PostgreSQL 基于 Patroni 方案的高可用启动流程分析

作者：谐云

2023-10-26
浙江
本文字数：18737 字
阅读完需：约 61 分钟

什么是 Patroni

在很多生产环境中，分布式数据库以高可用性、数据分布性、负载均衡等特性，被用户广泛应用。而作为高可用数据库的解决方案——Patroni，是专门为 PostgreSQL 数据库设计的，一款以 Python 语言实现的高可用架构模板。该架构模板，旨在通过外部共享存储软件（kubernetes、etcd、etcd3、zookeeper、aws 等），实现 PostgreSQL 集群的自动故障恢复、自动故障转移、自动备份等能力。主要特点：1.自动故障检测和恢复：Patroni 监视 PostgreSQL 集群的健康状态，一旦检测到主节点故障，它将自动执行故障恢复操作，将其中一个从节点晋升为主节点。2.自动故障转移：一旦 Patroni 定义了新的主节点，它将协调所有从节点和客户端，以确保它们正确地切换到新的主节点，从而实现快速、无缝的故障转移。3.一致性和数据完整性：Patroni 高度关注数据一致性和完整性。在故障切换过程中，它会确保在新主节点接管之前，数据不会丢失或受损。4.外部共享配置存储：Patroni 使用外部键值存储（如 ZooKeeper、etcd 或 Consul）来存储配置和集群状态信息。这确保了配置的一致性和可访问性，并支持多个 Patroni 实例之间的协作。5.支持多种云环境和物理硬件：Patroni 不仅可以在云环境中运行，还可以部署在物理硬件上，提供了广泛的部署选项。

Patroni 架构解析

●DCS（Distributed Configuration Store ）：是指分布式配置信息的存储位置，可支持 kubernetes、etcd、etcd3、zookeeper、aws 等存储媒介，由 Patroni 进行分布式配置信息的读写。●核心 Patroni：负责将分布式配置信息写入 DCS 中，并设置 PostgreSQL 节点的角色以及 PostgreSQL 配置信息，管理 PostgreSQL 的生命周期。●PostgreSQL 节点：各 PostgreSQL 节点，根据 Patroni 设置的 PostgreSQL 配置信息，生成主从关系链，以流复制的方式进行数据同步，最终生成一个 PostgreSQL 集群。Patroni 高可用源码分析 Patroni 高可用启动流程

流程说明：●加载集群信息，通过 DCS 支持的 API 接口，获取集群信息，主要内容如下：○config：记录 pg 集群 ID 以及配置信息（包括 pg 参数信息、一些超时时间配置等），用于集群校验、节点重建等；○leader：记录主节点选举时间、心跳时间、选举周期、最新的 lsn 等，用于主节点完成竞争后的信息记录；○sync: 记录主节点和同步节点信息，由主节点记录，用于主从切换、故障转移的同步节点校验；○failover: 记录最后一次故障转移的时间。●集群状态检测，主要检测集群配置信息的内容校验，当前集群的整体状态及节点状态，判断通过什么方式来启动 PostgreSQL；●启动 PostgreSQL，用于初始化 PostgreSQL 目录，根据集群信息设置相应的 PostgreSQL 配置信息，并启动；●生成 PostgreSQL 集群，指将完成启动的 PostgreSQL 节点，通过设置主从角色，关联不同角色的 PostgreSQL 节点，最终生成完整的集群。Patroni 高可用启动流程分析加载集群信息加载集群信息，是高可用流程启动的第一步，也是生成 PostgreSQL 集群的最关键信息。

第一步，记载集群信息

......try:self.load_cluster_from_dcs()self.state_handler.reset_cluster_info_state(self.cluster, self.patroni.nofailover)except Exception:self.state_handler.reset_cluster_info_state(None, self.patroni.nofailover)raise......

通过 DCS 接口加载集群信息

def load_cluster_from_dcs(self):cluster = self.dcs.get_cluster()

# We want to keep the state of cluster when it was healthyif not cluster.is_unlocked() or not self.old_cluster:    self.old_cluster = clusterself.cluster = cluster
if not self.has_lock(False):    self.set_is_leader(False)
self._leader_timeline = None if cluster.is_unlocked() else cluster.leader.timeline

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集群接口

def get_cluster(self, force=False):if force:self._bypass_caches()try:cluster = self._load_cluster()except Exception:self.reset_cluster()raise

self._last_seen = int(time.time())
with self._cluster_thread_lock:    self._cluster = cluster    self._cluster_valid_till = time.time() + self.ttl    return cluster

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@abc.abstractmethoddef _load_cluster(self):"""Internally this method should build Cluster object whichrepresents current state and topology of the cluster in DCS.this method supposed to be called only by get_cluster method.

   raise `~DCSError` in case of communication or other problems with DCS.   If the current node was running as a master and exception raised,   instance would be demoted."""

