亚马逊推出的Aurora数据库引擎，支持一份存储，一主多读的架构。这个架构和Oracle RAC类似，也是共享存储，但是只有一个实例可以执行写操作，其他实例只能执行读操作。相比传统的基于复制的一主多读，节约了存储和网络带宽的成本。
我们可以使用PostgreSQL的hot standby模式来模拟这种共享存储一主多读的架构，但是需要注意几点，hot standby也会对数据库有写的动作，例如recovery时，会修改控制文件，数据文件等等，这些操作是多余的。另外很多状态是存储在内存中的，所以内存状态也需要更新。
还有需要注意的是：
pg_xlogpg_logpg_clogpg_multixactpostgresql.confrecovery.confpostmaster.pid最终实现一主多备的架构，需要通过改PG内核来实现：
这些文件应该是每个实例对应一份。postgresql.conf, recovery.conf, postmaster.pid, pg_controlhot standby不执行实际的恢复操作，但是需要更新自己的内存状态，如当前的OID，XID等等，以及更新自己的pg_control。在多实例间，要实现主到备节点的OS脏页的同步，数据库shared buffer脏页的同步。模拟过程
不改任何代码，在同一主机下启多实例测试，会遇到一些问题。(后面有问题描述，以及如何修改代码来修复这些问题)
主实例配置文件：
# vi postgresql.conflisten_addresses='0.0.0.0'port=1921max_connections=100unix_socket_directories='.'ssl=onssl_ciphers='EXPORT40'shared_buffers=512MBhuge_pages=trymax_PRepared_transactions=0max_stack_depth=100kBdynamic_shared_memory_type=posixmax_files_per_process=500wal_level=logicalfsync=offsynchronous_commit=offwal_sync_method=open_datasyncfull_page_writes=offwal_log_hints=offwal_buffers=16MBwal_writer_delay=10mscheckpoint_segments=8archive_mode=offarchive_command='/bin/date'max_wal_senders=10max_replication_slots=10hot_standby=onwal_receiver_status_interval=1shot_standby_feedback=onenable_bitmapscan=onenable_hashagg=onenable_hashjoin=onenable_indexscan=onenable_material=onenable_mergejoin=onenable_nestloop=onenable_seqscan=onenable_sort=onenable_tidscan=onlog_destination='csvlog'logging_collector=onlog_directory='pg_log'log_truncate_on_rotation=onlog_rotation_size=10MBlog_checkpoints=onlog_connections=onlog_disconnections=onlog_duration=offlog_error_verbosity=verboselog_line_prefix='%ilog_statement='none'log_timezone='PRC'autovacuum=onlog_autovacuum_min_duration=0autovacuum_vacuum_scale_factor=0.0002autovacuum_analyze_scale_factor=0.0001datestyle='iso,timezone='PRC'lc_messages='C'lc_monetary='C'lc_numeric='C'lc_time='C'default_text_search_config='pg_catalog.english' # vi recovery.donerecovery_target_timeline='latest'standby_mode=onprimary_conninfo = 'host=127.0.0.1 port=1921 user=postgres keepalives_idle=60' # vi pg_hba.conflocal replication postgres trusthost replication postgres 127.0.0.1/32 trust启动主实例。
postgres@digoal-> pg_ctl start启动只读实例，必须先删除postmaster.pid，这点PostgreSQL新版本加了一个PATCH，如果这个文件被删除，会自动关闭数据库，所以我们需要注意，不要使用最新的PGSQL，或者把这个patch干掉先。
postgres@digoal-> cd $PGDATApostgres@digoal-> mv recovery.done recovery.confpostgres@digoal-> rm -f postmaster.pidpostgres@digoal-> pg_ctl start -o "-c log_directory=pg_log1922 -c port=1922"查看当前控制文件状态，只读实例改了控制文件，和前面描述一致。
postgres@digoal-> pg_controldata |grep stateDatabase cluster state: in archive recovery连到主实例，创建表，插入测试数据。
psql -p 1921postgres=# create table test1(id int);CREATE TABLEpostgres=# insert into test1 select generate_series(1,10);INSERT 0 10在只读实例查看插入的数据。
postgres@digoal-> psql -h 127.0.0.1 -p 1922postgres=# select * from test1; id---- 1 2 3 4 5 6 7 8 9 10(10 rows)主实例执行检查点后，控制文件状态会改回生产状态。
psql -p 1921postgres=# checkpoint;CHECKPOINTpostgres@digoal-> pg_controldata |grep stateDatabase cluster state: in production但是如果在只读实例执行完检查点，又会改回恢复状态。
