add constraint, comment

Author: smklein

nexus/types/src/deployment.rs

@@ -230,7 +230,8 @@ pub struct Blueprint {
     /// The generation of the active group of Nexuses
     ///
     /// If a Nexus instance notices it has a nexus_generation less than
-    /// this value, it will start to quiesce (see: RFD 588).
+    /// this value, it will start to quiesce in preparation for handing off
+    /// control to the newer generation (see: RFD 588).
     pub nexus_generation: Generation,
 
     /// CockroachDB state fingerprint when this blueprint was created

schema/crdb/dbinit.sql

@@ -4757,6 +4757,12 @@ CREATE TABLE IF NOT EXISTS omicron.public.bp_omicron_zone (
         OR
         (image_source != 'artifact'
             AND image_artifact_sha256 IS NULL)
+    ),
+
+    CONSTRAINT nexus_generation_for_nexus_zones CHECK (
+        (zone_type = 'nexus' AND nexus_generation IS NOT NULL)
+        OR
+        (zone_type != 'nexus' AND nexus_generation IS NULL)
     )
 );
 

schema/crdb/nexus-generation/up05.sql

@@ -0,0 +1,5 @@
+ALTER TABLE omicron.public.bp_omicron_zone ADD CONSTRAINT IF NOT EXISTS nexus_generation_for_nexus_zones CHECK (
+    (zone_type = 'nexus' AND nexus_generation IS NOT NULL)
+    OR
+    (zone_type != 'nexus' AND nexus_generation IS NULL)
+);