Microservices_with_Clojure.pdf

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1. Microservices with Clojure Microservices Meetup Berlin Michael Vitz & Joy Clark
2. Who are we? Michael Vitz michael.vitz@innoq.com @michaelvitz Joy Clark joy.clark@innoq.com @iamjoyclark
3. ~80 people in Germany (Monheim, Berlin, Offenbach, Munich) ~20 people in Switzerland (Zürich, Cham)
4. Who are you?
5. Microservices? Various aspects! http://martinfowler.com/articles/microservices.html
6. “Do one thing and do it well!”
7. Easier to understand $ m a n u n i q . . . D E S C R I P T I O N T h e u n i q u t i l i t y r e a d s t h e s p e c i f i e d i n p u t _ f i l e c o m p a r i n g a d j a c e n t l i n e s , a n d w r i t e s a c o p y o f e a c h u n i q u e i n p u t l i n e t o t h e o u t p u t _ f i l e . . . .
8. Understanding a monolith... $ m a n m y - h u g e - m o n o l i t h < e n d l e s s t e x t >
9. Composability p s - e f | g r e p p r o g r a m - i - d o n t - l i k e | g r e p - v g r e p | a w k ' { p r i n t $ 2 } ' | k i l l
10. “Do one thing and do it well!”? Yes!
11. “One thing” - which?
12. “Organize around business capabilities!”
13. Don’t build thinks like “frontend”, “api” and “database access” – instead, build “fulfillment” and “payment”.
14. “Organize around business capabilities!”? Yes!
15. All of this could be done with Libraries. Would that be Microservices?
16. Code Abstractions Function (Class) Library Component Service
17. Service Communicates via network Independently deployable Technologically independent Clear module boundaries
18. Microservices promise very loose coupling but you have to handle the complexity of distributed systems!
19. This requires rules and conventions across systems.
20. Good starting point: http://12factor.net/
21. Rules & conventions Project setup is done more than once  has to be easy Deployment is done all the time  must be automated Many apps are configured and write logs  should be simple for every app, common format, Splunk or ELK
22. Rules & conventions Many services are talking to each other  standardize on protocols and data formats The network is not reliable  asynchronous programming and stability patterns for decoupling and resilience It’s easy to lose track of what’s actually happening  gather metrics for monitoring
23. Given such rules, how to play nice using Clojure?
24. Clojure Crash Course ( p r i n t l n " H e l l o B e r l i n ! " ) Lisp + JVM Functional programming language Simple programming model
25. Clojure Crash Course ( p r i n t l n " H e l l o B e r l i n ! " ) Lisp + JVM Functional programming language Simple programming model
26. Clojure Crash Course { : n a m e " C l o j u r e " : f e a t u r e s [ : f u n c t i o n a l : j v m : p a r e n s ] : c r e a t o r " R i c h H i c k e y " : s t a b l e - v e r s i o n { : n u m b e r " 1 . 8 . 0 " : r e l e a s e " 2 0 1 6 / 0 1 / 1 9 " } }
27. Clojure Crash Course ( d e f n m a i n [ a r g s ] ( p r i n t l n " H e l l o W o r l d ! " ) ) vs. p u b l i c s t a t i c v o i d m a i n ( S t r i n g [ ] a r g s ) { S y s t e m . o u t . p r i n t l n ( " H e l l o W o r l d ! " ) ; }
28. Clojure Crash Course ( + 1 2 3 ) > 6 ( : c i t y { : n a m e " i n n o Q " : c i t y " B e r l i n " } ) > " B e r l i n " ( m a p i n c [ 1 2 3 ] ) > ( 2 3 4 )
29. Example contacts simple-calendar noti cations
30. Project setup derivative of by “Garuda Takeoff” Simon_sees (CC BY 2.0)
