github.com/mhilton/juju-juju@v0.0.0-20150901100907-a94dd2c73455/leadership/interface.go

// Copyright 2015 Canonical Ltd.
// Licensed under the AGPLv3, see LICENCE file for details.

package leadership

import (
	"time"

	"github.com/juju/errors"
)

// ErrClaimDenied is the error which will be returned when a
// leadership claim has been denied.
var ErrClaimDenied = errors.New("leadership claim denied")

// Claimer exposes leadership acquisition capabilities.
type Claimer interface {

	// ClaimLeadership claims leadership of the named service on behalf of the
	// named unit. If no error is returned, leadership will be guaranteed for
	// at least the supplied duration from the point when the call was made.
	ClaimLeadership(serviceId, unitId string, duration time.Duration) error

	// BlockUntilLeadershipReleased blocks until the named service is known
	// to have no leader, in which case it returns no error; or until the
	// manager is stopped, in which case it will fail.
	BlockUntilLeadershipReleased(serviceId string) (err error)
}

// Token represents a unit's leadership of its service.
//
// It seems to be generic enough (it could easily represent any fact) that it
// should find a more general home.
type Token interface {

	// Check returns an error if the condition it embodies no longer holds.
	// If you pass a non-nil value into Check, it must be a pointer to data
	// of the correct type, into which the token's content will be copied.
	//
	// The "correct type" is implementation-specific, and no implementation
	// is obliged to accept any non-nil parameter; but methods that return
	// Tokens should explain whether, and how, they will expose their content.
	//
	// In practice, most Token implementations will likely expect *[]txn.Op,
	// so that they can be used to gate mgo/txn-based state changes.
	Check(interface{}) error
}

// Checker exposes leadership testing capabilities.
type Checker interface {

	// LeadershipCheck returns a Token representing the supplied unit's
	// service leadership. The existence of the token does not imply
	// its accuracy; you need to Check() it.
	//
	// This method returns a token that accepts a *[]txn.Op, into which
	// it will (on success) copy mgo/txn operations that can be used to
	// verify the unit's continued leadership as part of another txn.
	LeadershipCheck(serviceName, unitName string) Token
}