Refactoring to theState Design Pattern

Jim Roepcke <jr@uvic.ca>CSC 578D Fall 2009

Motivation

• Improve the design of the GameStats application

• Make it easier to understand at a glance

• Make it easier to make improvements

PrimaryReplica

Backup

Replica

Backup

Replica

Backup

Replica

Backup

Replica

Primary-Backup• Bonjour can guarantee no more than one

Primary replica can publish its service

• Multiple Backup replicas communicate with the Primary to keep all replicas synchronized

• Writes are made to the Primary and propagated back to the Backup replicas

• If the Primary fails, a Backup can take over. The remaining backups sync to the new Primary

initial

trying to become primary

start

primary

failed to become primary

primary did start

primary failed to start

trying to connect to

primary

failed to connect to

primarybackup

stopping primary

stopping backup

stopped

error

stop stop

primary did stop backup did stop

backup did startbackup failed to start

Request()

Context

state->Handle()

Handle()

State

Handle()

ConcreteStateA

Handle()

ConcreteStateB

SetState(State s)Foo()Bar()Baz()....

Context

state->Foo()

TransitionTo(Context c, State s)Enter(Context c)Leave(Context c)

Foo(StateContext c)Bar(StateContext c)Baz(StateContext c)...

State

TransitionTo(Context c, State s)Enter(Context c)Leave(Context c)

Foo(StateContext c)Bar(StateContext c)Baz(StateContext c)...

BaseState

Enter(Context c)Baz(StateContext c)

ConcreteStateA

Foo(StateContext c)Bar(StateContext c)

ConcreteStateB

this->Leave(c)c->SetState(s)s->Enter(c)

TransitionTo(c, ConcreteStateA)

primary

stopping primary

stopped

error

stop

primary did stop

Original Code- (void) stop

{! if (self.state == GSGameControllerStatePrimary) {! ! self.state = GSGameControllerStateStopping;! ! [self uninstallServerTargets];! ! [_server stop];! ! self.state = GSGameControllerStateStopped;! } else if (self.state == GSGameControllerStateBackup) {! ! self.state = GSGameControllerStateStopping;! ! [_memberManager stopMonitoring:_primaryService];! ! [_clientToPrimary stop];! ! self.state = GSGameControllerStateStopped;! } else if (self.state != GSGameControllerStateStopped) {! ! self.state = GSGameControllerStateError;! }}

First Refactoring- (oneway void) stop

{ self.state = GSGameControllerStateStopping;}

The Devil(is in the details)

- (void) setState: (GSGameControllerState)newState{! GSGameControllerState oldState = _state;! _state = newState;! if ( _state == GSGameControllerStateTryingToFindPrimary) {! ! // [self findPrimary];! ! self.state = GSGameControllerStateTryingToBecomePrimary; // TODO: remove the findprimary state! } else if (_state == GSGameControllerStateTryingToBecomePrimary) {! ! [self startPrimaryServer];! } else if (_state == GSGameControllerStateFailedToBecomePrimary) {! ! [self tearDownPrimary];! ! // FIXME: this could be an endless loop of failing to become primary, put in a limit or something! ! self.state = GSGameControllerStateTryingToBecomePrimary;! } else if (_state == GSGameControllerStateTryingToConnectToPrimary) {! ! [self connectToPrimary];! } else if (_state == GSGameControllerStateFailedToConnectToPrimary) {! ! [self tearDownBackup];! ! self.state = GSGameControllerStateTryingToBecomePrimary;! } else if (_state == GSGameControllerStatePrimary) {! ! [self installServerTargets];! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerDidBecomePrimary:)];! } else if (_state == GSGameControllerStateBackup) {! ! [self tellPrimaryWhoIAm];! ! [self monitorPrimary];! ! [self synchronizeWithPrimaryFromVersion:_game.version];! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerDidBecomeBackup:)];! } else if (_state == GSGameControllerStateStopping) {! ! if (oldState == GSGameControllerStatePrimary) {! ! ! self.state = GSGameControllerStateStoppingPrimary;! ! } else if (oldState == GSGameControllerStateBackup) {! ! ! self.state = GSGameControllerStateStoppingBackup;

The Devil(is in the details)

! ! } else if (oldState != GSGameControllerStateError) {! ! ! self.state = GSGameControllerStateStopped;! ! }! } else if (_state == GSGameControllerStateStoppingPrimary) {! ! [self stopServicingBackups];! ! [_server stop];! ! // TODO: actually monitor the stop instead of just setting state to GSGameControllerStateStopped! ! self.state = GSGameControllerStateStopped;! } else if (_state == GSGameControllerStateStoppingBackup) {! ! [self stopMonitoringPrimary];! ! [_clientToPrimary stop];! ! // TODO: actually monitor the stop instead of just setting state to GSGameControllerStateStopped! ! self.state = GSGameControllerStateStopped;! } else if (_state == GSGameControllerStateStopped) {! ! if (oldState == GSGameControllerStatePrimary) {! ! ! [self tearDownPrimary];! ! } else ! if (oldState == GSGameControllerStateStoppingPrimary) {! ! ! [self tearDownPrimary];! ! } else if (oldState == GSGameControllerStateBackup) {! ! ! [self tearDownBackup];! ! } else if (oldState == GSGameControllerStateStoppingBackup) {! ! ! [self tearDownBackup];! ! }! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerDidStop:)];! } else if (_state == GSGameControllerStateError) {! ! if (oldState == GSGameControllerStateTryingToFindPrimary) {! ! ! [self tearDownPrimary];! ! } else if (oldState == GSGameControllerStateTryingToBecomePrimary) {! ! ! [self tearDownPrimary];! ! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerFailedToStart:)];! ! } else if (oldState == GSGameControllerStateTryingToConnectToPrimary) {

