<p>Reactive Systems Dave Farley http://www.davefarley.net @davefarley77 Dave Farley http://www.davefarley.net @davefarley77 http://www.continuous-delivery.co.uk Reactive Systems 21st Century Architecture for 21st Century Problems Our World Is Changing Large Applications circa 2005: • 10’s of Servers • Seconds of Response Time • Hours of Offline Maintenance • Gigabytes of Data
 Large Applications Now: • Handheld Devices to 1000’s of multi-core processors • Millisecond Response Time • 100% Uptime • Petabytes of Data Our World Is Changing The Reactive Manifesto Responsive Elastic Resilient Message Driven “21st Century Problems are not best solved with 20th Century Software Architectures” The Evolution of modern hardware has changed many of the common assumptions of software development Source: www.reactivemanifesto.org Reactive Systems Are: Responsive: • Responds in a Timely Manner • Cornerstone of Usability • Also Quick to Detect Problems Reactive Systems Are: Resilient: • Remains Responsive in the Face of Failure • Resilience Depends on - Replication, Containment, Isolation and Delegation Reactive Systems Are: Elastic: • Remains Responsive Under Varying Workload • Responds to Change in the Input Rate By Increasing or Decreasing Resources that Service the Input • Decentralised Architecture, No Contention Points, No Central Bottlenecks Reactive Systems Are: Message Driven: • Asynchronous Message Passing is the foundation for all of these properties • Loose-Coupling, Isolation, Location Transparency • Ability to Delegate Errors Properties of Reactive Systems • Flexible • Loosely-Coupled • Scalable • Easier to Develop • More Tolerant of Failure • Respond to Failure Gracefully • Responsive to Users Fractal Architecture • Large Systems Are Composed of Smaller Ones • They Depend on the Reactive Properties of Their Constituents • These Benefits Operate At All Scales • Such Systems are Composable Failure Modes in Synchronous Messaging Component ‘B’ Component ‘A’ Synch Messaging Breeds Complexity Component ‘B’ Component ‘A’ Synchronous Comms Increases Coupling in Location and Time Synch Messaging Breeds Complexity Component ‘B’ Component ‘A’ ? Component ‘A’ The Benefits of Asynchrony Component ‘B’ Single Thread ed! Single Thread ed! An Example Component ‘B’ Component ‘A’ BookStore Inve tory Order(“Continuous Delivery”) Reserve(“Continuous Delivery”) Order(“Continuous Delivery”) Reserve(“Continuous Delivery”) Ordered(“Continuous Delivery”) Ordered(“Continuous Delivery”) An Example BookStore Inventory res erv ing or de red res erv ing Order(“Continuous Delivery”) Reserve(“Continuous Delivery”) Ordered(“Continuous Delivery”) Ordered(“Continuous Delivery”) Bet er Aerobatics”) Bet er Aerobatics”) Bet er Aerobatics”) or de red Bet er Aerobatics”) 5 An Example of Idempotence Component ‘B’ 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 Expected(3) 3 4312 Component ‘A’ 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 Isolation • Decoupling in Time and Space • Time - Sender and Receiver have independent lifecycle • Space - Location Transparency • Share Nothing! • Built on Inter-Component Communication over Well Defined Protocols Isolation Component ‘A’ Component ‘B’ Share Nothing Component ‘A’ Component ‘B’ Back-Pressure • You Can’t Isolate Stress • The System as a Whole Needs to Respond Sensibly • Unacceptable For a Stressed Component to Fail Catastrophically or Loose Messages • Queues Represent An Unstable State - Load • Components Under Stress Need to Reflect This By Applying Back-Pressure, Slowing Upstream Inputs Queues Represent an Unstable State Component ‘B’ Component ‘A’ Queues are always full or always empty. Anything else is transitional, on its way to full or empty. Slightly Faster Slightly Slower Always Empty! Queues Represent an Unstable State Queues are always full or always empty. Anything else is transitional, on its way to full or empty. Component ‘B’ Component ‘A’ Slightly Faster Slightly Slower Always Full! ? Back-Pressure Component ‘B’ Component ‘A’ Slightly Faster Slightly Slower Always Full! Back-Pressure! Component ‘n’ Back-Pressure! Eventual Consistency Component ‘A’ Component ‘B’ Location Transparency • Elastic Systems Need To React To Changes In Demand • We Are All Doing Distributed Computing • Embracing This Means There Is No Difference Between Horizontal (Cluster) and Vertical (Multicore) Scalability • Components Should Be Mobile • One Pattern For Communications • Local Communications Is An Optimisation Linear Scalability Through Sharding Component ‘A’ Component ‘B’ Component ‘B2’ Component ‘B1’ Linear Scalability Through Sharding Component ‘A’ Modern Hardware Should Change Our Assumptions • For Efficient Software, The Biggest Cost is Shifting Data • RAM is Not Random Access • Disk is Not Random Access • SSD is Not Random Access • RAM is Slow, Network is Slow, Disk is Slower Conway’s Law Siloed Teams Rigid Architecture DB UI Specialists Middleware Specialists DB Specialists Conway’s Law Cross-Functional Teams Organised by Business Function Distributed Service Architecture Bounded Contexts • Each Component Is Autonomous and Isolated • It Only Communicates Through Well Defined Protocols • Works Best When Components Are Aligned With Bounded Contexts • Bounded Contexts Are A Concept From Domain Drive Design (DDD) - Eric Evans • Bounded Context: • The Context Within Which A Model Of the Problem Domain Make Sense • There Should Be ‘Translations’ Between Bounded Contexts - No “One Model To Rule Them All” • Bounded Contexts Tend To Align With Business Functions • Best Way To Decompose Organisations And Systems Example Reactive, MicroService architecture Public API Notification Service Market Management Application Customer Service Application TFX Application Public Web App Contact Service Trade Report Service Account Service Customer Service Market Management Payment Service Market Makers FIX Gateways Instrument Service Notification Service Market Data Consumers Clearing Gateways Public Message Bus Execution Venue Execution Venue Execution Venue (Markets & Matching) Execution Management Service Execution Management Service Execution M nagement Service (Accounts & Positions) Core Services General Services Gateway Services Gateway Services Control Message Bus Where to start? Q&A http://www.continuous-delivery.co.uk Dave Farley http://www.davefarley.net @davefarley77 </p>