Google Cloud True Time

Giriş

Dağıtık bir ağda zaman senkronizasyonunu sağlamak zor bir iş. Google bunu True Time ile sağlıyor. Google True Time, Google Spanner gibi veri tabanlarında kullanılıyor. Açıklaması şöyle

Google created a distributed SQL database called Spanner, it relies on something called True Time for very strong consistency of transactions across nodes. Google knows that time is uncertain, so True Time defines a bounded and small uncertainty of time window where transactions can not be ordered definitely. True Time works as a Global Time across Google datacenters.

True Time is expressed as a time interval [earliest, latest]. It exposes an API called now() whose value lies in this interval. The uncertainty interval varies between 1 ms to 7 ms — note that the maximum uncertainty has a tight upper bound.

The APIs TT.before(t) or TT.earliest() and TT.after(t) or TT.latest() take a timestamp as input and answers whether the given timestamp is before or after the current uncertainty interval.

The relation between TT.earliest(), TT.latest() and absolute time of an event is:

TT.earliest ≤ Absolute Time of current event ≤ TT.latest

Google bunu nasıl sağlıyor?

Açıklaması şöyle

Google does this magic by couple of tricks:

Optimized Infrastructure: Google infra runs on specially designed private network. They have optimized the network over time, it has a lot of redundancy of connections across datacenters and failure handling mechanisms built in. It does not mean network partition don’t happen or things don’t go wrong — however the possibility of such incidents and communication latency reduces a lot.

Using own clocks: True Time does not rely on external NTP pools or servers. Rather, Google datacenters are equipped with GPS receivers and Atomic clocks. See the below picture of such an installation:

AWS Time Sync Service

Açıklaması şöyle

Inspired from Google True Time, AWS also manages its own fleet of Atomic clocks and GPS clock receivers. Any EC2 server can connect to these time references via NTP using Chrony daemon for more accurate time rather than connecting to external NTP pools or time servers over NTP. More details can be found here. Leap second smearing is also handled by Amazon Time Sync Service.