# Domain modelling with state machines --- ## Who am I? Marco Perone [@marcoshuttle](https://twitter.com/marcoshuttle) [marcosh.github.io](http://marcosh.github.io/) working at [Tweag](https://www.tweag.io/) --- ## Working environment We will be using [Gitpod](https://gitpod.io/#https://github.com/marcosh/ddd-machines-dddeurope) https://github.com/marcosh/ddd-machines-dddeurope --- ## Domain modelling ---  --- ### <span style="color: orange">Domain events</span>  Events relevant for domain experts --- ### <span style="color: dodgerblue">Commands</span>  User intentions/actions/decisions --- ### <span style="color: lightyellow">Read models</span>  Data needed in order to make decisions --- ### <span style="color: yellow">Aggregates</span>  Decide what happens on commands --- ### <span style="color: orchid">Policies</span>  Reactive logic that takes place after an event --- ### <span style="color: lightgreen">Projections</span>  Aggregate data from events --- ## State machines --- ### Mealy machines ```haskell data Mealy state input output = Mealy { initialState :: state , action :: state -> input -> (state, output) } ```  --- ## Our domain --- ### Risk management We are building a lending application We collect information about the user and the loan in order to decide wheter to grant the user the requested loan --- ### Workflow ```mermaid graph TD CollectUserData --> CollectLoanDetails CollectUserData --> QueryCreditBureau CollectLoanDetails --> AllDataCollected QueryCreditBureau --> AllDataCollected ``` --- ### Event storming  --- ## Let's cook our ingredients together --- ## <span style="color: yellow">Aggregates</span>, <span style="color: lightgreen">projections</span> and <span style="color: orchid">policies</span> could be implemented as state machines --- ### <span style="color: yellow">Aggregates</span>  From <span style="color: dodgerblue">commands</span> to <span style="color: orange">events</span> --- Let's implement the <span style="color: yellow">aggregate</span> for our domain --- To compile the code and run the tests ```bash stack test --file-watch ``` --- ### Events ```haskell data RiskEvent = UserDataRegistered UserData | LoanDetailsProvided LoanDetails | CreditBureauDataReceived CreditBureauData ``` --- ### Commands ```haskell data RiskCommand = RegisterUserData UserData | ProvideLoanDetails LoanDetails | ProvideCreditBureauData CreditBureauData ``` --- What is the state space? ```haskell data RiskState = _ ``` --- Next we need to implement ```haskell action :: RiskState -> RiskCommand -> ([RiskEvent], RiskState) ``` and ```haskell initialState :: RiskState ``` --- ### <span style="color: lightgreen">Projections</span>  From <span style="color: orange">events</span> to <span style="color: lightyellow">read models</span> --- We want to project the received data, which could or could not be there ```haskell data ReceivedData = ReceivedData { userData :: Maybe UserData , loanDetails :: Maybe LoanDetails , creditBureauData :: Maybe CreditBureauData } ``` --- The projection is a state machine with <span style="color: #c7254e">`RiskEvent`</span> as input and <span style="color: #c7254e">`ReceivedData`</span> as output --- What is the state space? --- We can define it by implementing ```haskell action :: ReceivedData -> RiskEvent -> ReceivedData ``` and ```haskell initialState :: ReceivedData ``` --- ### <span style="color: orchid">Policies</span>  From <span style="color: orange">events</span> to <span style="color: dodgerblue">commands</span> --- <span style="color: orchid">Policies</span> are the only one allowing side effects --- A policy is an effectful state machine with <span style="color: #c7254e">`RiskEvent`</span> as inputs and <span style="color: #c7254e">`RiskCommand`</span> as outputs. --- Our policy needs to react to the <span style="color: #c7254e">`UserDataRegistered`</span> event and send the data to the credit bureau, using the <span style="color: #c7254e">`interactWithCreditBureau`</span> function --- To implement it, we just need to implement ```haskell action :: RiskEvent -> IO [RiskCommand] ``` --- How do we connect the pieces, now? --- ## Write side ```haskell feedback :: Mealy command [event] -> Mealy event [command] -> Mealy command [event] ``` Allows us to combine <span style="color: yellow">Aggregate</span> and <span style="color: orchid">Policy</span> --- ## Whole application ```haskell instance Category Mealy where (.) :: Mealy b c -> Mealy a b -> Mealy a c ``` Sequential composition Allows us to compose <span style="color: #c7254e">`Write`</span> and <span style="color: #c7254e">`Read`</span> sides --- Let's try to run it! ```haskell -- risk/Main.hs main :: IO () main = print =<< runApplication riskManagerApplication [ RegisterUserData myUserData , ProvideLoanDetails myLoanDetails ] ``` --- To execute it ```bash stack exec ddd-machines-dddeurope-risk ``` --- ## Requirements change Suppose we want to add another projection ```haskell userDataUpdatesCounter :: Projection RiskEvent (Sum Int) ``` Let's adapt our application to incorporate it --- We can use ```haskell (&&&) :: Projection a b -> Projection a c -> Projection a (b, c) ``` to run the two projections in parallel --- Time for question and remarks --- Thank you all!