postgres@digoal-> psql -h 127.0.0.1 -p 1922psql (9.4.4)postgres=# checkpoint;CHECKPOINTpostgres@digoal-> pg_controldata |grep stateDatabase cluster state: in archive recovery注意到，上面的例子有1个问题，用流复制的话，会从主节点通过网络拷贝XLOG记录，并覆盖同一份已经写过的XLOG记录的对应的OFFSET，这是一个问题，因为可能会造成主节点看到的数据不一致（比如一个数据块改了多次，只读实例在恢复时将它覆盖到老的版本了，在主实例上看到的就会变成老版本的BLOCK，后面再来改这个问题，禁止只读实例恢复数据）。
另一方面，我们知道PostgreSQL standby会从三个地方（流、pg_xlog、restore_command）读取XLOG进行恢复，所以在共享存储的环境中，我们完全没有必要用流复制的方式，直接从pg_xlog目录读取即可。修改recovery.conf参数，将以下注释
# primary_conninfo = 'host=127.0.0.1 port=1921 user=postgres keepalives_idle=60'重启只读实例。
pg_ctl stop -m fastpostgres@digoal-> pg_ctl start -o "-c log_directory=pg_log1922 -c port=1922"重新测试数据一致性。主实例：
postgres=# insert into test1 select generate_series(1,10);INSERT 0 10postgres=# insert into test1 select generate_series(1,10);INSERT 0 10postgres=# insert into test1 select generate_series(1,10);INSERT 0 10postgres=# insert into test1 select generate_series(1,10);INSERT 0 10只读实例：
postgres=# select count(*) from test1; count------- 60(1 row)问题分析和解决
截至目前，有几个问题未解决：
standby还是要执行recovery的操作，recovery产生的write操作会随着只读实例数量的增加而增加。另外recovery有一个好处，解决了脏页的问题，主实例shared buffer中的脏页不需要额外的同步给只读实例了。recovery还会带来一个严重的BUG，回放可能和当前主节点操作同一个data page；或者回放时将块回放到老的状态，而实际上主节点又更新了这个块，造成数据块的不一致。如果此时只读实例关闭，然后立即关闭主实例，数据库再起来时，这个数据块是不一致的；standby还是会改控制文件；在同一个$PGDATA下启动实例，首先要删除postmaster.pid；关闭实例时，已经被删除postmaster.pid的实例，只能通过找到postgres主进程的pid，然后发kill -s 15, 2或3的信号来关闭数据库；
static void set_mode(char *modeopt) { if (strcmp(modeopt, "s") == 0 || strcmp(modeopt, "smart") == 0) { shutdown_mode = SMART_MODE; sig = SIGTERM; } else if (strcmp(modeopt, "f") == 0 || strcmp(modeopt, "fast") == 0) { shutdown_mode = FAST_MODE; sig = SIGINT; } else if (strcmp(modeopt, "i") == 0 || strcmp(modeopt, "immediate") == 0) { shutdown_mode = IMMEDIATE_MODE; sig = SIGQUIT; } else { write_stderr(_("%s: unrecognized shutdown mode /"%s/"/n"), progname, modeopt); do_advice(); exit(1); } }当主节点删除rel page时，只读实例回放时，会报invalid xlog对应的rel page不存在的错误，这个也是只读实例需要回放日志带来的问题。非常容易重现这个问题，删除一个表即可。
2015-10-09 13:30:50.776 CST,,,2082,,561750ab.822,20,,2015-10-09 13:29:15 CST,1/0,0,WARNING,01000,"page 8 of relation base/151898/185251 does not exist",,,,,"xlog redo clean: rel 1663/151898/185251; blk 8 remxid 640632117",,,"report_invalid_page, xlogutils.c:67","" 2015-10-09 13:30:50.776 CST,,,2082,,561750ab.822,21,,2015-10-09 13:29:15 CST,1/0,0,PANIC,XX000,"WAL contains references to invalid pages",,,,,"xlog redo clean: rel 1663/151898/185251; blk 8 remxid 640632117",,,"log_invalid_page, xlogutils.c:91",""这个报错可以先注释这一段来绕过，从而可以演示下去。
src/backend/access/transam/xlogutils.c /* Log a reference to an invalid page */ static void log_invalid_page(RelFileNode node, ForkNumber forkno, BlockNumber blkno, bool present) { ////// /* * Once recovery has reached a consistent state, the invalid-page table * should be empty and remain so. If a reference to an invalid page is * found after consistency is reached, PANIC immediately. This might seem * aggressive, but it's better than letting the invalid reference linger * in the hash table until the end of recovery and PANIC there, which * might come only much later if this is a standby server. */ //if (reachedConsistency) //{ // report_invalid_page(WARNING, node, forkno, blkno, present); // elog(PANIC, "WAL contains references to invalid pages"); //}由于本例是在同一个操作系统中演示，所以没有遇到OS的dirty page cache的问题，如果是不同主机的环境，我们需要解决OS dirty page cache 的同步问题，或者消除dirty page cache，如使用direct IO。或者集群文件系统如gfs2。如果要产品化，至少需要解决以上问题。