31. Leiningen Alternative to Maven Describes Clojure project with generic data structures Maven repository compatibility Offers plugin system
32. Leiningen ( d e f p r o j e c t s i m p l e - c a l e n d a r " 0 . 1 . 0 - S N A P S H O T " : d e s c r i p t i o n " s i m p l e c a l e n d a r a p p " : u r l " h t t p s : / / g i t h u b . c o m / i n n o q / s i m p l e - c a l e n d a r " : d e p e n d e n c i e s [ [ o r g . c l o j u r e / c l o j u r e " 1 . 8 . 0 " ] [ c o m p o j u r e " 1 . 4 . 0 " ] [ r i n g " 1 . 4 . 0 " ] [ r i n g / r i n g - j s o n " 0 . 4 . 0 " ] . . . [ e n v i r o n " 1 . 0 . 2 " ] ] : p l u g i n s [ [ l e i n - r i n g " 0 . 9 . 7 " ] [ l e i n - e n v i r o n " 1 . 0 . 2 " ] ] : r i n g { : h a n d l e r s i m p l e - c a l e n d a r . c o r e / w e b a p p : i n i t s i m p l e - c a l e n d a r . c o r e / i n i t } : p r o f i l e s { : u b e r j a r { : a o t : a l l } } )
33. Automated deployment derivative of by “Launch Button -- SMASH Rocket Club 5-9-09 4” Steven Depolo (CC BY 2.0)
34. Automated deployment Not really a language thing ... ... but good tooling can help a lot Leiningen makes building fat JARs easy WAR files can be generated as well
35. Logging & configuration by “27 Jan 2007 (Flickr)” wonderferret (CC BY 2.0)
36. Logging Don’t let every app handle log files Just write everything to stdout Let some external tool handle storage Standardize on log format
37. org.clojure/tools.logging Macros delegating to different implementations Supports slf4j, Apache commons-logging, log4j and java.util.logging Logging levels: : t r a c e : d e b u g : i n f o : w a r n : e r r o r : f a t a l
38. org.clojure/tools.logging ( n s s i m p l e - c a l e n d a r . c o r e ( : r e q u i r e [ c l o j u r e . t o o l s . l o g g i n g : a s l o g ] ) ( d e f n u p d a t e - c o n t a c t ! [ u r l ] . . . ( l o g / i n f o " U p d a t e d u s e r " i d n e w - e m a i l ) ) p r o j e c t . c l j : : d e p e n d e n c i e s [ [ o r g . c l o j u r e / t o o l s . l o g g i n g " 0 . 3 . 1 " ] [ l o g 4 j " 1 . 2 . 1 7 " : e x c l u s i o n s [ j a v a x . m a i l / m a i l j a v a x . j m s / j m s c o m . s u n . j d m k / j m x t o o l s c o m . s u n . j m x / j m x r i ] ] [ o r g . s l f 4 j / s l f 4 j - l o g 4 j 1 2 " 1 . 7 . 1 8 " ] . . . ]
39. org.clojure/tools.logging l o g 4 j . p r o p e r t i e s : l o g 4 j . r o o t L o g g e r = I N F O , s t a n d a r d l o g 4 j . a p p e n d e r . s t a n d a r d = o r g . a p a c h e . l o g 4 j . C o n s o l e A p p e n d e r l o g 4 j . a p p e n d e r . s t a n d a r d . T a r g e t = S y s t e m . o u t l o g 4 j . a p p e n d e r . s t a n d a r d . l a y o u t = o r g . a p a c h e . l o g 4 j . P a t t e r n L a y o u t l o g 4 j . a p p e n d e r . s t a n d a r d . l a y o u t . C o n v e r s i o n P a t t e r n = % d { y y y y - m m - d d H H : m m : s s , S S S } [ % p ] % c - % m % n s t d o u t : 2 0 1 5 - 1 1 - 1 8 1 3 : 1 1 : 5 4 , 4 6 8 [ I N F O ] s i m p l e - c a l e n d a r . c o r e - U p d a t e d u s e r 5 f 5 6 5 0 4 0 e v e @ e x a m p l e . o r g 2 0 1 5 - 1 1 - 1 8 1 3 : 1 1 : 5 4 , 4 7 6 [ I N F O ] s i m p l e - c a l e n d a r . c o r e - U p d a t e d u s e r 7 8 6 4 9 4 e f b o b @ e x a m p l e . o r g
40. Configuration Store configuration in the environment, not in the codebase Use mechanism that works for every application, e.g., environment variables
