MySQL:MGR學(xué)習(xí)(2):Writeset(寫集合)的寫入過程
MySQL:MGR 學(xué)習(xí)(2):Write set(寫集合)的寫入過程
水平有限，有誤請諒解。
源碼版本5.7.22
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一、前文總結(jié)
前文 <<MySQL:MGR 學(xué)習(xí)(1):寫集合(Write set)>>中已經(jīng)說明了Write set的生成過程，但是Write set是需要封裝如下Transaction_context_log_event中進行廣播到其他節(jié)點進行認證的。本文就描述Write set的寫入和廣播的過程。
如前文所描述，整個事物的Write set在函數(shù)binlog_log_row中生成，對于5.7來講每一行的每個唯一鍵都會生成一個Write set(但是咨詢宋利兵老師得知8.0唯一鍵不會再記錄Write set了)，每個Write set實際上是一個8字節(jié)的uint64類型其通過hash函數(shù)生成，并且在Rpl_transaction_write_set_ctx存儲了一個vector數(shù)組和一個set集合來分別存儲，如果修改的行比較多那么可能需要一個更多內(nèi)存來存儲這些hash值，雖然8字節(jié)比較小，但是如果是大事物上千萬的表在一個事物里面做修改那么內(nèi)存可能消耗會上百兆。如下圖是事物執(zhí)行期間(commit之前)最終形成的Write set內(nèi)存空間示意圖。
image.png
二、Transaction_context_log_event生成的時機
在事物執(zhí)行期間會生成map event/query event/dml event等并且會源源不斷的寫入到binlog cache中，同時會將Write set 不斷的寫入到Rpl_transaction_write_set_ctx保存在內(nèi)存中，這些邏輯都在binlog_log_row中。但是Transaction_context_log_event的生成卻是在commit的時候，具體的位置是在MYSQL_BIN_LOG::prepare之后但是在MYSQL_BIN_LOG::ordered_commit之前，顯而易見這個時候的binlog event還在bing cache中，還沒有寫入binlog file中。所以MGR的事物全局認證的動作是發(fā)生在binlog event落地之前。下面是這個棧幀：
#0 group_replication_trans_before_commit (param=0x7ffff0e7b8d0) at /root/softm/percona-server-5.7.22-22/rapid/plugin/group_replication/src/observer_trans.cc:511#1 0x00000000014e4814 in Trans_delegate::before_commit (this=0x2e44800, thd=0x7fffd8000df0, all=false, trx_cache_log=0x7fffd8907a10, stmt_cache_log=0x7fffd8907858, cache_log_max_size=18446744073709547520) at /root/softm/percona-server-5.7.22-22/sql/rpl_handler.cc:325#2 0x000000000188a386 in MYSQL_BIN_LOG::commit (this=0x2e7b440, thd=0x7fffd8000df0, all=false) at /root/softm/percona-server-5.7.22-22/sql/binlog.cc:8974#3 0x0000000000f80623 in ha_commit_trans (thd=0x7fffd8000df0, all=false, ignore_global_read_lock=false) at /root/softm/percona-server-5.7.22-22/sql/handler.cc:1830#4 0x00000000016ddab9 in trans_commit_stmt (thd=0x7fffd8000df0) at /root/softm/percona-server-5.7.22-22/sql/transaction.cc:458#5 0x00000000015d1a8d in mysql_execute_command (thd=0x7fffd8000df0, first_level=true) at /root/softm/percona-server-5.7.22-22/sql/sql_parse.cc:5293#6 0x00000000015d3182 in mysql_parse (thd=0x7fffd8000df0, parser_state=0x7ffff0e7e600) at /root/softm/percona-server-5.7.22-22/sql/sql_parse.cc:5901#7 0x00000000015c6d16 in dispatch_command (thd=0x7fffd8000df0, com_data=0x7ffff0e7ed70, command=COM_QUERY) at /root/softm/percona-server-5.7.22-22/sql/sql_parse.cc:1490#8 0x00000000015c5aa3 in do_command (thd=0x7fffd8000df0) at /root/softm/percona-server-5.7.22-22/sql/sql_parse.cc:1021#9 0x000000000170ebb0 in handle_connection (arg=0x3cd32d0) at /root/softm/percona-server-5.7.22-22/sql/conn_handler/connection_handler_per_thread.cc:312#10 0x0000000001946140 in pfs_spawn_thread (arg=0x3c71630) at /root/softm/percona-server-5.7.22-22/storage/perfschema/pfs.cc:2190#11 0x00007ffff7bc7851 in start_thread () from /lib64/libpthread.so.0#12 0x00007ffff651290d in clone () from /lib64/libc.so.6
