ansible role
当一个配置管理的任务十分复杂时，playbook文件会十分庞大，这这种情况下将不利于扩展和复用。 这个时候可以使用ansible role将这个复杂的playbook模块化。Ansible role实际上是对playbook进行了逻辑上的划分，分成不同目录。 ansible已经规定好了目录的结构：
roles/
xxrole/
defaults/
handlers/
meta/
tasks/
vars/有了ansible role之后，来看一下一个ansible配置管理项目的目录结构：
production # inventory file for production servers
staging # inventory file for staging environment
group_vars/
group1 # here we assign variables to particular groups
group2 # ""
host_vars/
hostname1 # if systems need specific variables, put them here
hostname2 # ""
library/ # if any custom modules, put them here (optional)
filter_plugins/ # if any custom filter plugins, put them here (optional)
site.yml # master playbook
webservers.yml # playbook for webserver tier
dbservers.yml # playbook for dbserver tier
roles/
common/ # this hierarchy represents a "role"
tasks/ #
main.yml # <-- tasks file can include smaller files if warranted
handlers/ #
main.yml # <-- handlers file
templates/ # <-- files for use with the template resource
ntp.conf.j2 # <------- templates end in .j2
files/ #
bar.txt # <-- files for use with the copy resource
foo.sh # <-- script files for use with the script resource
vars/ #
main.yml # <-- variables associated with this role
defaults/ #
main.yml # <-- default lower priority variables for this role
meta/ #
main.yml # <-- role dependencies
library/ # roles can also include custom modules
lookup_plugins/ # or other types of plugins, like lookup in this case
webtier/ # same kind of structure as "common" was above, done for the webtier role
monitoring/ # ""
fooapp/ # ""当playbook需要引用某个role时：
- tasks/main.yml中的task都会被自动添加到该playbook中
- handlers/main.yml中的handler都会被自动添加到该playbook中
- vars/main.yml中的所有变量都会被自动添加到该playbook中
- meta/main.yml中的所有role依赖关系都会被自动添加到该playbook中
- defaults/main.yml中是一些默认变量值，如果在其他地方没有指定该变量的值时，才会用到默认变量
- task中的copy模块和script模块会自动从files目录中寻找文件
- task中的template模块会自动从templates目录中加载模块文件
- 通过include包含文件会自动从tasks目录中加载文件
在meta/main.yml中定义该role依赖于其他的role，同时可以向依赖的role传递变量：
---
dependencies:
- { role: common, some_parameter: 3 }
- { role: apache, apache_port: 80 }
- { role: postgres, dbname: blarg, other_parameter: 12 }