Add a comment warning about serialization format changes

1 files changed, 7 insertions, 0 deletions

diff --git a/node-repository/src/main/java/com/yahoo/vespa/hosted/provision/persistence/LoadBalancerSerializer.java b/node-repository/src/main/java/com/yahoo/vespa/hosted/provision/persistence/LoadBalancerSerializer.java
index a4b915a6128..fd2294c1b5d 100644
--- a/node-repository/src/main/java/com/yahoo/vespa/hosted/provision/persistence/LoadBalancerSerializer.java
+++ b/node-repository/src/main/java/com/yahoo/vespa/hosted/provision/persistence/LoadBalancerSerializer.java
@@ -27,6 +27,13 @@ import java.util.function.Function;
  */
 public class LoadBalancerSerializer {
 
+    // WARNING: Since there are multiple servers in a ZooKeeper cluster and they upgrade one by one
+    //          (and rewrite all nodes on startup), changes to the serialized format must be made
+    //          such that what is serialized on version N+1 can be read by version N:
+    //          - ADDING FIELDS: Always ok
+    //          - REMOVING FIELDS: Stop reading the field first. Stop writing it on a later version.
+    //          - CHANGING THE FORMAT OF A FIELD: Don't do it bro.
+
     private static final String idField = "id";
     private static final String hostnameField = "hostname";
     private static final String dnsZoneField = "dnsZone";