StefanRRichter commented on a change in pull request #6875: [FLINK-9808] [state 
backends] Migrate state when necessary in state backends
URL: https://github.com/apache/flink/pull/6875#discussion_r226669120
 
 

 ##########
 File path: 
flink-state-backends/flink-statebackend-rocksdb/src/main/java/org/apache/flink/contrib/streaming/state/RocksDBKeyedStateBackend.java
 ##########
 @@ -1389,6 +1388,149 @@ private void copyStateDataHandleData(
                return Tuple2.of(stateInfo.f0, newMetaInfo);
        }
 
+       private <N, S extends State, SV> 
RegisteredKeyValueStateBackendMetaInfo<N, SV> migrateStateIfNecessary(
+                       StateDescriptor<S, SV> stateDesc,
+                       TypeSerializer<N> namespaceSerializer,
+                       Tuple2<ColumnFamilyHandle, RegisteredStateMetaInfoBase> 
stateInfo,
+                       @Nullable StateSnapshotTransformer<SV> 
snapshotTransformer) throws Exception {
+
+               StateMetaInfoSnapshot restoredMetaInfoSnapshot = 
restoredKvStateMetaInfos.get(stateDesc.getName());
+
+               Preconditions.checkState(
+                       restoredMetaInfoSnapshot != null,
+                       "Requested to check compatibility of a restored 
RegisteredKeyedBackendStateMetaInfo," +
+                               " but its corresponding restored snapshot 
cannot be found.");
+
+               
Preconditions.checkState(restoredMetaInfoSnapshot.getBackendStateType()
+                               == 
StateMetaInfoSnapshot.BackendStateType.KEY_VALUE,
+                       "Incompatible state types. " +
+                               "Was [" + 
restoredMetaInfoSnapshot.getBackendStateType() + "], " +
+                               "registered as [" + 
StateMetaInfoSnapshot.BackendStateType.KEY_VALUE + "].");
+
+               Preconditions.checkState(
+                       Objects.equals(stateDesc.getName(), 
restoredMetaInfoSnapshot.getName()),
+                       "Incompatible state names. " +
+                               "Was [" + restoredMetaInfoSnapshot.getName() + 
"], " +
+                               "registered with [" + stateDesc.getName() + 
"].");
+
+               final StateDescriptor.Type restoredType =
+                       StateDescriptor.Type.valueOf(
+                               restoredMetaInfoSnapshot.getOption(
+                                       
StateMetaInfoSnapshot.CommonOptionsKeys.KEYED_STATE_TYPE));
+
+               if (!Objects.equals(stateDesc.getType(), 
StateDescriptor.Type.UNKNOWN)
+                       && !Objects.equals(restoredType, 
StateDescriptor.Type.UNKNOWN)) {
+
+                       Preconditions.checkState(
+                               stateDesc.getType() == restoredType,
+                               "Incompatible key/value state types. " +
+                                       "Was [" + restoredType + "], " +
+                                       "registered with [" + 
stateDesc.getType() + "].");
+               }
+
+               TypeSerializer<SV> stateSerializer = stateDesc.getSerializer();
+
+               RegisteredKeyValueStateBackendMetaInfo<N, SV> newMetaInfo = new 
RegisteredKeyValueStateBackendMetaInfo<>(
 
 Review comment:
   It seems that now both branches in the calling method 
`tryRegisterKvStateInformation` create a `newMetaInfo`. Why not always create 
it there and pass it as only argument to this function which can then transform 
it if needed? Could deduplicate and simplify some code.

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