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So I've recently played around with Juju, and I see quite a bit of potential. But there are a few things I don't exactly understand. Most notably, I'm not sure of the purpose of the "bootstrapping instance" (aka "bootstrapping node" or machine “0”).

Using the example from the user tutorial:

$ bin/juju status
  0: {dns-name:, instance-id: i-2a702745}
  1: {dns-name:, instance-id: i-227e294d}
  2: {dns-name:, instance-id: i-9c7e29f3}
    formula: local:mysql-11
    relations: {}
        machine: 1
        relations: {}
        state: started
    formula: local:wordpress-29
    relations: {}
        machine: 2
        relations: {}
        state: started

Machine 1 is the mysql instance, and machine 2 is the wordpress instance. What exactly is machine 0 up to?

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up vote 5 down vote accepted

The bootstrap instance runs two critical pieces of the infrastructure: Zookeeper and the provisioning agent. Here is the architecture overview:

Ensemble Architecture

Right now this is actually a single point of failure.. though that will be solved in the near term by expanding the "bootstrap" service to 2 units either on demand, or in response to deploying a second machine. Luckily, Zookeeper is designed for high availability and the provision agent will use Zookeeper for coordination so its not a complex problem to solve, just one that needs to be solved before Juju is ready for real production usage.

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