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A geo is one cluster in one place with its own Redis. Regions of the same world can be homed in different geos, and a player can walk from one to the other.
This is not the shape a rented network usually wants. If you are running a handful of servers on a panel host, everything is one geo and none of this applies. It is here because the design has to answer the question.

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What is local and what is global

Redis is per geo and carries the hot path: ghost frames, block deltas, transfer offers between neighbours. Every node subscribes only to its own geo’s Redis. A message for a node elsewhere is published onto that geo’s Redis instead. MongoDB is the one global thing: player records, the topology of who serves which region, and the block journal. That is what lets a proxy in one geo route a player to a shard in another.

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Two ways over the border

The cheap way is a bridge. Shards publish an address reachable from outside their own cluster, every proxy registers the far ones as ordinary servers, and a cross-geo crossing becomes a plain server switch. The client keeps its world and the border is invisible, but the player is now proxied through a machine on the wrong continent. The other way is a handover: the client is sent a transfer packet, disconnects, and reconnects to the other geo’s proxy. Seconds of reconnect, but afterwards they are talking to the proxy nearest them.

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Why a handover usually buys nothing

The ocean is in the path either way. It only moves:
A handover is only worth a reconnect when the player is nearer the other geo, not when the region is. Somebody sitting next to this proxy who walks into a region homed on another continent pays the same latency whichever proxy they are on. So the decision is made from the player’s measured ping to us, not from where the shard lives. A client on the same network reports single digits and is never handed over however long it stays. That was the original bug in this logic: it decided from where the shard was, which is the one thing that does not matter.

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When the handover happens

Bridge first, hand over later, and only once the player has settled. In order of preference:
1

They cross back

Nothing happens at all. No reconnect was ever paid for, and a border you pace over is free in both directions. This is the case the whole design exists for.
2

They cross again, deeper in, having settled

The reconnect replaces a server switch that was going to happen anyway, at a border, where a moment of hesitation is expected and already masked.
3

Neither, for long enough

It happens anyway. A whole session played across an ocean is worse than one loading screen.

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The hint, and why there are two of them

When a client is handed over, the target geo’s proxies are told directly and the same note is pinned on the client as a cookie. Either can be lost, so both are sent; the MongoDB record is always the authority and the hint only saves a database read at login. A cookie older than a minute is ignored, so a stale one cannot misroute anybody. On a single-geo network the cookie is never requested at all, because paying a client round trip on every login for a fallback that can never fire is pure cost.