Add a comment warning about serialization format changes

1 files changed, 7 insertions, 0 deletions

diff --git a/configserver/src/main/java/com/yahoo/vespa/config/server/tenant/ContainerEndpointSerializer.java b/configserver/src/main/java/com/yahoo/vespa/config/server/tenant/ContainerEndpointSerializer.java
index 379af7f71ea..91f9e3c8eed 100644
--- a/configserver/src/main/java/com/yahoo/vespa/config/server/tenant/ContainerEndpointSerializer.java
+++ b/configserver/src/main/java/com/yahoo/vespa/config/server/tenant/ContainerEndpointSerializer.java
@@ -18,6 +18,13 @@ import java.util.List;
  */
 public class ContainerEndpointSerializer {
 
+    // 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 clusterIdField = "clusterId";
     private static final String namesField = "names";