pool: add external load balancing methods support

[KaymeKaydex]

I added the ability to use custom balancing methods when connecting with a pool.

Related issues: #400

[oleg-jukovec]

Since all API is moved to the public scope you need to change a package to test from a test one package pool_test to a package one package pool. And test only the public API.

[oleg-jukovec]

Please, add comments to the public API methods.

[oleg-jukovec]

Since this is a public API now, we need to be careful with the names. Please, add a comments for the public API too.

Suggested change

	type BalancingPool interface {
	IsEmpty() bool
	GetConnection(string) *tarantool.Connection
	DeleteConnection(string) *tarantool.Connection
	AddConnection(id string, conn *tarantool.Connection)
	GetNextConnection() *tarantool.Connection
	GetConnections() map[string]*tarantool.Connection
	}
	type Balancer interface {
	IsEmpty() bool
	Add(id string, conn *tarantool.Connection)
	Get(string) *tarantool.Connection
	GetNext() *tarantool.Connection
	GetAll() map[string]*tarantool.Connection
	Delete(string) *tarantool.Connection
	}

[oleg-jukovec]

I suggest to remove the method from the interface since it could be replaced with:

GetNextConnection() == nil or len(GetConnections())== 0.

Suggested change

IsEmpty() bool

go-tarantool/pool/connection_pool.go

Lines 217 to 239 in 48c71d9

	// ConnectedNow gets connected status of pool.
	func (p *ConnectionPool) ConnectedNow(mode Mode) (bool, error) {
	p.poolsMutex.RLock()
	defer p.poolsMutex.RUnlock()
	if p.state.get() != connectedState {
	return false, nil
	}
	switch mode {
	case ANY:
	return !p.anyPool.IsEmpty(), nil
	case RW:
	return !p.rwPool.IsEmpty(), nil
	case RO:
	return !p.roPool.IsEmpty(), nil
	case PreferRW:
	fallthrough
	case PreferRO:
	return !p.rwPool.IsEmpty() || !p.roPool.IsEmpty(), nil
	default:
	return false, ErrNoHealthyInstance
	}
	}

[oleg-jukovec]

Please, rename the file to balancer.go.

[KaymeKaydex]

done

[oleg-jukovec]

Since we are talking about a general interface, this micro-optimization with size is only confusing.

Suggested change

	type BalancingMethod = func(int) BalancingPool
	type BalancingMethod = func() BalancingPool

[oleg-jukovec]

I prefer an interface instead of a method. An interface could be used as a method, but doing the opposite is more difficult. But this is debatable.

type BalancerFactory interface {
    Create() Balancer
}

As additional we could pass some useful info into the method:

type BalancerFactory interface {
    Create(role Role) Balancer
}

Or we can keep it simple.

[DerekBum]

Suggested change

	// BalancingMethod is how connections for the request will be selected
	// BalancingMethod determines how connections for the request will be selected.

Please make sure that all new comments end with a dot (this is part of a Tarantool code style).

[KaymeKaydex]

Thanks for the review, unfortunately, I was able to return to work only at the end of August

[oleg-jukovec]

Thanks for the review, unfortunately, I was able to return to work only at the end of August

Thank you, it's ok. We are not planning any work on the connector.