41. environ Manages environment settings following 12factor Reads values from Java system properties Environment variables . l e i n - e n v (created via Leiningen plugin from p r o f i l e s . c l j )
42. environ ( d e f c o n t a c t s - f e e d ( e n v : c o n t a c t s - f e e d ) ) > j a v a - D c o n t a c t s . f e e d = h t t p : / / c o n t a c t s . e x a m p l e . o r g / f e e d - j a r s t a n d a l o n e . j a r > C O N T A C T S _ F E E D = h t t p : / / c o n t a c t s . e x a m p l e . o r g / f e e d l e i n r i n g s e r v e r - h e a d l e s s > l e i n r i n g s e r v e r - h e a d l e s s p r o f i l e s . c l j : { : d e v { : e n v { : c o n t a c t s - f e e d " h t t p : / / c o n t a c t s . e x a m p l e . o r g / f e e d " } } }
43. Protocols & data formats derivative of by “Free ports” Markus Reinhardt (CC BY 2.0)
44. Protocols & data formats Avoid using different protocols everywhere Standardize on the outside: HTTP JSON XML Atom
45. HTTP server Ring for HTTP basics Compojure for routing Request & response are data A web app is a function which takes a request and returns a response https://github.com/ring-clojure/ring/blob/master/SPEC
46. Ring ( d e f e x a m p l e - r e q u e s t { : u r i " / c o n t a c t s " : r e q u e s t - m e t h o d : g e t : h e a d e r s { " A c c e p t " " t e x t / p l a i n " } } ) ( d e f n e x a m p l e - a p p [ r e q ] { : s t a t u s 2 0 0 : b o d y ( s t r " H e l l o a t " ( : u r i r e q ) ) } ) ( e x a m p l e - a p p e x a m p l e - r e q u e s t ) > { : s t a t u s 2 0 0 : b o d y " H e l l o a t / c o n t a c t s " }
47. Compojure ( d e f r o u t e s c o n t a c t s ( G E T " / c o n t a c t s / : i d " [ i d ] ( g e t - c o n t a c t i d ) ) ( P U T " / c o n t a c t s / : i d " [ i d : a s r e q u e s t ] ( u p d a t e - c o n t a c t ! i d ( : b o d y r e q u e s t ) ) ) ( P O S T " / c o n t a c t s " r e q u e s t ( a d d - c o n t a c t ! ( : b o d y r e q u e s t ) ) ) ( G E T " / f e e d " [ ] { : s t a t u s 2 0 0 : h e a d e r s { " C o n t e n t - T y p e " " a p p l i c a t i o n / a t o m + x m l " } : b o d y ( c o n t a c t s - f e e d ) } ) ) )
48. Frameworks Quickly get a web app up and running. Duct Luminus Pedestal Modularity tesla-microservices
49. org.clojure/data.json Clojure data structures are JSON superset ( j s o n / w r i t e - s t r { : n a m e " A l i c e M i l l e r " : t e a m s [ " T e a m A " , " T e a m B " ] } ) > " { \ " n a m e \ " : \ " A l i c e M i l l e r \ " , \ " t e a m s \ " : [ \ " T e a m A \ " , \ " T e a m B \ " ] } " ( j s o n / r e a d - s t r " { \ " i d \ " : 1 2 3 , \ " n a m e \ " : \ " A l i c e M i l l e r \ " } " : k e y - f n k e y w o r d ) > { : i d 1 2 3 , : n a m e " A l i c e M i l l e r " }
50. HTML HTML is represented with generic data structures This allows processing HTML with standard Clojure functions There are different formats, a popular one is called “hiccup”
51. HTML < h t m l > < b o d y > < h 1 > h e l l o ! < / h 1 > < i m g s r c = " s o m e . j p g " / > < i m g s r c = " a n o t h e r . j p g " / > < / b o d y > < / h t m l > [ : h t m l { } [ : b o d y { } [ : h 1 { } " h e l l o ! " ] [ : i m g { : s r c " s o m e . j p g " } ] [ : i m g { : s r c " a n o t h e r . j p g " } ] ] ]
52. Atom XML format for representing feeds of data Useful for doing pub/sub without middleware
