Introduction to Concurrency in Haskell

Introduction to Concurrency in Haskell

Transcript

1. Introduction to Concurrency in Haskell Abhinav Sarkar Functional Conf / Sep 12, 2015
2. I Am Abhinav Sarkar @abhin4v @nilenso
3. Haskell Pure functional programming Strict type system Immutable data structures Good for concurrent programs
4. Real World Scenarios Threads Locks/Synchronization Inter thread communication Asynchronous exceptions Transactions
5. Multiuser Chat
6. Features Users Channels Message and Tell
7. Feature Plan Accept user connections User to user chat User quitting and timeouts (maybe) Channels: joining, leaving and chatting
8. Start!
9. m o d u l e L i n k . T y p e s w h e r e d a t a U s e r = U s e r { u s e r N a m e : : S t r i n g } d e r i v i n g ( S h o w , E q , O r d ) d a t a C l i e n t = C l i e n t { c l i e n t U s e r : : U s e r , c l i e n t H a n d l e : : H a n d l e } d e r i v i n g ( S h o w , E q ) d a t a S e r v e r = S e r v e r { s e r v e r U s e r s : : M V a r ( M a p . M a p U s e r C l i e n t ) }
10. Accept User Connections
11. m o d u l e L i n k . S e r v e r w h e r e i m p o r t L i n k . T y p e s r u n S e r v e r : : I n t - > I O ( ) r u n S e r v e r p o r t = w i t h S o c k e t s D o $ d o h S e t B u f f e r i n g s t d o u t L i n e B u f f e r i n g s e r v e r < - n e w S e r v e r s o c k < - l i s t e n O n . P o r t N u m b e r . f r o m I n t e g r a l $ p o r t p r i n t f " L i s t e n i n g o n p o r t % d \ n " p o r t f o r e v e r $ d o ( h a n d l e , h o s t , p o r t ' ) < - a c c e p t s o c k p r i n t f " A c c e p t e d c o n n e c t i o n f r o m % s : % s \ n " h o s t ( s h o w p o r t ' ) f o r k I O $ c o n n e c t C l i e n t s e r v e r h a n d l e ` f i n a l l y ` h C l o s e h a n d l e
12. Threads Green threads Non blocking IO Memory efficient -threaded ghc option
13. MVar Mutable variable As a container for shared state As a blocking primitive For locks/synchronization
14. c o n n e c t C l i e n t : : S e r v e r - > H a n d l e - > I O ( ) c o n n e c t C l i e n t s e r v e r h a n d l e = d o h S e t N e w l i n e M o d e h a n d l e u n i v e r s a l N e w l i n e M o d e h S e t B u f f e r i n g h a n d l e L i n e B u f f e r i n g r e a d N a m e w h e r e
15. r e a d N a m e : : I O ( ) r e a d N a m e = d o n a m e < - h G e t L i n e h a n d l e i f n u l l n a m e t h e n r e a d N a m e e l s e d o l e t u s e r = U s e r n a m e o k < - c h e c k A d d C l i e n t s e r v e r u s e r h a n d l e c a s e o k o f N o t h i n g - > d o h P r i n t f h a n d l e " T h e n a m e % s i s i n u s e , p l e a s e c h o o s e a n o t h e r \ n " n a m e r e a d N a m e J u s t c l i e n t - > r u n C l i e n t s e r v e r c l i e n t ` f i n a l l y ` r e m o v e C l i e n t s e r v e r u s e r
