Client-Centric Consistency Models in Distributed Systems

Summary

The video explains client-centric consistency models, which constraint eventually consistent systems to provide intuitive behavior by ensuring clients see a reasonable view of the data. It covers guarantees like read your writes, monotonic reads, monotonic writes, and write follows read that reduce anomalies. The speaker explores differences between guarantees, like monotonic reads being about multiple reads within a session while consistent prefix reads apply system-wide. He also shows guarantees combine to provide causal consistency, though FIFO consistency alone doesn't ensure them.

Takeaways

• 😀 Session guarantees help provide reasonable behavior and reduce anomalies on top of weak consistency systems
• 😃 There are 5 main session guarantees: read your own writes, monotonic reads, monotonic writes, write follows reads, consistent prefix reads
• 🤔 Achieving session guarantees is relatively easy and cheap compared to strong consistency
• 😎 Read your own writes means writes done within a session will be seen in subsequent reads within that session
• 📝 Monotonic reads means successive reads within a session return non-decreasing sets of writes
• ✏️ Monotonic writes means writes within a session are seen by all later reads that see the later writes
• 📎 Write follows reads binds writes to include writes that were returned in previous reads in the session
• 📌 Consistent prefix reads means reads see a prefix of all writes in arbitration order
• 🔗 Combining some session guarantees gives causal consistency, but FIF consistency does not imply session guarantees
• 📚There are still more alternative consistency models to cover related to these session guarantees

Q & A

What are session guarantees and why are they needed?

Session guarantees are constraints applied on top of weakly consistent systems to make their behavior more reasonable and reduce anomalies. They are needed because weakly consistent systems on their own can exhibit unintuitive behavior and anomalies that are difficult for developers to handle.

What does the read your writes session guarantee ensure?

The read your writes session guarantee ensures that if a client makes a write and then a read within the same session, they will see their own write during the read.

What does monotonic reads guarantee?

Monotonic reads guarantees that if a client makes multiple reads in the same session, the reads will return a non-decreasing set of writes. So if the client has already seen a write, subsequent reads will also return that write.

What are the two constraints that make up the write follows read guarantee?

The two constraints are: 1) propagation order - the order in which writes propagate across replicas, and 2) write order - the global order all replicas will eventually reflect.

How does consistent prefix read differ from monotonic reads?

A system can satisfy monotonic reads but still violate consistent prefix reads. Monotonic reads only requires that each client sees a non-decreasing set of writes. But consistent prefix read requires that each replica state is an actual state that existed at some point in the real execution.

Can a system that ensures FIFO consistency provide session guarantees?

No, FIFO consistency does not ensure monotonic reads, monotonic writes, or read your writes session guarantees.

What relationship exists between session guarantees and causal consistency?

If a system provides monotonic reads, monotonic writes, read your writes, and write follows read guarantees, then it is causally consistent. And a causally consistent system will satisfy all those session guarantees.

Why are session guarantees important?

Session guarantees significantly improve the behavior of weakly consistent systems by reducing anomalies and making the systems more intuitive to use. This avoids difficult situations for developers.

What are some ways monotonic reads can be achieved?

Monotonic reads can be achieved through synchronous replication, having clients always connect to the same replica, or having clients provide a version with each read to ensure they get at least that version of data.

How do session guarantees relate to strong and weak consistency models?

Session guarantees act as a middle ground, improving weak consistency without all the limitations of strong consistency. So they provide more availability and performance than strong consistency, with more intuitive behavior than weak consistency.