Caching is an a very integral part of a mobile app since it allows less battery usage, quicker interactions and better experience overall. In this talk we’re going to explore the fundamentals of caching and learn how to compose different cache layers such as disk and network into one by defining each layer as a monoid which makes the composition of them effortless and more declarative.

1. COMPOSABLE CACHING
GEORGE TSIFRIKAS

2. WHY?
RAM
DISK
NETWORK
0 100 200 300 400

3. Ram
Disk
Network
Key Value
Error
Miss
Miss
Value
Value

4. IMAGE CACHING

5. Ram UIImage
Disk JPEG
Network Request
URL UIImage
Error
Miss
Miss
Value
Value
IMAGE CACHING

6. Ram Model
Disk JSON
Network Request
URL Model
Error
Miss
Miss
Value
Value
MODEL CACHING

7. CACHE
protocol Cache {
}

8. CACHE
protocol Cache {
associatedtype Key
associatedtype Value
}

9. CACHE
protocol Cache {
associatedtype Key
associatedtype Value
func get(_ key: Key) -> Observable<Value>
func set(_ value: Value, for key: Key) -> Observable<Void>
}

10. DISK CACHE
class DiskCache<K, V>: Cache
where K: StringConvertible, V: NSCoding {
typealias Key = K
typealias Value = V
func get(_ key: Key) -> Observable<Value> {}
func set(_ value: Value, for key: Key) -> Observable<Void> {}
}

11. NETWORK CACHE
class NetworkCache: Cache {
typealias Key = URLRequest
typealias Value = NSData
func get(_ key: URLRequest) -> Observable<NSData> {}
func set(_ value: Value, for key: Key) -> Observable<Void> {
return .just(())
}
}

12. NETWORK CACHE
protocol Fetcher: Cache {}
extension Fetcher {
func set(_ value: Value, for key: Key) -> Observable<Void> {
return .just(())
}
}

13. NETWORK CACHE
class NetworkCache: Cache {
typealias Key = URLRequest
typealias Value = NSData
func get(_ key: URLRequest) -> Observable<NSData> {}
func set(_ value: Value, for key: Key) -> Observable<Void> {
return .just(())
}
}

14. NETWORK FETCHER
class NetworkFetcher: Fetcher {
typealias Key = URLRequest
typealias Value = NSData
func get(_ key: URLRequest) -> Observable<NSData> {}
}

15. CACHE COMPOSITION

16. Ram
Disk
Network
Key Value
Error
Miss
Miss
Value
Value
CACHE

17. Ram
Disk
Network
Key Value
Error
Miss
Miss
Value
Value
CACHE

18. Ram
Disk
Network
Key Value
Error
Miss
Miss
Value
Value
CACHE

19. Ram
Disk
Network
Key Value
Error
Miss
Miss
Value
Value
CACHE

20. Ram
Disk
Network
Key Value
Error
Miss
Miss
Value
Value
CACHE

21. Ram
Disk
Network
Key Value
Error
Miss
Miss
Value
Value
CACHE

22. Ram
Disk
Miss Value
COMPOSITION
Network
Key Value
Error
Miss Value

23. COMPOSITION
extension Cache {
public func compose<B: Cache>(other: B)
-> CompositeCache<Key, Value> where Key == B.Key, Value == B.Value {
return CompositeCache(
get: { (key: Key) -> Observable<Value?> in
},
set: {value, key in
})
}
}

24. COMPOSITION
extension Cache {
public func compose<B: Cache>(other: B)
-> CompositeCache<Key, Value> where Key == B.Key, Value == B.Value {
return CompositeCache(
get: { (key: Key) -> Observable<Value?> in
return self.get(key)
},
set: {value, key in
})
}
}

25. COMPOSITION
extension Cache {
public func compose<B: Cache>(other: B)
-> CompositeCache<Key, Value> where Key == B.Key, Value == B.Value {
return CompositeCache(
get: { (key: Key) -> Observable<Value?> in
return self.get(key)
.flatMap { (value) -> Observable<Value?> in
if let foundValue = value {
return .just(foundValue)
}
}
},
set: {value, key in
})
}
}

26. COMPOSITION
extension Cache {
public func compose<B: Cache>(other: B)
-> CompositeCache<Key, Value> where Key == B.Key, Value == B.Value {
return CompositeCache(
get: { (key: Key) -> Observable<Value?> in
return self.get(key)
.flatMap { (value) -> Observable<Value?> in
if let foundValue = value {
return .just(foundValue)
}
return other.get(key)
.flatMap({ (valueFromOther) -> Observable<Value?> in
if let valueFromOther = valueFromOther {
_ = self.set(valueFromOther, for: key).subscribe()
}
return .just(valueFromOther)
})
}
},
set: {value, key in
})
}
}

27. COMPOSITION
extension Cache {
public func compose<B: Cache>(other: B)
-> CompositeCache<Key, Value> where Key == B.Key, Value == B.Value {
return CompositeCache(
get: { (key: Key) -> Observable<Value?> in
return self.get(key)
.flatMap { (value) -> Observable<Value?> in
if let foundValue = value {
return .just(foundValue)
}
return other.get(key)
.flatMap({ (valueFromOther) -> Observable<Value?> in
if let valueFromOther = valueFromOther {
_ = self.set(valueFromOther, for: key).subscribe()
}
return .just(valueFromOther)
})
}
},
set: {value, key in
return .zip(self.set(value, for: key),
other.set(value, for: key)) { _,_ in }
})
}
}

