Rethinking State Management currentState = foldl(pastEvents, initialState)

About Me Pragmatic, Passionate and Polyglot Programmer @tamizhvendan http://blog.tamizhvendan.in www.ajira.tech bit.ly/fsapplied

Rethinking ? State ??

CafeApp Courtesy: http://cqrs.nu/

Old School Backend Data Store Imperative Data Modeling!

{ "tables" : [{"id" : 1, "status" : "available"}] } { "tables" : [{"id" : 1, "status" : "waiting_for_order"}] } { "tables" : [{"id" : 1, "status" : "in_service"}] }

A Different Perspective

t0 t1 Open Tab t2 Place Order

t0 {"status" : "available"} t1 {"status" : "waiting_for_order"} t2 {"status" : "in_service"}

STATE is Transient

Serve Drink Prepare Food Serve Food Closed Tab Placed Order Opened Tab Order InProgress Served Order Open Tab Place Order Serve Drink Prepare Food Close Tab

State X State Y Command // State -> Command -> State let evolve state command = let newState = // ... newState

Closed Tab Opened Tab Open Tab initial state Tab Opened event // State -> Command -> Event list let execute state command = let events = // ... events

Closed Tab Placed Order Opened Tab Order InProgress Served Order Tab Opened Order Placed Drink Served Food Prepared Drink Served Food Prepared Food Served Tab Closed Order Served

// State -> Event -> State let apply state event = let newState = // ... newState Events Are Facts Yes, It’s like a Bank Statement! Closed Tab Opened Tab Tab Opened event current state new state

Closed Tab Placed Order Opened Tab Order InProgress Served Order Tab Opened Order Placed Drink Served Food Prepared Drink Served Food Prepared Food Served Order Served Tab Closed Tab Opened Order Placed Drink Served Food Prepared Food Served Order Served Tab Closed It’s append only!

Closed Tab Tab Opened Opened Tab Tab Opened Order Placed Drink Served Food Prepared Food Served Order Served Tab Closed past events Closed Tab initial state Opened Tab Order Placed Placed Order Placed Order Drink Served Order InProgress Food Prepared Order InProgress Order InProgress Food Served Order InProgress Order InProgress Order Served Served Order Order InProgress Tab Closed Closed Tab Served Order // State -> Event list -> State let computeState initialState pastEvents = List.fold apply initialState pastEvents // State -> Event -> State let apply state event = let newState = // ... newState Performance ??

// State -> Command -> State let evolve state command = let newState = // ... newState // State -> Command -> Event list let execute state command = let events = // ... events // State -> Event list -> State let computeState initialState pastEvents = List.fold apply initialState pastEvents // State -> Event -> State let apply state event = let newState = // ... newState // Event list -> Command -> Event list let evolve pastEvents command = let state = computeState TabClosed pastEvents execute state command

// State -> Command -> State let evolve state command = let newState = // ... newState // Event list -> Command -> Event list let evolve pastEvents command = let state = computeState TabClosed pastEvents execute state command currentState = foldl(pastEvents, initialState) Current state is a left-fold of past events Event Sourcing - Greg Young We started with this and evolved to this!

// Event list -> Command -> Event list let evolve = // ... // Command -> int let getTableId command = // ... // int -> Event list let getPastEvents tableId = // ... // Command -> Event list let handleCommand command = let pastEvents = getTableId command |> getPastEvents evolve pastEvents command Handling Commands

// Command -> Event list let handleCommand command = let pastEvents = getTableId command |> getPastEvents evolve pastEvents command // HttpRequest -> HttpResponse let commandHandler httpRequest = let command = // get Command from HTTP request let newEvents = handleCommand command saveEvents newEvents // return OK response Handling Commands

Serve Food Open Tab Place Order Serve Drink Prepare Food Close Tab Commands

Table Status Food(s) To Prepare Item(s) To Serve Amount To Pay Queries!!

Old School Backend Data Store Imperative Data Modeling!

{ "tables" : [{"id" : 1, "status" : "available"}] } { "tables" : [{"id" : 1, "status" : "waiting_for_order"}] } { "tables" : [{"id" : 1, "status" : "in_service"}] } Data Model Mutate during Commands Fetch during Queries

Single Responsibility Commands Queries Command Query Responsibility Separation

Event Projection Closed Tab Opened Tab Open Tab Tab Opened Publish ReadModel Projection Client Listen Populate Query API Server Read WebSocket Client Listen Push Pull Data Analytics Client Listen Populate Message Queue Tab Opened Kafka, RabbitMQ, etc., Uni directional Data flow (Flux?)

Read Model Projection { "tables" : [{"id" : 1, "status" : "available"}] } Tab Opened { "tables" : [{"id" : 1, "status" : "waiting_for_order"}] } Order Placed { "tables" : [{"id" : 1, "status" : "in_service"}] } Order Served

Error Handling User may post an invalid command Fetching Events may fail Saving Events may fail type Result<'T,'E> = | Success of 'T | Error of 'E

User may post an invalid command // Event list -> Command -> Event list let evolve = // ... // Command -> Event list let handleCommand command = let pastEvents = getTableId command |> getPastEvents evolve pastEvents command // State -> Command -> Event list let execute state command = let events = // ... events // State -> Command -> Result let execute state command = match command with | OpenTab -> match state with | ClosedTab -> Success [TabOpened] | _ -> Error "Tab already opened" // ... // Event list -> Command -> Result let evolve = // ... // Command -> Result let handleCommand command = // ...

User may post an invalid command // HttpRequest -> HttpResponse let commandHandler httpRequest = let command = // get Command from HTTP request let newEvents = handleCommand command saveEvents newEvents // return OK response // HttpRequest -> HttpResponse let commandHandler httpRequest = let command = //... match handleCommand command with | Success newEvents -> saveEvents newEvents // return OK response | Error message -> // return BAD_REQUEST Fetching Events may fail Saving Events may fail https://fsharpforfunandprofit.com/rop/

Performance Tab Opened Order Placed Drink Served Food Prepared Food Served Order Served Tab Closed Do I need to fetch all the events to compute the state every time? Need not to be! Snapshots

Parallel Update Optimistic Locking Tab Opened 0 Order Placed 1 Drink Served 2 Food Prepared 3 Food Served 4

It’s not a “Silver Bullet” Thinks To Consider A whole system based on Event Sourcing is an Anti-Pattern CQRS and Event Sourcing are not top-level Architectures

- ThoughtWorks Tech Radar 2012 https://github.com/tamizhvendan/CafeApp bit.ly/fsapplied

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