Cheating Gall's Law: MediterraneaJS edition

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Cheating Gall's Law

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C J Silverio director of engineering, @ceejbot

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Cheating Gall's Law How we split a monolith and lived to tell the tale

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Gall's Law A complex system that works is invariably found to have evolved from a simple system that worked.

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Gall's Law in full “A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over with a working simple system.”

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"A simple system may or may not work."

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Systemantics: How Systems Really Work and How They Fail

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starting simple often means starting with a monolith

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monolith everything in one process

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npm's monolith: embedded in couchdb

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monoliths work just ﬁne

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whatever it takes to build a system that satisﬁes your users

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success! now scale it.

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scaling monoliths many copies of the full thing

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Exponential growth of node resulted in exponential growth of the npm registry

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exponential monoliths were going to be expensive

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splitting the monolith

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yay microservices?

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just rewrite everything! what's the problem?

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Your monolith is complex. A split system is more complex.

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what did that Gall guy say about complex working systems?

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"If a system is working, leave it alone. Don't change anything."

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Gall's Law warns us about that rewrite

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how do you split a monolith successfully?

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Let's cheat.

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Q: How do you cheat? A: By not rewriting the whole thing.

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slice off a part of the system into a module with a clearly-deﬁned interface

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then write a second implementation of that interface

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send requests to that second implementation with a proxy

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now let's make our proxy smart Replace Varnish with a node service

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ﬁrst step just send everything through

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record what the proxy does how do people use your service? measure performance

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second step divide & conquer

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beat Gall's Law with modularity aka information hiding

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each service is a module a simple, testable system when viewed in isolation

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best part: now you can change everything

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your platform your database

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why stop there?

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the awesome

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Each piece is a simple system

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more scaling dials to turn ﬁner-grained perf data

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isolating tasks let us debug even without splitting the monolith

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the system overall is complex but loosely coupled

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you have a working system every step of the way

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a proxy service lets you ﬂip back & forth between implementations

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the not awesome

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now you've got a distributed system

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@architectclippy says “I see you have a poorly structured monolith. Would you like me to convert it into a poorly structured set of microservices?”