16. c h e c k A d d C l i e n t : : S e r v e r - > U s e r - > H a n d l e - > I O ( M a y b e C l i e n t ) c h e c k A d d C l i e n t S e r v e r { . . } u s e r @ U s e r { . . } h a n d l e = m o d i f y M V a r s e r v e r U s e r s $ \ c l i e n t M a p - > i f M a p . m e m b e r u s e r c l i e n t M a p t h e n r e t u r n ( c l i e n t M a p , N o t h i n g ) e l s e d o c l i e n t < - C l i e n t u s e r h a n d l e p r i n t f " N e w u s e r c o n n e c t e d : % s \ n " u s e r N a m e r e t u r n ( M a p . i n s e r t u s e r c l i e n t c l i e n t M a p , J u s t c l i e n t ) r e m o v e C l i e n t : : S e r v e r - > U s e r - > I O ( ) r e m o v e C l i e n t S e r v e r { . . } u s e r = m o d i f y M V a r _ s e r v e r U s e r s $ r e t u r n . M a p . d e l e t e u s e r
17. User to User Chat
18. m o d u l e L i n k . T y p e s w h e r e d a t a C l i e n t = C l i e n t { c l i e n t U s e r : : U s e r , c l i e n t H a n d l e : : H a n d l e , c l i e n t C h a n : : C h a n M e s s a g e } d a t a M e s s a g e = M s g U s e r S t r i n g d e r i v i n g ( S h o w , E q )
19. m o d u l e L i n k . P r o t o c o l w h e r e i m p o r t L i n k . T y p e s p a r s e C o m m a n d : : S t r i n g - > M a y b e M e s s a g e p a r s e C o m m a n d c o m m a n d = c a s e w o r d s c o m m a n d o f " M S G " : u s e r N a m e : m s g - > J u s t $ M s g ( U s e r u s e r N a m e ) ( u n w o r d s m s g ) _ - > N o t h i n g f o r m a t M e s s a g e : : M e s s a g e - > S t r i n g f o r m a t M e s s a g e ( M s g u s e r m s g ) = p r i n t f " M S G % s % s " ( u s e r N a m e u s e r ) m s g
20. Chans Unbounded blocking queues
21. r u n C l i e n t : : S e r v e r - > C l i e n t - > I O ( ) r u n C l i e n t S e r v e r { . . } C l i e n t { . . } = f o r e v e r $ d o r < - t r y $ r a c e r e a d C o m m a n d r e a d M e s s a g e c a s e r o f L e f t ( e : : S o m e E x c e p t i o n ) - > p r i n t f " E x c e p t i o n : % s \ n " ( s h o w e ) R i g h t c m - > c a s e c m o f L e f t m c o m m a n d - > c a s e m c o m m a n d o f N o t h i n g - > p r i n t f " C o u l d n o t p a r s e c o m m a n d \ n " J u s t c o m m a n d - > h a n d l e C o m m a n d c o m m a n d R i g h t m e s s a g e - > h a n d l e M e s s a g e m e s s a g e w h e r e
22. r e a d C o m m a n d : : I O ( M a y b e M e s s a g e ) r e a d C o m m a n d = d o c o m m a n d < - h G e t L i n e c l i e n t H a n d l e p r i n t f " < % s > : % s \ n " ( u s e r N a m e c l i e n t U s e r ) c o m m a n d r e t u r n $ p a r s e C o m m a n d c o m m a n d r e a d M e s s a g e : : I O M e s s a g e r e a d M e s s a g e = r e a d C h a n c l i e n t C h a n s e n d M e s s a g e : : M e s s a g e - > I O ( ) s e n d M e s s a g e m e s s a g e = w r i t e C h a n c l i e n t C h a n
23. h a n d l e C o m m a n d : : M e s s a g e - > I O ( ) h a n d l e C o m m a n d ( M s g u s e r m s g ) = c l i e n M a p < - r e a d M V a r s e r v e r U s e r s c a s e M a p . l o o k u p u s e r c l i e n t M a p o f N o t h i n g - > p r i n t f " N o s u c h u s e r : % s \ n " ( u s e r N a m e u s e r ) J u s t c l i e n t - > s e n d M e s s a g e ( M s g c l i e n t U s e r m s g ) c l i e n t h a n d l e M e s s a g e : : M e s s a g e - > I O ( ) h a n d l e M e s s a g e = p r i n t T o H a n d l e c l i e n t H a n d l e . f o r m a t M e s s a g e
