使用动态Ansible Playbook时，如何在主机间共享facts并对比变量？

问题 #

需要对比两台动态添加主机（无主机文件，无法通过主机名引用，只能用ansible_play_hosts）上的变量，但共享主机间变量时遇到问题。简化后的Playbook代码如下：

- set_fact:
    samplevar0: var_from_first_server_in_play
  when: inventory_hostname == ansible_play_hosts[0]
  
- set_fact:
    samplevar1: var_from_second_server_in_play
  when: inventory_hostname == ansible_play_hosts[1]
  
- ansible.builtin.debug: msg="echoing vars from both servers in play  {{ hostvars.ansible_play_hosts[0].samplevar0 }} {{ hostvars.ansible_play_hosts[1].samplevar1 }}"

- ansible.builtin.debug: msg="vars 0 and 1 are equal. "
  when: "{{ hostvars.ansible_play_hosts[0].samplevar0 }} == {{ hostvars.ansible_play_hosts[1].samplevar1 }}"

执行后报错，输出如下：

TASK [fact_check_role: set_fact] *************************************
ok: [host2255]

TASK [fact_check_role: set_fact] *************************************
ok: [host2892]

TASK [fact_check_role : ansible.builtin.debug] ************************
fatal: [llrp107d]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: "hostvars['ansible_play_hosts']" is undefined. "hostvars['ansible_play_hosts']" is undefined

The error appears to be in '/runner/project/roles/fact_check_role/tasks/preflight_host1.yml': line 195, column 3, but may
be elsewhere in the file depending on the exact syntax problem.

The offending line appears to be:

• ansible.builtin.debug: msg="echoing vars from both servers in play {{ hostvars.ansible_play_hosts[0].samplevar0 }} {{ hostvars.ansible_play_hosts[1].samplevar1 }}"
  ^ here
  We could be wrong, but this one looks like it might be an issue with
  missing quotes. Always quote template expression brackets when they
  start a value. For instance:

  with_items:
  - {{ foo }}

Should be written as:

with_items:
  - "{{ foo }}"

"}

疑问：是否是嵌套变量的Jinja格式使用错误导致的问题？

原因与解决方法 #

确实是Jinja2嵌套变量的语法错误导致的问题，核心问题有两个：

1. hostvars访问方式错误：hostvars是字典类型，当使用动态变量（比如ansible_play_hosts[0]这种运行时确定的主机名）作为键时，不能用点号.访问，必须用方括号[]形式。否则Ansible会把ansible_play_hosts当成主机名去hostvars里查找，而非解析这个变量的实际值。

2. when条件语法错误：Ansible的when参数本身处于Jinja2上下文，无需再用{{}}包裹表达式，直接写条件逻辑即可。

修正后的Playbook代码如下：

- set_fact:
    samplevar0: var_from_first_server_in_play
  when: inventory_hostname == ansible_play_hosts[0]
  
- set_fact:
    samplevar1: var_from_second_server_in_play
  when: inventory_hostname == ansible_play_hosts[1]
  
- ansible.builtin.debug: 
    msg: "echoing vars from both servers in play  {{ hostvars[ansible_play_hosts[0]].samplevar0 }} {{ hostvars[ansible_play_hosts[1]].samplevar1 }}"

- ansible.builtin.debug: 
    msg: "vars 0 and 1 are equal. "
  when: hostvars[ansible_play_hosts[0]].samplevar0 == hostvars[ansible_play_hosts[1]].samplevar1

关键修改点说明： #

• 将hostvars.ansible_play_hosts[0]改为hostvars[ansible_play_hosts[0]]：先解析ansible_play_hosts[0]得到第一个主机名，再用该主机名作为键去hostvars字典中获取对应主机的变量。
• 移除when条件中的{{}}：直接编写变量对比逻辑，Ansible会自动在Jinja2上下文中解析。

内容的提问来源于stack exchange，提问作者CaptainSkyfish