As a system administrator, a widely used concept is the “pets and cattle” analogy for machines. It implies that some machines are enduring pets, cared for and adjusted over time, while others are dispensable cattle, effortlessly swapped out and reproduced whenever necessary. This concept is instrumental for efficient system administration and makes it easier to manage machines with different customizations and functionalities.
Understanding Pet-like Machines
A pet-like machine is one that cannot be reproduced programmatically due to its idiosyncratic customizations, making it unique and challenging to replicate. Such systems have often been altered over an extended period, without proper documentation of the changes, making it hard for new administrators to discern the customizations without exposing them to significant risk.
Characteristics of Cattle-Like Machines
On the contrary, a cattle-like machine is designed to be reproducible programmatically. That is, it can be easily duplicated and cloned through automation tools such as scripts, configuration management, or virtualization technologies. Such machines are designed with the assumption that they will be replaced regularly, and all customizations will be reproducible by an automated tool or script.
Example of cattle-like architecture
A prime example of machine-like cattle is a web server architecture that uses replicas and load balancers. The replicas are designed with standard configurations, and the load balancers distribute the workload using various algorithms. The replicas are created in fast and fault-tolerant environments with automatic scaling abilities. Such systems can scale up to hundreds, or even thousands of replicas, and the administrator can replace any problematic replica in a matter of minutes.
Techniques for Making a System More Like Cattle
The easiest way to create a stateless system is to isolate the state by separating the stateless services from the stateful ones into their respective servers. For instance, separating the database server from the web server isolates the state from the web server, making it a stateless system, and allowing administrators to create and replace web servers as necessary. This way, customizations are only applied to the stateful servers, making it easier to reproduce the stateless servers when necessary.
Implementing standardized formats
To make the process of reproducing machines as consistent and straightforward as possible, it is essential to implement standardized formats. Standard formats should be used for naming conventions, structure, and everything else that might differentiate one machine from another. This way, the system administrator only needs to follow a standard process with as little variation as possible to reproduce any machine they need.
Advantages of Cattle-Like Systems
Due to their ease of reproduction and scalability, cattle-like systems are more cost-efficient and less labor-intensive than pet-like systems. Automated tools can replicate these machines in no time, minimizing downtime and increasing availability. This feature also makes upgrades or rollbacks easier since cattle-like systems can be quickly replicated or replaced. Cattle-like systems are also preferred in modern DevOps environments, where the concept of infrastructure as code (IAC) aims to optimize the entire lifecycle of IT assets, from hardening to deployment to decommissioning.
Understanding the “pets and cattle” concept in system administration is essential for efficient and scalable administration of machines. It is imperative to know the distinction between pet-like machines and cattle-like ones, as this will significantly improve system administrators’ ability to manage systems consistently and with minimal errors. Incorporating practices such as decoupling state and using standardized formats goes a long way in achieving the ideal cattle-like architecture, which is easy to reproduce and manage.