三、MGR全局認證發(fā)送內(nèi)容的生成過程
下面是我通過對源碼淺顯的理解得出過程：
1、獲取當(dāng)前的binlog cache內(nèi)容記錄為cache_log，這些就是已經(jīng)在執(zhí)行階段生成map/query/dml event等。
2、生成一個新的IO_CACHE作為臨時存儲為cache，目的在于存儲。Transaction_context_log_event 和Gtid_log_event。
3、將cache_log類型轉(zhuǎn)換為READ類型同時初始化各種輔助類容如偏移量。
4、初始化Transaction_context_log_event 。
5、掃描Rpl_transaction_write_set_ctx中的write_set_unique 集合的內(nèi)容，并且將其存儲到Transaction_write_set 定義的內(nèi)存空間中write_set中，注意這里只是用到了集合沒用到數(shù)組。這里也就是進行Write set的一個拷貝而已其考到write_set臨時變量中。
6、將write_set內(nèi)容填充到Transaction_context_log_event中，整個過程還會做base64的轉(zhuǎn)換，最終填充到event的是base64格式的Write set類容。完成后析構(gòu)write_set來臨時變量
7、 將Transaction_context_log_event寫入到第二步定義的cache中。
8、生成Gtid_log_event，只是做一些初始化動作，Gtid并沒有生成。
9、將Gtid_log_event寫入到第二步定義的cache中。
10、通過cache+cache_log的總和來對比 group_replication_transaction_size_limit 設(shè)置的值，也就是判斷整個事物的binlog event是否操作了參數(shù)設(shè)置。
11、將cache類型轉(zhuǎn)換為READ類型同時初始化各種輔助類容如偏移量。
12、將cache和cache_log分別寫入到到transaction_msg中。
13、流控相關(guān)，沒仔細看，如果有機會學(xué)習(xí)流控機制在仔細學(xué)習(xí)。
14、gcs_module負責(zé)發(fā)送transaction_msg到各個節(jié)點
15、掛起等待事物認證的結(jié)果。
那么整個過程大概就是：
經(jīng)過hash的Write set (集合)->拷貝到write_set變量(類數(shù)組)->通過base64算法寫入到Transaction_context_log_event ->合并其他binlog event到transaction_msg->gcs_module廣播transaction_msg到其他節(jié)點->等待認證結(jié)果
四、相關(guān)源碼
1、group_replication_trans_before_commit 函數(shù)相關(guān)內(nèi)容
if (trx_cache_log_position > 0 && stmt_cache_log_position == 0) //如果存在事物cache
{
cache_log= param->trx_cache_log; //設(shè)置到IO_cache
cache_log_position= trx_cache_log_position;
} else if (trx_cache_log_position == 0 && stmt_cache_log_position > 0)//如果存在語句cache
{
cache_log= param->stmt_cache_log;
cache_log_position= stmt_cache_log_position;
is_dml= false;
may_have_sbr_stmts= true;
} else
{ /* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "We can only use one cache type at a "
"time on session %u", param->thread_id);
shared_plugin_stop_lock->release_read_lock();
DBUG_RETURN(1); /* purecov: end */
}
applier_module->get_pipeline_stats_member_collector()
->increment_transactions_local();
DBUG_ASSERT(cache_log->type == WRITE_CACHE);
DBUG_PRINT("cache_log", ("thread_id: %u, trx_cache_log_position: %llu,"
" stmt_cache_log_position: %llu",
param->thread_id, trx_cache_log_position,
stmt_cache_log_position)); /*
Open group replication cache.
Reuse the same cache on each session for improved performance.
*/
cache= observer_trans_get_io_cache(param->thread_id,
param->cache_log_max_size); //獲取一個新的IO_CACHE系統(tǒng)
if (cache == NULL) //錯誤處理
{ /* purecov: begin inspected */
error= pre_wait_error; goto err; /* purecov: end */
} // Reinit binlog cache to read.
if (reinit_cache(cache_log, READ_CACHE, 0)) ////將IO_CACHE類型進行轉(zhuǎn)換 并且位置還原
{ /* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "Failed to reinit binlog cache log for read "
"on session %u", param->thread_id);
error= pre_wait_error; goto err; /* purecov: end */
}
/*
After this, cache_log should be reinit to old saved value when we
are going out of the function scope.
