Visor

Visor is a library which provides an abstract interface over a global process state.

Usage

To understand how Visor works, we need to understand how it works with time. Each of the Visor data-types File, App, Revision and Instance are snapshots of a specific point in time in the coordinator. When a mutating operation is successfully performed on one of these data-types, a new snapshot is returned, representing the state of the coordinator after the operation. If the operation would fail, the old snapshot is returned with an error.

With the new snapshot, we can perform an operation on this new state, and so on with every new snapshot. Here's an example to illustrate:

snapshot, err := visor.Dial("localhost:8046", "/") // snapshot.Rev == 42

file1, err := visor.Get(snapshot, "/path", codec)  // file1.Value == "billy",    file1.Rev == 42
file2, err := file1.Update("bob")                  // file2.Value == "bob",      file2.Rev == 43
file3, err := file2.Update("thornton")             // file3.Value == "thornton", file3.Rev == 44
...

Working with snapshots

// Get a snapshot of the latest coordinator state
snapshot, err := visor.Dial("coordinator:8046", "/")

// Get the list of applications at snapshot
apps, _ := visor.Apps(snapshot)
app := apps[0] // app.Rev == snapshot.Rev == 1

// Set some environment vars on *app*. Every time state is
// changed in the coordinator, a new App snapshot is returned.
app, _ = app.SetEnvironmentVar("cow", "moo")  // app.Rev == 2
app, _ = app.SetEnvironmentVar("cat", "meow") // app.Rev == 3

// Attempt to get a recently set environment var from an old snapshot (apps[0].Rev == 1)
apps[0].GetEnvironmentVar("cat") // "", ErrKeyNotFound

// Get a recently set environment var from the latest snapshot (app.Rev == 3)
app.GetEnvironmentVar("cat")     // "meow", nil

Advancing in time

// Get a snapshot of the latest coordinator state
snapshot, err := visor.Dial("coordinator:8046", "/")

apps, _ := visor.Apps(snapshot) // len(apps) == 0

app, _ := NewApp("soundcloud.com", "git://github.com/sc/soundcloud.com", "mystack", snapshot)
app.Register()

// *snapshot* still refers to the old state, so apps is still empty
apps, _ := visor.Apps(snapshot) // len(apps) == 0

// Get a snapshot of the latest coordinator state
snapshot = snapshot.FastForward(-1)

// Now that snapshot reflects the latest state, apps contains our registered app
apps, _ := visor.Apps(snapshot) // len(apps) == 1

Watching for events

package main

import "soundcloud/visor"

func main() {
  snapshot, err := visor.Dial("coordinator:8046", "/")
  if err != nil {
    panic(err)
  }

  c := make(chan *visor.Event)

  go visor.WatchEvent(snapshot, c)

  // Read one event from the channel
  fmt.Println(<-c)
}

Development

Setup

Dependencies

• Go (weekly) execute this and add it to your ~/.bashrc:

  export GOROOT=<somepath>                 
  export PATH=$PATH:$GOROOT/bin
  alias updatego="cd $GOROOT; hg pull; hg update weekly; cd src; ./all.bash"
  

  then do the following:

  hg clone -u weekly https://go.googlecode.com/hg/ go
  hg update weekly
  cd src
  ./all.bash
  

  the alias updatego will update your go version to weekly when you call it.

• doozer (implicit)

  go get github.com/ha/doozer
  

  if you run in trouble with the protobuf, do a:

  cd src/pkg/code.google.com/p/goprotobuf
  hg pull
  hg update 
  make install
  

• Doozerd (testing)

  go get github.com/ha/doozerd
  

  if this fails, do the following

  cd $GOROOT/src/pkg/github.com/ha/doozerd 
  git remote add soundcloud git@github.com:soundcloud/doozerd
  git pull soundcloud master
  ./make.sh
  go install
  

Installation

Mac Os X:

brew install https://raw.github.com/soundcloud/visor/master/visor.rb

or

brew upgrade https://raw.github.com/soundcloud/visor/master/visor.rb

Debian in our internal network:

apt-get install visor

Compile yourself:

• install golang and make install

Testing

First start doozerd with default configuration. If listening run:

go test

Conventions

This repository follows the code conventions dictated by gofmt. To automate the formatting process install this pre-commit hook, which runs gofmt and adds the files. Don't forget to make the file executable: chmod +x .git/hooks/pre-commit.

Branching

See Guide.

Versioning

This project is versioned with the help of the Semantic Versioning Specification using 0.0.0 as the initial version. Please make sure you have read the guidelines before increasing a version number either for a release or a hotfix.