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Published: May 1, 2021 License: MIT Imports: 2 Imported by: 727


concurrent map Build Status

As explained here and here, the map type in Go doesn't support concurrent reads and writes. concurrent-map provides a high-performance solution to this by sharding the map with minimal time spent waiting for locks.

Prior to Go 1.9, there was no concurrent map implementation in the stdlib. In Go 1.9, sync.Map was introduced. The new sync.Map has a few key differences from this map. The stdlib sync.Map is designed for append-only scenarios. So if you want to use the map for something more like in-memory db, you might benefit from using our version. You can read more about it in the golang repo, for example here and here


Import the package:

import (

go get ""

The package is now imported under the "cmap" namespace.


	// Create a new map.
	m := cmap.New()

	// Sets item within map, sets "bar" under key "foo"
	m.Set("foo", "bar")

	// Retrieve item from map.
	if tmp, ok := m.Get("foo"); ok {
		bar := tmp.(string)

	// Removes item under key "foo"

For more examples have a look at concurrent_map_test.go.

Running tests:

go test ""

guidelines for contributing

Contributions are highly welcome. In order for a contribution to be merged, please follow these guidelines:

  • Open an issue and describe what you are after (fixing a bug, adding an enhancement, etc.).
  • According to the core team's feedback on the above mentioned issue, submit a pull request, describing the changes and linking to the issue.
  • New code must have test coverage.
  • If the code is about performance issues, you must include benchmarks in the process (either in the issue or in the PR).
  • In general, we would like to keep concurrent-map as simple as possible and as similar to the native map. Please keep this in mind when opening issues.



MIT (see LICENSE file)




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var SHARD_COUNT = 32


This section is empty.


type ConcurrentMap

type ConcurrentMap []*ConcurrentMapShared

A "thread" safe map of type string:Anything. To avoid lock bottlenecks this map is dived to several (SHARD_COUNT) map shards.

func New

func New() ConcurrentMap

Creates a new concurrent map.

func (ConcurrentMap) Clear

func (m ConcurrentMap) Clear()

Clear removes all items from map.

func (ConcurrentMap) Count

func (m ConcurrentMap) Count() int

Count returns the number of elements within the map.

func (ConcurrentMap) Get

func (m ConcurrentMap) Get(key string) (interface{}, bool)

Get retrieves an element from map under given key.

func (ConcurrentMap) GetShard

func (m ConcurrentMap) GetShard(key string) *ConcurrentMapShared

GetShard returns shard under given key

func (ConcurrentMap) Has

func (m ConcurrentMap) Has(key string) bool

Looks up an item under specified key

func (ConcurrentMap) IsEmpty

func (m ConcurrentMap) IsEmpty() bool

IsEmpty checks if map is empty.

func (ConcurrentMap) Items

func (m ConcurrentMap) Items() map[string]interface{}

Items returns all items as map[string]interface{}

func (ConcurrentMap) Iter deprecated

func (m ConcurrentMap) Iter() <-chan Tuple

Iter returns an iterator which could be used in a for range loop.

Deprecated: using IterBuffered() will get a better performence

func (ConcurrentMap) IterBuffered

func (m ConcurrentMap) IterBuffered() <-chan Tuple

IterBuffered returns a buffered iterator which could be used in a for range loop.

func (ConcurrentMap) IterCb

func (m ConcurrentMap) IterCb(fn IterCb)

Callback based iterator, cheapest way to read all elements in a map.

func (ConcurrentMap) Keys

func (m ConcurrentMap) Keys() []string

Keys returns all keys as []string

func (ConcurrentMap) MSet

func (m ConcurrentMap) MSet(data map[string]interface{})

func (ConcurrentMap) MarshalJSON

func (m ConcurrentMap) MarshalJSON() ([]byte, error)

Reviles ConcurrentMap "private" variables to json marshal.

func (ConcurrentMap) Pop

func (m ConcurrentMap) Pop(key string) (v interface{}, exists bool)

Pop removes an element from the map and returns it

func (ConcurrentMap) Remove

func (m ConcurrentMap) Remove(key string)

Remove removes an element from the map.

func (ConcurrentMap) RemoveCb

func (m ConcurrentMap) RemoveCb(key string, cb RemoveCb) bool

RemoveCb locks the shard containing the key, retrieves its current value and calls the callback with those params If callback returns true and element exists, it will remove it from the map Returns the value returned by the callback (even if element was not present in the map)

func (ConcurrentMap) Set

func (m ConcurrentMap) Set(key string, value interface{})

Sets the given value under the specified key.

func (ConcurrentMap) SetIfAbsent

func (m ConcurrentMap) SetIfAbsent(key string, value interface{}) bool

Sets the given value under the specified key if no value was associated with it.

func (ConcurrentMap) Upsert

func (m ConcurrentMap) Upsert(key string, value interface{}, cb UpsertCb) (res interface{})

Insert or Update - updates existing element or inserts a new one using UpsertCb

type ConcurrentMapShared

type ConcurrentMapShared struct {
	sync.RWMutex // Read Write mutex, guards access to internal map.
	// contains filtered or unexported fields

A "thread" safe string to anything map.

type IterCb

type IterCb func(key string, v interface{})

Iterator callback,called for every key,value found in maps. RLock is held for all calls for a given shard therefore callback sess consistent view of a shard, but not across the shards

type RemoveCb

type RemoveCb func(key string, v interface{}, exists bool) bool

RemoveCb is a callback executed in a map.RemoveCb() call, while Lock is held If returns true, the element will be removed from the map

type Tuple

type Tuple struct {
	Key string
	Val interface{}

Used by the Iter & IterBuffered functions to wrap two variables together over a channel,

type UpsertCb

type UpsertCb func(exist bool, valueInMap interface{}, newValue interface{}) interface{}

Callback to return new element to be inserted into the map It is called while lock is held, therefore it MUST NOT try to access other keys in same map, as it can lead to deadlock since Go sync.RWLock is not reentrant

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