*/
reinit_cache_log_required= true; // Create transaction context.
tcle= new Transaction_context_log_event(param->server_uuid, Rpl_transaction_write_set_ctx
is_dml,
param->thread_id,
is_gtid_specified); //初始化 Transaction_context_log_event
if (!tcle->is_valid())
{ /* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "Failed to create the context of the current "
"transaction on session %u", param->thread_id);
error= pre_wait_error; goto err; /* purecov: end */
} if (is_dml)
{
Transaction_write_set* write_set= get_transaction_write_set(param->thread_id);// 獲取前期得到write set 并且放回到一個臨時內(nèi)存空間 write_set中
/*
When GTID is specified we may have empty transactions, that is,
a transaction may have not write set at all because it didn't
change any data, it will just persist that GTID as applied.
*/
if ((write_set == NULL) && (!is_gtid_specified))
{
log_message(MY_ERROR_LEVEL, "Failed to extract the set of items written "
"during the execution of the current "
"transaction on session %u", param->thread_id);
error= pre_wait_error; goto err;
} if (write_set != NULL)
{ if (add_write_set(tcle, write_set))//將整個wirte_set內(nèi)容復(fù)制到event Transaction_context_log_event中 此時就進入了event了
{ /* purecov: begin inspected */
cleanup_transaction_write_set(write_set); //write set已經(jīng)完成了它的功能需要析構(gòu)
log_message(MY_ERROR_LEVEL, "Failed to gather the set of items written "
"during the execution of the current "
"transaction on session %u", param->thread_id);
error= pre_wait_error; goto err; /* purecov: end */
}
cleanup_transaction_write_set(write_set); //如果add_write_set函數(shù)調(diào)用出現(xiàn) 有問題 也需要析構(gòu)掉
DBUG_ASSERT(is_gtid_specified || (tcle->get_write_set()->size() > 0));
} else
{ /*
For empty transactions we should set the GTID may_have_sbr_stmts. See
comment at binlog_cache_data::may_have_sbr_stmts().
*/
may_have_sbr_stmts= true;
} Log_event::write
} // Write transaction context to group replication cache.
tcle->write(cache); //寫入到MGR CACHE 寫入 TCLE的header(virtual) body(virtual) footer
// Write Gtid log event to group replication cache.
gle= new Gtid_log_event(param->server_id, is_dml, 0, 1,
may_have_sbr_stmts,
gtid_specification);
gle->write(cache); //寫入GTID event到MGR CACHE 占位
transaction_size= cache_log_position + my_b_tell(cache); if (is_dml && transaction_size_limit &&
transaction_size > transaction_size_limit)
{
log_message(MY_ERROR_LEVEL, "Error on session %u. "
"Transaction of size %llu exceeds specified limit %lu. "
"To increase the limit please adjust group_replication_transaction_size_limit option.",
param->thread_id, transaction_size,
transaction_size_limit); //group_replication_transaction_size_limit 事物大小參數(shù)
error= pre_wait_error; goto err;
} // Reinit group replication cache to read.
if (reinit_cache(cache, READ_CACHE, 0))//將IO_CACHE類型進行轉(zhuǎn)換 并且位置還原
{ /* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "Error while re-initializing an internal "
"cache, for read operations, on session %u",
param->thread_id);
error= pre_wait_error; goto err; /* purecov: end */
} // Copy group replication cache to buffer.
if (transaction_msg.append_cache(cache)) //加入到transaction_msg
{ /* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "Error while appending data to an internal "
"cache on session %u", param->thread_id);
error= pre_wait_error; goto err; /* purecov: end */
} // Copy binlog cache content to buffer.
if (transaction_msg.append_cache(cache_log))//加入到transaction_msg
{ /* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "Error while writing binary log cache on "
"session %u", param->thread_id);
error= pre_wait_error; goto err; /* purecov: end */
}
DBUG_ASSERT(certification_latch != NULL); if (certification_latch->registerTicket(param->thread_id))
{ /* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "Unable to register for getting notifications "
"regarding the outcome of the transaction on "
"session %u", param->thread_id);
error= pre_wait_error; goto err; /* purecov: end */
}#ifndef DBUG_OFF
DBUG_EXECUTE_IF("test_basic_CRUD_operations_sql_service_interface",
{
DBUG_SET("-d,test_basic_CRUD_operations_sql_service_interface");
DBUG_ASSERT(!sql_command_check());
};);
DBUG_EXECUTE_IF("group_replication_before_message_broadcast",
{ const char act[]= "now wait_for waiting";
DBUG_ASSERT(!debug_sync_set_action(current_thd, STRING_WITH_LEN(act)));
});#endif
/*
Check if member needs to throttle its transactions to avoid
cause starvation on the group.
