Add a comment warning about serialization format changes

1 files changed, 7 insertions, 0 deletions

diff --git a/config-provisioning/src/main/java/com/yahoo/config/provision/AllocatedHosts.java b/config-provisioning/src/main/java/com/yahoo/config/provision/AllocatedHosts.java
index de4f3a555bd..96942c53a12 100644
--- a/config-provisioning/src/main/java/com/yahoo/config/provision/AllocatedHosts.java
+++ b/config-provisioning/src/main/java/com/yahoo/config/provision/AllocatedHosts.java
@@ -26,6 +26,13 @@ import java.util.Set;
  */
 public class AllocatedHosts {
 
+    // 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 mappingKey = "mapping";
     private static final String hostSpecKey = "hostSpec";
     private static final String hostSpecHostNameKey = "hostName";