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119 lines
4.3 KiB
Go
119 lines
4.3 KiB
Go
// Copyright 2013 Prometheus Team
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package retrieval
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import (
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"github.com/prometheus/prometheus/utility"
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"math"
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"time"
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)
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const (
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// The default increment for exponential backoff when querying a target.
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DEFAULT_BACKOFF_VALUE = 2
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// The base units for the exponential backoff.
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DEFAULT_BACKOFF_VALUE_UNIT = time.Second
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// The maximum allowed backoff time.
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MAXIMUM_BACKOFF_VALUE = 30 * time.Minute
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)
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// scheduler is an interface that various scheduling strategies must fulfill
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// in order to set the scheduling order for a target.
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//
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// Target takes advantage of this type by embedding an instance of scheduler
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// in each Target instance itself. The emitted scheduler.ScheduledFor() is
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// the basis for sorting the order of pending queries.
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//
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// This type is described as an interface to maximize testability.
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type scheduler interface {
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// ScheduledFor emits the earliest time at which the given object is allowed
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// to be run. This time may or not be a reflection of the earliest parameter
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// provided in Reschedule; that is up to the underlying strategy
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// implementations.
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ScheduledFor() time.Time
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// Instruct the scheduled item to re-schedule itself given new state data and
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// the earliest time at which the outside system thinks the operation should
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// be scheduled for.
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Reschedule(earliest time.Time, future TargetState)
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}
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// healthScheduler is an implementation of scheduler that uses health data
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// provided by the target field as well as unreachability counts to determine
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// when to next schedule an operation.
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//
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// The type is almost capable of being used with default initialization, except
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// that a target field must be provided for which the system compares current
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// health against future proposed values.
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type healthScheduler struct {
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scheduledFor time.Time
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target healthReporter
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time utility.Time
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unreachableCount int
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}
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func (s healthScheduler) ScheduledFor() time.Time {
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return s.scheduledFor
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}
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// Reschedule, like the protocol described in scheduler, uses the current and
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// proposed future health state to determine how and when a given subject is to
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// be scheduled.
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//
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// If a subject has been at given moment marked as unhealthy, an exponential
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// backoff scheme is applied to it. The reason for this backoff is to ensure
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// that known-healthy targets can consume valuable request queuing resources
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// first. Depending on the retrieval interval and number of consecutive
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// unhealthy markings, the query of these unhealthy individuals may come before
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// the healthy ones for a short time to help ensure expeditious retrieval.
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// The inflection point that drops these to the back of the queue is beneficial
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// to save resources in the long-run.
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//
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// If a subject is healthy, its next scheduling opportunity is set to
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// earliest, for this ensures fair querying of all remaining healthy targets and
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// removes bias in the ordering. In order for the anti-bias value to have any
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// value, the earliest opportunity should be set to a value that is constant
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// for a given batch of subjects who are to be scraped on a given interval.
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func (s *healthScheduler) Reschedule(e time.Time, f TargetState) {
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currentState := s.target.State()
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// XXX: Handle metrics surrounding health.
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switch currentState {
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case UNKNOWN, UNREACHABLE:
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switch f {
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case ALIVE:
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s.unreachableCount = 0
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break
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case UNREACHABLE:
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s.unreachableCount++
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break
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}
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case ALIVE:
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switch f {
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case UNREACHABLE:
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s.unreachableCount++
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}
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}
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if s.unreachableCount == 0 {
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s.scheduledFor = e
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} else {
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backoff := MAXIMUM_BACKOFF_VALUE
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exponential := time.Duration(math.Pow(DEFAULT_BACKOFF_VALUE, float64(s.unreachableCount))) * DEFAULT_BACKOFF_VALUE_UNIT
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if backoff > exponential {
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backoff = exponential
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}
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s.scheduledFor = s.time.Now().Add(backoff)
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}
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}
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