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以 Kubernetes 作为 DCS 为例 def _load_cluster(self):stop_time = time.time() + self._retry.deadlineself._api.refresh_api_servers_cache()try:with self._condition:self._wait_caches(stop_time)

        members = [self.member(pod) for pod in self._pods.copy().values()]        nodes = self._kinds.copy()
    config = nodes.get(self.config_path)    metadata = config and config.metadata    annotations = metadata and metadata.annotations or {}
    # get initialize flag    initialize = annotations.get(self._INITIALIZE)
    # get global dynamic configuration    config = ClusterConfig.from_node(metadata and metadata.resource_version,                                     annotations.get(self._CONFIG) or '{}',                                     metadata.resource_version if self._CONFIG in annotations else 0)
    # get timeline history    history = TimelineHistory.from_node(metadata and metadata.resource_version,                                        annotations.get(self._HISTORY) or '[]')
    leader = nodes.get(self.leader_path)    metadata = leader and leader.metadata    self._leader_resource_version = metadata.resource_version if metadata else None    annotations = metadata and metadata.annotations or {}
    # get last known leader lsn    last_lsn = annotations.get(self._OPTIME)    try:        last_lsn = 0 if last_lsn is None else int(last_lsn)    except Exception:        last_lsn = 0
    # get permanent slots state (confirmed_flush_lsn)    slots = annotations.get('slots')    try:        slots = slots and json.loads(slots)    except Exception:        slots = None
    # get leader    leader_record = {n: annotations.get(n) for n in (self._LEADER, 'acquireTime',                     'ttl', 'renewTime', 'transitions') if n in annotations}    if (leader_record or self._leader_observed_record) and leader_record != self._leader_observed_record:        self._leader_observed_record = leader_record        self._leader_observed_time = time.time()
    leader = leader_record.get(self._LEADER)    try:        ttl = int(leader_record.get('ttl')) or self._ttl    except (TypeError, ValueError):        ttl = self._ttl
    if not metadata or not self._leader_observed_time or self._leader_observed_time + ttl < time.time():        leader = None
    if metadata:        member = Member(-1, leader, None, {})        member = ([m for m in members if m.name == leader] or [member])[0]        leader = Leader(metadata.resource_version, None, member)
    # failover key    failover = nodes.get(self.failover_path)    metadata = failover and failover.metadata    failover = Failover.from_node(metadata and metadata.resource_version,                                  metadata and (metadata.annotations or {}).copy())
    # get synchronization state    sync = nodes.get(self.sync_path)    metadata = sync and sync.metadata    sync = SyncState.from_node(metadata and metadata.resource_version,  metadata and metadata.annotations)
    return Cluster(initialize, config, leader, last_lsn, members, failover, sync, history, slots)except Exception:    logger.exception('get_cluster')    raise KubernetesError('Kubernetes API is not responding properly')