53. Creating Atom Feeds ( d e f n e n t r y [ e v e n t ] [ : e n t r y [ : t i t l e ( - > e v e n t : t y p e n a m e ) ] [ : u p d a t e d ( : t i m e s t a m p e v e n t ) ] [ : a u t h o r [ : n a m e " c o n t a c t s s e r v i c e " ] ] [ : i d ( s t r " u r n : c o n t a c t s : f e e d : e v e n t : " ( : i d e v e n t ) ) ] [ : c o n t e n t { : t y p e " j s o n " } ( j s o n / g e n e r a t e - s t r i n g e v e n t ) ] ] ) ( d e f n a t o m - f e e d [ e v e n t s u r l ] ( c l o j u r e . d a t a . x m l / e m i t - s t r ( c l o j u r e . d a t a . x m l / s e x p - a s - e l e m e n t [ : f e e d { : x m l n s " h t t p : / / w w w . w 3 . o r g / 2 0 0 5 / A t o m " } [ : i d " u r n : c o n t a c t s : f e e d " ] [ : u p d a t e d ( - > e v e n t s l a s t : t i m e s t a m p ) ] [ : t i t l e { : t y p e " t e x t " } " c o n t a c t s e v e n t s " ] [ : l i n k { : r e l " s e l f " : h r e f u r l } ] ( m a p e n t r y e v e n t s ) ] ) ) )
54. adamwynne/feedparser-clj Retrieves and Parses RSS/Atom feeds ( d e f f ( f e e d p a r s e r / p a r s e - f e e d " h t t p s : / / w w w . i n n o q . c o m / d e / p o d c a s t . r s s " ) ) ( : t i t l e f ) > " i n n o Q P o d c a s t " ( c o u n t ( : e n t r i e s f ) ) > 1 8 Library for consuming feeds based on feedparser: Feedworker
55. Lightweight messaging Just a side note: Kafka (clj-kafka) RabbitMQ (Langohr) ...
56. Problems of distributed systems: Stability derivative of by “the Jenga” Ed Garcia (CC BY 2.0)
57. Cascading failures dash- board app web service data- base
58. Cascading failures dash- board app web service data- base
59. Cascading failures dash- board app web service data- base
60. Cascading failures dash- board app web service data- base
61. Cascading failures dash- board app web service data- base
62. How to achieve stability?
63. Bulkheads derivative of by (Public Domain) “Titanic sinking” Willy Stöwer
64. Asynchronous communication Helps to decouple and separate different application parts E-mail notifications do not need to be sent synchronously
65. org.clojure/core.async Supports asynchronous programming and communications Messages can be sent to and read from channels Channels can be buffered or unbuffered Blocking and non-blocking operations possible
66. org.clojure/core.async ( d e f n o t i f i c a t i o n s ( c h a n 1 0 0 0 ) )
67. org.clojure/core.async ( d e f n o t i f i c a t i o n s ( c h a n 1 0 0 0 ) )
68. org.clojure/core.async ( d e f n o t i f i c a t i o n s ( c h a n 1 0 0 0 ) ) ( d e f n s e n d - n o t i f i c a t i o n [ e m a i l e v e n t - l i n k ] ( g o ( > ! n o t i f i c a t i o n s { : e m a i l e m a i l : e v e n t - l i n k e v e n t - l i n k } ) ) )
69. org.clojure/core.async ( d e f n o t i f i c a t i o n s ( c h a n 1 0 0 0 ) ) ( d e f n s e n d - n o t i f i c a t i o n [ e m a i l e v e n t - l i n k ] ( g o ( > ! n o t i f i c a t i o n s { : e m a i l e m a i l : e v e n t - l i n k e v e n t - l i n k } ) ) )
70. org.clojure/core.async ( d e f n o t i f i c a t i o n s ( c h a n 1 0 0 0 ) ) ( d e f n s e n d - n o t i f i c a t i o n [ e m a i l e v e n t - l i n k ] ( g o ( > ! n o t i f i c a t i o n s { : e m a i l e m a i l : e v e n t - l i n k e v e n t - l i n k } ) ) )
71. org.clojure/core.async ( d e f n o t i f i c a t i o n s ( c h a n 1 0 0 0 ) ) ( d e f n s e n d - n o t i f i c a t i o n [ e m a i l e v e n t - l i n k ] ( g o ( > ! n o t i f i c a t i o n s { : e m a i l e m a i l : e v e n t - l i n k e v e n t - l i n k } ) ) ) ( d e f n n o t i f y - u s e r [ e m a i l e v e n t - l i n k ] . . . ) ( d e f n s t a r t - n o t i f i e r [ ] ( g o - l o o p [ m e s s a g e ( < ! n o t i f i c a t i o n s ) ] ( n o t i f y - u s e r ( : e m a i l m e s s a g e ) ( : e v e n t - l i n k m e s s a g e ) ) ( r e c u r ( < ! n o t i f i c a t i o n s ) ) ) )
