Implements simple rolling deploy logic.

Requires the latest ansible to use (you may need to remove and add it again if
you are in a virtualenv)

See `edxapp_rolling_example.yml`

A new module is necessary for ELB information since that is not
available in EC2 metadata (which is what ec2_facts fetches).

ec2_facts grabs the instance ids and other meta information
ec2_elb_facts fetches ELB information
elb_reg.py takes an elb and instance id and will either register or
deregister. (called as pre and post).

`Serial` should be set to the number of instances that you want to
operate on simultaneously, currently set to 1.

playbooks/edxapp_rolling_example.yml

+# ansible-playbook -v --user=ubuntu  edxapp_rolling_example.yml -i ./ec2.py  --private-key=/path/to/deployment.pem --module-path /path/to/repo/modules
+- hosts: tag_Group_anothermulti
+  serial: 1
+  vars_files:
+    - "vars/secure/edxapp_stage_vars.yml"
+    - "vars/secure/users.yml"
+    - "vars/secure/edxapp_stage_users.yml"
+  pre_tasks:
+    - name: Gathering ec2 facts 
+      ec2_facts: 
+    - name: Gathering ELB facts
+      local_action: ec2_elb_facts
+    - local_action: command util/elb_reg.py -e {{ ",".join(elbs[ansible_ec2_instance_id]) }} -i {{ ansible_ec2_instance_id }} deregister
+  roles:
+    - common
+    - nginx 
+    - lms
+  post_tasks:
+    - local_action: command util/elb_reg.py -e {{ ",".join(elbs[ansible_ec2_instance_id]) }} -i {{ ansible_ec2_instance_id }} register

playbooks/modules/ec2_elb_facts

+#!/usr/bin/python
+# -*- coding: utf-8 -*-
+
+# This file is part of Ansible
+#
+# Ansible is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Ansible is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Ansible.  If not, see <http://www.gnu.org/licenses/>.
+
+DOCUMENTATION="""
+---
+module: ec2_elb_facts
+short_description: Creates a fact with instance to ELB mappings
+version_added: "1.0"
+options: {}
+description:
+     - This module fetches data from AWS using the boto api and returns 
+       a hash as a fact that maps instances to the ELB(s) that they belong to.
+examples:
+    - code: ansible all -m ec2_elb_facts
+      description: Obtain ELB info using boto
+requirements: [ "boto" ]
+author: "John Jarvis <john@jarv.org>"
+"""
+
+try:
+    import boto
+except ImportError:
+    print "failed=True msg='boto required for this module'"
+    sys.exit(1)
+
+
+from collections import defaultdict
+
+def get_elb_info(module):
+
+    try:
+        elb = boto.connect_elb()
+    except boto.exception.NoAuthHandlerFound, e:
+        module.fail_json(msg = str(e))
+
+    elbs = elb.get_all_load_balancers()
+    elb_info = defaultdict(set)
+    for lb in elbs:
+        for info in lb.instances:
+            elb_info[info.id].add(lb.name)
+    elb_info = {k: list(v) for k,v in elb_info.iteritems()}
+    return elb_info
+
+
+def main():
+
+
+    module = AnsibleModule(argument_spec = dict())
+    elb_info = get_elb_info(module)
+    ec2_facts_result = dict(changed=False, ansible_facts={'elbs': elb_info})
+    module.exit_json(**ec2_facts_result)
+
+# this is magic, see lib/ansible/module_common.py
+#<<INCLUDE_ANSIBLE_MODULE_COMMON>>
+
+main()

playbooks/util/elb_reg.py

+#!/usr/bin/env python
+
+from argparse import ArgumentParser
+import time
+import boto
+
+
+def await_elb_instance_state(lb, instance_id, awaited_state):
+    """blocks until the ELB reports awaited_state
+    for instance_id.
+    lb = loadbalancer object
+    instance_id : instance_id (string)
+    awaited_state : state to poll for (string)"""
+
+    start_time = time.time()
+    while True:
+        state = lb.get_instance_health([instance_id])[0].state
+        if state == awaited_state:
+            print "Load Balancer {lb} is in awaited state " \
+                  "{awaited_state}, proceeding.".format(
+                  lb=lb.dns_name,
+                  awaited_state=awaited_state)
+            break
+        else:
+            print "Checking again in 2 seconds. Elapsed time: {0}".format(
+                time.time() - start_time)
+            time.sleep(2)
+
+
+def deregister():
+    """Deregister the instance from all ELBs and wait for the ELB
+    to report them out-of-service"""
+
+    for lb in active_lbs:
+        lb.deregister_instances([args.instance])
+        await_elb_instance_state(lb, args.instance, 'OutOfService')
+
+
+def register():
+    """Register the instance for all ELBs and wait for the ELB
+    to report them in-service"""
+    for lb in active_lbs:
+        lb.register_instances([args.instance])
+        await_elb_instance_state(lb, args.instance, 'InService')
+
+
+def parse_args():
+    parser = ArgumentParser()
+    subparsers = parser.add_subparsers(dest="sp_action")
+    subparsers.add_parser('register', help='register an instance')
+    subparsers.add_parser('deregister', help='deregister an instance')
+
+    parser.add_argument('-e', '--elbs', required=True,
+                        help="Comma separated list of ELB names")
+    parser.add_argument('-i', '--instance', required=True,
+                        help="Single instance to operate on")
+    return parser.parse_args()
+
+if __name__ == '__main__':
+    args = parse_args()
+
+    elb = boto.connect_elb()
+    elbs = elb.get_all_load_balancers()
+    active_lbs = sorted(
+        lb
+        for lb in elbs
+        if lb.name in args.elbs.split(','))
+
+    print "ELB : " + str(args.elbs.split(','))
+    print "Instance: " + str(args.instance)
+    if args.sp_action == 'deregister':
+        print "Deregistering an instance"
+        deregister()
+    elif args.sp_action == 'register':
+        print "Registering an instance"
+        register()