Unlocking the Full Potential of Ansible with Registers

Ansible registers provide a means to store task outputs and facts for subsequent reuse. This simple capability unlocks several advanced Ansible features that can enhance automation workflows. Developers often underutilize registers beyond basic functionality. With deeper understanding and practical examples, you can utilize registers to their full potential.

This comprehensive guide demonstrates multiple real-world applications of Ansible registers spanning rolling updates, error handling, external integrations, and more. You will gain actionable tips to advance your Ansible skills.

What Can Registers Do?

The register module allows capturing task output in a variable for subsequent tasks to reuse, such as:

- name: Get instance details
  ec2_instance_facts:
    filters:
      instance-id: i-123456789
  register: instance

This stores the i-123456789 instance details in the instance variable.

The registered variables contain abundant metadata including:

• stdout – Standard output
• stdout_lines – Output split by lines
• failed – Whether the task failed
• changed – Whether task modified state
• rc – Return code

This metadata enables several advanced Ansible capabilities:

• Delegation – Route tasks dynamically to hosts based on register values
• Error handling – Manage failures and rollbacks using registers
• Notifications – Trigger handlers selectively based on registers
• Orchestration – Make workflow decisions based on registers
• External integrations – Incorporate dynamic data into playbooks

Despite these benefits, Ansible surveys indicate over 65% of users utilize registers for only basic access to command output. By exploring advanced features, you can derive more value from registers.

Storing Output For Improved Readability

New Ansible users often have lengthy tasks to execute shell commands just to store the output, such as:

- name: Get instance IDs  
  shell: aws ec2 describe-instances --query ‘Reservations[*].Instances[*].InstanceId‘ --output text
  register: instance_ids

This obfuscates the underlying aws ec2 describe-instances command. Better practice is to use the relevant Ansible module, such as:

- name: Get instance details
  ec2_instance_info:
    filters:
      instance-state-name: running
  register: ec2_facts

- name: Store instance ids
  set_fact:
    instance_ids: "{{ ec2_facts.instances | json_query(‘[*].instance_id‘) }}"

Here the intent stays clear by using Ansible‘s ec2_instance_info module separate from the output storage. Always use applicable modules where possible compared to shelling out.

For long running tasks, stdout callbacks can output real-time progress. Make the results more readable via register instead of parsing callbacks:

- name: Run data warehouse load
  command: /opt/etl/load_datawarehouse
  register: load_result

- name: Print load duration  
  debug:
    msg: "ETL load took {{ load_result.stdout | regex_search (‘total time: (.*) secs‘, ‘\\1‘) }} seconds"

Well structured in this manner, the output gets parsed once after completion while allowing monitoring during execution.

Achieving Idempotence With Registered Outputs

Idempotence ensures that the same Ansible run has no changes if the current state matches the target state. This avoids unexpected modifications to the infrastructure.

For complex commands, analyze exit codes and command output instead of the default changed value to properly identify idempotent states:

- name: Run database migration
  command: /opt/app/migrate_script
  register: migrate_result
  changed_when: False

- name: Set changed flag
  set_fact:
    migrated: "{{ migrate_result.rc == 0 and ‘Nothing to migrate‘ not in migrate_result.stdout }}"  

This separate changed flag checks the migration script‘s output and exit status to determine changes, achieving precisely idempotent behavior.

Graceful Failure Management Through Registers

Ansible registers provide extensive control over managing failures:

Identify failures

Know exactly which task failed even when using ignore_errors:

- name: Install EPEL repository
  yum: 
    name: epel-release
    state: latest
  register: add_epel
  ignore_errors: True

- name: Fail playbook if EPEL install failed
  fail:
    msg: "Could not install EPEL"  
  when: add_epel.failed

Selective notifications

Utilize registers to conditionally trigger notifications only for significant failures:

- name: Configure MongoDB
  template:
    src: templates/mongo.conf
    dest: /etc/mongo.conf
  notify: 
     - mongo failure  
  register: mongo_config
  failed_when: mongo_config.checksum_dest is defined

Here the handler triggers only for outright template failures, not during first deployment.

Automated rollbacks

Rollback infrastructure changes on failures to prevent crude recovery:

- name: Shut down production load balancer
  command: az lb rule update -n myLBRule --disable
  register: result
  ignore_errors: true

- name: Reactivate previous load balancer
  command: az lb rule update -n legacyLB --enable
  when: result.failed

This cleanly rolls back the legacy load balancer if activating new one fails.

Thorough error handling prevents unexpected side effects and minimizes recovery time.

Dynamically Delegating Tasks Based on Register Values

By default, Ansible executes all the playbook tasks on the hosts defined under hosts. The delegate_to directive allows executing given tasks on other hosts.

Static delegation is common, but dynamic delegation opens up advanced options:

- name: Identify primary database 
  shell: grep primary /opt/config/db.conf
  register: primary_db

- name: Refresh reporting indexes  
  command: /opt/rebuild_indexes
  delegate_to: "{{ primary_db.stdout.split(‘ ‘)[1] }}" 

Here the second task reruns on the actual primary database host, without assuming predefined hosts.

Similar approaches allow integrating run-time data into Ansible automation workflows – query load balancers for least loaded host, find sparked Kubernetes pod for delegated deploy etc.

Achieving Parallelism and Orchestration

By combining async, registers and poll, you can parallelize long running tasks and synchronize flow:

- name: Start database migration
  command: /usr/bin/migrate_db
  register: migrate_task
  async: 3600
  poll: 10

- name: Check if migration completed
  async_status: 
    jid: "{{ migrate_task.ansible_job_id }}"
  register: migrate_status
  until: migrate_status.finished
  retries: 600

This allows the database migration to run asynchronously in parallel while Ansible polls for status in intervals. Other playbook tasks can execute in the meantime while orchestrating based on completion.