28. Ram
Disk
Miss Value
COMPOSITION
Network
Key Value
Error
Miss Value

29. COMPOSITION
let ramDiskCache = ramCache.compose(other: diskCache)

30. Ram
Disk
Miss Value
COMPOSITION
Network
Key Value
Error
Miss Value

31. COMPOSITION
let ramDiskCache = ramCache.compose(other: diskCache)
.compose(other: networkFetcher)

32. COMPOSITION
let ramDiskCache = (ramCache + diskCache) + networkFetcher

33. COMPOSITION
let ramDiskCache = (ramCache + diskCache) + networkFetcher
let ramDiskCache = ramCache + (diskCache + networkFetcher)
// vs

34. COMPOSITION
let ramDiskCache = (ramCache + diskCache) + networkFetcher
let ramDiskCache = ramCache + (diskCache + networkFetcher)
// Equivalent to
Associative

35. COMPOSITION
Identity
Foo cache
Miss Value
Always misses

36. Identity
Foo cache
Miss Value
COMPOSITION
Always misses
Foo cache
Identity
Miss Value
Always misses
=

37. COMPOSITION
Associativity
Identity element
} Monoid
https://en.wikipedia.org/wiki/Monoid

38. MONOID?? WHAT?

40. COMPOSITION
extension Cache {
public func compose<B: Cache>(other: B)
-> CompositeCache<Key, Value> where Key == B.Key, Value == B.Value {
return CompositeCache(
get: { (key: Key) -> Observable<Value?> in
},
set: {value, key in
})
}
}

41. Ram
Disk
Miss Value
COMPOSITION
Network
Key Value
Error
Miss Value

42. Ram
Disk
Miss Value
COMPOSITION
Network
Key Value
Error
Miss Value

43. ADAPT

44. ADAPT
extension Cache {
func mapValues<V2>(f: @escaping (Value) throws -> V2,
fInv: @escaping (V2) throws -> Value)
-> CompositeCache<Key, V2> {}
}

45. ADAPT
let imageDiskCache = diskCache.mapValues(
f: { (data) -> UIImage in
return UIImage(data: data as Data)!
}) { (image) -> NSData in
return UIImagePNGRepresentation(image) as! NSData
}

46. Ram
Disk
Network
Key Value
Error
ADAPT, USER PROFILE EXAMPLE

47. Ram
Disk
Network
ADAPT, USER PROFILE EXAMPLE

48. Ram
Disk
Network
ADAPT, USER PROFILE EXAMPLE

49. Ram
Disk
Network
ADAPT, USER PROFILE EXAMPLE
f( ) =
Data -> JSON

50. Ram
Disk
Network
ADAPT, USER PROFILE EXAMPLE
f( ) =
Data -> JSON
f( ) =
JSON -> User
Profile Model

51. Ram
Disk
Network
ADAPT, USER PROFILE EXAMPLE
f( ) =
Data -> JSON
f( ) =
JSON -> User
Profile Model
fInv( ) =
fInv( ) =
JSON ->
Data
User Profile
Model -> JSON

52. Ram
Disk
Network
ADAPT, USER PROFILE EXAMPLE
f( ) =
f( ) =fInv( ) =
fInv( ) =

53. ADAPT
extension Cache {
func mapKeys<K2>(fInv: @escaping (K2) throws -> Key) ->
CompositeCache<K2, Value> {}
}

54. Ram
Disk
Network
ADAPT, USER PROFILE EXAMPLE
userId -> url
string
representation
fInv( ) =
fInv( ) =
url string
representation ->
URLRequest

55. ADAPT
let ramCache = RamCache<URL, User>()
let diskCache = DiskCache<URL, NSData>()
let networkFetcher = NetworkFetcher()
.mapKeys { (url: URL) -> URLRequest in
return URLRequest(url: url)
}
let diskNetworkCache = diskCache + networkFetcher
let userDiskNetworkCache = diskNetworkCache
.mapValues(f: { (userJSONData) -> User in
try JSONDecoder().decode(User.self, from: userJSONData)
}) { (user) -> NSData in
try JSONEncoder().encode(user)
}
let userCache = ramCache + userDiskNetworkCache

56. OPTIMISATION

57. REUSE-IN-FLIGHT
extension Cache where Key: Hashable {
func reuseInFlight() -> CompositeCache<Key, Value> {
let dictionary = Dictionary<Key, Observable<Value?>>()
return CompositeCache(
get: { (key) in
return (dictionary[key] ?? ({
let newRequest: Observable<Value?> = self.get(key)
dictionary[key] = newRequest
return newRequest
})())!
},
set: set,
)
}
}

58. REUSE-IN-FLIGHT
let avatarCache = ramCache + diskCache + networkFetcher.reuseInFlight()

59. REUSE-IN-FLIGHT
let avatarCache = ramCache + (diskCache + networkFetcher).reuseInFlight()

60. .forwardRequest
.capRequests
.skipWhile
.switchCache
.. and more

61. https://github.com/spring-media/Carlos
https://github.com/gtsifrikas/Droste
LIBRARIES
Compatible with RxSwift

62. WE ARE HIRING!
careers.workable.com

63. QUESTIONS?
George Tsifrikas
END OF PRESENTATION

64. THANK YOU!
George Tsifrikas
END OF PRESENTATION