etcd v3.4 makes ETCDCTL_API=3 by default

[gyuho]

v3.0.0 was out 2 years ago https://github.com/coreos/etcd/releases/tag/v3.0.0. v2 is officially deprecated in Container Linux https://coreos.com/blog/toward-etcd-v3-in-container-linux.html. While we still do keep all v2 features plus v2 API wrapper backed by v3 storage, default API should now be v3.

Before (< v3.4.0)

# "ETCDCTL_API=2" is set by default
$ ETCDCTL_API=2 ./bin/etcdctl set foo bar
bar

$ ./bin/etcdctl set foo bar
bar

$ ETCDCTL_API=3 ./bin/etcdctl set foo bar
Error: unknown command "set" for "etcdctl"

$ ETCDCTL_API=2 ./bin/etcdctl put foo bar
No help topic for 'put'

$ ./bin/etcdctl put foo bar
No help topic for 'put'

# for v3, "ETCDCTL_API=3" needs be set explicitly
$ ETCDCTL_API=3 ./bin/etcdctl put foo bar
OK

After (>= v3.4.0), we want to set "ETCDCTL_API=3" by default

$ ETCDCTL_API=2 ./bin/etcdctl set foo bar
bar

# "ETCDCTL_API=3" is set by default
$ ./bin/etcdctl set foo bar
Error: unknown command "set" for "etcdctl"

$ ETCDCTL_API=3 ./bin/etcdctl set foo bar
Error: unknown command "set" for "etcdctl"

# for v2, "ETCDCTL_API=2" needs be set explicitly
$ ETCDCTL_API=2 ./bin/etcdctl put foo bar
No help topic for 'put'

$ ./bin/etcdctl put foo bar
OK

$ ETCDCTL_API=3 ./bin/etcdctl put foo bar
OK

In general, community feedback is positive:

• https://groups.google.com/forum/#!topic/etcd-dev/u1vkn4a9Sz0

Please let us know if anybody has other concerns.

[vimalk78]

@joelegasse , if you are not working on this i would like to take it up.

[joelegasse]

@vimalk78 All yours. :-) Thank you