synchronized-start.yaml

Overview

synchronized-start.yaml is an automation playbook designed to coordinate the timing of operations by synchronizing the start time across multiple hosts or processes. It uses timestamp comparison and conditional waiting to ensure that subsequent tasks execute simultaneously or after a predetermined synchronized timestamp (SYNCHRONIZED_START). This is important in scenarios requiring coordinated execution, such as load testing, batch processing, or distributed orchestration.

Playbook Tasks and Their Functionality

The file consists of three main tasks executed in sequence, each running only once (due to the run_once: true directive) to avoid redundant operations on multiple hosts:

1. Get current timestamp

- name: Get current timestamp
  ansible.builtin.shell: date +%s
  register: current_timestamp
  run_once: true

• Purpose:
  Executes a shell command to retrieve the current Unix timestamp (seconds since the Unix epoch) on the control node or the first host.

• Key Points:

  • Uses the date +%s command to get an integer timestamp.

  • The result is stored in the variable current_timestamp, which can be referenced in subsequent tasks.

  • run_once: true ensures this command is executed a single time and not on all hosts.

• Usage Example:
  This timestamp serves as a reference point for calculating how long to wait until the synchronized start time.

2. Print timestamps

- name: Print timestamps
  debug:
    msg: "Current timestamp: {{ current_timestamp.stdout }}, Synchronized start: {{ SYNCHRONIZED_START }}, need to wait {{ SYNCHRONIZED_START - current_timestamp.stdout | int }} seconds"
  run_once: true

• Purpose:
  Outputs diagnostic information that includes:

  • The current timestamp obtained above.

  • The target synchronized start time (SYNCHRONIZED_START), which must be provided externally (e.g., as an extra variable or inventory variable).

  • The computed wait time (difference between SYNCHRONIZED_START and current_timestamp).

• Parameters:

  • Uses Jinja2 templating to interpolate values into the debug message.

  • The subtraction operation converts the timestamps to integers before calculation.

• Context:
  Useful for verifying that the synchronization logic is working as expected before the actual wait.

3. Wait for synchronized start

- name: Wait for synchronized start
  run_once: true
  ansible.builtin.pause:
    seconds: "{{ SYNCHRONIZED_START - current_timestamp.stdout | int }}"
  when:
    - current_timestamp.stdout | int < SYNCHRONIZED_START

• Purpose:
  Pauses execution until the synchronized start time is reached, effectively delaying further playbook tasks or roles.

• Implementation Details:

  • Uses Ansible's pause module to halt execution for a calculated number of seconds.

  • The pause duration is the difference between the target synchronized start time and the current timestamp.

  • The when conditional ensures that the pause only happens if the current time is earlier than the target start time to avoid negative pauses.

• Practical Use:
  Ensures that all hosts or processes begin their subsequent operations at the same global time.

Important Implementation Details

• The playbook relies on the external provision of the SYNCHRONIZED_START variable. This should be an integer representing a Unix timestamp in the future.

• The run_once: true directive is critical to avoid redundant timestamp fetching and waiting logic on multiple hosts.

• The calculation SYNCHRONIZED_START - current_timestamp.stdout uses Jinja2 filters to ensure integer arithmetic.

• If the current time is already past the synchronized start time, no waiting occurs.

Interaction With Other System Components

• This playbook is typically integrated as a prelude or synchronization step within a larger automation workflow or orchestration pipeline.

• It depends on upstream components or user input to define the SYNCHRONIZED_START timestamp.

• Downstream tasks triggered after this playbook will execute in a synchronized manner relative to the centralized start time.

• It may be combined with distributed workload execution, where coordinated timing is crucial for correctness or performance measurement.

Visual Diagram

flowchart TD
A[Get current timestamp] --> B[Print timestamps]
B --> C{Is current time < synchronized start?}
C -- Yes --> D[Pause until synchronized start]
C -- No --> E[Proceed immediately]

• Explanation:
  This flowchart illustrates the sequential workflow:

  1. Retrieve the current timestamp.

  2. Output timing information.

  3. Conditionally wait if the current time is earlier than the synchronized start.

  4. Otherwise, proceed without delay.

Note: For detailed understanding of playbook execution and Ansible modules used, refer to [Ansible Playbooks](/ansible-playbooks) and [Ansible Modules](/ansible-modules). For timestamp management concepts, see [Unix Timestamp](/unix-timestamp) and [Time Synchronization](/time-synchronization).