consistent

Documentation

Overview

Package consistent provides a consistent hashing function.

Consistent hashing is often used to distribute requests to a changing set of servers. For example, say you have some cache servers cacheA, cacheB, and cacheC. You want to decide which cache server to use to look up information on a user.

You could use a typical hash table and hash the user id to one of cacheA, cacheB, or cacheC. But with a typical hash table, if you add or remove a server, almost all keys will get remapped to different results, which basically could bring your service to a grinding halt while the caches get rebuilt.

With a consistent hash, adding or removing a server drastically reduces the number of keys that get remapped.

Read more about consistent hashing on wikipedia: http://en.wikipedia.org/wiki/Consistent_hashing

Index

• Variables
• type Consistent
  • func New(osdMap *pb.OsdMap, policy pb.DistributePolicy) *Consistent
  • func (c *Consistent) Add(osd *pb.Osd)
  • func (c *Consistent) Get(name string) (*pb.Osd, error)
  • func (c *Consistent) GetN(name string, n int) ([]*pb.Osd, error)
  • func (c *Consistent) GetTwo(name string) (*pb.Osd, *pb.Osd, error)
  • func (c *Consistent) Members() []string
  • func (c *Consistent) Remove(osd *pb.Osd)
  • func (c *Consistent) Set(osdMap *pb.OsdMap)

Variables

var ErrEmptyCircle = errors.New("empty circle")

ErrEmptyCircle is the error returned when trying to get an element when nothing has been added to hash.

Types

type Consistent

type Consistent struct {
	sync.RWMutex
	// contains filtered or unexported fields
}

Consistent holds the information about the members of the consistent hash circle.

func New

func New(osdMap *pb.OsdMap, policy pb.DistributePolicy) *Consistent

New creates a new Consistent object with a default setting of 10 replicas for weight 1.

To change the number of replicas, set NumberOfReplicas before adding entries.

func (*Consistent) Add

func (c *Consistent) Add(osd *pb.Osd)

Add inserts a osd in the consistent hash.

func (*Consistent) Get

func (c *Consistent) Get(name string) (*pb.Osd, error)

Get returns an element close to where name hashes to in the circle.

func (*Consistent) GetN

func (c *Consistent) GetN(name string, n int) ([]*pb.Osd, error)

GetN returns the N closest distinct elements to the name input in the circle.

func (*Consistent) GetTwo

func (c *Consistent) GetTwo(name string) (*pb.Osd, *pb.Osd, error)

GetTwo returns the two closest distinct elements to the name input in the circle.

func (*Consistent) Members

func (c *Consistent) Members() []string

func (*Consistent) Remove

func (c *Consistent) Remove(osd *pb.Osd)

Remove removes an element from the hash.

func (*Consistent) Set

func (c *Consistent) Set(osdMap *pb.OsdMap)

Set sets all the elements in the hash. If there are existing elements not present in elts, they will be removed.