Re: Clock-skew management in logical replication

Nisha Moond <nisha(dot)moond412(at)gmail(dot)com> writes:
> Thoughts? Looking forward to hearing others' opinions!

Had a productive conversation with Amit Kaplia today about time skew
in distributed systems, and wanted to share some thoughts.
Essentially, we're grappling with the classic distributed snapshot
problem. In a multi-active environment, where multiple nodes can
independently process transactions, it becomes crucial to determine
the visibility of these transactions across the system. Time skew,
where different machines have different timestamps make it a hard
problem. How can we ensure consistent transaction ordering and
visibility when time itself is unreliable?

As you mentioned, there are several ways to tackle the time skew
problem in distributed systems. These approaches generally fall into
three main categories:

1. Centralized Timestamps (Timestamp Oracle)

Mechanism: A dedicated server acts as a single source of truth for
time, eliminating skew by providing timestamps to all nodes. Google
Percolator and TiDB use this approach.
Consistency level: Serializable
Pros: Simple to implement.
Cons: High latency for cross-geo transactions due to reliance on a
central server. Can become a bottleneck.

2. Atomic Clocks (True Time)

Mechanism: Utilizes highly accurate atomic clocks to provide a
globally consistent view of time, as seen in Google Spanner.
Consistency level: External Serializable
Pros: Very high consistency level (externally consistent).
Cons: Requires specialized and expensive hardware. Adds some latency
to transactions, though less than centralized timestamps.

3. Hybrid Logical Clocks

Mechanism: CombinesNTP for rough time synchronization with logical
clocks for finer-grained ordering. Yugabyte and CockroachDB employ
this strategy.
Consistency level: Serializable
Pros: Avoids the need for specialized hardware.
Cons: Can introduce significant latency to transactions.

4 Local Clocks

Mechanism: Just use logical clock
Consistency level: Eventual Consistency
Pros: Simple implementation
Cons: The consistency level is very low

Of the four implementations considered, only local clocks and the HLC
approach offer a 'pure database' solution. Given PostgreSQL's
practical use cases, I recommend starting with a local clock
implementation. However, recognizing the increasing prevalence of
distributed clock services, we should also implement a pluggable time
access method. This allows users to integrate with different time
services as needed.

In the mid-term, implementing the HLC approach would provide highly
consistent snapshot reads. This offers a significant advantage for
many use cases.

Long-term, we should consider integrating with a distributed time
service like AWS Time Sync Service. This ensures high accuracy and
scalability for demanding applications.

Thanks,
Shihao