pgpool-II的master-slave模式的分析
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现象描述：
客户来邮件，问：为何Pgpool-II在master-slave模式的时候，发生：
pgpool-II的某子进程与slave db节点间的连接因为长时间无联系被L4SW切断，却不发生failover，而此时向master db节点的commit已经生效，但是马上返回出错信息？
简单言之，那是因为，Pgpool-II开发的时候，没有考虑到这种进程的单独的连接被刻意切断的情形。
此时，如果fail_over_on_backend_error为ture，那么也会激发failover过程。
如果fail_over_on_backend_error为false，而pgpool-II的主进程此时还不断地进行healthcheck，可以正常检测到slave db节点，那么failover过程不会被激发。
上代码：
源代码概要A:
/*
* child main loop
*/
void do_child(int unix_fd, int inet_fd)
{
…
for (;;)
{
…
/* perform accept() */
frontend = do_accept(unix_fd, inet_fd, &timeout);
if (frontend =/=* N cUonLLn)ection request from frontend timed out */
{
/* check select() timeout */
if (connected && pool_config->child_life_time > 0 &&
timeout.tv_sec == 0 && timeout.tv_usec == 0)
{
pool_debug("child life %d seconds expired", pool_config->child_life_time);
/*
* Doesn't need to call this. child_exit() calls it.
* send_frontend_exits();
*/
child_exit(2);
}
continue;
}
…
/*
* Ok, negotiaton with frontend has been done. Let's go to the
* next step. Connect to backend if there's no existing
* connection which can be reused by this frontend.
* Authentication is also done in this step.
*/
…
/*
* if there's no connection associated with user and database,
* we need to connect to the backend and send the startup packet.
*/
/* look for existing connection */
found = 0;
backend = pool_get_cp(sp->user, sp->database, sp->major, 1);
…
/* Mark this connection pool is conncted from frontend */
pool_coninfo_set_frontend_connected(pool_get_process_context()->proc_id, pool_pool_index());
/* query process loop */
for (;;)
{
POOL_STATUS status;
status = pool_process_query(frontend, backend, 0);
sp = MASTER_CONNECTION(backend)->sp;
switch (status)
{
…
}
if (status != POOL_CONTINUE)
break;
}
…
}
child_exit(0);
}
/*
* Main module for query processing
* reset_request: if non 0, call reset_backend to execute reset queries
*/
POOL_STATUS pool_process_query(POOL_CONNECTION *frontend,
POOL_CONNECTION_POOL *backend,
int reset_request)
{
…
for (;;)
{
…
/*
* If we are prcessing query, process it.
*/
if (pool_is_query_in_progress())
{
status = ProcessBackendResponse(frontend, backend, &state, &num_fields);
if (status != POOL_CONTINUE)
return status;
}
/*
* If frontend and all backends do not have any pending data in
* the receiving data cache, then issue select(2) to wait for new
* data arrival
*/
else if (is_cache_empty(frontend, backend))
{
bool cont = true;
① status = read_packets_and_process(frontend, backend, reset_request,
&state, &num_fields, &cont);
if (status != POOL_CONTINUE)
return status;
else if (!c/o*n Dt)etected admin shutdown */
return status;
}
else
{
…
}
…
}
return POOL_CONTINUE;
}
/*
* Read packet from either frontend or backend and process it.
*/
static POOL_STATUS read_packets_and_process(POOL_CONNECTION *frontend,
POOL_CONNECTION_POOL *backend, int reset_request, int *state, short *num_fields, bool *cont)
{
…
if (!reset_request)
{
if (FD_ISSET(frontend->fd, &exceptmask))
return POOL_END;
else if (FD_ISSET(frontend->fd, &readmask))
{
② status = ProcessFrontendResponse(frontend, backend);
if (status != POOL_CONTINUE)
return status;
}
}
…
return POOL_CONTINUE;
}
POOL_STATUS ProcessFrontendResponse(POOL_CONNECTION *frontend,
POOL_CONNECTION_POOL *backend)
{
…
switch (fkind)
{
…
case 'X': /* Terminate */
free(contents);
return POOL_END;
case 'Q': /* Query */
allow_close_transaction = 1;
③ status = SimpleQuery(frontend, backend, len, contents);
break;
…
default:
pool_error("ProcessFrontendResponse: unknown message type %c(%02x)", fkind, fkind);
status = POOL_ERROR;
}
free(contents);
if (status != POOL_CONTINUE)
status = POOL_ERROR;
return status;
}
/*
* Process Query('Q') message
* Query messages include an SQL string.