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上述集群信息中，主要以 xxx-config、xxx-leader、xxx-failover、xxx-sync 作为配置信息，具体内容如下：●xxx-config% kubectl get cm pg142-1013-postgresql-config -oyamlapiVersion: v1kind: ConfigMapmetadata:annotations:config: '{"loop_wait":10,"maximum_lag_on_failover":33554432,"postgresql":{"parameters":{"archive_command":"/bin/true","archive_mode":"on","archive_timeout":"1800s","autovacuum":"on","autovacuum_analyze_scale_factor":0.02,"autovacuum_max_workers":"3","autovacuum_naptime":"5min","autovacuum_vacuum_cost_delay":"2ms","autovacuum_vacuum_cost_limit":"-1","autovacuum_vacuum_scale_factor":0.05,"autovacuum_work_mem":"128MB","backend_flush_after":"0","bgwriter_delay":"200ms","bgwriter_flush_after":"256","bgwriter_lru_maxpages":"100","bgwriter_lru_multiplier":"2","checkpoint_completion_target":"0.9","checkpoint_flush_after":"256kB","checkpoint_timeout":"5min","commit_delay":"0","constraint_exclusion":"partition","datestyle":"iso,mdy","deadlock_timeout":"1s","default_text_search_config":"pg_catalog.english","dynamic_shared_memory_type":"posix","effective_cache_size":"32768","fsync":"on","full_page_writes":"on","hot_standby":"on","hot_standby_feedback":"off","huge_pages":"off","idle_in_transaction_session_timeout":"600000","lc_messages":"en_US.UTF-8","lc_monetary":"en_US.UTF-8","lc_numeric":"en_US.UTF-8","lc_time":"en_US.UTF-8","listen_addresses":"*","log_autovacuum_min_duration":"0","log_checkpoints":"on","log_connections":"off","log_disconnections":"off","log_error_verbosity":"default","log_line_prefix":"%t[%p]: [%l-1] %c %x %d %u %a %h","log_lock_waits":"on","log_min_duration_statement":"500","log_rotation_size":"0","log_statement":"none","log_temp_files":0,"log_timezone":"Asia/Shanghai","maintenance_work_mem":"32768","max_connections":"170","max_parallel_maintenance_workers":"2","max_parallel_workers":"2","max_parallel_workers_per_gather":"2","max_replication_slots":"10","max_standby_archive_delay":"30s","max_standby_streaming_delay":"30s","max_wal_senders":"10","max_wal_size":"2048","max_worker_processes":"8","old_snapshot_threshold":"-1","pg_stat_statements.max":"10000","pg_stat_statements.save":"on","pg_stat_statements.track":"all","pgaudit.log":"NONE","pgaudit.log_catalog":"on","pgaudit.log_client":"off","pgaudit.log_level":"log","pgaudit.log_parameter":"off","pgaudit.log_relation":"off","pgaudit.log_rows":"off","pgaudit.log_statement":"on","pgaudit.log_statement_once":"off","pgaudit.role":"","random_page_cost":"4","restart_after_crash":"on","synchronous_commit":"on","tcp_keepalives_count":"0","tcp_keepalives_idle":"900","tcp_keepalives_interval":"100","temp_buffers":"8MB","timezone":"Asia/Shanghai","track_activity_query_size":"1kB","track_functions":"all","track_io_timing":"off","unix_socket_directories":"/var/run/postgresql","vacuum_cost_delay":"0ms","vacuum_cost_limit":"200","wal_buffers":"2048","wal_compression":"on","wal_keep_segments":"128","wal_keep_size":"2048MB","wal_level":"replica","wal_log_hints":"on","wal_receiver_status_interval":"10s","wal_sender_timeout":"1min","wal_writer_delay":"200ms","wal_writer_flush_after":"1MB","work_mem":"4MB"},"use_pg_rewind":true,"use_slots":true},"retry_timeout":10,"synchronous_mode":true,"ttl":30}'initialize: "7289263672843878470"creationTimestamp: "2023-10-13T02:25:51Z"labels:application: spilocluster-name: pg142-1013-postgresqlname: pg142-1013-postgresql-confignamespace: defaultresourceVersion: "22858249"uid: dfa64d28-e939-4bdd-8db1-a3485fa09637

上述例子中，下有和 2 个参数，1.定义集群的整体配置信息，这里包含了 PostgreSQL 配置参数以及集群参数（选举等待时间、允许的最大 WAL 延迟量、是否开启同步模式等）等；2.定义了集群的 ID，该值对应 pg_controldata 命令内的值，因此，所有集群内的 PostgreSQL 节点有相同的 sys_id。

root@pg142-1013-postgresql-1:/home/postgres# pg_controldata | grep "Database system identifier"Database system identifier: 7289263672843878470●xxx-leader% kubectl get cm pg142-1013-postgresql-leader -oyamlapiVersion: v1kind: ConfigMapmetadata:annotations:acquireTime: "2023-10-13T02:26:06.973552+00:00"leader: pg142-1013-postgresql-0optime: "67109192"renewTime: "2023-10-16T07:02:57.418940+00:00"transitions: "0"ttl: "30"creationTimestamp: "2023-10-13T02:26:07Z"labels:application: spilocluster-name: pg142-1013-postgresqlname: pg142-1013-postgresql-leadernamespace: defaultresourceVersion: "23286847"uid: cb235c85-6a21-454d-8320-222205eaa77f

上述下，各参数含义：1.acquireTime：获取集群 leader 锁时间；2.leader：集群 leader 锁的拥有者，这里表示某个 PostgreSQL 节点名称；3.optime：集群 leader 的最新 LSN 的十进制数，这里;4.renewTime：集群 leader 锁的拥有者心跳时间，心跳周期与 xxx-config 中的对应；5.transitions：集群 leader 锁占用次数，一般发生在主从切换或故障转移场景，依次累加；6.ttl：故障转移前的选举时间，即超过 TTL 时间下，没有获取到 renewTime 值更新，便触发选举，由新的节点占用 leader 锁。

●xxx-sync% kubectl get cm pg142-1013-postgresql-sync -oyamlapiVersion: v1kind: ConfigMapmetadata:annotations:leader: pg142-1013-postgresql-1sync_standby: pg142-1013-postgresql-0creationTimestamp: "2023-10-16T06:54:39Z"labels:application: spilocluster-name: pg142-1013-postgresqlname: pg142-1013-postgresql-syncnamespace: defaultresourceVersion: "23288352"uid: 1c46e63b-8b90-4fc6-9596-8e2f71fba2ab

