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，节约了存储的成本，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_xlog
pg_log
pg_clog
pg_multixact
postgresql.conf
recovery.conf
postmaster.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_control - hot 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.conf
listen_addresses='0.0.0.0'
port=1921
max_connections=100
unix_socket_directories='.'
ssl=on
ssl_ciphers='EXPORT40'
shared_buffers=512MB
huge_pages=try
max_prepared_transactions=0
max_stack_depth=100kB
dynamic_shared_memory_type=posix
max_files_per_process=500
wal_level=logical
fsync=off
synchronous_commit=off
wal_sync_method=open_datasync
full_page_writes=off
wal_log_hints=off
wal_buffers=16MB
wal_writer_delay=10ms
checkpoint_segments=8
archive_mode=off
archive_command='/bin/date'
max_wal_senders=10
max_replication_slots=10
hot_standby=on
wal_receiver_status_interval=1s
hot_standby_feedback=on
enable_bitmapscan=on
enable_hashagg=on
enable_hashjoin=on
enable_indexscan=on
enable_material=on
enable_mergejoin=on
enable_nestloop=on
enable_seqscan=on
enable_sort=on
enable_tidscan=on
log_destination='csvlog'
logging_collector=on
log_directory='pg_log'
log_truncate_on_rotation=on
log_rotation_size=10MB
log_checkpoints=on
log_connections=on
log_disconnections=on
log_duration=off
log_error_verbosity=verbose
log_line_prefix='%i
log_statement='none'
log_timezone='PRC'
autovacuum=on
log_autovacuum_min_duration=0
autovacuum_vacuum_scale_factor=0.0002
autovacuum_analyze_scale_factor=0.0001
datestyle='iso,
timezone='PRC'
lc_messages='C'
lc_monetary='C'
lc_numeric='C'
lc_time='C'
default_text_search_config='pg_catalog.english'
# vi recovery.done
recovery_target_timeline='latest'
standby_mode=on
primary_conninfo = 'host=127.0.0.1 port=1921 user=postgres keepalives_idle=60'
# vi pg_hba.conf
local replication postgres trust
host 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 $PGDATA
postgres@digoal-> mv recovery.done recovery.conf
postgres@digoal-> rm -f postmaster.pid
postgres@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 state
Database cluster state: in archive recovery连到主实例，创建表，插入测试数据。
psql -p 1921
postgres=# create table test1(id int);
CREATE TABLE
postgres=# 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 1922
postgres=# 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 1921
postgres=# checkpoint;
CHECKPOINT
postgres@digoal-> pg_controldata |grep state
Database 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 1922
psql (9.4.4)
postgres=# checkpoint;
CHECKPOINT
postgres@digoal-> pg_controldata |grep state
Database 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 fast
postgres@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 10
postgres=# insert into test1 select generate_series(1,10);
INSERT 0 10
postgres=# insert into test1 select generate_series(1,10);
INSERT 0 10
postgres=# insert into test1 select generate_series(1,10);
INSERT 0 10只读实例
postgres=# select count(*) from test1;
count
-------
60
(1 row)截至目前，有几个问题未解决，
1 . 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。
2 . standby还是会改控制文件。
3 . 在同一个$PGDATA下启动实例，首先要删除postmaster.pid。
4 . 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);
}
}5 . 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");
//}6 . 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。
1 . 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
},2 . 新增变量
# vi src/include/postmaster/postmaster.h
extern bool aurora;3 . 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;4 . 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);
}
}5 . 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;
//////6 . 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]
1 . https://aws.amazon.com/cn/rds/aurora/
2 . 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.
*-------
*/