Query请求的执行流程分析
我们以
httpd/handler.go中的serverQuery为入口来分析;在前面我们有专门讲解 httpd/handler 的一篇文章;
我们不会分析查询结果是如何通过tsm tree和倒排索引得到的，重点放在查询的上层流程上;
本章我们将主要精力放在
query.Executor的分析上。
Executor的定义和创建
定义：
type Executor struct {
// Used for executing a statement in the query.
// 具体的查询操作
StatementExecutor StatementExecutor // Used for tracking running queries.
// 每个query都会对应一个task, 由TaskManager统一管理
TaskManager *TaskManager // Logger to use for all logging.
// Defaults to discarding all log output.
Logger *zap.Logger // expvar-based stats.
stats *Statistics
}对应的New函数:
func NewExecutor() *Executor { return &Executor{
TaskManager: NewTaskManager(),
Logger: zap.NewNop(),
stats: &Statistics{},
}
}创建和初始化在
run/server.go中的NewServer函数， 其中包括TaskManager和StatementExecutor的初始化
s.QueryExecutor = query.NewExecutor()
s.QueryExecutor.StatementExecutor = &coordinator.StatementExecutor{
MetaClient: s.MetaClient,
TaskManager: s.QueryExecutor.TaskManager,
TSDBStore: s.TSDBStore,
ShardMapper: &coordinator.LocalShardMapper{
MetaClient: s.MetaClient,
TSDBStore: coordinator.LocalTSDBStore{Store: s.TSDBStore},
},
Monitor: s.Monitor,
PointsWriter: s.PointsWriter,
MaxSelectPointN: c.Coordinator.MaxSelectPointN,
MaxSelectSeriesN: c.Coordinator.MaxSelectSeriesN,
MaxSelectBucketsN: c.Coordinator.MaxSelectBucketsN,
}
s.QueryExecutor.TaskManager.QueryTimeout = time.Duration(c.Coordinator.QueryTimeout)
s.QueryExecutor.TaskManager.LogQueriesAfter = time.Duration(c.Coordinator.LogQueriesAfter)
s.QueryExecutor.TaskManager.MaxConcurrentQueries = c.Coordinator.MaxConcurrentQueriesTaskManager相关内容解析
Task分析
首先我们先来看
Task, 它被定义在queyr/executor.go, 每个Query请求都会对应一个Task,交由TaskManager统一管理
type Task struct {
query string //Query请求的string
database string //当前Query需要操作的db
status TaskStatus //task运行的状态： RunningTask或KilledTask
startTime time.Time // task开始时间
closing chan struct{} // task结束时，通过这个closing chan来通知
monitorCh chan error
err error
mu sync.Mutex
}monitor监控函数，用来也监控task来发生的事情，比如慢请求
func (q *Task) monitor(fn MonitorFunc) { if err := fn(q.closing); err != nil {
select { case <-q.closing: case q.monitorCh <- err:
}
}
}2.1 这个MonitorFunc是一个函数类型，定义为type MonitorFunc func(<-chan struct{}) error, 它用来检查当前task对应的query的健康情况，如果当前query被某些错误中断，它将返回err;
2.2 如果fn MonitorFunc返回了err, 则将此err写到q.monitorCh这个chan中;
close函数，结束掉一个task
q.mu.Lock() if q.status != KilledTask { // Set the status to killed to prevent closing the channel twice.
q.status = KilledTask //通过q.closing这个chan作通知
close(q.closing)
}
q.mu.Unlock()kill函数，task自杀
q.mu.Lock() if q.status == KilledTask {
q.mu.Unlock() return ErrAlreadyKilled
}
q.status = KilledTask
close(q.closing)
q.mu.Unlock() return nilExecutionContext分析
定义在
execution_context.go中, 跟踪当前query的执行状态
type ExecutionContext struct {
// 匿名字段 Context
context.Context // The statement ID of the executing query.
statementID int
// The query ID of the executing query.
QueryID uint64 // The query task information available to the StatementExecutor.
task *Task // Output channel where results and errors should be sent.
Results chan *Result // Options used to start this query.