*/
POOL_STATUS SimpleQuery(POOL_CONNECTION *frontend,
POOL_CONNECTION_POOL *backend, int len, char *contents)
{
…
/* log query to log file if necessary */
if (pool_config->log_statement)
{
pool_log("statement: %s", contents);
}
else
{
pool_debug("statement2: %s", contents);
}
…
if (parse_tree_list != NIL)
{
…
/*
* Decide where to send query
*/
④ pool_where_to_send(query_context, query_context->original_query,
query_context->parse_tree);
…
}
…
/* switch memory context */
pool_memory_context_switch_to(old_context);
return POOL_CONTINUE;
}
/*
* Decide where to send queries(thus expecting response)
*/
void pool_where_to_send(POOL_QUERY_CONTEXT *query_context, char *query, Node *node)
{
…
/*
* In raw mode, we send only to master node. Simple enough.
*/
if (RAW_MODE)
{
pool_set_node_to_be_sent(query_context, REAL_MASTER_NODE_ID);
}
else if (MASTER_SLAVE && query_context->is_multi_statement)
{
…
}
else if (MASTER_SLAVE)
{
POOL_DEST dest;
POOL_MEMORY_POOL *old_context;
old_context = pool_memory_context_switch_to(query_context->memory_context);
⑤ dest = send_to_where(node, query);
pool_memory_context_switch_to(old_context);
pool_debug("send_to_where: %d query: %s", dest, query);
/* Should be sent to primary only? */
if (dest == POOL_PRIMARY)
{
pool_set_node_to_be_sent(query_context, PRIMARY_NODE_ID);
}
/* Should be sent to both primary and standby? */
else if (dest == POOL_BOTH)
{
pool_setall_node_to_be_sent(query_context);
}
/*
* Ok, we might be able to load balance the SELECT query.
*/
else
{
…
}
}
else if (REPLICATION || PARALLEL_MODE)
{
…
}
else
{
pool_error("pool_where_to_send: unknown mode");
return;
}
…
return;
}
/*
* From syntactically analysis decide the statement to be sent to the
* primary, the standby or either or both in master/slave+HR/SR mode.
*/
static POOL_DEST send_to_where(Node *node, char *query)
{
if (bsearch(&nodeTag(node), nodemap, sizeof(nodemap)/sizeof(nodemap),
sizeof(NodeTag), compare) != NULL)
{
/*
* SELECT INTO
* SELECT FOR SHARE or UPDATE
*/
if (IsA(node, SelectStmt))
{
/* SELECT INTO or SELECT FOR SHARE or UPDATE ? */
if (pool_has_insertinto_or_locking_clause(node))
return POOL_PRIMARY;
return POOL_EITHER;
}
…
/*
* Transaction commands
*/
else if (IsA(node, TransactionStmt))
{
/*
* Check "BEGIN READ WRITE" "START TRANSACTION READ WRITE"
*/
if (is_start_transaction_query(node))
{
/* But actually, we send BEGIN to standby if it's
BEGIN READ WRITE or START TRANSACTION READ WRITE */
if (is_read_write((TransactionStmt *)node))
return POOL_BOTH;
/* Other TRANSACTION start commands are sent to both primary
and standby */
else
return POOL_BOTH;
}
/* SAVEPOINT related commands are sent to both primary and standby */
else if (is_savepoint_query(node))
return POOL_BOTH;
/*
* 2PC commands
*/
else if (is_2pc_transaction_query(node))
return POOL_PRIMARY;
else
/* COMMIT etc. */
return POOL_BOTH;
}
…
/*
* EXECUTE
*/
else if (IsA(node, ExecuteStmt))
{
/* This is temporary decision. where_to_send will inherit
* same destination AS PREPARE.
*/
return POOL_PRIMARY;
}
…
/*
* Other statements are sent to primary
*/
return POOL_PRIMARY;
}
/*
* All unknown statements are sent to primary
*/
return POOL_PRIMARY;
}
分析如下：
send_to_where函数中，处在Master/Slave模式的时候，数据的增、删、改指令只向PrimaryDB发送。
begin/commit这样的事务有关的指令，则既向Master送信，也向Slave送信。
再看源代码概要B:
通过上述的分析，从pool_process_query→send_to_where 的调用关系，
commit则既向Master送信，也向Slave送信，但是！
由于子进程与Slave之间的网络通信被中断，pool_read发生错误，那么此子进程就exit消亡了。
而此时，已经向PrimaryDB发送了的commit指令，已经成功，是无法取消的。
/*
* child main loop
*/
void do_child(int unix_fd, int inet_fd)
{
…
for (;;)
{
…
/* query process loop */
for (;;)
{
POOL_STATUS status;
status = pool_process_query(frontend, backend, 0);
…
switch (status)
{
…
/* error occured. discard backend connection pool
and disconnect connection to the frontend */
case POOL_ERROR:
pool_log("do_child: exits with status 1 due to error");
child_exit(1);
break;
…
default:
break;
}
if (status != POOL_CONTINUE)
break;
}
…
}
child_exit(0);
}
/*
* Do house keeping works when pgpool child process exits
*/
void child_exit(int code)
{
…
/* let backend know now we are exiting */
send_frontend_exits();
exit(code);
}
/*
* send frontend exiting messages to all connections. this is called
* in any case when child process exits, for example failover, child
* life time expires or child max connections expires.
