etcd focuses just on the key/value store with consensus to enable other systems. And there are a number of systems that are now built on top of etcd now. Here are a handful that I talked about in a recent talk at ContainerCon[1].
- locksmith: a scheduler for host reboots in a cluster. Designed to ensure a cluster can do OS upgrades unattended.
- skydns: a DNS server built on top of etcd.
- confd: a configuration file templating system designed to watch changes and rewrite configuration on disk.
- vulcand: a HTTP load balancer with rate limiting, and dynamic balancing algos.
- kubernetes: a system to manage clusters of containers which backs its service discovery, scheduling, and election with etcd.
One interesting thing is that kubernetes handles load balancing, DNS and configuration packaged together which many people enjoy using. While other people like to just have DNS or configuration so there are tools that focus on just that to tie together existing systems.
Okay sounds good if that is the solution, but if you have 5 nodes and end up with partitions A and B where A has 2 nodes and B has 3 nodes. Couldn't B have enough for a quorum, but still possibly have stale data?
Since A doesn't have quorum, writes on the A side of the partition are impossible, and the data in B cannot go stale (but operations can continue through the new master in B). That's the essence of the CAP theorem's consistency-availability trade off.
This assumes all reads and writes go through Raft.
That isn't possible and would break the consistency guarantee of using something like raft. A quorum read or a write are serialized through the raft state machine and thus has to be acknowledged by every member of the cluster.
- locksmith: a scheduler for host reboots in a cluster. Designed to ensure a cluster can do OS upgrades unattended.
- skydns: a DNS server built on top of etcd.
- confd: a configuration file templating system designed to watch changes and rewrite configuration on disk.
- vulcand: a HTTP load balancer with rate limiting, and dynamic balancing algos.
- kubernetes: a system to manage clusters of containers which backs its service discovery, scheduling, and election with etcd.
One interesting thing is that kubernetes handles load balancing, DNS and configuration packaged together which many people enjoy using. While other people like to just have DNS or configuration so there are tools that focus on just that to tie together existing systems.
[1] https://github.com/philips/hacks/tree/master/etcd-demos