Squid vs Varnish: What are the differences?

Introduction

In this article, we will discuss the key differences between Squid and Varnish, two popular web proxy caching servers.

1. Architecture: Squid and Varnish have different architectures. Squid is a forward proxy server that primarily focuses on accelerating requests from clients to servers. It supports HTTP, FTP, and other protocols. On the other hand, Varnish is a reverse proxy server that mainly focuses on accelerating requests from clients to backend servers. It is specifically designed for HTTP and is highly optimized for handling high traffic websites.

2. Caching Mechanism: Squid and Varnish have different caching mechanisms. Squid uses a hierarchical caching system where it stores the cached objects in a local disk or memory. It checks the freshness of objects using time-based expiration or validation with the origin server. Varnish, on the other hand, uses an object-based caching system. It stores the objects in memory and uses a highly efficient caching algorithm called LRU (Least Recently Used) to manage the cache. Varnish also supports Edge Side Includes (ESI) which allows assembling dynamic pages from multiple sources.

3. Configuration Language: Squid and Varnish use different configuration languages. Squid uses a configuration language known as Squid.conf, which is a text-based syntax for defining various settings and parameters. It requires manual configuration and understanding of the configuration file structure. Varnish, on the other hand, uses a configuration language called VCL (Varnish Configuration Language). VCL is a domain-specific language that allows developers to write custom rules and policies to control the behavior of Varnish. VCL provides greater flexibility and control over caching logic.

4. Concurrency Model: Squid and Varnish have different concurrency models. Squid follows a multithreaded concurrency model where it uses multiple threads to handle incoming requests concurrently. It creates a separate thread for each request, which can consume more system resources in high traffic scenarios. Varnish, on the other hand, follows an event-driven concurrency model. It uses an event-driven architecture and a single worker process to handle multiple incoming requests asynchronously. This makes Varnish highly scalable and efficient in handling high concurrency.

5. SSL/TLS Support: Squid and Varnish have different levels of support for SSL/TLS. Squid supports SSL/TLS by acting as an SSL/TLS termination point and encrypting the traffic between clients and servers. It can handle both HTTP and HTTPS requests. Varnish, on the other hand, does not natively support SSL/TLS. However, it can be combined with other tools, such as HAProxy or Nginx, to provide SSL/TLS termination and encryption capabilities.

6. Community and Ecosystem: Squid and Varnish have different community support and ecosystems. Squid has a large and active community with extensive documentation, user forums, and a wide range of third-party plugins and extensions. It has been around for a longer time and is widely adopted. Varnish also has an active community and good documentation, but it may not have the same level of third-party plugins and extensions as Squid. However, Varnish is known for its performance and has gained popularity in the web caching space.

In summary, Squid and Varnish differ in their architecture, caching mechanisms, configuration languages, concurrency models, SSL/TLS support, and community ecosystems. Squid is a forward proxy server with a hierarchical caching system, while Varnish is a reverse proxy server with an object-based caching system.