上述内容记录了 2 个信息：1.leader：显示 leader 节点的名称；2.sync_standby：显示同步节点的名称，多个同步节点以逗号分隔。

●xxx-failover% kubectl get cm pg142-1013-postgresql-failover -oyamlapiVersion: v1kind: ConfigMapmetadata:creationTimestamp: "2023-10-16T07:16:03Z"labels:application: spilocluster-name: pg142-1013-postgresqlmanagedFields:

apiVersion: v1fieldsType: FieldsV1fieldsV1:f:metadata:f:labels:.: {}f:application: {}f:cluster-name: {}manager: Patronioperation: Updatetime: "2023-10-16T07:36:56Z"name: pg142-1013-postgresql-failovernamespace: defaultresourceVersion: "23290596"uid: 72d50c58-bc65-4b77-8870-93d0b8f8b7a2

上述内容，主要记录最后一次故障转移发生的时间。集群状态检测

if self.is_paused():self.watchdog.disable()self._was_paused = Trueelse:if self._was_paused:self.state_handler.schedule_sanity_checks_after_pause()self._was_paused = False

if not self.cluster.has_member(self.state_handler.name):self.touch_member()

cluster has leader key but not initialize key

if not (self.cluster.is_unlocked() or self.sysid_valid(self.cluster.initialize)) and self.has_lock():self.dcs.initialize(create_new=(self.cluster.initialize is None), sysid=self.state_handler.sysid)

if not (self.cluster.is_unlocked() or self.cluster.config and self.cluster.config.data) and self.has_lock():self.dcs.set_config_value(json.dumps(self.patroni.config.dynamic_configuration, separators=(',', ':')))self.cluster = self.dcs.get_cluster()

if self._async_executor.busy:return self.handle_long_action_in_progress()

msg = self.handle_starting_instance()if msg is not None:return msg

we've got here, so any async action has finished.

if self.state_handler.bootstrapping:return self.post_bootstrap()

if self.recovering:self.recovering = False

  if not self._rewind.is_needed:      # Check if we tried to recover from postgres crash and failed      msg = self.post_recover()      if msg is not None:          return msg
  # Reset some states after postgres successfully started up  self._crash_recovery_executed = False  if self._rewind.executed and not self._rewind.failed:      self._rewind.reset_state()
  # The Raft cluster without a quorum takes a bit of time to stabilize.  # Therefore we want to postpone the leader race if we just started up.  if self.cluster.is_unlocked() and self.dcs.__class__.__name__ == 'Raft':      return 'started as a secondary'

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检测集群是否暂停集群暂停，是指集群中的 PostgreSQL 节点不由 Patroni 管理，当集群异常时，不再出发故障转移等措施。集群暂停一般由用户主动出发，可以用在单个 PostgreSQL 节点的维护上，触发方式：root@pg142-1013-postgresql-0:/home/postgres# patronictl list

Cluster: pg142-1013-postgresql (7289263672843878470) ---+---------+----+-----------+| Member | Host | Role | State | TL | Lag in MB |+-------------------------+----------------+--------------+---------+----+-----------+| pg142-1013-postgresql-0 | 10.244.117.143 | Leader | running | 3 | || pg142-1013-postgresql-1 | 10.244.165.220 | Sync Standby | running | 3 | 0 |+-------------------------+----------------+--------------+---------+----+-----------+root@pg142-1013-postgresql-0:/home/postgres# patronictl pauseSuccess: cluster management is pausedroot@pg142-1013-postgresql-0:/home/postgres# patronictl list
Cluster: pg142-1013-postgresql (7289263672843878470) ---+---------+----+-----------+| Member | Host | Role | State | TL | Lag in MB |+-------------------------+----------------+--------------+---------+----+-----------+| pg142-1013-postgresql-0 | 10.244.117.143 | Leader | running | 3 | || pg142-1013-postgresql-1 | 10.244.165.220 | Sync Standby | running | 3 | 0 |+-------------------------+----------------+--------------+---------+----+-----------+Maintenance mode: on上述，即表示当前集群已停止。此时，PostgreSQL进程仍然存活，如果故障，将需要用户自行启动。集群暂停恢复方式：root@pg142-1013-postgresql-0:/home/postgres# patronictl list
Cluster: pg142-1013-postgresql (7289263672843878470) ---+---------+----+-----------+| Member | Host | Role | State | TL | Lag in MB |+-------------------------+----------------+--------------+---------+----+-----------+| pg142-1013-postgresql-0 | 10.244.117.143 | Leader | running | 3 | || pg142-1013-postgresql-1 | 10.244.165.220 | Sync Standby | running | 3 | 0 |+-------------------------+----------------+--------------+---------+----+-----------+Maintenance mode: onroot@pg142-1013-postgresql-0:/home/postgres# patronictl resumeSuccess: cluster management is resumedroot@pg142-1013-postgresql-0:/home/postgres# patronictl list
Cluster: pg142-1013-postgresql (7289263672843878470) ---+---------+----+-----------+| Member | Host | Role | State | TL | Lag in MB |+-------------------------+----------------+--------------+---------+----+-----------+| pg142-1013-postgresql-0 | 10.244.117.143 | Leader | running | 3 | || pg142-1013-postgresql-1 | 10.244.165.220 | Sync Standby | running | 3 | 0 |+-------------------------+----------------+--------------+---------+----+-----------+
通过命令，即可恢复集群。在恢复集群后，需要对集群中 PostgreSQL 节点进行处理：1.重新配置 PostgreSQL 的参数；2.根据 xxx-sync 中最后一次记录的主、同步节点名称信息，在主节点上设置同步复制槽信息；3.检测恢复后的 PostgreSQL 节点的是否变更，与最后一次 xxx-config 中的值，是否一致，否则将无法恢复集群。集群初始化检测

