Ansible的handler

环境

• 管理节点：Ubuntu 22.04
• 控制节点：CentOS 8
• Ansible：2.15.6

handler

当任务结果产生变化时，通过 notify 通知handler做相应处理。

一个简单例子

---
- hosts: all
  tasks:
    - name: task1
      debug:
        msg: "I am task1"
      changed_when: true
      notify:
        - handler1

  handlers:
    - name: handler1
      debug:
        msg: "I am handler1"

运行结果如下：

TASK [task1] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task1"
}

RUNNING HANDLER [handler1] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler1"
}

注意：只有task产生变化时，才会触发notify。由于debug语句不产生变化，所以指定 changed_when: true ，强制task1产生变化。

一个复杂例子

---
- hosts: all
  tasks:
    - name: task1
      debug:
        msg: "I am task1"

    - name: task2
      debug:
        msg: "I am task2"
      changed_when: true
      notify:
        - handler2
        - handler1

    - name: task3
      debug:
        msg: "I am task3"

  handlers:
    - name: handler1
      debug:
        msg: "I am handler1"
    
    - name: handler2
      debug:
        msg: "I am handler2"

运行结果如下：

TASK [task1] ***************************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am task1"
}

TASK [task2] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task2"
}

TASK [task3] ***************************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am task3"
}

RUNNING HANDLER [handler1] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler1"
}

RUNNING HANDLER [handler2] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler2"
}

可见：

1. 一个task可以nofiy多个handler
2. 多个handler的运行顺序和notify时的顺序无关，只和handler本身的顺序有关（本例中，notify的顺序是handler2在handler1之前，但运行时是按handler本身的顺序，handler1在handler2之前）
3. handler在所有task运行结束之后才运行（本例中，task2 notify了handler，但handler是在task3之后才运行的）

---
- hosts: all
  tasks:
    - name: task1
      debug:
        msg: "I am task1"
      changed_when: true
      notify:
        - handler1

    - name: task2
      debug:
        msg: "I am task2"
      changed_when: true
      notify:
        - handler1

  handlers:
    - name: handler1
      debug:
        msg: "I am handler1"

运行结果如下：

TASK [task1] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task1"
}

TASK [task2] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task2"
}

RUNNING HANDLER [handler1] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler1"
}

可见，若多个task notify同一个handler，该handler只会运行一次。

通过 listen 匹配topic

前面的例子，在notify时，是通过handler的 name 来匹配topic的。也可以通过handler的 listen 来匹配：

---
- hosts: all
  tasks:
    - name: task1
      debug:
        msg: "I am task1"
      changed_when: true
      notify:
        - task1 changed

    - name: task2
      debug:
        msg: "I am task2"
      changed_when: true
      notify:
        - task2 changed

  handlers:
    - name: handler1
      debug:
        msg: "I am handler1"
      listen:
        - "task2 changed"
        - "task1 changed"

运行结果如下：

TASK [task1] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task1"
}

TASK [task2] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task2"
}

RUNNING HANDLER [handler1] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler1"
}

可见，通过 listen ，一个handler可以监听多个nofiy的topic。

考一考

下面的play，运行结果是什么？

---
- hosts: all
  tasks:
    - name: task1
      debug:
        msg: "I am task1"
      changed_when: true
      notify:
        - task1 changed

    - name: task2
      debug:
        msg: "I am task2"
      changed_when: true
      notify:
        - task2 changed

  handlers:
    - name: handler1
      debug:
        msg: "I am handler1"
      listen: "task2 changed"

    - name: handler2
      debug:
        msg: "I am handler2"
      listen:
        - "task2 changed"
        - "task1 changed"

    - name: handler3
      debug:
        msg: "I am handler3"
      listen: "task1 changed"

答：运行结果为：

TASK [task1] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task1"
}

TASK [task2] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task2"
}

RUNNING HANDLER [handler1] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler1"
}

RUNNING HANDLER [handler2] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler2"
}

RUNNING HANDLER [handler3] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler3"
}

立即运行handler

前面提到，handler是在所有task运行结束后才运行的。如果想要立即运行handler，可用 meta: flush_handlers ：

---
- hosts: all
  tasks:
    - name: task1
      debug:
        msg: "I am task1"
      changed_when: true
      notify: handler2

    - name: flush
      meta: flush_handlers

    - name: task2
      debug:
        msg: "I am task2"
      changed_when: true
      notify: handler1

    - name: task3
      debug:
        msg: "I am task3"

  handlers:
    - name: handler1
      debug:
        msg: "I am handler1"

    - name: handler2
      debug:
        msg: "I am handler2"

运行结果如下：

TASK [task1] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task1"
}

TASK [flush] ***************************************************************************************

RUNNING HANDLER [handler2] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler2"
}

TASK [task2] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task2"
}

TASK [task3] ***************************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am task3"
}

RUNNING HANDLER [handler1] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler1"
}

可见，flush之后，handler2立即运行了，而handler1则是在所有task都运行结束之后才运行的。

注意：flush触发运行的handler，如果随后又被notify，还会再次运行：

---
- hosts: all
  tasks:
    - name: task1
      debug:
        msg: "I am task1"
      changed_when: true
      notify: handler1

    - name: flush
      meta: flush_handlers

    - name: task2
      debug:
        msg: "I am task2"
      changed_when: true
      notify: handler1

  handlers:
    - name: handler1
      debug:
        msg: "I am handler1"

运行结果如下：

TASK [task1] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task1"
}

TASK [flush] ***************************************************************************************

RUNNING HANDLER [handler1] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler1"
}

TASK [task2] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task2"
}

RUNNING HANDLER [handler1] *************************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler1"
}

可见，handler1运行了两次。

handler的命名最好不要包含变量

---
- hosts: all
  vars:
    var1: "aaa"
  tasks:
    - name: task1
      debug:
        msg: "I am task1"
      changed_when: true
      notify: handler {{ var1 }}

  handlers:
    - name: handler {{ var1 }}
      debug:
        msg: "I am handler1"

运行结果如下：

TASK [task1] ***************************************************************************************
changed: [192.168.1.55] => {
    "msg": "I am task1"
}

RUNNING HANDLER [handler aaa] **********************************************************************
ok: [192.168.1.55] => {
    "msg": "I am handler1"
}

没有问题。但是，这里有个潜在的问题：

---
- hosts: all
  #vars:
  #  var1: "aaa"
  tasks:
    - name: task0
      set_fact:
        var1: "aa"

    - name: task1
      debug:
        msg: "I am task1"
      changed_when: true
      notify: handler {{ var1 }} # 可以识别

  handlers:
    - name: handler {{ var1 }} # 无法识别
      debug:
        msg: "I am handler1"

运行结果如下：

TASK [task0] ***************************************************************************************
ok: [192.168.1.55]

TASK [task1] ***************************************************************************************
[WARNING]: Handler 'handler {{ var1 }}' is unusable because it has no listen topics and the name
could not be templated (host-specific variables are not supported in handler names). The error:
'var1' is undefined. 'var1' is undefined
ERROR! The requested handler 'handler aa' was not found in either the main handlers list nor in the listening handlers list

可见，对于动态定义的变量，notify可以识别，而handler命名处无法识别（未定义）。

类似的，如果已定义的变量，如果动态改变其值，notify可以识别，而handler命名处仍然是原值。

所以，最好不要在handler命名处使用变量。

参考

• https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_handlers.html

代码007普通

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