How can roles be distributed across servers step by step without changing all at once?

For phased distribution of roles across servers without simultaneously changing all at once, you can use a rolling deployment strategy or a canary release:

1. Define server groups (pools). Divide the infrastructure into several groups, for example, 10-20% of the total servers.

2. Update roles by groups. First, update the role on the first server group, and verify correct operation.

3. Monitoring and testing. After updating each group, conduct monitoring and testing to ensure no issues.

4. Proceed to the next group. If everything is stable, move on to updating the next server group.

5. Automation. Use configuration management tools (Ansible, Puppet, Chef) or orchestration tools (Kubernetes, Terraform) to automate the process.

This approach minimizes risks and allows quick rollback if something goes wrong.

Example with Ansible:

- hosts: webservers_group1
  roles:
    - my_role

- hosts: webservers_group2
  roles:
    - my_role

# Run the playbook on groups sequentially