cluster has leader key but not initialize key

集群初始化检测，主要检测 2 个方面的信息：●集群当前存在 leader 节点，但 xxx-config 中的不存在，此时，需要将 leader 节点上 PostgreSQL 的 sysid 设置到 xxx-config 中；●集群当前存在 leader 节点，但未获取到 xxx-config 信息，需要将 leader 节点上的配置信息和 sysid 都设置到 xxx-config 中，并重新获取集群信息。该步骤的用途是，防止 xxx-config 文件被删除，导致从节点加载集群信息失败。节点状态检测

检测当前 PostgreSQL 的进程启动到了什么阶段

if self._async_executor.busy:return self.handle_long_action_in_progress()

msg = self.handle_starting_instance()if msg is not None:return msg

节点状态检测，是通过检测 PostgreSQL 节点的当前运行状态，来确定是否需要进行具体的操作，节点状态检测的方式可分为 2 种：1.通过 PostgreSQL 的运行状态确定；2.通过异步进程（_async_executor）监听，当前节点处于什么阶段。

节点检测通过后基础操作

we've got here, so any async action has finished.

if self.state_handler.bootstrapping:return self.post_bootstrap()

if self.recovering:self.recovering = False

if not self._rewind.is_needed:    # Check if we tried to recover from postgres crash and failed    msg = self.post_recover()    if msg is not None:        return msg
# Reset some states after postgres successfully started upself._crash_recovery_executed = Falseif self._rewind.executed and not self._rewind.failed:    self._rewind.reset_state()
# The Raft cluster without a quorum takes a bit of time to stabilize.# Therefore we want to postpone the leader race if we just started up.if self.cluster.is_unlocked() and self.dcs.__class__.__name__ == 'Raft':    return 'started as a secondary'

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节点状态检测通过后，需要对 PostgreSQL 进行操作：1.PostgreSQL 启动后操作 def post_bootstrap(self):with self._async_response:result = self._async_response.result# bootstrap has failed if postgres is not runningif not self.state_handler.is_running() or result is False:self.cancel_initialization()

if result is None:    if not self.state_handler.is_leader():        return 'waiting for end of recovery after bootstrap'
    self.state_handler.set_role('master')    ret = self._async_executor.try_run_async('post_bootstrap', self.state_handler.bootstrap.post_bootstrap,                                             args=(self.patroni.config['bootstrap'], self._async_response))    return ret or 'running post_bootstrap'
self.state_handler.bootstrapping = Falseif not self.watchdog.activate():    logger.error('Cancelling bootstrap because watchdog activation failed')    self.cancel_initialization()self._rewind.ensure_checkpoint_after_promote(self.wakeup)self.dcs.initialize(create_new=(self.cluster.initialize is None), sysid=self.state_handler.sysid)self.dcs.set_config_value(json.dumps(self.patroni.config.dynamic_configuration, separators=(',', ':')))self.dcs.take_leader()self.set_is_leader(True)self.state_handler.call_nowait(ACTION_ON_START)self.load_cluster_from_dcs()
return 'initialized a new cluster'

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上述操作，包括 pg_rewind 后的 checkpoint 检测、初始化 DCS 的 xxx-config 资源、生成 xxx-leader 资源、加载集群信息等。2.恢复中的 PostgreSQL 检测是否需要执行 pg_rewindif self.recovering:self.recovering = False

if not self._rewind.is_needed:    # Check if we tried to recover from postgres crash and failed    msg = self.post_recover()    if msg is not None:        return msg
# Reset some states after postgres successfully started upself._crash_recovery_executed = Falseif self._rewind.executed and not self._rewind.failed:    self._rewind.reset_state()

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pg_rewind 命令用于将从节点的 WAL 与主节点的 WAL 拉齐，一般用于从节点 WAL 因异常后滞后于主节点 WAL。启动 PostgreSQL

is data directory empty?

if self.state_handler.data_directory_empty():self.state_handler.set_role('uninitialized')self.state_handler.stop('immediate', stop_timeout=self.patroni.config['retry_timeout'])# In case datadir went away while we were master.self.watchdog.disable()

