vlc and application brief description.md

vlc and application brief description

Prerequisites

Lamport clock

The Lamport clock is a logical clock used to order events in a distributed system. It provides a way to capture the "happens-before" relationship between events, which is essential for understanding causality.

Examples

Consider two processes, P1 and P2:

• P1 starts with a clock value of 1, performs an event (clock becomes 2), and sends a message to P2 with the timestamp 2.
• P2 receives the message with timestamp 2, and updates its clock to max(current clock, 2) + 1 (e.g., if P2's current clock was 1, it becomes 3).

Conclusions

• Cannot capture concurrent events (events that are not causally related).
  • As shown in the above picture, events a and b
• It constructs fail to work in an open network with Byzantine participants.

Vector clock

Vector clocks are an extension of Lamport clocks that provide more detailed information about the causality of events. Each process maintains a vector of clock values, one for each process in the system.

Examples

Consider two processes, P1 and P2:

• P1's vector clock is initially [0, 0]. It performs an event, updating to [1, 0], and sends a message to P2.
• P2's vector clock is [0, 0]. Upon receiving the message with vector clock [1, 0], it updates to [1, 1] (assuming it performs an event after receiving).

Conclusions

• It constructs fail to work in an open network with Byzantine participants.

VLC (Verifiable Logical Clock)

Without reliable logical clocks, systems and applications are forced to use expensive BFT SMR protocols which enforces total order of requests, even though their application semantics may only require partial ordering guarantees.

Chrono overview

So, vlc (verifiable logical clock) is construct atop on vector clock, and add the verifiable proof feature. The chrono is as a concrete instance of VLC. More detail could read the paper of Building a Verifiable Logical Clock for P2P Networks

Validator backends

Applications & use cases

Two use cases, causally Consistent Data Store and mutual exclusion, are introduced in detail in paper.

Regular logical clock use case

Regular logical clock have been applied in many scenarios. As follows:

• Weakly consistent storage systems
  • Cops: causally Consistent Data Store
• Causally ordered broadcast
  • Reference: vlc + p2p combined
• Deadlock detection
  • Mutual exclusion of shared resources in a distributed system
  • Bakery algorithm
• Distributed snapshots
• Distributed system debugging.

For sure, the verifiable logical clock is an enhanced version of a regular logical clock that can do everything a regular clock can do.