[ztlpn]

(ClickHouse dev here)

Yes, replication in ClickHouse is asynchronous by default. For intended use cases (OLAP queries aggregating data from many rows) data that is a few seconds stale is usually okay. In a serious production deployment you absolutely should enable replication, otherwise you risk losing all your data, not just last couple of seconds of inserts.

That said, sometimes synchronous replication is necessary despite the latency penalty that comes with it. This feature is actually implemented but not yet considered ready for prime time.

We have several years of production experience with ClickHouse (as a DBMS powering Yandex.Metrica - second largest web analytics system in the world). If you have questions - just ask.