dcapwell commented on code in PR #4879:
URL: https://github.com/apache/cassandra/pull/4879#discussion_r3416453901
##########
src/java/org/apache/cassandra/service/accord/txn/TxnCondition.java:
##########
@@ -698,27 +797,60 @@ public long serializedSize(BooleanGroup condition,
TableMetadatas tables)
public static final ParameterisedUnversionedSerializer<TxnCondition,
TableMetadatas> serializer = new ParameterisedUnversionedSerializer<>()
{
+ // TOP_BIT is used to differentiate between Value.Serializer and
Reference.Serialzer.
+ // This is so done to preserve upgrade compatibility with the prior
serializer.
+ // Nodes that are not yet upgraded can still deserialize all values
modulo those that are
Review Comment:
```
In order for Accord to make changes to the serialization it needs to support
V2 first which requires global configs, which is not in scope in the immediate
future; given this we need work-arounds to support users who have been
deploying off trunk to upgrade without breaking them; this is why we have the
TOP_BIT concept, to enable this type of upgrade for users.
This change is safe under the following assumptions
1) `ref op ref` feature is only used after all nodes have been upgraded
2) cluster can be mixed mode as long as `ref op ref` is not used
If a user tries to use `ref op ref` in a mixed mode this will lead to
undefined errors where the only recovery process is to force older nodes to
upgrade
```
Something like that. Its very detailed on why this is the case...
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