Continuation, MessagePassing & NoSQL

The Greater Helsinki Area F# User Group

http://www.meetup.com/FSharpHelsinki/

© Tuomas Hietanen, 2012

Forecast…

The number of different data producers increases

The number of processors and communication increases

We don’t know the runtime environment…

(Could be e.g. cloud; Azure)

Turing, state machines and objects

Nouns vs. verbs• Who is the owner of the activity?• Object versus event

The problems of state machine:• Complexity: What’s going on?• Who caused the present state? How to reproduce?• Asynchrony-problems, locks

Encapsulation of the state: Type vs Class vs Monad

Type

• No state

Class

• State may be directly exposed

• The state is also revealed through the methods

Monad

• Like no state• The state is

revealed when leaving out

Ease up programming in “a particular context”

The role of a Domain Model

Intended for developers• The computer itself doesn’t get any benefits

Attemps to model the playfield• Advantages vs. disadvantages?

Roles• Active / anemice• Can the model evolve?

Set-operations

x x x

x x y x x

y y yx = y

x = okFilter:

Projection:(“mapping”)

Set-operations

x x x

x x y x x

y y yx = y

x = okFilter:

Projection:(“mapping”)

SQL: Select … where

C#: LINQ

Other languages: Map, Filter

Cloud: MapReduce

Reactive programming

Observe the surrounding environment

Set-operations to environment events

Event sourcing (CQRS)

Event history

Aggregate the Domain Model

Notice the similarity to reactive programming

Message passing

Transport messages (Message may also be an event.)

Set-operations to messages

Actors

Don’t reveal their state

Uses message passing• May call others• …or itself

Agents

Don’t reveal their state• Handles requests

Uses message passing• May call others• …or itself

Continuation

Reify: How long to dream, and when to execute?

Possible actions: continue, complete or exception

Church vs. Call-with-current-continuation (call/cc)

NoSQL: Data without schema

Document databases• No IDs• Dualism with SQL-databases• RavenDB, MongoDB

Big Data• Log-files, GPS-data, etc• Hadoop, MapReduce