backend_tx_begin_ro
int backend_tx_begin_ro(backend_type_t, backend_tx_t **);
res = backend_tx_begin_ro(backend, &tx);
rc_ro = backend_tx_begin_ro(BACKEND_TYPE_NONPERSIST, &tx_ro);
rc_ro = backend_tx_begin_ro(BACKEND_TYPE_NORMAL, &tx_ro);