3 ms·

If you run etcd as a singleton you already get this behavior - consensus is shortcircuited and you only have to pay the cost to write to durable storage (which

If you run etcd as a singleton you already get this behavior - consensus is shortcircuited and you only have to pay the cost to write to durable storage (which is already heavily batched). And you need durable writes so you can crash recover (kube has a gaping hole today in that a restore from an earlier time point breaks many controllers until a reconcile is performed). Note that Kubernetes requires a total ordering of writes (which simplifies how hard it is for us puny humans to reason about) AND requires strong consistency in order to provide guarantees like “this pod only runs on one machine at a time” and “PVs aren’t released until the pods are really stopped”. Leaderless is a simple tradeoff - singleton or three instances. That’s the best possible choice in the world and it’s etcd and it’s relative simplicity that make it possible. I’ve never seen a production Kube system with HA etcd go down due to non-human error, so I don’t believe single instance is going to give you better availability when single machine faults happen (they happen frequently; about 0.5-1% of the machines in the OpenShift fleet - cloud and on-premise - are down at any one time due to power, software, or hardware issues). Almost all of those clusters tick along fine when they lose that machine.

peterwwillis 6y ago
> I’ve never seen a production Kube system with HA etcd go down due to non-human error I have, with OpenShift, several times. The worst one took hours to get it partially back online, days to fully recover. At least I got to have one free drink before I had to leave our holiday party to start fixing it. (Ok, technically it was non-prod; luckily prod was ECS... long story) > I don’t believe single instance is going to give you better availability when single machine faults happen Depends on your definition and context of availability. It is possibly the simplest thing to destroy any existing nodes (and in the event that a node can't be destroyed, null-route it and/or disable its network port) and bring up a new node. So single instance is fairly easy to recover. But with multiple nodes, for each function of each instance that is required to reach consensus, the likelihood of consensus failure increases; you actually need more nodes and variety in the cluster to resist consensus failure. Yet ironically, the more nodes you have, the higher the probability of failure. In the end, the real-world reliability of the system is based on additional factors besides the network model.

peterwwillis 6y ago

> I’ve never seen a production Kube system with HA etcd go down due to non-human error I have, with OpenShift, several times. The worst one took hours to get it partially back online, days to fully recover. At least I got to have one free drink before I had to leave our holiday party to start fixing it. (Ok, technically it was non-prod; luckily prod was ECS... long story) > I don’t believe single instance is going to give you better availability when single machine faults happen Depends on your definition and context of availability. It is possibly the simplest thing to destroy any existing nodes (and in the event that a node can't be destroyed, null-route it and/or disable its network port) and bring up a new node. So single instance is fairly easy to recover. But with multiple nodes, for each function of each instance that is required to reach consensus, the likelihood of consensus failure increases; you actually need more nodes and variety in the cluster to resist consensus failure. Yet ironically, the more nodes you have, the higher the probability of failure. In the end, the real-world reliability of the system is based on additional factors besides the network model.