doc/dev/mon: info on auth mon disk format

Signed-off-by: Joao Eduardo Luis <joao@suse.de>

doc/dev/mon-on-disk-formats.rst

@@ -0,0 +1,91 @@
+##############
+ON-DISK FORMAT
+##############
+
+
+************
+UPGRADE PATH
+************
+
+On-disk formats, or even data structure formats, may be changed during an
+upgrade. Services wishing to do so, may so do it via the
+`PaxosService::upgrade_format()` call path. There is no formalized, unified
+format versioning; the `PaxosService` class keeps track of its
+`format_version` through a key in the store, assumed an `unsigned int`,  but
+it will be the service's responsibility to give meaning to those versions.
+
+AUTH MONITOR
+============
+
+versions
+--------
+
+versions are represented with a single `unsigned int`. By default, the value
+zero represents the absence of a formal upgraded format. The first format
+version was introduced in Dumpling; clusters upgrading to Dumpling saw their
+format version being increased from zero to one.::
+
+  0 to 1 - introduced in v0.65, dev release for v0.67 dumpling
+  1 to 2 - introduced in v12.0.2, dev release for luminous
+  2 to 3 - introduced in mimic
+
+  0 - all clusters pre-dumpling
+  1 - all clusters dumpling+ and pre-luminous
+  2 - all clusters luminous+ and pre-mimic
+  3 - all clusters mimic+
+
+  version 1: introduces new-style monitor caps (i.e., profiles)
+  version 2: introduces mgr caps and bootstrap-mgr key
+  version 3: creates all bootstrap and admin keys if they don't yet exist
+
+callstack
+---------
+
+format_version set on `PaxosService::refresh()`:::
+
+  - initially called from Monitor::refresh_from_paxos
+    - initially called from Monitor::init_paxos()
+      - initially called from Monitor::preinit()
+
+AuthMonitor::upgrade_format() called by `PaxosService::_active()`:::
+
+  - called from C_Committed callback, from PaxosService::propose_pending()
+  - called from C_Active callback, from PaxosService::_active()
+  - called from PaxosService::election_finished()
+
+  - on a freshly deployed cluster, upgrade_format() will be first called
+    *after* create_initial().
+  - on an existing cluster, upgrade_format() will be called after the first
+    election.
+
+  - upgrade_format() is irrelevant on a freshly deployed cluster, as there is
+    no format to upgrade at this point.
+
+boil down
+---------
+
+* if `format_version >= current_version` then format is uptodate, return.
+* if `features doesn't contain LUMINOUS` then `current_version = 1`
+* else if `features doesn't contain MIMIC` then `current_version = 2`
+* else `current_version = 3`
+
+if `format_version == 0`:::
+
+  - upgrade to format version 1
+    - move to new-style monitor caps (i.e., profiles):
+      - set daemon profiles for existing entities
+      - set profile for existing bootstrap keys
+
+if `format_version == 1`:::
+
+  - upgrade to format version 2
+    - for existing entities:
+      - add new cap for mgr
+    - for existing 'mgr' entities, fix 'mon' caps due to bug from kraken
+      setting 'allow \*', and set 'allow profile mgr' instead.
+    - add bootstrap-mgr key.
+
+if `format_version == 2`:::
+
+  - upgrade to format version 3
+    - create all bootstrap keys if they don't currently exist