Howto execute ansible one host group at a time?

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Summary

The problem at hand is executing an Ansible playbook on multiple host groups, but only one host group at a time, due to dependencies between services. The ANSIBLE_FORKS setting allows for parallel execution up to a certain limit, but this can cause issues when trying to execute host groups sequentially.

Root Cause

The root cause of this issue is the lack of a built-in feature in Ansible to execute host groups one at a time, while still utilizing the ANSIBLE_FORKS setting for parallel execution within each group. Some possible causes include:

  • Insufficient documentation: The official Ansible documentation does not provide a clear solution for this specific use case.
  • Limitations of existing features: The serial keyword, –limit CLI parameter, and delegate keyword do not provide the desired functionality.
  • Execution strategies: The default execution strategies available in Ansible do not support executing host groups one at a time.

Why This Happens in Real Systems

This issue can occur in real systems when:

  • Dependencies between services: Hosts in different groups have dependencies that require sequential execution.
  • Large-scale infrastructure: Managing a large number of hosts requires efficient execution strategies to minimize downtime and optimize resource utilization.
  • Version limitations: Being stuck with an older version of Ansible, such as 2.16.3, can limit access to newer features and solutions.

Real-World Impact

The impact of this issue can be significant, including:

  • Increased downtime: Executing host groups in parallel can cause unexpected behavior and errors, leading to increased downtime and maintenance windows.
  • Inefficient resource utilization: Not utilizing the ANSIBLE_FORKS setting efficiently can result in underutilization of resources, leading to longer execution times and decreased productivity.
  • Complex workarounds: Implementing custom solutions, such as bash scripts or multiple playbooks, can add complexity and maintenance overhead.

Example or Code

# Example Ansible playbook
---
- name: Execute host groups sequentially
  hosts: HG1:HG2:HG3
  serial: 1  # Not a viable solution, as it only serializes execution within each group

How Senior Engineers Fix It

Senior engineers can address this issue by:

  • Developing a custom execution strategy: Creating a custom execution strategy that executes host groups one at a time, while still utilizing the ANSIBLE_FORKS setting for parallel execution within each group.
  • Utilizing Ansible Galaxy: Leveraging existing solutions and modules on Ansible Galaxy to find a suitable execution strategy or plugin.
  • Implementing a tagging system: Using tags to control the execution of host groups and ensure sequential execution.

Why Juniors Miss It

Junior engineers may miss this solution due to:

  • Lack of experience: Limited experience with Ansible and its features can make it difficult to identify the root cause and develop a suitable solution.
  • Insufficient knowledge of execution strategies: Not being familiar with the different execution strategies available in Ansible can lead to overlooking potential solutions.
  • Overreliance on documentation: Relying too heavily on official documentation can cause junior engineers to miss alternative solutions and workarounds.

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