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Co-authored-by: Israel <[email protected]>
This commit is contained in:
Alexander Kukushkin
2023-08-24 11:14:19 +02:00
committed by GitHub
co-authored by Israel
parent 4d26435244
commit 79b409847d
3 changed files with 34 additions and 19 deletions
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patroni/postgresql/__init__.py
+1 -1
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@@ -170,7 +170,7 @@ class Postgresql(object):
@property
def supports_quorum_commit(self) -> bool:
""":returns: `True` if quorum commit is supported by Postgres."""
"""``True`` if quorum commit is supported by Postgres."""
return self._major_version >= 100000
@property
patroni/postgresql/sync.py
+3 -3
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@@ -156,14 +156,14 @@ def parse_sync_standby_names(value: str) -> _SSN:
class _SyncState(NamedTuple):
"""Class representing the current synchronous state.
:ivar sync_type: possible values: 'off', 'priority', 'quorum'
:ivar sync_type: possible values: ``off``, ``priority``, ``quorum``
:ivar numsync: how many nodes are required to be synchronous (according to ``synchronous_standby_names``).
Is ``0`` if ``synchronous_standby_names`` value is invalid or contains ``*``.
:ivar numsync_confirmed: how many nodes are known to be synchronous according to the ``pg_stat_replication``
view. Only nodes that caught up with the ``SyncHandler._primary_flush_lsn` are counted.
view. Only nodes that caught up with the :attr:`SyncHandler._primary_flush_lsn` are counted.
:ivar sync: collection of synchronous node names. In case of quorum commit all nodes listed
in ``synchronous_standby_names``, otherwise nodes that are confirmed to be synchronous according
to the `pg_stat_replication` view.
to the ``pg_stat_replication`` view.
:ivar active: collection of node names that are streaming and have no restrictions to become synchronous.
"""
sync_type: str
patroni/quorum.py
+30 -15
View File
@@ -19,10 +19,10 @@ class Transition(NamedTuple):
* ``quorum`` - indicates that we need to update ``/sync`` key in DCS.
* ``restart`` - caller should stop iterating over transitions and restart :class:`QuorumStateResolver`.
:ivar leader: the new value of the ``leader`` field in the ``/sync`` key.
:ivar num: the new value of the synchronous nodes in ``synchronous_standby_names`` or value of the ``quorum``
field in the ``/sync`` key for *transition_type* values ``sync`` and ``quorum`` respectively.
:ivar num: the new value of the synchronous nodes count in ``synchronous_standby_names`` or value of the ``quorum``
field in the ``/sync`` key for :attr:`transition_type` values ``sync`` and ``quorum`` respectively.
:ivar names: the new value of node names listed in ``synchronous_standby_names`` or value of ``voters``
field in the ``/sync`` key for *transition_type* values ``sync`` and ``quorum`` respectively.
field in the ``/sync`` key for :attr:`transition_type` values ``sync`` and ``quorum`` respectively.
"""
transition_type: str
@@ -38,17 +38,22 @@ class QuorumError(Exception):
class QuorumStateResolver(object):
"""Calculates a list of state transition tuples of the form `('sync'/'quorum'/'restart',leader,number,set_of_names)`
Synchronous replication state is set in two places. PostgreSQL configuration sets how many and which nodes are
needed for a commit to succeed, abbreviated as ``numsync`` and ``sync`` set here. DCS contains information about how
many and which nodes need to be interrogated to be sure to see an wal position containing latest confirmed commit,
abbreviated as ``quorum`` and ``voters`` set. Both pairs have the meaning "ANY n OF set".
Synchronous replication state is set in two places:
The number of nodes needed for commit to succeed, ``numsync``, is also called the replication factor.
* PostgreSQL configuration sets how many and which nodes are needed for a commit to succeed, abbreviated as
``numsync`` and ``sync`` set here;
* DCS contains information about how many and which nodes need to be interrogated to be sure to see an wal position
containing latest confirmed commit, abbreviated as ``quorum`` and ``voters`` set.
.. note::
Both of above pairs have the meaning "ANY n OF set".
