How to set target hosts in Fabric file

Question

I want to use Fabric to deploy my web app code to development, staging and production servers. My fabfile:

def deploy_2_dev():
  deploy('dev')

def deploy_2_staging():
  deploy('staging')

def deploy_2_prod():
  deploy('prod')

def deploy(server):
  print 'env.hosts:', env.hosts
  env.hosts = [server]
  print 'env.hosts:', env.hosts

Sample output:

host:folder user$ fab deploy_2_dev
env.hosts: []
env.hosts: ['dev']
No hosts found. Please specify (single) host string for connection:

When I create a set_hosts() task as shown in the Fabric docs, env.hosts is set properly. However, this is not a viable option, neither is a decorator. Passing hosts on the command line would ultimately result in some kind of shell script that calls the fabfile, I would prefer having one single tool do the job properly.

It says in the Fabric docs that 'env.hosts is simply a Python list object'. From my observations, this is simply not true.

Can anyone explain what is going on here ? How can I set the host to deploy to ?

Answer 1

Was stuck on this myself, but finally figured it out. You simply can't set the env.hosts configuration from within a task. Each task is executed N times, once for each Host specified, so the setting is fundamentally outside of task scope.

Looking at your code above, you could simply do this:

@hosts('dev')
def deploy_dev():
    deploy()

@hosts('staging')
def deploy_staging():
    deploy()

def deploy():
    # do stuff...

Which seems like it would do what you're intending.

Or you can write some custom code in the global scope that parses the arguments manually, and sets env.hosts before your task function is defined. For a few reasons, that's actually how I've set mine up.

Answer 2

You need to set host_string an example would be:

from fabric.context_managers import settings as _settings

def _get_hardware_node(virtualized):
    return "localhost"

def mystuff(virtualized):
    real_host = _get_hardware_node(virtualized)
    with _settings(
        host_string=real_host):
        run("echo I run on the host %s :: `hostname -f`" % (real_host, ))

Answer 3

You can assign to env.hoststring before executing a subtask. Assign to this global variable in a loop if you want to iterate over multiple hosts.

Unfortunately for you and me, fabric is not designed for this use case. Check out the main function at http://github.com/bitprophet/fabric/blob/master/fabric/main.py to see how it works.

Answer 4

You need to modify env.hosts at the module level, not within a task function. I made the same mistake.

from fabric.api import *

def _get_hosts():
    hosts = []
    ... populate 'hosts' list ...
    return hosts

env.hosts = _get_hosts()

def your_task():
    ... your task ...