String batchMemberId() {
            if (batchState == null) {
                throw new IllegalStateException("The batch member id is not available as the offset state is maintained");
            }
            return batchState.memberId;
        }
The InFlightBatch maintains the in-memory state of the fetched records i.e. in-flight records.
core/src/main/java/kafka/server/share/SharePartition.java