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Multi-Machine
Vagrant is able to define and control multiple guest machines per Vagrantfile. This is known as a "multi-machine" environment.
These machines are generally able to work together or are somehow associated with each other. Here are some use-cases people are using multi-machine environments for today:
- Accurately modeling a multi-server production topology, such as separating a web and database server.
- Modeling a distributed system and how they interact with each other.
- Testing an interface, such as an API to a service component.
- Disaster-case testing: machines dying, network partitions, slow networks, inconsistent world views, etc.
Historically, running complex environments such as these was done by flattening them onto a single machine. The problem with that is that it is an inaccurate model of the production setup, which can behave far differently.
Using the multi-machine feature of Vagrant, these environments can be modeled in the context of a single Vagrant environment without losing any of the benefits of Vagrant.
Defining Multiple Machines
Multiple machines are defined within the same project Vagrantfile
using the config.vm.define method call. This configuration directive
is a little funny, because it creates a Vagrant configuration within a
configuration. An example shows this best:
Vagrant.configure("2") do |config|
config.vm.provision :shell, :inline => "echo Hello"
config.vm.define :web do |web|
web.vm.box = "apache"
end
config.vm.define :db do |db|
db.vm.box = "mysql"
end
end
As you can see, config.vm.define takes a block with another variable. This
variable, such as web above, is the exact same as the config variable,
except any configuration of the inner variable applies only to the machine
being defined. Therefore, any configuration on web will only affect the
:web machine.
And importantly, you can continue to use the config object as well. The
configuration object is loaded and merged before the machine-specific configuration,
just like other Vagrantfiles within the
Vagrantfile load order.
If you're familiar with programming, this is similar to how languages have different variable scopes.
Controlling Multiple Machines
The moment more than one machine is defined within a Vagrantfile, the
usage of the various vagrant commands changes slightly. The change should
be mostly intuitive.
Most commands, such as vagrant up, begin requiring the name of the machine
to control. Using the example above, you could say vagrant up web, or
vagrant up db. If no name is specified, it is assumed you mean to perform
that operation on every machine. Therefore, vagrant up alone will bring
up both the web and DB machine.
Additionally, you can specify a regular expression for matching only
certain machines. This is useful in some cases where you specify many similar
machines, for example if you're testing a distributed service you may have
a master machine as well as a slave0, slave1, slave2, etc. If you
want to bring up all the slaves but not the master, you can just do
vagrant up /slave[0-9]/. If Vagrant sees a machine name within forward
slashes, it assumes you're using a regular expression.
Communication Between Machines
In order to faciliate communication within machines in a multi-machine setup, the various networking options should be used. In particular, the private network can be used to make a private network between multiple machines and the host.