Exchange ex = schemaTreeExchange(session);
try {
ex.clear().append(S_K_ONLINE).append(onlineSession.id).append(S_K_ERROR);
ex.fetch();
// write skew
ex.lock();
return ex.getValue().isDefined() ? ex.getValue().getString() : null;
} catch(PersistitException | RollbackException e) {
if(ex != null) {
treeService.releaseExchange(session, ex);
}