We have already defined all the constant etcd versions in the
centralized place api/version/version.go. So we should replace all
the versions with the centralized definitions.
Reason to store CI and term in backend was to make db fully independent
snapshot, it was never meant to interfere with apply logic. Skip of CI
was introduced for v2->v3 migration where we wanted to prevent it from
decreasing when replaying wal in
https://github.com/etcd-io/etcd/pull/5391. By mistake it was added to
apply flow during refactor in
https://github.com/etcd-io/etcd/pull/12855#commitcomment-70713670.
Consistency index and term should only be negotiated and used by raft to make
decisions. Their values should only driven by raft state machine and
backend should only be responsible for storing them.
Previously the SetConsistentIndex() is called during the apply workflow,
but it's outside the db transaction. If a commit happens between SetConsistentIndex
and the following apply workflow, and etcd crashes for whatever reason right
after the commit, then etcd commits an incomplete transaction to db.
Eventually etcd runs into the data inconsistency issue.
In this commit, we move the SetConsistentIndex into a txPostLockHook, so
it will be executed inside the transaction lock.
Motivation is as follows:
- etcdctl we only depend on clientv3 APIs, no dependencies of bolt, backend, mvcc, file-layout
- etcdctl can be officially supported across wide range of versions, while etcdutl is pretty specific to file format at particular version.
it's step towards desired modules layout, documented in: https://etcd.io/docs/next/dev-internal/modules/
