Eventual Consistency
A guarantee that replicas converge given no further updates, without guaranteeing when.
Eventual consistency is the weakest useful consistency guarantee: in the absence of new updates, all replicas will eventually agree. It says nothing about how long that takes or what anyone sees meanwhile.
It exists because of the CAP theorem's trade: a distributed system experiencing a network partition must choose between remaining available and remaining consistent. Choosing availability means accepting that different participants temporarily see different things.
For many systems that is obviously correct. A cached page, a feed, a counter, a search index — all tolerate being briefly behind. For others it is obviously wrong; a balance check that can be stale is not a balance check.
The design work is in making inconsistency legible rather than pretending it away. Showing when something was last updated, reflecting a local change immediately while it propagates, and reconciling honestly when the answer arrives are all better than an interface that implies certainty it does not have.
A specific, easily-missed instance: a cache whose scope is a single process behaves like a shared cache in development and like nothing at all across several instances, so a value written by one worker is invisible to the rest. That is eventual consistency with an eventuality of never. See Cache Invalidation.
See also4
Hand-picked in the note itself — the neighbours worth reading next.
Conflict-Free Replicated Data Type
A data structure whose replicas can be edited independently and always converge without coordination.
Networks & Distribution5 connections
Cache Invalidation
Deciding when a cached value has stopped being correct — famously one of the hard problems.
Web Platform11 connections
Peer-to-Peer
An architecture where participants communicate directly rather than through a central server.
Networks & Distribution5 connections
Idempotence
The property that performing an operation twice has the same effect as performing it once.
Systems & Tooling12 connections
Related3
Nearby in the graph rather than deliberately chosen. Looser, sometimes surprising.
Linked from5
Notes elsewhere in the wiki that reach for this one.
- Conflict-Free Replicated Data TypeNetworks & Distribution
A data structure whose replicas can be edited independently and always converge without coordination.
- IdempotenceSystems & Tooling
The property that performing an operation twice has the same effect as performing it once.
- Peer-to-PeerNetworks & Distribution
An architecture where participants communicate directly rather than through a central server.
- Signaling ServerNetworks & Distribution
The small coordination service peers use to find each other before connecting directly.
- WebRTCNetworks & Distribution
The browser standard for direct peer connections carrying audio, video, and arbitrary data.