The number of nodes needed for commit to succeed, ``numsync``, is also called the replication factor.
To guarantee zero transaction loss on failover we need to keep the invariant that at all times any subset of
nodes that can acknowledge a commit overlaps with any subset of nodes that can achieve quorum to promote a new
leader. Given a desired replication factor and a set of nodes able to participate in sync replication there
is one optimal state satisfying this condition. Given the node set ``active``, the optimal state is:
is one optimal state satisfying this condition. Given the node set ``active``, the optimal state is::
sync = voters = active
@@ -60,18 +65,25 @@ class QuorumStateResolver(object):
other arbitrary state given arbitrary changes is node availability, configuration and interrupted transitions.
To keep the invariant the rule to follow is that when increasing ``numsync`` or ``quorum``, we need to perform the
increasing operation first. When decreasing either, the decreasing operation needs to be performed later.
increasing operation first. When decreasing either, the decreasing operation needs to be performed later. In other
words:
* If a user increases ``synchronous_node_count`` configuration, first we increase ``synchronous_standby_names``
(``numsync``), then we decrease ``quorum`` field in the ``/sync`` key;
* If a user decreases ``synchronous_node_count`` configuration, first we increase ``quorum`` field in the ``/sync``
key, then we decrease ``synchronous_standby_names`` (``numsync``).
Order of adding or removing nodes from ``sync`` and ``voters`` depends on the state of
``synchronous_standby_names``.
When adding new nodes:
When adding new nodes::
if ``sync`` (``synchronous_standby_names``) is empty:
add new nodes first to ``sync`` and then to ``voters`` when ``numsync_confirmed`` > ``0``.
else:
add new nodes first to ``voters`` and then to ``sync``.
When removing nodes:
When removing nodes::
if ``sync`` (``synchronous_standby_names``) will become empty after removal:
first remove nodes from ``voters`` and then from ``sync``.
@@ -91,7 +103,7 @@ class QuorumStateResolver(object):
:ivar active: set of node names that are replicating from the primary (according to ``pg_stat_replication``)
and are eligible to be listed in ``synchronous_standby_names``.
:ivar sync_wanted: desired number of synchronous nodes (``synchronous_node_count`` from the global configuration).
:ivar leader_wanted: the desired leader (could be different from the *leader* right after a failover).
:ivar leader_wanted: the desired leader (could be different from the :attr:`leader` right after a failover).
"""
def __init__(self, leader: str, quorum: int, voters: Collection[str],
@@ -126,8 +138,10 @@ class QuorumStateResolver(object):
self.leader_wanted = leader_wanted
def check_invariants(self) -> None:
"""Checks invatiant of ``synchronous_standby_names`` and ``/sync`` key in DCS.
"""Checks invariant of ``synchronous_standby_names`` and ``/sync`` key in DCS.
.. seealso::
Check :class:`QuorumStateResolver`'s docstring for more information.
:raises:
:exc:`QuorumError`: in case of broken state"""
voters = CaseInsensitiveSet(self.voters | CaseInsensitiveSet([self.leader]))
@@ -135,6 +149,7 @@ class QuorumStateResolver(object):
# We need to verify that subset of nodes that can acknowledge a commit overlaps
# with any subset of nodes that can achieve quorum to promote a new leader.
# ``+ 1`` is required because the leader is included in the set.
if self.voters and not (len(voters | sync) <= self.quorum + self.numsync + 1):
raise QuorumError("Quorum and sync not guaranteed to overlap: nodes %d >= quorum %d + sync %d" %
(len(voters | sync), self.quorum, self.numsync))
@@ -146,7 +161,7 @@ class QuorumStateResolver(object):
def quorum_update(self, quorum: int, voters: CaseInsensitiveSet, leader: Optional[str] = None,
adjust_quorum: Optional[bool] = True) -> Iterator[Transition]:
"""Updates quorum, voters and optionally leader fields.
"""Updates :attr:`quorum`, :attr:`voters` and optionally :attr:`leader` fields.
:param quorum: the new value for :attr:`quorum`, could be adjusted depending
on values of :attr:`numsync_confirmed` and *adjust_quorum*.