//在 httpd.Handler.go中生成的ExecutionOptions
ExecutionOptions
mu sync.RWMutex
done chan struct{}
err error
}watch函数: 开一个新的goroutine, 等待task.closing chan的通知，Contex.Done完成的通知和ExecutionOptions.AbortCh的通知
func (ctx *ExecutionContext) watch() {
ctx.done = make(chan struct{}) if ctx.err != nil {
close(ctx.done) return
}
go func() {
defer close(ctx.done)
var taskCtx <-chan struct{} if ctx.task != nil {
taskCtx = ctx.task.closing
}
select { case <-taskCtx:
ctx.err = ctx.task.Error() if ctx.err == nil {
ctx.err = ErrQueryInterrupted
} case <-ctx.AbortCh:
ctx.err = ErrQueryAborted case <-ctx.Context.Done():
ctx.err = ctx.Context.Err()
}
}()
}TaskManager分析
定义在
query/taks_manager.go
type TaskManager struct {
// Query 执行的超时时长，超时请求的执行将被中断
QueryTimeout time.Duration // Log queries if they are slower than this time.
// If zero, slow queries will never be logged.
// 慢请求的阈值
LogQueriesAfter time.Duration // Maximum number of concurrent queries.
// 并发处理的query数量
MaxConcurrentQueries int
// Logger to use for all logging.
// Defaults to discarding all log output.
Logger *zap.Logger // Used for managing and tracking running queries.
// Task id和task组成的map
queries map[uint64]*Task
nextID uint64
mu sync.RWMutex
shutdown bool}TaskManager.AttachQuery: 将query封装成task交由TaskManager管理
func (t *TaskManager) AttachQuery(q *influxql.Query, opt ExecutionOptions, interrupt <-chan struct{}) (*ExecutionContext, func(), error) {
... // 超过设置的并发Query数量后，Attach失败
if t.MaxConcurrentQueries > 0 && len(t.queries) >= t.MaxConcurrentQueries { return nil, nil, ErrMaxConcurrentQueriesLimitExceeded(len(t.queries), t.MaxConcurrentQueries)
} // 生成 Task和 TaskId
qid := t.nextID
query := &Task{
query: q.String(),
database: opt.Database,
status: RunningTask,
startTime: time.Now(),
closing: make(chan struct{}),
monitorCh: make(chan error),
}
t.queries[qid] = query //新开一个goroutine, 等待query超时，http连接断后工等各种情况,后面详述
go t.waitForQuery(qid, query.closing, interrupt, query.monitorCh)
if t.LogQueriesAfter != 0 { // 遇到慢查询打log
go query.monitor(func(closing <-chan struct{}) error {
timer := time.NewTimer(t.LogQueriesAfter)
defer timer.Stop()
select { case <-timer.C:
t.Logger.Warn(fmt.Sprintf("Detected slow query: %s (qid: %d, database: %s, threshold: %s)",
query.query, qid, query.database, t.LogQueriesAfter)) case <-closing:
} return nil
})
}
t.nextID++ // 生成ExcutionContext
ctx := &ExecutionContext{
Context: context.Background(),
QueryID: qid,
task: query,
ExecutionOptions: opt,
}
// 开如watch这个query的执行状态
ctx.watch() return ctx, func() { t.DetachQuery(qid) }, nil}分析Query执行过程中可能遇到的几种情况
前提
其实还是从
results := h.QueryExecutor.ExecuteQuery(q, opts, closing)说起
AttachQuery失败
在执行Query前，先要将Query生成Task交由
TaskManager管理，AttachQuery失败有两种情况
// query/task_manager.go:AttachQuery
if t.shutdown { return nil, nil, ErrQueryEngineShutdown
} if t.MaxConcurrentQueries > 0 && len(t.queries) >= t.MaxConcurrentQueries { return nil, nil, ErrMaxConcurrentQueriesLimitExceeded(len(t.queries), t.MaxConcurrentQueries)
}这将反回相应的err, 如果处理这个err呢？
// query/executor.go:executeQueryfunc (e *Executor) executeQuery(query *influxql.Query, opt ExecutionOptions, closing <-chan struct{}, results chan *Result) {
...
ctx, detach, err := e.TaskManager.AttachQuery(query, opt, closing) if err != nil {
select { case results <- &Result{Err: err}: case <-opt.AbortCh:
} return
}
...
