Flexible consistency in XDN

Tip

Developers can specify the intended consistency model, or even implement a custom replication protocol with the provided API.

XDN replicates a service under the consistency model the developer declares. Each model below links to its formal definition, including the original definitions from the literature.

Predefined consistency models

Linearizable

Every request appears to take effect instantaneously, in one global order consistent with real time: a read always reflects every previously acknowledged write. The strongest model XDN offers, and the default. Formal definition.

Sequential

All replicas apply writes in one agreed global order, but reads may observe a stale prefix of it: linearizability without the real-time requirement. Formal definition.

Causal

Writes that may have influenced one another are applied in that order at every replica; concurrent writes may be applied in different orders. Formal definition.

PRAM

Each replica applies any single client's writes in the order they were issued; writes from different sources may interleave differently at different replicas. Formal definition.

Eventual

Replicas may serve any applied state; once writes stop arriving, all replicas converge to the same state. Formal definition.

Read your writes

A session always observes its own earlier writes. Formal definition.

Writes follow reads

A session's write is ordered, everywhere, after the writes its earlier reads observed. Formal definition.

Monotonic reads

A session never observes a state older than one it has already observed. Formal definition.

Monotonic writes

A session's writes are applied everywhere in the order the session issued them. Formal definition.

Custom replication protocol

Note

Informally, a consistency model provides guarantees on what are valid values observable from the read requests. A replication protocol is the one that ensures the guarantee is satisfied and never broken by preventing invalid observations (i.e., upholding the safety property).

TBD

Note

Replication protocol is commonly also referred to as a Coordination or Synchronization protocol. It is a class of Distributed Protocol that manages the state in all the replicas.