先解决Aurora实例写数据文件、控制文件、检查点的问题。
增加一个启动参数，表示这个实例是否为Aurora实例（即只读实例）
# vi src/backend/utils/misc/guc.c /******** option records follow ********/ static struct config_bool ConfigureNamesBool[] = { { {"aurora", PGC_POSTMASTER, CONN_AUTH_SETTINGS, gettext_noop("Enables advertising the server via Bonjour."), NULL }, &aurora, false, NULL, NULL, NULL },新增变量
# vi src/include/postmaster/postmaster.h extern bool aurora;禁止Aurora实例更新控制文件
# vi src/backend/access/transam/xlog.c #include "postmaster/postmaster.h" bool aurora; void UpdateControlFile(void) { if (aurora) return;禁止Aurora实例启动bgwriter进程
# vi src/backend/postmaster/bgwriter.c #include "postmaster/postmaster.h" bool aurora; /* * Main entry point for bgwriter process * * This is invoked from AuxiliaryProcessMain, which has already created the * basic execution environment, but not enabled signals yet. */ void BackgroundWriterMain(void) { ////// pg_usleep(1000000L); /* * If an exception is encountered, processing resumes here. * * See notes in postgres.c about the design of this coding. */ if (!aurora && sigsetjmp(local_sigjmp_buf, 1) != 0) { ////// /* * Do one cycle of dirty-buffer writing. */ if (!aurora) { can_hibernate = BgBufferSync(); ////// } pg_usleep(1000000L); } }禁止Aurora实例启动checkpointer进程
# vi src/backend/postmaster/checkpointer.c #include "postmaster/postmaster.h" bool aurora; ////// /* * Main entry point for checkpointer process * * This is invoked from AuxiliaryProcessMain, which has already created the * basic execution environment, but not enabled signals yet. */ void CheckpointerMain(void) { ////// /* * Loop forever */ for (;;) { bool do_checkpoint = false; int flags = 0; pg_time_t now; int elapsed_secs; int cur_timeout; int rc; pg_usleep(100000L); /* Clear any already-pending wakeups */ if (!aurora) ResetLatch(&MyProc->procLatch); /* * Process any requests or signals received recently. */ if (!aurora) AbsorbFsyncRequests(); if (!aurora && got_SIGHUP) { got_SIGHUP = false; ProcessConfigFile(PGC_SIGHUP); /* * Checkpointer is the last process to shut down, so we ask it to * hold the keys for a range of other tasks required most of which * have nothing to do with checkpointing at all. * * For various reasons, some config values can change dynamically * so the primary copy of them is held in shared memory to make * sure all backends see the same value. We make Checkpointer * responsible for updating the shared memory copy if the * parameter setting changes because of SIGHUP. */ UpdateSharedMemoryConfig(); } if (!aurora && checkpoint_requested) { checkpoint_requested = false; do_checkpoint = true; BgWriterStats.m_requested_checkpoints++; } if (!aurora && shutdown_requested) { /* * From here on, elog(ERROR) should end with exit(1), not send * control back to the sigsetjmp block above */ ExitOnAnyError = true; /* Close down the database */ ShutdownXLOG(0, 0); /* Normal exit from the checkpointer is here */ proc_exit(0); /* done */ } /* * Force a checkpoint if too much time has elapsed since the last one. * Note that we count a timed checkpoint in stats only when this * occurs without an external request, but we set the CAUSE_TIME flag * bit even if there is also an external request. */ now = (pg_time_t) time(NULL); elapsed_secs = now - last_checkpoint_time; if (!aurora && elapsed_secs >= CheckPointTimeout) { if (!do_checkpoint) BgWriterStats.m_timed_checkpoints++; do_checkpoint = true; flags |= CHECKPOINT_CAUSE_TIME; } /* * Do a checkpoint if requested. */ if (!aurora && do_checkpoint) { bool ckpt_performed = false; bool do_restartpoint; /* use volatile pointer to prevent code rearrangement */ volatile CheckpointerShmemStruct *cps = CheckpointerShmem; /* * Check if we should perform a checkpoint or a restartpoint. As a * side-effect, RecoveryInProgress() initializes TimeLineID if * it's not set yet. */ do_restartpoint = RecoveryInProgress(); /* * Atomically fetch the request flags to figure out what kind of a * checkpoint we should perform, and increase the started-counter * to acknowledge that we've started a new checkpoint. */ SpinLockAcquire(&cps->ckpt_lck); flags |= cps->ckpt_flags; cps->ckpt_flags = 0; cps->ckpt_started++; SpinLockRelease(&cps->ckpt_lck); /* * The end-of-recovery checkpoint is a real checkpoint that's * performed while we're still in recovery. */ if (flags & CHECKPOINT_END_OF_RECOVERY) do_restartpoint = false; ////// ckpt_active = false; } /* Check for archive_timeout and switch xlog files if necessary. */ if (!aurora) CheckArchiveTimeout(); /* * Send off activity statistics to the stats collector. (The reason * why we re-use bgwriter-related code for this is that the bgwriter * and checkpointer used to be just one process. It's probably not * worth the trouble to split the stats support into two independent * stats message types.) */ if (!aurora) pgstat_send_bgwriter(); /* * Sleep until we are signaled or it's time for another checkpoint or * xlog file switch. */ now = (pg_time_t) time(NULL); elapsed_secs = now - last_checkpoint_time; if (elapsed_secs >= CheckPointTimeout) continue; /* no sleep for us ... */ cur_timeout = CheckPointTimeout - elapsed_secs; if (!aurora && XLogArchiveTimeout > 0 && !RecoveryInProgress()) { elapsed_secs = now - last_xlog_switch_time; if (elapsed_secs >= XLogArchiveTimeout) continue; /* no sleep for us ... */ cur_timeout = Min(cur_timeout, XLogArchiveTimeout - elapsed_secs); } if (!aurora) rc = WaitLatch(&MyProc->procLatch, WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH, cur_timeout * 1000L /* convert to ms */ ); /* * Emergency bailout if postmaster has died. This is to avoid the * necessity for manual cleanup of all postmaster children. */ if (rc & WL_POSTMASTER_DEATH) exit(1); } } ////// /* SIGINT: set flag to run a normal checkpoint right away */ static void ReqCheckpointHandler(SIGNAL_ARGS) { if (aurora) return; int save_errno = errno; checkpoint_requested = true; if (MyProc) SetLatch(&MyProc->procLatch); errno = save_errno; } ////// /* * AbsorbFsyncRequests * Retrieve queued fsync requests and pass them to local smgr. * * This is exported because it must be called during CreateCheckPoint; * we have to be sure we have accepted all pending requests just before * we start fsync'ing. Since CreateCheckPoint sometimes runs in * non-checkpointer