Web micro-services in Go

Task

Test how web micro-services can be written/developed in Go.

Features to check

• provide RESTful API for a fake "Todo" service
• deploy service to Cloud Foundry
• provide WebSocket endpoint that reverses the string passed
• use RabbitMQ with publish/subscribe; implement something like when I POST at a REST endpoint, inform through a RabbitMQ queue on this ORM change
• deploy micro-service and link it with RabbitMQ services in Cloud Foundry
• check out gorm for ORM support
• use PostgreSQL as a database backend
• deploy micro-service and link it with MySQL services in Cloud Foundry: so we change the used database backend when deploying - how do we do it?
• use OAuth2 and authenticate and authorize against Cloud Foundry's UAA

Components used

On my machine I used

• Windows 10.0.17134.137
• Erlang 10.0.1
• RabbitMQ 3.7.7
• Go 1.10.3 x64
• cf CLI cf version 6.36.2+18ceab10f.2018-05-16
• PCF Dev version 0.30.0 (CLI: 850ae45, OVA: 0.549.0)
• PCF Dev has a p-rabbitmq service deployed under the name rabbitmq
• GoLand as IDE

AMQP integration

• everytime a Todo is added or updated using POST /todo/{id} end-point, this Todo is piped into a Go channel chan Todo called todoChanged
• AMQP connection is established and queue orm:todos is created in the go routine Connect2AMQPAndSetupQueue that is executed at start
• in its body an endless loop waits for new Todo that arrive in the todoChanged chan and finally publishes them in the queue orm:todos upon arrival

Lessons learned

• how GOPATH works
• how dep works to store dependencies and resolve them to a vendor directory - needed to deploy external dependencies when staging in Cloud Foundry
• how the manifest.yml needs to written (env e.g. needs to contain GOPACKAGENAME, command contains the name of the binary to execute)
• Cloud Foundry buildpacks
  • how to use a remote buildpack in cf push with -b https://....
  • how to update a buildpack with cf update-buildpack in a PCF Dev deployment
• for i,v := range slice returns the index i - for updating the element - and a copy v of the element - that can not be changed!
• we can write "execute an operation with error result" and "check for actual error" as one-liner in go using if err := call(); err != nil {...}
• WebSocket can simply be tested out of any recent browser:
  • open the URL were our service runs, e.g. localhost:8080
  • press F12 to open the developer console
  • enter var ws = new WebSocket('ws://localhost:8080/ws'); ws.onmessage = function(data) { console.log(data); }
  • send stuff entering ws.send("שלום") and check the console for the response
• if a REST call changed a Todo we can not use this Todo directly in the AMQP part as this will cause an Access violation ; use a Go routine instead to pipe the Todo
• if a rabbitmq service is bound to an application in Cloud Foundry, the VCAP_SERVICES environment variable will contain a service definition that can be identified by its amqp tag and whose credentials.uri field contains the full amqp://... URI