24. r a c e : : I O a - > I O b - > I O ( E i t h e r a b ) r a c e i o a i o b = d o m < - n e w E m p t y M V a r b r a c k e t ( f o r k I O ( f m a p L e f t i o a ) ( p u t M V a r m ) ) k i l l T h r e a d $ \ _ - > b r a c k e t ( f o r k I O ( f m a p R i g h t i o b ) ( p u t M V a r m ) ) k i l l T h r e a d $ \ _ - > r e a d M V a r m
25. MVar For inter thread communication
26. Chatting in Channels
29. Software Transactional Memory
30. Why Transactions? Atomic changes to multiple variables Read from multiple channels efficiently Robustness in the presence of failure
31. m o d u l e L i n k . T y p e s w h e r e t y p e U s e r N a m e = S t r i n g t y p e C h a n n e l N a m e = S t r i n g d a t a U s e r = U s e r { u s e r N a m e : : ! U s e r N a m e } d e r i v i n g ( S h o w , E q , O r d ) d a t a C h a n n e l = C h a n n e l { c h a n n e l N a m e : : C h a n n e l N a m e , c h a n n e l U s e r s : : T V a r ( S e t . S e t U s e r ) , c h a n n e l C h a n : : T C h a n M e s s a g e }
32. d a t a S e r v e r = S e r v e r { s e r v e r U s e r s : : M V a r ( M a p . M a p U s e r C l i e n t ) , s e r v e r C h a n n e l s : : T V a r ( M a p . M a p C h a n n e l N a m e C h a n n e l ) } d a t a C l i e n t = C l i e n t { c l i e n t U s e r : : ! U s e r , c l i e n t H a n d l e : : ! H a n d l e , c l i e n t C h a n : : T C h a n M e s s a g e , c l i e n t C h a n n e l C h a n s : : T V a r ( M a p . M a p C h a n n e l N a m e ( T C h a n M e s s a g e ) ) }
33. d a t a M e s s a g e = M s g U s e r S t r i n g | T e l l C h a n n e l N a m e S t r i n g | J o i n C h a n n e l N a m e | L e a v e C h a n n e l N a m e | M s g R e p l y U s e r S t r i n g | T e l l R e p l y C h a n n e l N a m e U s e r S t r i n g | J o i n e d C h a n n e l N a m e U s e r | L e a v e d C h a n n e l N a m e U s e r d e r i v i n g ( S h o w , E q )
34. STM Monad A monad of its own Type system prevents doing IO
35. TVar TVar: Transactional variable n e w T V a r : : a - > S T M ( T V a r a ) r e a d T V a r : : T V a r a - > S T M a w r i t e T V a r : : T V a r a - > a - > S T M ( ) a t o m i c a l l y : : S T M a - > I O a r e t r y : : S T M a
36. TChan Transactional channels Efficient merging n e w T C h a n : : S T M ( T C h a n a ) w r i t e T C h a n : : T C h a n a - > a - > S T M ( ) r e a d T C h a n : : T C h a n a - > S T M a o r E l s e : : S T M a - > S T M a - > S T M a
37. r u n C l i e n t : : S e r v e r - > C l i e n t - > I O ( ) r u n C l i e n t S e r v e r { . . } c l i e n t @ C l i e n t { . . } = d o c o m m a n d T h r e a d < - f o r k I O $ r e a d C o m m a n d s r u n ` f i n a l l y ` d o k i l l T h r e a d c o m m a n d T h r e a d c l i e n t C h a n n e l M a p < - r e a d T V a r I O c l i e n t C h a n n e l C h a n s f o r M _ ( M a p . k e y s c l i e n t C h a n n e l M a p ) $ \ c h a n n e l N a m e - > h a n d l e M e s s a g e ( L e a v e c h a n n e l N a m e ) w h e r e
