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 healthy if not cluster.is_unlocked() or not self.old_cluster: self.old_cluster = cluster self.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# 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@abc.abstractmethoddef _load_cluster(self): """Internally this method should build `Cluster` object which represents 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."""以Kubernetes作为DCS为例
def _load_cluster(self): stop_time = time.time() + self._retry.deadline self._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')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: spilo cluster-name: pg142-1013-postgresql name: pg142-1013-postgresql-config namespace: default resourceVersion: "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-0 optime: "67109192" renewTime: "2023-10-16T07:02:57.418940+00:00" transitions: "0" ttl: "30" creationTimestamp: "2023-10-13T02:26:07Z" labels: application: spilo cluster-name: pg142-1013-postgresql name: pg142-1013-postgresql-leader namespace: default resourceVersion: "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-1 sync_standby: pg142-1013-postgresql-0 creationTimestamp: "2023-10-16T06:54:39Z" labels: application: spilo cluster-name: pg142-1013-postgresql name: pg142-1013-postgresql-sync namespace: default resourceVersion: "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: spilo cluster-name: pg142-1013-postgresql managedFields: - apiVersion: v1 fieldsType: FieldsV1 fieldsV1: f:metadata: f:labels: .: {} f:application: {} f:cluster-name: {} manager: Patroni operation: Update time: "2023-10-16T07:36:56Z" name: pg142-1013-postgresql-failover namespace: default resourceVersion: "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 = True else: 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'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 |+-------------------------+----------------+--------------+---------+----+-----------+集群初始化检测
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。
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()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()# 检测启动中的PostgreSQL是否出现异常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 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'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 running if 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 = False if 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'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_rewind
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()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 nodeelse: # check if we are allowed to join data_sysid = self.state_handler.sysid if not self.sysid_valid(data_sysid): # data directory is not empty, but no valid sysid, cluster must be broken, suggest reinit return ("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)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 leader clone_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 initialization self.state_handler.bootstrapping = True with 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' else: create_replica_methods = self.get_standby_cluster_config().get('create_replica_methods', []) \ if self.is_standby_cluster() else None if 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 ' + msg return '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.failover if 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') 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 elections if 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')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())...... 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' else: logger.debug('does not have lock') lock_owner = self.cluster.leader and self.cluster.leader.name if 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.html
Patroni基于2.1.5分支源码https://github.com/zalando/patroni/tree/v2.1.5