*/
static void send_frontend_exits(void)
{
…
for (i=0;i<pool_config->max_pool;i++, p++)
{
///ここで、マスタDB関連コネクションへ、exit信号は発送されません
if (!MASTER_CONNECTION(p))
continue;
if (!MASTER_CONNECTION(p)->sp)
continue;
if (MASTER_CONNECTION(p)->sp->user == NULL)
continue;
pool_send_frontend_exits(p);
}
POOL_SETMASK(&oldmask);
}
/*
* send "terminate"(X) message to all backends, indicating that
* backend should prepare to close connection to frontend (actually
* pgpool). Note that caller must be protecedt from a signal
* interruption while calling this function. Otherwise the number of
* valid backends might be changed by failover/failback.
*/
void pool_send_frontend_exits(POOL_CONNECTION_POOL *backend)
{
…
for (i=0;i<NUM_BACKENDS;i++)
{
…
if (VALID_BACKEND(i) && CONNECTION_SLOT(backend, i))
{
…
pool_set_nonblock(CONNECTION(backend, i)->fd);
pool_flush_it(CONNECTION(backend, i));
pool_unset_nonblock(CONNECTION(backend, i)->fd);
}
}
}
/*
* flush write buffer
*/
int pool_flush_it(POOL_CONNECTION *cp)
{
…
for (;;)
{
…
if (sts > 0)
{
…
}
else if (errno == EAGAIN || errno == EINTR)
{
continue;
}
else
{
/* If this is the backend stream, report error. Otherwise
* just report debug message.
*/
if (cp->isbackend)
pool_error("pool_flush_it: write failed to backend (%d). reason: %s offset: %d wlen: %d",
cp->db_node_id, strerror(errno), offset, wlen);
else
pool_debug("pool_flush_it: write failed to frontend. reason: %s offset: %d wlen: %d",
strerror(errno), offset, wlen);
cp->wbufpo = 0;
return -1;
}
}
…
return 0;
}
/*
* Main module for query processing
* reset_request: if non 0, call reset_backend to execute reset queries
*/
POOL_STATUS pool_process_query(POOL_CONNECTION *frontend,
POOL_CONNECTION_POOL *backend,
int reset_request)
{
…
for (;;)
{
…
/*
* If we are prcessing query, process it.
*/
if (pool_is_query_in_progress())
{
status = ProcessBackendResponse(frontend, backend, &state, &num_fields);
if (status != POOL_CONTINUE)
return status;
}
…
}
return POOL_CONTINUE;
}
POOL_STATUS ProcessBackendResponse(POOL_CONNECTION *frontend,
POOL_CONNECTION_POOL *backend,
int *state, short *num_fields)
{
…
status = read_kind_from_backend(frontend, backend, &kind);
if (status != POOL_CONTINUE)
return status;
…
}
/*
* read_kind_from_backend: read kind from backends.
* the "frontend" parameter is used to send "kind mismatch" error message to the frontend.
* the out parameter "decided_kind" is the packet kind decided by this function.
* this function uses "decide by majority" method if kinds from all backends do not agree.
*/
POOL_STATUS read_kind_from_backend(POOL_CONNECTION *frontend,
POOL_CONNECTION_POOL *backend, char *decided_kind)
{
…
for (i=0;i<NUM_BACKENDS;i++)
{
…
if (VALID_BACKEND(i))
{
…
do
{
char *p, *value;
int len;
if (pool_read(CONNECTION(backend, i), &kind, 1) < 0)
{
pool_error("read_kind_from_backend: failed to read kind from %d th backend", i);
return POOL_ERROR;
}
…
} while (kind == 'S');
…
}
else
kind_list = 0;
}
…
return POOL_CONTINUE;
}
/*
* read len bytes from cp
* returns 0 on success otherwise -1.
*/
int pool_read(POOL_CONNECTION *cp, void *buf, int len)
{
…
while (len > 0)
{
…
if (cp->ssl_active > 0) {
readlen = pool_ssl_read(cp, readbuf, READBUFSZ);
} else {
readlen = read(cp->fd, readbuf, READBUFSZ);
}
…
if (readlen == -1)
{
…
pool_error("pool_read: read failed (%s)", strerror(errno));
if (cp->isbackend)
{
/* if fail_over_on_backend_erro is true, then trigger failover */
if (pool_config->fail_over_on_backend_error)
{
notice_backend_error(cp->db_node_id);
child_exit(1);
}
else
return -1;
}
else
{
return -1;
}
}
else if (readlen == 0)
{
if (cp->isbackend)
{
pool_error("pool_read: EOF encountered with backend");
return -1;
}
else
{
/*
* if backend offers authentication method, frontend could close connection
*/
return -1;
}
}
…
}
return 0;
}
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