import (
"net/http"
+ "sync"
+ "time"
+
+ "github.com/prometheus/client_golang/prometheus"
)
-// RequestCounter is an http.Handler that tracks the number of
-// requests in progress.
-type RequestCounter interface {
- http.Handler
+// RequestLimiter wraps http.Handler, limiting the number of
+// concurrent requests being handled by the wrapped Handler. Requests
+// that arrive when the handler is already at the specified
+// concurrency limit are queued and handled in the order indicated by
+// the Priority function.
+//
+// Caller must not modify any RequestLimiter fields after calling its
+// methods.
+type RequestLimiter struct {
+ Handler http.Handler
+
+ // Maximum number of requests being handled at once. Beyond
+ // this limit, requests will be queued.
+ MaxConcurrent int
+
+ // Maximum number of requests in the queue. Beyond this limit,
+ // the lowest priority requests will return 503.
+ MaxQueue int
+
+ // Priority determines queue ordering. Requests with higher
+ // priority are handled first. Requests with equal priority
+ // are handled FIFO. If Priority is nil, all requests are
+ // handled FIFO.
+ Priority func(req *http.Request, queued time.Time) int64
+
+ // "concurrent_requests", "max_concurrent_requests",
+ // "queued_requests", and "max_queued_requests" metrics are
+ // registered with Registry, if it is not nil.
+ Registry *prometheus.Registry
+
+ setupOnce sync.Once
+ mtx sync.Mutex
+ handling int
+ queue heap
+}
+
+type qent struct {
+ queued time.Time
+ priority int64
+ heappos int
+ ready chan bool // true = handle now; false = return 503 now
+}
- // Current() returns the number of requests in progress.
- Current() int
+type heap []*qent
- // Max() returns the maximum number of concurrent requests
- // that will be accepted.
- Max() int
+func (h heap) Swap(i, j int) {
+ h[i], h[j] = h[j], h[i]
+ h[i].heappos, h[j].heappos = i, j
}
-type limiterHandler struct {
- requests chan struct{}
- handler http.Handler
+func (h heap) Less(i, j int) bool {
+ pi, pj := h[i].priority, h[j].priority
+ return pi > pj || (pi == pj && h[i].queued.Before(h[j].queued))
}
-// NewRequestLimiter returns a RequestCounter that delegates up to
-// maxRequests at a time to the given handler, and responds 503 to all
-// incoming requests beyond that limit.
-func NewRequestLimiter(maxRequests int, handler http.Handler) RequestCounter {
- return &limiterHandler{
- requests: make(chan struct{}, maxRequests),
- handler: handler,
+func (h heap) Len() int {
+ return len(h)
+}
+
+// Move element i to a correct position in the heap. When the heap is
+// empty, fix(0) is a no-op (does not crash).
+func (h heap) fix(i int) {
+ // If the initial position is a leaf (i.e., index is greater
+ // than the last node's parent index), we only need to move it
+ // up, not down.
+ uponly := i > (len(h)-2)/2
+ // Move the new entry toward the root until reaching a
+ // position where the parent already has higher priority.
+ for i > 0 {
+ parent := (i - 1) / 2
+ if h.Less(i, parent) {
+ h.Swap(i, parent)
+ i = parent
+ } else {
+ break
+ }
+ }
+ // Move i away from the root until reaching a position where
+ // both children already have lower priority.
+ for !uponly {
+ child := i*2 + 1
+ if child+1 < len(h) && h.Less(child+1, child) {
+ // Right child has higher priority than left
+ // child. Choose right child.
+ child = child + 1
+ }
+ if child < len(h) && h.Less(child, i) {
+ // Chosen child has higher priority than i.
+ // Swap and continue down.
+ h.Swap(i, child)
+ i = child
+ } else {
+ break
+ }
}
}
-func (h *limiterHandler) Current() int {
- return len(h.requests)
+func (h *heap) add(ent *qent) {
+ ent.heappos = len(*h)
+ *h = append(*h, ent)
+ h.fix(ent.heappos)
}
-func (h *limiterHandler) Max() int {
- return cap(h.requests)
+func (h *heap) removeMax() *qent {
+ ent := (*h)[0]
+ if len(*h) == 1 {
+ *h = (*h)[:0]
+ } else {
+ h.Swap(0, len(*h)-1)
+ *h = (*h)[:len(*h)-1]
+ h.fix(0)
+ }
+ ent.heappos = -1
+ return ent
}
-func (h *limiterHandler) ServeHTTP(resp http.ResponseWriter, req *http.Request) {
+func (h *heap) remove(i int) {
+ // Move the last leaf into i's place, then move it to a
+ // correct position.