processes, do nothing if not checkpointer. */ void AbsorbFsyncRequests(void) { CheckpointerRequest *requests = NULL; CheckpointerRequest *request; int n; if (!AmCheckpointerProcess() || aurora) return; //////禁止Aurora实例手工调用checkpoint命令
# vi src/backend/tcop/utility.c #include "postmaster/postmaster.h" bool aurora; ////// void standard_ProcessUtility(Node *parsetree, const char *queryString, ProcessUtilityContext context, ParamListInfo params, DestReceiver *dest, char *completionTag) { ////// case T_CheckPointStmt: if (!superuser() || aurora) ereport(ERROR, (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE), errmsg("must be superuser to do CHECKPOINT")));改完上面的代码，重新编译一下，现在接近一个DEMO了。现在Aurora实例不会更新控制文件，不会写数据文件，不会执行checkpoint，是我们想要的结果。启动只读实例时，加一个参数aurora=true，表示启动Aurora实例。
pg_ctl start -o "-c log_directory=pg_log1922 -c port=1922 -c aurora=true"不过要产品化，还有很多细节需要考虑，这只是一个DEMO。阿里云RDS的小伙伴们加油！
还有一种更保险的玩法，共享存储多读架构，需要存储两份数据。其中一份是主实例的存储，它自己玩自己的，其他实例不对它做任何操作；另一份是standby的，这部作为共享存储，给多个只读实例来使用。
参考
https://aws.amazon.com/cn/rds/aurora/src/backend/access/transam/xlog.c
/* * Open the WAL segment containing WAL position 'RecPtr'. * * The segment can be fetched via restore_command, or via walreceiver having * streamed the record, or it can already be present in pg_xlog. Checking * pg_xlog is mainly for crash recovery, but it will be polled in standby mode * too, in case someone copies a new segment directly to pg_xlog. That is not * documented or recommended, though. * * If 'fetching_ckpt' is true, we're fetching a checkpoint record, and should * prepare to read WAL starting from RedoStartLSN after this. * * 'RecPtr' might not point to the beginning of the record we're interested * in, it might also point to the page or segment header. In that case, * 'tliRecPtr' is the position of the WAL record we're interested in. It is * used to decide which timeline to stream the requested WAL from. * * If the record is not immediately available, the function returns false * if we're not in standby mode. In standby mode, waits for it to become * available. * * When the requested record becomes available, the function opens the file * containing it (if not open already), and returns true. When end of standby * mode is triggered by the user, and there is no more WAL available, returns * false. */ static bool WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess, bool fetching_ckpt, XLogRecPtr tliRecPtr) { ////// static pg_time_t last_fail_time = 0; pg_time_t now; /*------- * Standby mode is implemented by a state machine: * * 1. Read from either archive or pg_xlog (XLOG_FROM_ARCHIVE), or just * pg_xlog (XLOG_FROM_XLOG) * 2. Check trigger file * 3. Read from primary server via walreceiver (XLOG_FROM_STREAM) * 4. Rescan timelines * 5. Sleep 5 seconds, and loop back to 1. * * Failure to read from the current source advances the state machine to * the next state. * * 'currentSource' indicates the current state. There are no currentSource * values for "check trigger", "rescan timelines", and "sleep" states, * those actions are taken when reading from the previous source fails, as * part of advancing to the next state. *------- */
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