seek

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Published: Oct 17, 2019 License: Apache-2.0 Imports: 9 Imported by: 0

Documentation

Overview

Proxy-Seeking alogirthm

Premise: An unknown number of proxies exist behind a "fair" load-balancer. Proxies will share their peerset via gossip, but this gossip is asynchronous and may suffer from ordering/timing issues. Furthermore, rotations may cause a complete and permanent loss of contact with all known proxies.

Goals: Ensure that we have one agent managing a connection to each available proxy. Minimize unnecessary discovery attempts. Recover from bad state (e.g. due to full rotations) in a timely manner. Mitigate resource drain due to failing or unreachable proxies.

Each known proxy has an associated entry which stores its seek state (seeking | claimed | backoff).

When an agent discovers (connects to) a proxy, it attempts to acquire an exclusive claim to that proxy. If sucessful, the agent takes responsibility for the proxy, releasing its claim when the connection terminates (regardless of reason). If another agent has already claimed the proxy, the connection is dropped.

Unclaimed entries are subject to expiry. Expiration timers are refreshed by gossip messages.

If a claim is released within a very short interval after being acquired, termination is said to be premature. Premature termination triggers a backoff phase which pauses discovery attempts for the proxy. The length of the backoff phase is determined by an incrementing multiplier. If backoff is entered too often to allow the counter to reset, the backoff phase will grow beyond the expiry limit and the associated entry will be removed.

+---------+
|         |                  acquire
|  START  +------------------------------------------------+
|         |                                                |
+----+----+                                                v
     |                                               +-----+-----+
     |      refresh               release (ok)       |           |
     +-----+--------+   +----------------------------+  Claimed  |
           ^        |   |                            |           |
           |        v   v                            +--+-----+--+
           |    +---+---+---+                           ^     |
           |    |           |          acquire          |     |
           +----+  Seeking  +---------------------------+     |
                |           |                                 |
+--------+      +---+---+---+                                 |
|        |          |   ^        +-----------+                |
|  STOP  |          |   |  done  |           |  release (err) |
|        |          |   +--------+  Backoff  +<---------------+
+---+----+          |            |           |
    ^               |            +-----+-----+
    |               v   expire         |
    +---------------+------------------+

Index

Constants

This section is empty.

Variables

This section is empty.

Functions

This section is empty.

Types

type Config

type Config struct {
	// TickRate defines the maximum amount of time between expiry & seek checks.
	// Shorter tick rates reduce discovery delay.  Longer tick rates reduce resource
	// consumption (default: ~4s).
	TickRate time.Duration
	// EntryExpiry defines how long a seeker entry should be held without successfully
	// establishing a healthy connection.  This value should be reasonably long
	// (default: 3m).
	EntryExpiry time.Duration
	// BackoffInterval defines the basline backoff amount observed by seekers.  This value
	// should be reasonably short (default: 256ms)
	BackoffInterval time.Duration
	// BackoffThreshold defines the minimum amount of time that a connection is expected to last
	// if the conencted peer is generally healthy.  Connections which fail before BackoffThreshold
	// cause the seekstate to enter backoff (default: 30s)
	BackoffThreshold time.Duration
}

Config describes the various parameters related to a seek operation

func (*Config) Check

func (s *Config) Check() error

func (*Config) CheckAndSetDefaults

func (s *Config) CheckAndSetDefaults() error

type GroupHandle

type GroupHandle struct {
	// contains filtered or unexported fields
}

GroupHandle is a handle to an ongoing seek process. Each seek process manages a group of related proxies. This handle allows agents to claim exclusive "locks" for individual proxies and to broadcast gossip to the process.

func (*GroupHandle) Gossip

func (s *GroupHandle) Gossip() chan<- string

Gossip channel must be informed whenever a proxy's identity becomes known via gossip messages.

func (*GroupHandle) Status

func (s *GroupHandle) Status() <-chan Status

Status channel is regularly updated with the most recent status value. Consuming status values is optional.

func (*GroupHandle) WithProxy

func (s *GroupHandle) WithProxy(do func(), principals ...string) (did bool)

WithProxy is used to wrap the connection-handling logic of an agent, ensuring that it is run if and only if no other agent is already handling this proxy.

type Key

type Key struct {
	Cluster string
	Type    string
	Addr    utils.NetAddr
}

Key uniquely identifies a seek group

type Pool

type Pool struct {
	// contains filtered or unexported fields
}

Pool manages a collection of group-level seek operations.

func NewPool

func NewPool(ctx context.Context, conf Config) (*Pool, error)

NewPool configures a seek pool.

func (*Pool) Group

func (p *Pool) Group(key Key) GroupHandle

Group gets a handle to the seek manager for the specified group. If none exists, one will be started.

func (*Pool) Seek

func (p *Pool) Seek() <-chan Key

Seek channel yields stream of keys indicating which groups are seeking.

func (*Pool) Shutdown

func (p *Pool) Shutdown()

Shutdown stops all seek operations

func (*Pool) Stop

func (p *Pool) Stop(group Key, groups ...Key)

Stop stops one or more group-level seek operations

type Status

type Status struct {
	Seeking int
	Claimed int
	Backoff int
}

Status is a summary of the status of a collection of proxy seek states.

func (*Status) ShouldSeek

func (s *Status) ShouldSeek() bool

ShouldSeek checks if we should be seeking connections.

func (*Status) Sum

func (s *Status) Sum() int

Sum returns the sum of all known proxies.

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