background task comment could mention other Nexus instances (#9738)

Author: davepacheco

nexus/src/app/background/mod.rs

@@ -103,12 +103,14 @@
 //! * Each activation of the reconciler accepts no input.  That is, even when we
 //!   think we know what changed, we do not use that information.  This ensures
 //!   that the reconciler really is idempotent and its actions are based solely
-//!   on the state that it's watching.  Put differently: having reconcilers
-//!   accept an explicit hint about what changed (and then doing something
-//!   differently based on that) bifurcates the code: there's the common case
-//!   where that hint is available and the rarely-exercised case when it's not
-//!   (e.g., because Nexus crashed and it's the subsequent periodic activation
-//!   that's propagating this change).  This is what we're trying to avoid.
+//!   on the state that it's watching.  If the background task accepted input to
+//!   determine what to do, then it would separately have to handle two cases
+//!   where that input isn't available: (1) after a Nexus restart, and (2) when
+//!   there's more than one Nexus and the input is only seen in one of the Nexus
+//!   instances.  Thus, having reconcilers accept an explicit hint about what
+//!   changed (and then doing something differently based on that) bifurcates
+//!   the code: there's the common case where that hint is available and the
+//!   rarely-exercised case when it's not.  This is what we're trying to avoid.
 //! * We do allow reconcilers to be triggered by a `tokio::sync::watch` channel
 //!   -- but again, not using the _data_ from that channel.  There are two big
 //!   advantages here: (1) reduced latency from when a change is made to when