Middleware all the things

Middleware all the things

At Trainline, we sell train tickets, across EU and UK.

How does it work? Frontends Rails API Connections API aka connection with carriers APIs

A bunch of middlewares Requests are processed by generated apps on the fly, which are stacks of middlewares.

Request (an environment Hash) Connections API env[:params][:action] Base::Builder.app do use DoSomething use AnotherThing # insert a bunch of cool stuff here end

Connections API An action (an app) Middleware 1 ① down ↓ ↑ ④ up Middleware 2 ② down ↓ ↑ ③ up Middleware 3 ⃔ → ⑤ env[:callback] ⃔

% tree lib/pacon/actions -L 1 lib/pacon/actions ├── book.rb ├── book_subscription.rb ├── cancel.rb ├── emit.rb ├── emit_subscription.rb ├── estimate_exchange.rb ├── estimate_refund.rb ├── exchange.rb ├── exchange_pay.rb ├── exchange_search.rb ├── fetch_pnr.rb ├── fetch_subscription.rb ├── fetch_travel_document.rb ├── immediate_cancel.rb ├── load_timetable.rb ├── pay.rb ├── refund.rb └── search.rb

def self.search Base::Builder.app do use Base::LogAction use Base::VolatileStations use Base::InitResultCache use Base::LogSearchFruitfulness use Base::AvoidFruitlessSearch use Base::ReportAndBlockCombinations use Base::TryAgain use Base::SearchWindow use Base::FilterUnsellableFolderMix use Base::FilterTimetableResults use Base::TooManyPassengers use Base::WarnDifferentStation use Base::Parallel end end

def self.search Base::Builder.app do use Base::LogAction use Base::VolatileStations use Base::InitResultCache use Base::LogSearchFruitfulness use Base::AvoidFruitlessSearch use Base::ReportAndBlockCombinations use Base::TryAgain use Base::SearchWindow use Base::FilterUnsellableFolderMix use Base::FilterTimetableResults use Base::TooManyPassengers use Base::WarnDifferentStation use Base::Parallel end end Will call SNCF.search, DB.search…

module SNCF class << self def search Base::Builder.app do use SNCF::Stations use SNCF::ExtractFoldersFromJourneys # tons of very secret sauce use SNCF::HttpRequest end end end end

Event Machine FTW The whole is based on Event Machine to coordinate all the parallel requests made to carriers APIs.