The Devil(is in the details)

! ! ! [self tearDownBackup];! ! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerFailedToStart:)];! ! } else if (oldState == GSGameControllerStatePrimary) {! ! ! [self tearDownPrimary];! ! ! // TODO: should this tellDelegate gameControllerDidStop: ?! ! } else if (oldState == GSGameControllerStateBackup) {! ! ! [self tearDownBackup];! ! ! // TODO: should this tellDelegate gameControllerDidStop: ?! ! } else if (oldState == GSGameControllerStateStopping) {! ! ! [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerFailedToStop:)];! ! ! // TODO: should this tellDelegate gameControllerDidStop: ?! ! } else if (oldState == GSGameControllerStateStoppingPrimary) {! ! ! [self tearDownPrimary];! ! ! // TODO: should this tellDelegate gameControllerDidStop: ?! ! } else if (oldState == GSGameControllerStateStoppingBackup) {! ! ! [self tearDownBackup];! ! ! // TODO: should this tellDelegate gameControllerDidStop: ?! ! }! ! /* else ??? */ [self tellDelegate:_delegate performSelectorWithSelf:@selector(gameControllerErrorOccurred:)];! }}

First Refactoring

- (oneway void) stop { self.state = GSGameControllerStateStopping; }

- (void) setState: (GSGameControllerState)newState{! GSGameControllerState oldState = _state;! _state = newState;

} else if (_state == GSGameControllerStateStopping) { if (oldState == GSGameControllerStatePrimary) { self.state = GSGameControllerStateStoppingPrimary; } else if (oldState == GSGameControllerStateBackup) { self.state = GSGameControllerStateStoppingBackup; } else if (oldState != GSGameControllerStateError) { self.state = GSGameControllerStateStopped; }}

} else if (_state == GSGameControllerStateStoppingPrimary) { [self stopServicingBackups]; [_server stop]; self.state = GSGameControllerStateStopped;}

} else if (_state == GSGameControllerStateStopped) { if (oldState == GSGameControllerStatePrimary) { [self tearDownPrimary]; } else if (oldState == GSGameControllerStateStoppingPrimary) { [self tearDownPrimary]; } else if (oldState == GSGameControllerStateBackup) { [self tearDownBackup]; } else if (oldState == GSGameControllerStateStoppingBackup) { [self tearDownBackup]; } [self tellDelegate:_delegate performSelectorWithSelf: @selector(gameControllerDidStop:)];}

State Refactoring

- (oneway void) stop{ [self.state stop: self];}

Stopping a primary@implementation GSGameControllerStatePrimary

- (void) enter: (GSGameController *)gc{! [gc installServerTargets];}

- (void) error: (GSGameController *)gc{ ! [gc tearDownPrimary];! [super error: gc];}

- (void) incrementIntegerForKey: (id)aKey context: (GSGameController *)gc{! [gc primaryIncrementIntegerForKey: aKey];}

- (void) stop: (GSGameController *)gc{! [self transition: gc to: [GSGameControllerStateStoppingPrimary state]];}

@end

Only I know how@implementation GSGameControllerStateStoppingPrimary

- (void) enter: (GSGameController *)gc{! [gc stopServicingBackups];}

- (void) error: (GSGameController *)gc{ ! [gc tearDownPrimary];! [super error: gc];}

- (void) primaryDidStop: (GSGameController *)gc{! [gc tearDownPrimary];! [self transition: gc to: [GSGameControllerStateStopped state]];}

@end

Transition to stopped@implementation GSGameControllerStateStoppingPrimary

- (void) enter: (GSGameController *)gc{! [gc stopServicingBackups];}

- (void) error: (GSGameController *)gc{ ! [gc tearDownPrimary];! [super error: gc];}

- (void) primaryDidStop: (GSGameController *)gc{! [gc tearDownPrimary];! [self transition: gc to: [GSGameControllerStateStopped state]];}

@end

Done@implementation GSGameControllerStateStopped

- (void) enter: (GSGameController *)gc{ [gc tellDelegate:gc.delegate performSelectorWithSelf: @selector(gameControllerDidStop:)];}

@end

Same result on stop} else if (_state == GSGameControllerStateStopped) { if (oldState == GSGameControllerStatePrimary) { [self tearDownPrimary]; } else if (oldState == GSGameControllerStateStoppingPrimary) { [self tearDownPrimary]; } else if (oldState == GSGameControllerStateBackup) { [self tearDownBackup]; } else if (oldState == GSGameControllerStateStoppingBackup) { [self tearDownBackup]; } [self tellDelegate:_delegate performSelectorWithSelf: @selector(gameControllerDidStop:)];}

Result

• GSGameController only knows its own operations

• Not states or transitions

• Separation of concerns

• Each state is a black box

• Doesn’t know details of other states

Motivation

• Improve the design of the GameStats application

• Make it easier to understand at a glance

• Make it easier to make improvements

Conclusion

The State Design Pattern made complex code easier to understand and modify

Questions?

The State Design Pattern made complex code easier to understand and modify