docs: replicating the system database with a constrained topology (5.2)

reference/replication/overview.md

@@ -198,7 +198,36 @@ replication:
 
 In this example, the local node only receives from `node-two` (one-way inbound) and only sends to `node-three` (one-way outbound).
 
-> **Note**: When using controlled flow replication, avoid replicating the `system` database. The `system` database contains node configurations, so replicating it would cause all nodes to have identical (and incorrect) route configurations.
+You can also scope flow per database, so different databases flow in different directions between the same two nodes. Use `sendsTo` / `receivesFrom` entries with a `database`:
+
+```yaml
+replication:
+  databases:
+    - cardata
+    - config
+    - system
+  routes:
+    - host: node-two
+      replicates:
+        sendsTo:
+          - database: config # push central config downstream
+        receivesFrom:
+          - database: cardata # aggregate telemetry upstream
+```
+
+#### Replicating the `system` database with controlled flow
+
+<VersionBadge type="changed" version="v5.2.0" />
+
+Before v5.2, replicating the `system` database under controlled flow was discouraged: because `hdb_nodes` (the node registry) lives in `system` and each node advertised itself as a full-mesh participant, replicating `system` caused every node to discover and directly connect to every other node — collapsing a constrained topology into a full mesh.
+
+As of v5.2 you can replicate `system` while keeping a constrained topology. When a node has directional routes, it advertises a **directional** registry record derived from those routes (which neighbors it sends to / receives from) instead of a blanket "connect to everyone." Discovered non-neighbor nodes therefore do not open direct connections. This lets central configuration — users, roles, and schemas — propagate transitively across the whole cluster while user-database connections stay on the routes you configured. For example, in a `roadside → middle → core` aggregation tree, a role created on a roadside node reaches the core through the middle tier, yet the core never opens a direct socket to a roadside node.
+
+Notes and current limitations:
+
+- This applies only when a node has **directional** routes (`replicates` with `sends`/`receives` or `sendsTo`/`receivesFrom`). A node with no directional routes keeps the legacy full-mesh advertisement.
+- Central visibility of every node is not guaranteed: an aggregation node may not list every distant leaf in its `hdb_nodes` registry (the registry relay differs from data relay). This does not open a connection either way.
+- Route changes to a node's own directionality take effect on restart.
 
 ### Explicit Subscriptions
 

release-notes/v5-lincoln/5.2.md

@@ -13,3 +13,23 @@ All patch release notes for 5.2.x are available on the [releases page](https://g
 ### Replicated `set_configuration`
 
 The `set_configuration` operation now accepts `"replicated": true` to apply a configuration change to all cluster nodes in a single Operations API call, with per-node outcomes reported in the response's `replicated` array. Only cluster-appropriate parameters should be replicated — see [Configuration Operations](/reference/v5/configuration/operations#set-configuration).
+
+## Replication
+
+### Replicating the System Database with a Constrained Topology
+
+Controlled-flow replication can now include the `system` database while keeping a constrained (non-mesh) topology.
+
+Previously, replicating `system` was discouraged when using [controlled replication flow](/reference/v5/replication/overview#controlling-replication-flow): because the node registry (`hdb_nodes`) lives in `system` and each node advertised itself as a full-mesh participant, replicating `system` caused every node to discover and directly connect to every other node — defeating the point of a constrained topology.
+
+As of 5.2, a node with directional routes advertises a **directional** registry record derived from those routes (the neighbors it sends to / receives from) instead of a blanket "connect to everyone." Discovered non-neighbor nodes therefore never open direct connections. This lets central configuration — users, roles, and schemas — propagate transitively across the entire cluster while user-database connections stay on the routes you configured.
+
+For example, in a `roadside → middle → core` aggregation tree, replicating `system` now lets a role created on a roadside node reach the core (through the middle tier) without the core ever opening a direct socket to a roadside node.
+
+Behavior notes:
+
+- This applies only to nodes that have **directional** routes. A node with no directional routes keeps the legacy full-mesh advertisement, so existing full-mesh clusters are unaffected.
+- `add_node` / `set_node` with database-scoped `sendsTo` / `receivesFrom` entries now restrict replication to those databases (consistent with the config-route behavior). To replicate all databases while excluding specific tables, use a wildcard entry (`excludeTables` with no `database`).
+- Central visibility of every node is not guaranteed: an aggregation node may not list every distant leaf in its `hdb_nodes` registry. This does not open a connection either way.
+
+See [Controlling Replication Flow](/reference/v5/replication/overview#controlling-replication-flow) for configuration details.