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Etcd is a key-value storage based on a distributed consensus algorithm. This (sort of) means that you can have a source of cluster truth in the db and make sure

Etcd is a key-value storage based on a distributed consensus algorithm. This (sort of) means that you can have a source of cluster truth in the db and make sure it lives through certain distributed system bad scenarios. Sqlite is a (relatively) simple in-process sql rdbms. You get a proper reliable sql on a single computer, which is practically useful for many more cases. It is possible to build distributed dbs on top of a bunch of sqlite instances but one would need to solve these very distributed system problems that etcd solves. In fact, some of the popular distributed dbs use etcd or zookeeper or custom implementations of the same algos o top of traditional rdmss. K8s is a distributed system that needs to have a reliable view of its nodes. Clearly, a concensus-based db is necessary.

gigatexal 4y ago
So that would make etcd consistent and available (CP of the CAP)? edit: almost -- available and partition tolerant at least in the default config: https://github.com/cloudfoundry-attic/etcd-release/blob/master/README.md#fault-tolerance-and-data-durability https://github.com/cloudfoundry-attic/etcd-release/blob/mast...
- kevincox 4y ago
  CP doesn't exist. CAP is about what happens in case of a partition (P). You can either remain available (AP) our you can remain consistent (CP). If you don't have partitions then CAP doesn't apply. But I wouldn't recommend depending on that. For example Google's Spanner is CP, but they work really hard to make partitions rare, giving them 99.999% availability.
- remram 4y ago
  etcd is strongly consistent, it will only be available if you have a majority of nodes up and reaching each other (CP over AP). If you don't have a majority it will not be available (but won't get inconsistent). This is more resilient than SQLite of course, which runs on a single node, and therefore can't remain available if any node (= the only node) fails.

gigatexal 4y ago

So that would make etcd consistent and available (CP of the CAP)? edit: almost -- available and partition tolerant at least in the default config: https://github.com/cloudfoundry-attic/etcd-release/blob/master/README.md#fault-tolerance-and-data-durability https://github.com/cloudfoundry-attic/etcd-release/blob/mast...

kevincox 4y ago
CP doesn't exist. CAP is about what happens in case of a partition (P). You can either remain available (AP) our you can remain consistent (CP). If you don't have partitions then CAP doesn't apply. But I wouldn't recommend depending on that. For example Google's Spanner is CP, but they work really hard to make partitions rare, giving them 99.999% availability.
remram 4y ago
etcd is strongly consistent, it will only be available if you have a majority of nodes up and reaching each other (CP over AP). If you don't have a majority it will not be available (but won't get inconsistent). This is more resilient than SQLite of course, which runs on a single node, and therefore can't remain available if any node (= the only node) fails.