*/
applier_module->get_flow_control_module()->do_wait(); //流控相關(guān)
//Broadcast the Transaction Message
send_error= gcs_module->send_message(transaction_msg); //gcs廣播
if (send_error == GCS_MESSAGE_TOO_BIG)
{ /* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "Error broadcasting transaction to the group "
"on session %u. Message is too big.",
param->thread_id);
error= pre_wait_error; goto err; /* purecov: end */
} else if (send_error == GCS_NOK)
{ /* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "Error while broadcasting the transaction to "
"the group on session %u", param->thread_id);
error= pre_wait_error; goto err; /* purecov: end */
}
shared_plugin_stop_lock->release_read_lock();
DBUG_ASSERT(certification_latch != NULL); if (certification_latch->waitTicket(param->thread_id)) //等待認證結(jié)果
{ /* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "Error while waiting for conflict detection "
"procedure to finish on session %u",
param->thread_id);
error= post_wait_error; goto err; /* purecov: end */
}
2、add_write_set函數(shù)
int add_write_set(Transaction_context_log_event *tcle,
Transaction_write_set *set){
DBUG_ENTER("add_write_set"); int iterator= set->write_set_size; //將循環(huán)次數(shù)設(shè)置為 set的長度 也就是有多少個write sets
for (int i = 0; i < iterator; i++)
{
uchar buff[BUFFER_READ_PKE];
int8store(buff, set->write_set[i]); //逐字節(jié)復(fù)制到buffer中
uint64 const tmp_str_sz= base64_needed_encoded_length((uint64) BUFFER_READ_PKE); char *write_set_value= (char *) my_malloc(PSI_NOT_INSTRUMENTED, static_cast<size_t>(tmp_str_sz), MYF(MY_WME)); //13bytes (gdb) p tmp_str_sz $2 = 13
if (!write_set_value)//分配內(nèi)存錯誤
{
/* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "No memory to generate write identification hash");
DBUG_RETURN(1); /* purecov: end */
} if (base64_encode(buff, (size_t) BUFFER_READ_PKE, write_set_value)) //做base64算法
{ /* purecov: begin inspected */
log_message(MY_ERROR_LEVEL, "Base 64 encoding of the write identification hash failed");
DBUG_RETURN(1); /* purecov: end */
}
tcle->add_write_set(write_set_value); //最終將base64格式的write set寫入到event中
}
DBUG_RETURN(0);
}
3、get_transaction_write_set函數(shù)
Transaction_write_set* get_transaction_write_set(unsigned long m_thread_id){
DBUG_ENTER("get_transaction_write_set");
THD *thd= NULL;
Transaction_write_set *result_set= NULL; Find_thd_with_id find_thd_with_id(m_thread_id, false);
thd= Global_THD_manager::get_instance()->find_thd(&find_thd_with_id); if (thd)
{ std::set<uint64> *write_set= thd->get_transaction()
->get_transaction_write_set_ctx()->get_write_set(); //Rpl_transaction_write_set_ctx std::set<uint64> *get_write_set();
unsigned long write_set_size= write_set->size(); //返回集合大小
if (write_set_size == 0)
{
mysql_mutex_unlock(&thd->LOCK_thd_data);
DBUG_RETURN(NULL);
}
result_set= (Transaction_write_set*)my_malloc(key_memory_write_set_extraction, sizeof(Transaction_write_set),
MYF(0));//這里為其Transaction_write_set分配內(nèi)存空間
result_set->write_set_size= write_set_size; //獲取size
result_set->write_set=
(unsigned long long*)my_malloc(key_memory_write_set_extraction,
write_set_size * sizeof(unsigned long long),
MYF(0));//分配內(nèi)存
int result_set_index= 0; for (std::set<uint64>::iterator it= write_set->begin();//完成復(fù)制注意是從set中復(fù)制到簡單的內(nèi)存中
it != write_set->end();
++it)
{
uint64 temp= *it;
result_set->write_set[result_set_index++]=temp;
}
mysql_mutex_unlock(&thd->LOCK_thd_data);
}
DBUG_RETURN(result_set);
}
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