38. r u n : : I O ( ) r u n = f o r e v e r $ d o r < - t r y . a t o m i c a l l y $ d o c l i e n t C h a n n e l M a p < - r e a d T V a r c l i e n t C h a n n e l C h a n s f o l d r ( o r E l s e . r e a d T C h a n ) r e t r y $ c l i e n t C h a n : M a p . e l e m s c l i e n t C h a n n e l M a p c a s e r o f L e f t ( e : : S o m e E x c e p t i o n ) - > p r i n t f " E x c e p t i o n : % s \ n " ( s h o w e ) R i g h t m e s s a g e - > h a n d l e M e s s a g e m e s s a g e r e a d C o m m a n d s : : I O ( ) r e a d C o m m a n d s = f o r e v e r $ d o c o m m a n d < - h G e t L i n e c l i e n t H a n d l e p r i n t f " < % s > : % s \ n " ( u s e r N a m e c l i e n t U s e r ) c o m m a n d c a s e p a r s e C o m m a n d c o m m a n d o f N o t h i n g - > p r i n t f " C o u l d n o t p a r s e c o m m a n d : % s \ n " c o m m a n d J u s t c - > s e n d M e s s a g e I O c l i e n t c
39. h a n d l e M e s s a g e : : M e s s a g e - > I O ( ) h a n d l e M e s s a g e ( J o i n c h a n n e l N a m e ) = a t o m i c a l l y $ d o c l i e n t C h a n n e l M a p < - r e a d T V a r c l i e n t C h a n n e l C h a n s - - g e t u s e r ' s c h a n n e l s - - i f u s e r h a s n o t a l r e a d y j o i n e d t h e c h a n n e l u n l e s s ( M a p . m e m b e r c h a n n e l N a m e c l i e n t C h a n n e l M a p ) $ d o c h a n n e l M a p < - r e a d T V a r s e r v e r C h a n n e l s - - g e t s e r v e r c h a n n e l s c h a n n e l @ C h a n n e l { c h a n n e l C h a n } < - c a s e M a p . l o o k u p c h a n n e l N a m e c h a n n e l M a p o f J u s t ( c h a n n e l @ C h a n n e l { c h a n n e l U s e r s } ) - > d o - - i f t h e c h a n n e l a l r e a d y e x i s t s o n t h e s e r v e r , a d d u s e r t o i t m o d i f y T V a r ' c h a n n e l U s e r s $ S e t . i n s e r t c l i e n t U s e r r e t u r n c h a n n e l N o t h i n g - > d o - - e l s e c r e a t e a n e w c h a n n e l w i t h t h i s u s e r i n i t a n d a d d i t t o t h e s e r v e r c h a n n e l < - n e w C h a n n e l c h a n n e l N a m e $ S e t . s i n g l e t o n c l i e n t U s e r m o d i f y T V a r ' s e r v e r C h a n n e l s $ M a p . i n s e r t c h a n n e l N a m e c h a n n e l r e t u r n c h a n n e l - - d u p l i c a t e c h a n n e l T C h a n f o r t h i s u s e r a n d a d d i t t o t h e u s e r s ' s c h a n n e l s c l i e n t C h a n n e l C h a n < - d u p T C h a n c h a n n e l C h a n m o d i f y T V a r ' c l i e n t C h a n n e l C h a n s $ M a p . i n s e r t c h a n n e l N a m e c l i e n t C h a n n e l C h a n - - s e n d a J O I N E D m e s s a g e t o t h e c h a n n e l f o r t h i s u s e r t e l l M e s s a g e c h a n n e l $ J o i n e d c h a n n e l N a m e c l i e n t U s e r
40. High Level Design
41. High Level Design Threads are cheap. Don't be afraid to launch new threads. Non blocking IO. No need of event loops. Small components connected through channels Write small functions in STM and compose them
42. High Level Design Structure programs like assembly lines Processing functions read from and write to channels Channels connect processing functions like conveyor belts Channels can be duplicated and merged
43. That's All Folks
44. References Parallel and Concurrent Programming in Haskell - Simon Marlow Real World Haskell - O'Sullivan, Stewart, Goerzen
45. https://github.com/abhin4v/link https://tinyurl.com/introconchs @abhin4v abhinav@nilenso.com
46. Questions?