StarRocks 中如何做到查询超时(QueryTimeout)
背景
本文基于 StarRocks 3.1.7
主要是分析以下两种超时设置的方式:
- SESSION 级别
SET query_timeout = 10;
SELECT sleep(20);
- SQL 级别
select /*+ SET_VAR(query_timeout=10) */ sleep(20);
通过本文的分析大致可以了解到在Starrocks的FE端是如何进行Command的交互以及数据流走向，其他的命令也是可以举一反三
分析
query_timeout 命令解析
和Spark以及hive等但是解析一样，StarRocks也是采用的Anltr4进行语法的解析，
对于StarRocks来说， 对应的语法解析文件为 StarRocks.g4文件，那么其set query_time在如下的位置
setStatement
: SET setVar (',' setVar)*
;
setVar
: (CHAR SET | CHARSET | CHARACTER SET) (identifierOrString | DEFAULT) #setNames
| NAMES (charset = identifierOrString | DEFAULT)
(COLLATE (collate = identifierOrString | DEFAULT))? #setNames
| PASSWORD '=' (string | PASSWORD '(' string ')') #setPassword
| PASSWORD FOR user '=' (string | PASSWORD '(' string ')') #setPassword
| userVariable '=' expression #setUserVar
| varType? identifier '=' setExprOrDefault #setSystemVar
| systemVariable '=' setExprOrDefault #setSystemVar
| varType? TRANSACTION transaction_characteristics #setTransaction
;
继而可以找到对应的语法解析部分为 AstBuilder.java 中
@Override
public ParseNode visitSetSystemVar(StarRocksParser.SetSystemVarContext context) {
NodePosition pos = createPos(context);
if (context.systemVariable() != null) {
VariableExpr variableDesc = (VariableExpr) visit(context.systemVariable());
Expr expr = (Expr) visit(context.setExprOrDefault());
return new SystemVariable(variableDesc.getSetType(), variableDesc.getName(), expr, pos);
} else {
Expr expr = (Expr) visit(context.setExprOrDefault());
String variable = ((Identifier) visit(context.identifier())).getValue();
if (context.varType() != null) {
return new SystemVariable(getVariableType(context.varType()), variable, expr, pos);
} else {
return new SystemVariable(SetType.SESSION, variable, expr, pos);
}
}
}
从以上所示，SET query_timeout = 10; 就会在语法层面解析为 new SystemVariable(SetType.SESSION, variable, expr, pos)
数据流向
以上只是说到了 SET query_timeout = 10 只会被解析为SystemVariable对应的java数据结构，但是一条SQL从客户端发送过来，是怎么一个数据流呢？
我们大概的捋一下：
StarRocksFE中新建QeService对象
||
\/
new NMysqlServer(port, scheduler, sslContext)
||
\/
new AcceptListener(connectScheduler, sslContext)
||
\/
AcceptListener.handleEvent
||
\/
context.startAcceptQuery(processor)
||
\/
NMysqlChannel.startAcceptQuery
||
\/
conn.getSourceChannel().setReadListener(new ReadListener(nConnectContext, connectProcessor))
||
\/
ReadListener.handleEvent
||
\/
connectProcessor.processOnce()
||
\/
connectProcessor.dispatch
||
\/
connectProcessor.handleQuery
||
\/
stmts = com.starrocks.sql.parser.SqlParser.parse(originStmt, ctx.getSessionVariable());
||
\/
StmtExecutor.execute()
||
\/
StatementPlanner.plan(parsedStmt, context)
||
\/
StmtExecutor.handleSetStmt()
||
\/
SetExecutor.execute // 会设置到变量的keyValue到`ConnectContext`的`SystemVariable`变量中，后续会或获取对应的值
query_timeout 怎么生效
还是定位到StarRocksFE.java中：
ExecuteEnv.setup();
该方法是整个执行环境的基础设置。其中会有ConnectScheduler的初始化：
public ConnectScheduler(int maxConnections) {
this.maxConnections = new AtomicInteger(maxConnections);
numberConnection = new AtomicInteger(0);
nextConnectionId = new AtomicInteger(0);
// Use a thread to check whether connection is timeout. Because
// 1. If use a scheduler, the task maybe a huge number when query is messy.
// Let timeout is 10m, and 5000 qps, then there are up to 3000000 tasks in scheduler.
// 2. Use a thread to poll maybe lose some accurate, but is enough to us.
