public Transaction tx() {
        if (null == this.connection)
            return this.graph.tx();
        else {
            throw new UnsupportedOperationException("TinkerPop 4 does not yet support remote transactions");
        }

    }
Proxies calls through to the underlying {@link Graph#tx()} or to the {@link RemoteConnection#tx()}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/dsl/graph/GraphTraversalSource.java