Ansible fetch Module: Retrieving Files from Remote Hosts

Introduction

The fetch module in Ansible allows you to retrieve files from remote hosts and copy them to the control machine. This module is particularly useful for gathering log files, configuration files, or any other data generated on remote hosts during playbook execution. This blog post explores the usage of the fetch module, its parameters, and real-world examples to demonstrate its practicality in retrieving files from remote hosts.

Understanding the Concept of fetch Module

During the execution of Ansible playbooks, remote hosts may generate log files or produce other data that can provide insights into the system’s state. The fetch module enables you to securely collect these files from remote hosts and bring them to the control machine for analysis or further processing.

How to Use the Ansible fetch Module

The fetch module is straightforward to use and provides a convenient way to retrieve files from remote hosts. Let’s explore its usage through practical examples:

Syntax and Parameters

The basic syntax of the fetch module is as follows:

The module accepts the following parameters:

• src: (required) Specifies the path of the file on the remote host that needs to be fetched.
• dest: (required) Specifies the local directory path on the control machine where the file should be copied.
• flat: (optional) If set to yes, the fetched file will be copied into the destination directory without creating intermediate directories.
• validate_checksum: (optional) If set to yes, Ansible will verify the checksum of the fetched file against the remote file’s checksum to ensure its integrity.
• remote_src: (optional) If set to yes, the src parameter will be treated as a URL instead of a local file path on the remote host.

Fetching a File from a Remote Host

Let’s start with a basic example of using the fetch module to retrieve a file from a remote host:

In this example, the fetch module will retrieve the file /var/log/syslog from the remote host and copy it to the local directory /tmp/fetched_logs.

Flattening Directory Structure

When fetching files from remote hosts, you may encounter scenarios where the source file resides in a directory structure, and you want to copy only the file without preserving the directory structure. This can be achieved using the flat parameter:

In this case, the fetch module will copy the file config.ini from the remote host’s /etc/my_app/ directory to the local directory /tmp/fetched_configs without preserving the my_app directory.

Validating Checksum

To ensure the integrity of the fetched file, you can use the validate_checksum parameter:

In this example, Ansible will verify the checksum of the fetched file important.txt against the remote file’s checksum, ensuring that the file is not corrupted during transfer.

Real-World Examples

Let’s explore some real-world scenarios where the fetch module proves useful.

Example 1: Collecting Log Files

In a playbook designed to monitor multiple web servers, you may want to fetch log files generated on each server and store them on the control machine for further analysis:

In this example, the fetch module will retrieve the access.log file from the /var/log/apache2/ directory on each web server (defined in the webservers group) and copy it to the local directory /tmp/fetched_logs.

Example 2: Backing Up Configuration Files

Before making changes to configuration files on remote hosts, it’s a good practice to back them up. The fetch module can help in this scenario:

In this case, the fetch module will copy the app_config.conf file from the /etc/ directory on each application server (defined in the application_servers group) and store it in the local directory /tmp/config_backups.

Conclusion

The Ansible fetch module provides a convenient way to retrieve files from remote hosts and bring them to the control machine. By fetching log files, configuration files, or other generated data, you can gain insights into the system’s state and ensure efficient troubleshooting and analysis. Throughout this blog post, we explored the concept of the fetch module, its parameters, and provided real-world examples to demonstrate its practical applications.