# is this instance the leader?if self.has_lock():    self.release_leader_key_voluntarily()    return 'released leader key voluntarily as data dir empty and currently leader'
if self.is_paused():    return 'running with empty data directory'return self.bootstrap()  # new node

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else:# check if we are allowed to joindata_sysid = self.state_handler.sysidif not self.sysid_valid(data_sysid):# data directory is not empty, but no valid sysid, cluster must be broken, suggest reinitreturn ("data dir for the cluster is not empty, ""but system ID is invalid; consider doing reinitialize")

if self.sysid_valid(self.cluster.initialize):    if self.cluster.initialize != data_sysid:        if self.is_paused():            logger.warning('system ID has changed while in paused mode. Patroni will exit when resuming'                           ' unless system ID is reset: %s != %s', self.cluster.initialize, data_sysid)            if self.has_lock():                self.release_leader_key_voluntarily()                return 'released leader key voluntarily due to the system ID mismatch'        else:            logger.fatal('system ID mismatch, node %s belongs to a different cluster: %s != %s',                         self.state_handler.name, self.cluster.initialize, data_sysid)            sys.exit(1)elif self.cluster.is_unlocked() and not self.is_paused():    # "bootstrap", but data directory is not empty    if not self.state_handler.cb_called and self.state_handler.is_running() \            and not self.state_handler.is_leader():        self._join_aborted = True        logger.error('No initialize key in DCS and PostgreSQL is running as replica, aborting start')        logger.error('Please first start Patroni on the node running as master')        sys.exit(1)    self.dcs.initialize(create_new=(self.cluster.initialize is None), sysid=data_sysid)

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无数据目录启动无数据目录启动，是指在执行初始化目录异常、恢复节点异常、WAL 拉齐异常等场景下，会触发的流程：1.设置角色，用于后续重新初始化集群；2.立即停止当前 PostgreSQL 进程；3.判断当前节点是否为主节点，主动释放主节点锁；4.执行启动操作。def bootstrap(self):if not self.cluster.is_unlocked(): # cluster already has leaderclone_member = self.cluster.get_clone_member(self.state_handler.name)member_role = 'leader' if clone_member == self.cluster.leader else 'replica'msg = "from {0} '{1}'".format(member_role, clone_member.name)ret = self._async_executor.try_run_async('bootstrap {0}'.format(msg), self.clone, args=(clone_member, msg))return ret or 'trying to bootstrap {0}'.format(msg)

no initialize key and node is allowed to be master and has 'bootstrap' section in a configuration file

elif self.cluster.initialize is None and not self.patroni.nofailover and 'bootstrap' in self.patroni.config:if self.dcs.initialize(create_new=True): # race for initializationself.state_handler.bootstrapping = Truewith self._async_response:self._async_response.reset()

      if self.is_standby_cluster():          ret = self._async_executor.try_run_async('bootstrap_standby_leader', self.bootstrap_standby_leader)          return ret or 'trying to bootstrap a new standby leader'      else:          ret = self._async_executor.try_run_async('bootstrap', self.state_handler.bootstrap.bootstrap,                                                   args=(self.patroni.config['bootstrap'],))          return ret or 'trying to bootstrap a new cluster'  else:      return 'failed to acquire initialize lock'

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else:create_replica_methods = self.get_standby_cluster_config().get('create_replica_methods', [])

if self.is_standby_cluster() else Noneif self.state_handler.can_create_replica_without_replication_connection(create_replica_methods):msg = 'bootstrap (without leader)'return self.async_executor.try_run_async(msg, self.clone) or 'trying to ' + msgreturn 'waiting for {0}leader to bootstrap'.format('standby' if self.is_standby_cluster() else '')

上述代码，表示启动的几种方式：1.当前集群已有 leader 节点，当前 PostgreSQL 将以从节点从主节点上同步数据启动；2.当前集群没有 leader 节点，当前 PostgreSQL 将以主节点启动，如果是备用集群，将以备用集群主节点启动；3.当前集群为备用集群且没有主节点，从节点通过方式，一般通过协议流方式从主集群上进行数据同步。有数据目录启动有数据目录启动，主要校验集群 ID 与 PostgreSQL 节点 sysid 的一致性，触发的主要流程：1.校验 PostgreSQL 节点 sysid 是否有效，如果无效，表示 PostgreSQL 出现了异常需要重启；2.校验校验集群 ID 与 PostgreSQL 节点 sysid 是否一致，不一致将无法加入集群，如果集群已暂停，将会释放 leader 锁占用；3.检验集群没有 leader 节点，当前节点将重新初始化集群，将 sysid 作为新的集群 ID 启动。