}这个err被封装到Result中写入到results这个chan中. 接下来呢？
results := h.QueryExecutor.ExecuteQuery(q, opts, closing)for r := range results {
...
}调用返回，results就是上面写入的chan（httpd/henalder.go:serveQuery(), 读取出包含err信息的Result返回给客户端
Query被正确执行并返回给客户端
AttachQuery成功后返回了ExecutionContext,并且将返回Query结果的Results chan赋值给ExcutionContext.Results备用;一个Query可能包含多个statement, 逐一执行
for ; i < len(query.Statements); i++ {
...
}Query语句过滤, 针对
system measurements（_fieldKeys，_measurements,_series,_tagkeys,_tags）的 select操作，是不被允许的， 将含err信息的Result写入Results
results <- &Result{ Err: fmt.Errorf("unable to use system source '%s': use %s instead", s.Name, command),
}改变query statement, 主要是针对
meta的show ...，改写成针对system measurement的select语句执行具体的Query:
err = e.StatementExecutor.ExecuteStatement(stmt, ctx)(coordinator/statement_executor.go),特别是针对select query, 调用func (e *StatementExecutor) executeSelectStatement(stmt *influxql.SelectStatement, ctx *query.ExecutionContext) error
关于StatementExecutor， 我们这里不详细分析，只需要知道它会将查询结果封装成query.Result里写入到上面提到的contex.Results chan中
Query执行过程中Http连接中断
在
httpd/henalder.go:serveQuery中
closing = make(chan struct{}) if notifier, ok := w.(http.CloseNotifier); ok { done := make(chan struct{})
defer close(done)
notify := notifier.CloseNotify()
go func() {
// Wait for either the request to finish
// or for the client to disconnect
select { case <-done: case <-notify:
close(closing)
}
}()
opts.AbortCh = done
} else {
defer close(closing)
}如果http 连接断掉，则会close(closing)，关掉这个chosing chan;
在
TaskManager.AttachQuery时，会TaskManager.waitForQuery:
select { case <-closing:
t.queryError(qid, ErrQueryInterrupted)
....
}
t.KillQuery(qid)上面的select会返回，KillQuery被调用，它又会调用Task.kill()
func (q *Task) kill() error {
...
q.status = KilledTask
close(q.closing)
...
}将q.closing这个chan关掉，让我们再次回到AttachQuery的最后是ExcutionContext.watch:
func (ctx *ExecutionContext) watch() {
ctx.done = make(chan struct{})
...
go func() {
defer close(ctx.done)
var taskCtx <-chan struct{} if ctx.task != nil {
taskCtx = ctx.task.closing
}
select { case <-taskCtx:
ctx.err = ctx.task.Error() if ctx.err == nil {
ctx.err = ErrQueryInterrupted
}
...
}
}()
}上面的select将被触发,将ctx.err = ErrQueryInterrupted并调用close(ctx.done)，关掉这个ctx.donw chan,这个chan很关键，让我们回到执行具体query的coordinator/statement_executor.go:executeSelectStatement:
func (e *StatementExecutor) executeSelectStatement(stmt *influxql.SelectStatement, ctx *query.ExecutionContext) error {
... for {
select {
... case <-ctx.Done(): return ctx.Err() default:
}
...
}
... if err := ctx.Send(result); err != nil { return err
}
}上面的两处都会返回err, executeSelectStatement调用结束，返回err -> ErrQueryInterrupted, 最终被封装在query.Result里写入到Results chan中;
Query执行超时
如果超时，
TaskManager::waitForQuery中下面的代码将被触发:
var timerCh <-chan time.Time if t.QueryTimeout != 0 {
timer := time.NewTimer(t.QueryTimeout)
timerCh = timer.C
defer timer.Stop()
}
select {
... case <-timerCh:
t.queryError(qid, ErrQueryTimeoutLimitExceeded)
...
}
t.KillQuery(qid)往下的流程就和http断开后的流程一样了，最后返回的err -> ErrQueryTimeoutLimitExceeded
作者：扫帚的影子
链接：https://www.jianshu.com/p/c07c27bb1280
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