72. org.clojure/core.async ( d e f n o t i f i c a t i o n s ( c h a n 1 0 0 0 ) ) ( d e f n s e n d - n o t i f i c a t i o n [ e m a i l e v e n t - l i n k ] ( g o ( > ! n o t i f i c a t i o n s { : e m a i l e m a i l : e v e n t - l i n k e v e n t - l i n k } ) ) ) ( d e f n n o t i f y - u s e r [ e m a i l e v e n t - l i n k ] . . . ) ( d e f n s t a r t - n o t i f i e r [ ] ( g o - l o o p [ m e s s a g e ( < ! n o t i f i c a t i o n s ) ] ( n o t i f y - u s e r ( : e m a i l m e s s a g e ) ( : e v e n t - l i n k m e s s a g e ) ) ( r e c u r ( < ! n o t i f i c a t i o n s ) ) ) )
73. org.clojure/core.async ( d e f n o t i f i c a t i o n s ( c h a n 1 0 0 0 ) ) ( d e f n s e n d - n o t i f i c a t i o n [ e m a i l e v e n t - l i n k ] ( g o ( > ! n o t i f i c a t i o n s { : e m a i l e m a i l : e v e n t - l i n k e v e n t - l i n k } ) ) ) ( d e f n n o t i f y - u s e r [ e m a i l e v e n t - l i n k ] . . . ) ( d e f n s t a r t - n o t i f i e r [ ] ( g o - l o o p [ m e s s a g e ( < ! n o t i f i c a t i o n s ) ] ( n o t i f y - u s e r ( : e m a i l m e s s a g e ) ( : e v e n t - l i n k m e s s a g e ) ) ( r e c u r ( < ! n o t i f i c a t i o n s ) ) ) )
74. Circuit Breaker derivative of by “switch-fuse” Mark_K_ (CC BY 2.0)
75. com.netflix.hystrix/hystrix-clj Idiomatic Clojure wrapper for Hystrix ( h y s t r i x / d e f c o m m a n d n o t i f y - u s e r [ e m a i l e v e n t - l i n k ] ( c l i e n t / p o s t n o t i f i c a t i o n - s e r v i c e { : c o n t e n t - t y p e : j s o n : b o d y . . . } ) ) Will throw exception in case of timeout or other failure
76. com.netflix.hystrix/hystrix-clj ; r e t u r n s t r u e i f s u c e s s f u l , f a l s e i f c i r c u i t - b r e a k e r o p e n ( h y s t r i x / d e f c o m m a n d n o t i f y - u s e r { : h y s t r i x / f a l l b a c k - f n n o t i f y - f a l l b a c k } [ e m a i l e v e n t - l i n k ] ( c l i e n t / p o s t n o t i f i c a t i o n - s e r v i c e { : c o n t e n t - t y p e : j s o n : b o d y . . . } ) t r u e ) ; r e t u r n s f a l s e i f c i r c u i t - b r e a k e r o p e n ( d e f n n o t i f y - f a l l b a c k [ e m a i l e v e n t - l i n k ] ( l e t [ i s O p e n ( . i s C i r c u i t B r e a k e r O p e n h y s t r i x / * c o m m a n d * ) ] ; a d d m e s s a g e t o q u e u e a g a i n ( s e n d - n o t i f i c a t i o n e m a i l e v e n t - l i n k ) ( n o t i s O p e n ) ) )
77. com.netflix.hystrix/hystrix-clj ( d e f n s t a r t - n o t i f i e r [ ] ( g o - l o o p [ m e s s a g e ( < ! n o t i f i c a t i o n s ) ] ( i f - n o t ( n o t i f y - u s e r ( : e m a i l m e s s a g e ) ( : e v e n t - l i n k m e s s a g e ) ) ( d o ( l o g / e r r o r " C a n n o t r e a c h n o t i f i c a t i o n s e r v i c e " " - w i l l w a i t u n t i l n e x t t r y " ) ( < ! ( t i m e o u t 5 0 0 0 ) ) ) ) ( r e c u r ( < ! n o t i f i c a t i o n s ) ) ) ) hystrix-event-stream-clj available as well
78. Monitoring derivative of by “Space Shuttle Endeavour's Control Panels” Steve Jurvetson (CC BY 2.0)
79. Questions How many HTTP errors are occurring? Are database queries failing? Is that backend service slow again? How many jobs are in the queue?
