear! Feel free to reach us out if there are questions! And I encourage every reader to check WizardCell’s articles.

Enjoy, WizardCell and vori.