1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
|
.. _handlers:
Handlers: running operations on change
======================================
Sometimes you want a task to run only when a change is made on a machine. For example, you may want to restart a service if a task updates the configuration of that service, but not if the configuration is unchanged. Ansible uses handlers to address this use case. Handlers are tasks that only run when notified. Each handler should have a globally unique name.
.. contents::
:local:
Handler example
---------------
This playbook, ``verify-apache.yml``, contains a single play with a handler::
---
- name: Verify apache installation
hosts: webservers
vars:
http_port: 80
max_clients: 200
remote_user: root
tasks:
- name: Ensure apache is at the latest version
ansible.builtin.yum:
name: httpd
state: latest
- name: Write the apache config file
ansible.builtin.template:
src: /srv/httpd.j2
dest: /etc/httpd.conf
notify:
- Restart apache
- name: Ensure apache is running
ansible.builtin.service:
name: httpd
state: started
handlers:
- name: Restart apache
ansible.builtin.service:
name: httpd
state: restarted
In this example playbook, the second task notifies the handler. A single task can notify more than one handler::
- name: Template configuration file
ansible.builtin.template:
src: template.j2
dest: /etc/foo.conf
notify:
- Restart memcached
- Restart apache
handlers:
- name: Restart memcached
ansible.builtin.service:
name: memcached
state: restarted
- name: Restart apache
ansible.builtin.service:
name: apache
state: restarted
Controlling when handlers run
-----------------------------
By default, handlers run after all the tasks in a particular play have been completed. This approach is efficient, because the handler only runs once, regardless of how many tasks notify it. For example, if multiple tasks update a configuration file and notify a handler to restart Apache, Ansible only bounces Apache once to avoid unnecessary restarts.
If you need handlers to run before the end of the play, add a task to flush them using the :ref:`meta module <meta_module>`, which executes Ansible actions::
tasks:
- name: Some tasks go here
ansible.builtin.shell: ...
- name: Flush handlers
meta: flush_handlers
- name: Some other tasks
ansible.builtin.shell: ...
The ``meta: flush_handlers`` task triggers any handlers that have been notified at that point in the play.
Using variables with handlers
-----------------------------
You may want your Ansible handlers to use variables. For example, if the name of a service varies slightly by distribution, you want your output to show the exact name of the restarted service for each target machine. Avoid placing variables in the name of the handler. Since handler names are templated early on, Ansible may not have a value available for a handler name like this::
handlers:
# This handler name may cause your play to fail!
- name: Restart "{{ web_service_name }}"
If the variable used in the handler name is not available, the entire play fails. Changing that variable mid-play **will not** result in newly created handler.
Instead, place variables in the task parameters of your handler. You can load the values using ``include_vars`` like this:
.. code-block:: yaml+jinja
tasks:
- name: Set host variables based on distribution
include_vars: "{{ ansible_facts.distribution }}.yml"
handlers:
- name: Restart web service
ansible.builtin.service:
name: "{{ web_service_name | default('httpd') }}"
state: restarted
Handlers can also "listen" to generic topics, and tasks can notify those topics as follows::
handlers:
- name: Restart memcached
ansible.builtin.service:
name: memcached
state: restarted
listen: "restart web services"
- name: Restart apache
ansible.builtin.service:
name: apache
state: restarted
listen: "restart web services"
tasks:
- name: Restart everything
ansible.builtin.command: echo "this task will restart the web services"
notify: "restart web services"
This use makes it much easier to trigger multiple handlers. It also decouples handlers from their names,
making it easier to share handlers among playbooks and roles (especially when using 3rd party roles from
a shared source like Galaxy).
.. note::
* Handlers always run in the order they are defined, not in the order listed in the notify-statement. This is also the case for handlers using `listen`.
* Handler names and `listen` topics live in a global namespace.
* Handler names are templatable and `listen` topics are not.
* Use unique handler names. If you trigger more than one handler with the same name, the first one(s) get overwritten. Only the last one defined will run.
* You can notify a handler defined inside a static include.
* You cannot notify a handler defined inside a dynamic include.
When using handlers within roles, note that:
* handlers notified within ``pre_tasks``, ``tasks``, and ``post_tasks`` sections are automatically flushed at the end of section where they were notified.
* handlers notified within ``roles`` section are automatically flushed at the end of ``tasks`` section, but before any ``tasks`` handlers.
* handlers are play scoped and as such can be used outside of the role they are defined in.
|
