[The content of this post is being edited, and is not yet approved plan of record ]

Background

In 3.4 storeV2 is still extensively used:

• User can opt in --enable-v2 to unable V2 API that writes solely data to storeV2 files.

• Membership information is both stored in V2 & V3 (backend) stores

• Membership information is being read (recovered) from storeV2

• Publishing membership happens through StoreV2 raft operation.

  • The new ClusterMemberAttrSet applier was implemented in etcd 3.5 so cannot be used by default in 3.5: to allow 3.4->3.5 rollback.

• During startup etcd assumes each WAL log snapshot is accompanied by storev2 snapshot.

• Note: etcdctl snapshot restore is not restoring V2 content (producing fake storeV2 with membership information)

Plan:

3.5 release (updated by ahrtr )

Both V2 and V3 state are supported, but V3 becomes the source of truth. TODO:

• --experimental-enable-v2v3= promoted to --enable-v2v3=... or deprecated. [see https://github.com/3.5 decision making for alpha/beta/experimental feature stability #12905]
• --enable-v2v3=... should allow mounting as root ?
• --enable-v2 prints deprecation warning [optional: when not combined with --enable-v2v3=... ]
• ClusterMemberAttrSet is implemented.
• Membership is only read from backend v2 (No-storeV2: Read membership information from the backend (Part5) #12914)
• --experimental-no-v2=PHASE_1_WRITE_ONLY mode that:
  • cannot be used when --enable-v2 is ON
  • if the storev2 snapshot is found accompanying the (used to be used) WAL log snapshot,
    its validated whether it has no used-data (apart of membership & version)
  • storev2 is content created on demand using 'etcdctl snapshot restore' logic based on the last WAL snapshot
• [3.5] Add offline command to check whether the v2store contains custom content #18993
• [3.5] Print warning message for deprecated v2 flags if set #18999

3.6 release (updated by serathius@, ahrtr )

V3 is the only meaningful state. V2 state is generated from V3 state to maintained backward compability. TODO:

• --enable-v2 is docomissioned
• PublishV3 is used solely
• Remove v2 commands from etcdctl (etcdctl: Remove V2 API commands #13643)
• Remove v2 client library (client: Hide v2 client package #15591)
• Discovery v3 is implemented Implement discovery protocol on v3 api #13624
• experimental-no PHASE_1_WRITE_ONLY becomes the default.
• Discovery v2 is deprecated
• Tests for backward v2 compatibility are implemented tests: Add tests for snapshot compatibility and recovery between versions #13756
• V2 snapshots are generated from V3 data
• V2 apply code is removed
• V2 no longer stores membership data
• Cleanup V2 code to leave only code for generating snapshots.
• Decide on Expected v2 deprecation behavior in HA clusters #17009 and implement --v2-deprecation=write-only-drop-data
• Don't allow users to change the value for --v2-deprecation #18994
• Mark --snapshot-count as deprecated and to be decommissioned in 3.7 #18995
• The document will be covered in Document for 3.6 release website#926

3.7 release (updated by serathius@, ahrtr )

• V2 code is totally removed. TODO:
  • Discovery v2 is removed
  • client v2 is removed
  • All V2 store code is removed
  • Only load data (i.e. members) from v3store (bbolt)
  • Validate membership applying using v3store
• Do not generate v2 snapshot files at all
• Bootstrap etcdserver from v3store(bbolt) instead of v2 snapshot: Bootstrap etcdserver from v3store (consistent_index) #20187
• Change the default value for --v2-deprecation to V2Depr1WriteOnlyDrop
• rewrite TestV2DeprecationWriteOnlyWAL and TestV2DeprecationWriteOnlySnapshot, refer to Bump version to 3.7 #19961

3.8 release

Remove --v2-deprecation. Completely cleanup anything related to v2store.

Docs

• https://docs.google.com/document/d/14PYLcdTyX3jhdBDc6q6WWRceTaIZePauHTWgjHJ0_ZE/edit?tab=t.0
• https://docs.google.com/document/d/1dpvBrOvBquZ8QozJcT6xcwVUVXYwO3hvjmVBd7ixIJU/edit