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, its value is nil or it's expired.
	Get(ctx context.Context, key interface{}) (interface{}, error)

	// GetOrSet retrieves and returns the value of <key>, or sets <key>-<value> pair and
	// returns <value> if <key> does not exist in the cache. The key-value pair expires
	// after <duration>.
	//
	// It does not expire if <duration> == 0.
	// It deletes the <key> if <duration> < 0 or given <value> is nil, but it does nothing
	// if <value> is a function and the function result is nil.
	GetOrSet(ctx context.Context, key interface{}, value interface{}, duration time.Duration) (interface{}, error)

	// GetOrSetFunc retrieves and returns the value of <key>, or sets <key> with result of
	// function <f> and returns its result if <key> does not exist in the cache. The key-value
	// pair expires after <duration>.
	//
	// It does not expire if <duration> == 0.
	// It deletes the <key> if <duration> < 0 or given <value> is nil, but it does nothing
	// if <value> is a function and the function result is nil.
	GetOrSetFunc(ctx context.Context, key interface{}, f func() (interface{}, error), duration time.Duration) (interface{}, error)

	// GetOrSetFuncLock retrieves and returns the value of <key>, or sets <key> with result of
	// function <f> and returns its result if <key> does not exist in the cache. The key-value
	// pair expires after <duration>.
	//
	// It does not expire if <duration> == 0.
	// It does nothing if function <f> returns nil.
	//
	// Note that the function <f> should be executed within writing mutex lock for concurrent
	// safety purpose.
	GetOrSetFuncLock(ctx context.Context, key interface{}, f func() (interface{}, error), duration time.Duration) (interface{}, error)

	// Contains returns true if <key> exists in the cache, or else returns false.
	Contains(ctx context.Context, key interface{}) (bool, error)

	// GetExpire retrieves and returns the expiration of <key> in the cache.
	//
	// It returns 0 if the <key> does not expire.
	// It returns -1 if the <key> does not exist in the cache.
	GetExpire(ctx context.Context, key interface{}) (time.Duration, error)

	// Remove deletes one or more keys from cache, and returns its value.
	// If multiple keys are given, it returns the value of the last deleted item.
	Remove(ctx context.Context, keys ...interface{}) (value interface{}, err error)

	// Update updates the value of <key> without changing its expiration and returns the old value.
	// The returned value <exist> is false if the <key> does not exist in the cache.
	//
	// It deletes the <key> if given <value> is nil.
	// It does nothing if <key> does not exist in the cache.
	Update(ctx context.Context, key interface{}, value interface{}) (oldValue interface{}, exist bool, err error)

	// UpdateExpire updates the expiration of <key> and returns the old expiration duration value.
	//
	// It returns -1 and does nothing if the <key> does not exist in the cache.
	// It deletes the <key> if <duration> < 0.
	UpdateExpire(ctx context.Context, key interface{}, duration time.Duration) (oldDuration time.Duration, err error)

	// Size returns the number of items in the cache.
	Size(ctx context.Context) (size int, err error)

	// Data returns a copy of all key-value pairs in the cache as map type.
	// Note that this function may leads lots of memory usage, you can implement this function
	// if necessary.
	Data(ctx context.Context) (map[interface{}]interface{}, error)

	// Keys returns all keys in the cache as slice.
	Keys(ctx context.Context) ([]interface{}, error)

	// Values returns all values in the cache as slice.
	Values(ctx context.Context) ([]interface{}, error)

	// Clear clears all data of the cache.
	// Note that this function is sensitive and should be carefully used.
	Clear(ctx context.Context) error

	// Close closes the cache if necessary.
	Close(ctx context.Context) error
}