Breaking down problems - Speaker Deck

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Breaking down problems Building systems of systems with microservices at Monzo   @mattheath

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Hi, I’m Matt

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@mattheath

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16:02 Wednesday 29 May

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Nov 2015 May 2019 Fastest growing UK Bank  2m customers 40k a week

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LICENCE WITH RESTRICTIONS WE ARE HERE A UK banking licence is authorised by the PRA and regulated by the PRA and FCA1, allowing deposit- taking and balance sheet lending. Once granted, it allows firms to passport across Europe, accessing This is followed by a “mobilisation” phase during which final capital is raised and IT systems are completed, before launching to the public. We received a UK banking licence in August 2016! FEB 2015 JAN 2016 JAN 2017 MAR FEB FEB APR MAR MAR APR MAY JUN JUL MAY APR JUN MAY JUL JUN AUG JUL SEP AUG NOV SEP DEC NOV DEC PRE APPLICATION APPLICATION MOBILISATION LAUNCH LICENCE WITH RESTRICTIONS 50K MAX DEPOSIT WE ARE HERE A UK banking licence is authorised by the PRA and regulated by the PRA and FCA1, allowing deposit- taking and balance sheet lending. Once granted, it allows firms to passport across Europe, accessing This is followed by a “mobilisation” phase during which final capital is raised and IT systems are completed, before launching to the public. We received a UK banking licence in August 2016! JAN 2016 JAN 2017 MAR FEB FEB APR MAR MAR APR MAY JUN JUL MAY APR JUN MAY JUL JUN AUG JUL SEP AUG NOV SEP DEC NOV DEC E APPLICATION APPLICATION MOBILISATION LAUNCH LICENCE WITH RESTRICTIONS WE ARE HERE AUG  2016 JAN  2016 Feb  2015 APR  2017 A UK banking licence is authorised by the PRA and regulated by the PRA and FCA1, allowing deposit- taking and balance sheet lending. Once granted, it allows firms to passport across Europe, accessing This is followed by a “mobilisation” phase during which final capital is raised and IT systems are completed, before launching to the public. We received a UK banking licence in August 2016! FEB 2015 JAN 2016 JAN 2017 MAR FEB FEB APR MAR MAR APR MAY APR JUN MAY JUL JUN AUG JUL SEP AUG NOV SEP DEC NOV DEC PRE APPLICATION APPLICATION MOBILISATION LICENCE WITH RESTRICTIONS WE ARE HERE

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Henrik Kniberg

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Henrik Kniberg

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James Leah Matt

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James Leah Matt

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James Leah Matt

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X X X

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☕☕

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☕☕

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Payment API Ledger

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Payment API Ledger Database

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Transactions Payment API Ledger

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Transactions Payment API Ledger Accounts

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Payment API Transactions Ledger Accounts Cassandra Database

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Payment API Transactions Ledger Accounts Cassandra Database ? ? ? ? ? ? ? ? ?

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Henrik Kniberg

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Payment API Transactions Ledger Accounts Cassandra Database

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Card API Payment API Transactions Ledger Accounts Cassandra Database

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IT’S REAL!

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Feb 2015 May 2019 monzo services

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1136 Feb 2015 May 2019 monzo services

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Why do we break down problems?

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Why do we break down problems? Manage risk , Scale organisations Use the best tools for the job

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Why do we break down problems? Manage risk , Scale organisations Use the best tools for the job

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Managing risk ⚠ Are we building the right thing? ⚠ Will we hit deadlines?

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Henrik Kniberg

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Guiding Principle:   Ship it, and iterate.

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Guiding Principle:   Make changes small, make them often

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Why do we break down problems? Manage risk Use the best tools for the job , Scale organisations

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Why do we break down problems? Manage risk Use the best tools for the job , Scale organisations

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How do we break down problems?

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Dividing features up

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Henrik Kniberg

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Split by domain

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Split by domain users transactions faster payments card payments cards accounts addresses profiles feature flags points pots direct debits

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Split by responsibility

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Split by responsibility card management customer  signup clearing  processing reconciliation pots accounts authentication authorisation bank  statements message signing

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Guiding Principle:   Do one thing, do it well.

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Examples:  Building APIs

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API

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API

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API Service API Service API Service API Service API Service API Service API Service API Service Edge Gateway

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Accounts Cards Pots Feed Payments … Edge Gateway

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Edge Gateway Accounts Cards Pots Payments … New  Exciting  API!! Feed

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Cards Pots Payments … Accounts New  Exciting  API!! Feed Edge Gateway

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Cards Pots Payments … Accounts New  Exciting  API!! Feed Edge Gateway

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Edge Gateway Accounts Cards Pots Payments … New  Exciting  API!! Feed

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Isolate failure Make changes small, make them often Ship it, and iterate , Scale teams and organisations

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Single responsibility

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Load Balancer Edge Gateway

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Load Balancer Edge Gateway API Service

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Service A Load Balancer Edge Gateway API Service

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Service A Service B Load Balancer Edge Gateway API Service

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Service A Service B Load Balancer Edge Gateway API Service Responsible for external interface Responsible for domain logic Responsible for routing

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Manage complexity

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Account  Service Load Balancer Edge Gateway Account  API

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Account  Service Load Balancer Edge Gateway Account  API Account  Number  Service

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Account  Service Load Balancer Edge Gateway Account  API Account  Number  Service IBAN  Service

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Account  Service Load Balancer Edge Gateway Account  API Account  Number  Service IBAN  Service Account service responsibility remains unchanged, even as overall system complexity increases

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Account  Service Load Balancer Edge Gateway Account  API Account  Number  Service IBAN  Service Additional, simple, services solve related but separate problems

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Account  Service Load Balancer Edge Gateway Account  API Account  Number  Service IBAN  Service

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Account  Service Load Balancer Edge Gateway Account  API Account  Number  Service IBAN  Service Euro  Payment  Services 5

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Account  Service Account  Number  Service IBAN  Service Euro  Payment  Services 5 Decoupling more specialised behaviour keeps services simple 6

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Account  Service Account  Number  Service IBAN  Service Euro  Payment  Services 5 Faster  Payment  Services 6

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Account  Service Account  Number  Service IBAN  Service Euro  Payment  Services 5 Faster  Payment  Services 6

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Account  Service Account  Number  Service IBAN  Service Euro  Payment  Services 5 Faster  Payment  Services 6 Isolating failure along domain boundaries improves availability

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Managing Complexity

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Guiding Principle:   Simplicity

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https://xkcd.com/927/

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Feb 2015 May 2019 monzo services

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Feb 2015 May 2019 monzo services The Refactathon https://monzo.com/blog/2018/12/04/refactoring-old-code/

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8 Identify areas of complexity and debt ⏱ Make time to improve these Teams move faster as a result

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Guiding Principle:   Standardisation

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Build on abstractions

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instance instance instance instance instance instance

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instance instance instance instance instance instance Kubernetes

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kubernetes

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kubernetes service service service service service service service service service service service service service

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kubernetes service Prometheus service service Kafka envoy Kafka service service Elasticsearch calico API service

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Service Service Reliable inter-service  communication ofﬂoaded to Envoy, simplifying services

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API Service Service A Service B Load Balancer Edge Gateway Service D Service D Service C Event based communication decouples services, lets us build reliable pipelines, and helps isolate failure

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Simple systems are more reliable Small systems are faster to iterate Failure can be isolated and constrained

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Standardisation makes everything easier Onboard new people quickly People feel empowered and productive Build reliable systems, faster.