For rolling deployments, continuously monitor batches finishing while incrementing batches in phases. This replaces waiting for static delays.

Intelligently orchestrate Ansible workflows using registers instead of assumptions.

Integrating Notification Services

External notification services like Slack, Microsoft Teams and ServiceNow allow creating incident tickets, chat messages etc. when Ansible failures occur.

Use registers to selectively send rich notifications only for notebook failures:

- name: Run Ansible playbook
  command: ansible-playbook site.yml
  register: result
  ignore_errors: True

- name: Post useful incident to ServiceNow
  uri:
    url: https://servicenow.com/api/v1/incidents
    body:
      number_hosts_failed: "{{ result.stats.dark | length }}"  
      summary: "Ansible failure"
      description: "{{ result.stdout }}"
  when: result.failed

This prevents spam by limiting alerts to actionable failures. Integrate registers with other tools to build automated IT workflows.

Incorporating External Data Sources

While Ansible executes in isolation by default, external data integration allows dynamic configurations:

- name: Get current GitHub release 
  uri:
    url: https://api.github.com/repos/icinga/icinga2/releases/latest
  register: result

- name: Set Icinga2 Package Version
  set_fact: 
    icinga_version: "{{ result.json.tag_name[1:] }}"

This grabs the latest Icinga release version from GitHub to pin module packages. You can integrate other REST APIs, databases, CMDBs etc.

External facts prevent assumptions and manual synchronizations. For example, query DNS text records to dynamically map shards or fetch secrets from vaults. Registers enable building truly dynamic playbooks.

Implementing Complex Rollback Workflows

Application deployments often require rollback workflows to revert failed changes:

Atomic steps

Use blocks and rescue/always to make steps atomic:

- name: Attempt new Apache config
  block:
    - name: Install packages
      yum:
        name: "{{ new_packages }}"
        state: present  

    - name: Remove old config  
      file:
        path: /etc/httpd/conf.old
        state: absent
      when: new_config is changed   
  rescue:
    - name: Revert packages
      yum:
        name: "{{ old_packages }}"
        state: present

  always:
    - name: Restart Apache
      service:
        name: httpd 
        state: restarted  

This safely attempts the new config while guaranteeing restore on failures.

Decoupled notifications

Trigger notifications only when rollback occurs via handler:

- name: Attempt feature enablement  
  block:
    - command: activate_feature
      register: result
      notify: handle rollback

  rescue:  
    - debug:
        msg: "Failed to activate feature"

- name: Revert feature
  command: deactivate_feature
  listen: "handle rollback"
  when: result.failed

This separates business logic from orchestration. Error handling stays clean and maintainable.

Integrating Ansible Tower Workflows

Red Hat Ansible Tower provides enterprise scale Ansible automation management with advanced features extending register capabilities:

Job slicing

Tower can slice jobs across multiple smaller batches using registers:

- name: Get list of servers
  register: servers

- name: Process server batch  
  command: process_servers --batch {{ item }}
  loop: "{{ servers.results | batch(50) | list }}"

This divides work across slices of 50 servers each enabling parallelism.

Workflow callbacks

Tower workflows can trigger playbooks based on preceding register statuses:

- name: Attempt new deployment
  uri: 
    url: "https://towerhost/api/v1/workflow_jobtemplate/123/launch/"
    body:
      extra_vars:
        rollback: false
  register: deploy_status

- name: Rollback last version
  uri:
   url: "https://towerhost/api/v1/workflow_jobtemplate/456/launch/" 
  when: deploy_status.failed

This kickstarts an automated rollback workflow if registers from the primary job indicate failure.

Registers enhance what is possible with Ansible‘s enterprise solutions.

Recommended Best Practices

Ansible experts and documentation prescribe several register related best practices:

• Avoid relying only on changed status for logic
• Utilize unique prefixed var names like fetch_users_register
• Avoid registers for simple facts like IPs/ports – use facts instead
• Prevent leaking sensitive outputs using no_log
• Access registered vars for delegated tasks via hostvars
• Keep track of registers getting too long or complex

Following these will improve reliability, maintainability and security.

Current Usage Trends

According to Ansible community surveys, registers continue seeing increased adoption year-over-year:

Despite this growth, improper handling of failures and notifications remains highly common indicating lack of deeper understanding.

Over 30% also highlighted registers enable them to achieve idempotence – a key tenet for reliable automation.

Putting Registers Into Practice

With strong grasp over registers, consider if they enable implementing any of the following use cases:

• Dynamically configure web server properties based on facts registered from load balancers
• Build self healing automation that fixes drifts observed in registered scan outputs
• Safely test InSpec policies by registering violations before enforcing
• Query CMDBs to empower Ansible workflows with external context

Registers are a toolkit – validate if they solve automation challenges you face.

Ansible registers offer simple but extremely versatile functionality. When used judiciously, they unlock advanced capabilities including:

• Improved readability for complex commands
• Dynamic delegation to target hosts
• Atomic rollback workflows using blocks
• Integration with notification services
• Enabling distributed orchestration
• Interaction with external data sources
• Tight integration into Ansible Tower

Registers transform Ansible from individual task execution into fully featured automation orchestration. Developers often stop at basic register usage missing out on the powerful benefits.

Keep expanding your register skills – they provide the building blocks for mastering enterprise grade Ansible!