code: private void precheckSchemaAgreement(Client client) throws Exception {
          Map<String, List<String>> schemas = client.describe_schema_versions();
          if (schemas.size() > 1) {
              throw new SchemaDisagreementException("Can't change schema due to pending schema agreement");
          }
      }
docs: Do a quick check to see if there is a schema disagreement.  This is done as an extra precaution
  to reduce the chances of putting the cluster into a bad state.  This will not gurantee however, that
  by the time a schema change is made the cluster will be in the same state.
  @param client
  @throws Exception