+ h.Swap(i, len(*h)-1)
+ *h = (*h)[:len(*h)-1]
+ if i < len(*h) {
+ h.fix(i)
+ }
+}
+
+func (rl *RequestLimiter) setup() {
+ if rl.Registry != nil {
+ rl.Registry.MustRegister(prometheus.NewGaugeFunc(
+ prometheus.GaugeOpts{
+ Namespace: "arvados",
+ Name: "concurrent_requests",
+ Help: "Number of requests in progress",
+ },
+ func() float64 {
+ rl.mtx.Lock()
+ defer rl.mtx.Unlock()
+ return float64(rl.handling)
+ },
+ ))
+ rl.Registry.MustRegister(prometheus.NewGaugeFunc(
+ prometheus.GaugeOpts{
+ Namespace: "arvados",
+ Name: "max_concurrent_requests",
+ Help: "Maximum number of concurrent requests",
+ },
+ func() float64 { return float64(rl.MaxConcurrent) },
+ ))
+ rl.Registry.MustRegister(prometheus.NewGaugeFunc(
+ prometheus.GaugeOpts{
+ Namespace: "arvados",
+ Name: "queued_requests",
+ Help: "Number of requests in queue",
+ },
+ func() float64 {
+ rl.mtx.Lock()
+ defer rl.mtx.Unlock()
+ return float64(len(rl.queue))
+ },
+ ))
+ rl.Registry.MustRegister(prometheus.NewGaugeFunc(
+ prometheus.GaugeOpts{
+ Namespace: "arvados",
+ Name: "max_queued_requests",
+ Help: "Maximum number of queued requests",
+ },
+ func() float64 { return float64(rl.MaxQueue) },
+ ))
+ }
+}
+
+// caller must have lock
+func (rl *RequestLimiter) runqueue() {
+ // Handle entries from the queue as capacity permits
+ for len(rl.queue) > 0 && (rl.MaxConcurrent == 0 || rl.handling < rl.MaxConcurrent) {
+ rl.handling++
+ ent := rl.queue.removeMax()
+ ent.heappos = -1
+ ent.ready <- true
+ }
+}
+
+// If the queue is too full, fail and remove the lowest-priority
+// entry. Caller must have lock. Queue must not be empty.
+func (rl *RequestLimiter) trimqueue() {
+ if len(rl.queue) <= rl.MaxQueue {
+ return
+ }
+ min := 0
+ for i := range rl.queue {
+ if i == 0 || rl.queue.Less(min, i) {
+ min = i
+ }
+ }
+ rl.queue[min].heappos = -1
+ rl.queue[min].ready <- false
+ rl.queue.remove(min)
+}
+
+func (rl *RequestLimiter) enqueue(req *http.Request) *qent {
+ rl.mtx.Lock()
+ defer rl.mtx.Unlock()
+ qtime := time.Now()
+ var priority int64
+ if rl.Priority != nil {
+ priority = rl.Priority(req, qtime)
+ }
+ ent := &qent{
+ queued: qtime,
+ priority: priority,
+ ready: make(chan bool, 1),
+ heappos: -1,
+ }
+ if rl.MaxConcurrent == 0 || rl.MaxConcurrent > rl.handling {
+ // fast path, skip the queue
+ rl.handling++
+ ent.ready <- true
+ return ent
+ }
+ rl.queue.add(ent)
+ rl.trimqueue()
+ return ent
+}
+
+func (rl *RequestLimiter) remove(ent *qent) {
+ rl.mtx.Lock()
+ defer rl.mtx.Unlock()
+ if ent.heappos >= 0 {
+ rl.queue.remove(ent.heappos)
+ ent.heappos = -1
+ ent.ready <- false
+ }
+}
+
+func (rl *RequestLimiter) ServeHTTP(resp http.ResponseWriter, req *http.Request) {
+ rl.setupOnce.Do(rl.setup)
+ ent := rl.enqueue(req)
+ var ok bool
select {
- case h.requests <- struct{}{}:
- default:
- // reached max requests
+ case <-req.Context().Done():
+ rl.remove(ent)
+ // we still need to wait for ent.ready, because
+ // sometimes runqueue() will have already decided to
+ // send true before our rl.remove() call, and in that
+ // case we'll need to decrement rl.handling below.
+ ok = <-ent.ready
+ case ok = <-ent.ready:
+ }
+ if !ok {
resp.WriteHeader(http.StatusServiceUnavailable)
return
}
- h.handler.ServeHTTP(resp, req)
- <-h.requests
+ defer func() {
+ rl.mtx.Lock()
+ defer rl.mtx.Unlock()
+ rl.handling--
+ // unblock the next waiting request
+ rl.runqueue()
+ }()
+ rl.Handler.ServeHTTP(resp, req)
}
projects / arvados.git / blobdiff