ScheduledExecutorService checkTimer = ThreadPoolManager.newDaemonScheduledThreadPool(1,
"Connect-Scheduler-Check-Timer", true);
checkTimer.scheduleAtFixedRate(new TimeoutChecker(), 0, 1000L, TimeUnit.MILLISECONDS);
}
这里有个定时线程池去进行timeout的检查，间隔是一秒。具体的检查机制在TimeoutChecker类中：
private class TimeoutChecker extends TimerTask {
@Override
public void run() {
try {
long now = System.currentTimeMillis();
synchronized (ConnectScheduler.this) {
//Because unregisterConnection will be callback in NMysqlChannel's close,
//unregisterConnection will remove connectionMap (in the same thread)
//This will result in a concurrentModifyException.
//So here we copied the connectionIds to avoid removing iterator during operate iterator
ArrayList<Long> connectionIds = new ArrayList<>(connectionMap.keySet());
for (Long connectId : connectionIds) {
ConnectContext connectContext = connectionMap.get(connectId);
connectContext.checkTimeout(now);
}
}
} catch (Throwable e) {
//Catch Exception to avoid thread exit
LOG.warn("Timeout checker exception, Internal error : " + e.getMessage());
}
}
}
主要逻辑就是从connectionMap中获取对应的ConnectContext,从而调用ConnectContext.checkTimeout方法检查是否超时。
checkTimeout方法主要是通过sessionVariable.getQueryTimeoutS()获取设置的超时时间，如果超时，则调用StmtExecutor.cancel,继而调用Coordinator.cancel
所以现在就存在一个问题： 当前连接的ConnectContext什么时候被集成到 connectionMap中去的？
还是回到流程 AcceptListener.handleEvent 中去：
connectScheduler.submit(context);
...
if (connectScheduler.registerConnection(context)) {
MysqlProto.sendResponsePacket(context);
connection.setCloseListener(
streamConnection -> connectScheduler.unregisterConnection(context));
} else {
...
这里的submit 方法会生成context的conectionId.
registerConnection方法会把当前 ConnectionContext 的id和 ConnectionContext 组成KeyValue对并放置到connectionMap中
至此 SET query_timeout = 10 整体的数据流就结束了，待在同一个连接中进行select 操作的时候，就会根据执行的长短进行超时处理了。
注意：
对于 select /*+ SET_VAR(query_timeout=10) */ sleep(20); 这种情况的解析，是通过 HintCollector来解析的。
词法解析是在StarRocksLex.g4 中，
OPTIMIZER_HINT
: '/*+' .*? '*/' -> channel(2)
;
对于获取hint是通过HintCollector的extractHintToRight获取的：
private void extractHintToRight(ParserRuleContext ctx) {
Token semi = ctx.start;
int i = semi.getTokenIndex();
List<Token> hintTokens = tokenStream.getHiddenTokensToRight(i, HINT_CHANNEL);
if (hintTokens != null) {
contextWithTokenMap.computeIfAbsent(ctx, e -> new ArrayList<>()).addAll(hintTokens);
}
}
对应的解析在：SqlParser.parseWithStarRocksDialect 方法中:
HintCollector collector = new HintCollector((CommonTokenStream) parser.getTokenStream());
collector.collect(singleStatementContexts.get(idx));
AstBuilder astBuilder = new AstBuilder(sessionVariable.getSqlMode(), collector.getContextWithHintMap());
AstBuilder 中会存储 hint到 hintMap 变量中，而在 visitQuerySpecification方法中
selectList.setOptHints(extractVarHints(hintMap.get(context)));
从而在StmtExecutor.execute中会调用 optHints = selectRelation.getSelectList().getOptHints();获取对应的hint，
if (isQuery &&
((QueryStatement) parsedStmt).getQueryRelation() instanceof SelectRelation) {
SelectRelation selectRelation = (SelectRelation) ((QueryStatement) parsedStmt).getQueryRelation();
optHints = selectRelation.getSelectList().getOptHints();
}
if (optHints != null) {
LOG.error("optHints: parsedStmt:" + parsedStmt.getOrigStmt() +" "+ optHints.size());
});
SessionVariable sessionVariable = (SessionVariable) sessionVariableBackup.clone();
for (String key : optHints.keySet()) {
VariableMgr.setSystemVariable(sessionVariable,
new SystemVariable(key, new StringLiteral(optHints.get(key))), true);
}
context.setSessionVariable(sessionVariable);
这样 hint相关的变量就设置到ConnectContext的SessionVariable中了，后续的流程和之前的一致。