生成 PostgreSQL 集群 try:if self.cluster.is_unlocked():ret = self.process_unhealthy_cluster()else:msg = self.process_healthy_cluster()ret = self.evaluate_scheduled_restart() or msgfinally:# we might not have a valid PostgreSQL connection here if another thread# stops PostgreSQL, therefore, we only reload replication slots if no# asynchronous processes are running (should be always the case for the master)if not self._async_executor.busy and not self.state_handler.is_starting():create_slots = self.state_handler.slots_handler.sync_replication_slots(self.cluster,self.patroni.nofailover)if not self.state_handler.cb_called:if not self.state_handler.is_leader():self._rewind.trigger_check_diverged_lsn()self.state_handler.call_nowait(ACTION_ON_START)if create_slots and self.cluster.leader:err = self._async_executor.try_run_async('copy_logical_slots',self.state_handler.slots_handler.copy_logical_slots,args=(self.cluster.leader, create_slots))if not err:ret = 'Copying logical slots {0} from the primary'.format(create_slots)生成 PostgreSQL 集群，主要根据当前集群是否存在主节点，判断走健康的集群流程还是非健康的集群流程。非健康的集群流程 def process_unhealthy_cluster(self):"""Cluster has no leader key"""

if self.is_healthiest_node():if self.acquire_lock():failover = self.cluster.failoverif failover:if self.is_paused() and failover.leader and failover.candidate:logger.info('Updating failover key after acquiring leader lock...')self.dcs.manual_failover('', failover.candidate, failover.scheduled_at, failover.index)else:logger.info('Cleaning up failover key after acquiring leader lock...')self.dcs.manual_failover('', '')self.load_cluster_from_dcs()

      if self.is_standby_cluster():          # standby leader disappeared, and this is the healthiest          # replica, so it should become a new standby leader.          # This implies we need to start following a remote master          msg = 'promoted self to a standby leader by acquiring session lock'          return self.enforce_follow_remote_master(msg)      else:          return self.enforce_master_role(              'acquired session lock as a leader',              'promoted self to leader by acquiring session lock'          )  else:      return self.follow('demoted self after trying and failing to obtain lock',                         'following new leader after trying and failing to obtain lock')

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else:# when we are doing manual failover there is no guaranty that new leader is ahead of any other node# node tagged as nofailover can be ahead of the new leader either, but it is always excluded from electionsif bool(self.cluster.failover) or self.patroni.nofailover:self._rewind.trigger_check_diverged_lsn()time.sleep(2) # Give a time to somebody to take the leader lock

  if self.patroni.nofailover:      return self.follow('demoting self because I am not allowed to become master',                         'following a different leader because I am not allowed to promote')  return self.follow('demoting self because i am not the healthiest node',                     'following a different leader because i am not the healthiest node')

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非健康的集群流程，是确定 leader 节点的候选，首要条件必须找到一个健康的节点，如何判断健康的节点，主要有以下几个条件：1.PostgreSQL 集群状态非暂停；2.PostgreSQL 节点状态非启动中；3.PostgreSQL 节点允许故障转移；4.PostgreSQL 节点 WAL 与集群缓存中的（最后一次主节点同步的 lsn 值）的滞后量在允许的范围内。def is_healthiest_node(self):if time.time() - self._released_leader_key_timestamp < self.dcs.ttl:logger.info('backoff: skip leader race after pre_promote script failure and releasing the lock voluntarily')return False

    if self.is_paused() and not self.patroni.nofailover and \            self.cluster.failover and not self.cluster.failover.scheduled_at:        ret = self.manual_failover_process_no_leader()        if ret is not None:  # continue if we just deleted the stale failover key as a master            return ret
    if self.state_handler.is_starting():  # postgresql still starting up is unhealthy        return False
    if self.state_handler.is_leader():        # in pause leader is the healthiest only when no initialize or sysid matches with initialize!        return not self.is_paused() or not self.cluster.initialize\                or self.state_handler.sysid == self.cluster.initialize
    if self.is_paused():        return False
    if self.patroni.nofailover:  # nofailover tag makes node always unhealthy        return False
    if self.cluster.failover:        # When doing a switchover in synchronous mode only synchronous nodes and former leader are allowed to race        if self.is_synchronous_mode() and self.cluster.failover.leader and \                self.cluster.failover.candidate and not self.cluster.sync.matches(self.state_handler.name):            return False        return self.manual_failover_process_no_leader()
    if not self.watchdog.is_healthy:        logger.warning('Watchdog device is not usable')        return False
    # When in sync mode, only last known master and sync standby are allowed to promote automatically.    all_known_members = self.cluster.members + self.old_cluster.members    if self.is_synchronous_mode() and self.cluster.sync and self.cluster.sync.leader:        if not self.cluster.sync.matches(self.state_handler.name):            return False        # pick between synchronous candidates so we minimize unnecessary failovers/demotions        members = {m.name: m for m in all_known_members if self.cluster.sync.matches(m.name)}    else:        # run usual health check        members = {m.name: m for m in all_known_members}
    return self._is_healthiest_node(members.values())

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......