80. Metrics Logging provides a stream of events Metrics provide aggregated state Popular library: Dropwizard Metrics Two steps: collect metrics, then publish them
81. Gauges by “Old gauge, old style” speredenn (CC BY 2.0)
82. Gauges Current state of one single value Number of jobs in the queue Some configured value Ratio of cache hits to misses
83. Gauges ( d e f m e t r i c s - r e g i s t r y ( n e w - r e g i s t r y ) ) ( g a u g e - f n m e t r i c s - r e g i s t r y " j o b s - r e a d y " # ( q u e r y d a t a b a s e " s e l e c t c o u n t ( * ) f r o m j o b s w h e r e s t a t u s = ' r e a d y ' " ) )
84. Counters derivative of by “The US National Debt clock / counter, New York” Ben Sutherland (CC BY 2.0)
85. Counters Single numeric value For example, number of logged in users
86. Counters ( d e f l o g g e d - i n - u s e r s ( c o u n t e r m e t r i c s - r e g i s t r y " l o g g e d - i n - u s e r s " ) ) ( P O S T " / l o g i n " [ n a m e p w d ] . . . ( i n c ! l o g g e d - i n - u s e r s ) . . . ) ( P O S T " / l o g o u t " [ n a m e ] . . . ( d e c ! l o g g e d - i n - u s e r s ) . . . )
87. Histograms derivative of by “Number of cat posts on Metafilter per month, as a percentage of the total number of posts” Steven Taschuk (CC BY 2.0)
88. Histograms Distribution of numerical data (min, max, mean, standard deviation, quantiles) For example, number of search results Different value “reservoirs”, e.g., Entire application lifetime Last N searches Last N minutes
89. Histograms ( d e f n u m b e r - o f - r e s u l t s ( h i s t o g r a m m e t r i c s - r e g i s t r y " n u m b e r - o f - s e a r c h - r e s u l t s " ) ) ( d e f n s e a r c h [ q u e r y ] ( l e t [ r e s u l t s ( e x e c u t e q u e r y ) ] ( u p d a t e ! n u m b e r - o f - r e s u l t s ( c o u n t r e s u l t s ) ) r e s u l t s ) )
90. Meters derivative of by “Hosp_1011” Justin Taylor (CC BY 2.0)
91. Meters Rate of an event (per second) Total count of events Average rate over application lifetime Rate in the last 1, 5 and 15 minutes For example, incoming requests
92. Meters ( d e f i n c - r e q u e s t s - m e t e r ( m e t e r s / m e t e r m e t r i c s - r e g i s t r y " i n c o m i n g - r e q u e s t s - m e t e r " ) ) ( d e f n r e q u e s t s - m e t e r [ h a n d l e r ] ( f n [ r e q ] ( m e t e r s / m a r k ! i n c - r e q u e s t s - m e t e r ) ( h a n d l e r r e q ) ) )
93. Timers by “Stopwatch” William Warby (CC BY 2.0)
94. Timers Histogram & meter Histogram of duration of an activity, meter of occurrence of the activity For example, specific type of database query
95. Timers ( d e f i n c - r e q u e s t s - t i m e r ( t i m e r s / t i m e r ! m e t r i c s - r e g i s t r y " i n c o m i n g - r e q u e s t s - t i m e r " ) ) ( d e f n r e q u e s t s - t i m e r [ h a n d l e r ] ( f n [ r e q ] ( t i m e r s / t i m e ! i n c - r e q u e s t s - t i m e r ( h a n d l e r r e q ) ) ) )
96. Reporting Metrics
97. Reporting Metrics stores current state in its registry Should be reported to external tool Visualization Historical data Reporters for console, JMX, Ganglia, Graphite, CSV etc. included
98. Conclusion Organize around business capabilities Rules & conventions needed Standardize interfaces, logging and configuration Different solutions to improve stability Monitor your systems Good support in Clojure
99. Example Microservices Simple Calendar: Simple Contacts: https://github.com/innoq/simple- calendar https://github.com/innoq/simple- contacts
100. Thank you! Questions? https://www.innoq.com/de/talks/2016/04/microservice-meetup-clojure/