def _is_healthiest_node(self, members, check_replication_lag=True):"""This method tries to determine whether I am healthy enough to became a new leader candidate or not."""

    my_wal_position = self.state_handler.last_operation()    if check_replication_lag and self.is_lagging(my_wal_position):        logger.info('My wal position exceeds maximum replication lag')        return False  # Too far behind last reported wal position on master
    if not self.is_standby_cluster() and self.check_timeline():        cluster_timeline = self.cluster.timeline        my_timeline = self.state_handler.replica_cached_timeline(cluster_timeline)        if my_timeline < cluster_timeline:            logger.info('My timeline %s is behind last known cluster timeline %s', my_timeline, cluster_timeline)            return False
    # Prepare list of nodes to run check against    members = [m for m in members if m.name != self.state_handler.name and not m.nofailover and m.api_url]
    if members:        for st in self.fetch_nodes_statuses(members):            if st.failover_limitation() is None:                if not st.in_recovery:                    logger.warning('Master (%s) is still alive', st.member.name)                    return False                if my_wal_position < st.wal_position:                    logger.info('Wal position of %s is ahead of my wal position', st.member.name)                    # In synchronous mode the former leader might be still accessible and even be ahead of us.                    # We should not disqualify himself from the leader race in such a situation.                    if not self.is_synchronous_mode() or st.member.name != self.cluster.sync.leader:                        return False                    logger.info('Ignoring the former leader being ahead of us')    return True

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当前节点为健康节点，因当前集群没有主节点，需要执行 leader 锁抢占。如果当前节点抢占 leader 锁失败，将作为从节点加入到集群中。当前节点为异常节点，则会一直等待 PostgreSQL 节点正常后，参与集群的选举行为。健康的集群流程 def process_healthy_cluster(self):if self.has_lock():if self.is_paused() and not self.state_handler.is_leader():if self.cluster.failover and self.cluster.failover.candidate == self.state_handler.name:return 'waiting to become master after promote...'

      if not self.is_standby_cluster():          self._delete_leader()          return 'removed leader lock because postgres is not running as master'
  if self.update_lock(True):      msg = self.process_manual_failover_from_leader()      if msg is not None:          return msg
      # check if the node is ready to be used by pg_rewind      self._rewind.ensure_checkpoint_after_promote(self.wakeup)
      if self.is_standby_cluster():          # in case of standby cluster we don't really need to          # enforce anything, since the leader is not a master.          # So just remind the role.          msg = 'no action. I am ({0}), the standby leader with the lock'.format(self.state_handler.name) \                if self.state_handler.role == 'standby_leader' else \                'promoted self to a standby leader because i had the session lock'          return self.enforce_follow_remote_master(msg)      else:          return self.enforce_master_role(              'no action. I am ({0}), the leader with the lock'.format(self.state_handler.name),              'promoted self to leader because I had the session lock'          )  else:      # Either there is no connection to DCS or someone else acquired the lock      logger.error('failed to update leader lock')      if self.state_handler.is_leader():          if self.is_paused():              return 'continue to run as master after failing to update leader lock in DCS'          self.demote('immediate-nolock')          return 'demoted self because failed to update leader lock in DCS'      else:          return 'not promoting because failed to update leader lock in DCS'

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else:logger.debug('does not have lock')lock_owner = self.cluster.leader and self.cluster.leader.nameif self.is_standby_cluster():return self.follow('cannot be a real primary in a standby cluster','no action. I am ({0}), a secondary, and following a standby leader ({1})'.format(self.state_handler.name, lock_owner), refresh=False)return self.follow('demoting self because I do not have the lock and I was a leader','no action. I am ({0}), a secondary, and following a leader ({1})'.format(self.state_handler.name, lock_owner), refresh=False)

@健康的集群流程，是指当前的集群存在 leader 节点，对该流程的处理，主要有 2 个方向：1.检测当前节点为主节点，进行更新 leader 锁操作，保持主节点心跳，避免从节点竞争锁，如果更新锁失败，将立即释放锁，让其他从节点抢占；2.检测当前节点非主节点，作为从节点加入集群。

总结

综上所述，Patroni 是一个用于管理 PostgreSQL 数据库集群的高可用性（HA）管理工具，旨在确保数据库系统的连续可用性，以应对节点故障和维护操作等挑战。Patroni 提供了一系列关键功能和特点，使得它成为强大的高可用性解决方案。总之，在很多场景中，Patroni 能够保持 PostgreSQL 集群友好的运行，保证在集群异常的情况下，通过自动故障转移、数据同步和备份策略等功能，确保数据库集群的稳定性和可用性，使得应用程序能够持续访问数据，即使在节点故障或维护时也不会中断服务。

参考资源

Patroni 配置参数https://patroni.readthedocs.io/en/latest/patroni_configuration.htmlPatroni基于 2.1.5 分支源码https://github.com/zalando/patroni/tree/v2.1.5

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