Squid

Squid is a high‑performance, open‑source caching proxy that sits between clients and web servers to accelerate HTTP(S), FTP, and other protocols. From a developer’s standpoint it is a lightweight daemon written in C that can be compiled on virtually any POSIX‑compatible system, with experimental Windows ports available. Its core function is to intercept outgoing requests, determine whether a cached copy exists, and serve that content if it remains fresh. If not, Squid forwards the request to the origin server, caches the response, and then returns it to the client. This simple yet powerful loop is what enables Internet Service Providers and large enterprises to reduce bandwidth consumption and improve latency.

Architecture

• Language & Runtime: C (≈ 200k LOC) compiled with GCC or Clang; the codebase is modular, exposing a set of hooks for extensions.
• Data Store: Uses a flat‑file cache on disk (or memory‑only mode) with an LRU eviction policy. The cache index is a binary B‑tree for fast lookups.
• Configuration: Driven by a single text file (squid.conf) parsed at startup. The parser supports includes, ACL definitions, and dynamic reloading (-K).
• Networking: Implements a non‑blocking event loop (select/poll/epoll) to handle thousands of concurrent connections. TLS termination is optional via ssl_bump or external libraries.
• Extensibility: Plugins are shared objects (.so/.dll) loaded at runtime; the API exposes hooks for authentication, ACL evaluation, and request/response manipulation.

Core Capabilities

• Fine‑grained Access Control: ACLs can filter by IP, URL, HTTP method, or even custom authentication tokens.
• Hierarchical Caching: Supports parent/child relationships (cache_peer) for multi‑tiered cache networks, ideal for ISP backbones or CDN edge nodes.
• Content Routing & Load Balancing: cache_peer and url_rewrite_program allow dynamic routing to multiple upstream servers based on URL patterns or headers.
• SSL/TLS Support: ssl_bump can intercept HTTPS traffic for inspection or caching, while preserving end‑to‑end encryption when configured as a forward proxy.
• Monitoring & Metrics: Built‑in statistics daemon exposes metrics via HTTP (/stats), and logs can be rotated or forwarded to syslog.

Deployment & Infrastructure

Squid is designed for self‑hosting; it runs as a daemon under systemd or legacy init scripts. For production, the recommended stack includes:

• Containerization: Official Docker images are available; a single container can run a full cache or be part of a cluster with cache_peer directives.
• Scalability: Horizontal scaling is achieved by deploying multiple cache nodes and configuring them in a hierarchy. Each node can be tuned for memory, disk space, or network bandwidth.
• High Availability: squidGuard or custom scripts can monitor health and automatically redirect traffic to healthy peers.

Integration & Extensibility

Developers can extend Squid through:

• Custom ACL Modules: Written in C, compiled as shared objects, and loaded via acl_type.
• External Authentication: auth_param supports PAM, LDAP, OAuth, or custom HTTP auth servers.
• Webhooks & APIs: While Squid lacks a REST API out of the box, its squidclient utility and log hooks can be used to trigger external services (e.g., Slack alerts on cache misses).
• Event Hooks: The cache_manager interface allows integration with external monitoring tools like Prometheus or Grafana.

Developer Experience

• Documentation: The official manual (squid.conf reference) is exhaustive, though the learning curve can be steep for newcomers. Community forums and mailing lists provide rapid support.
• Configuration: Declarative syntax with include files keeps large setups manageable. Dynamic reloading (squid -k reconfigure) avoids downtime.
• Community & Licensing: GPLv2 ensures freedom to modify and redistribute. A vibrant developer community contributes patches, plugins, and performance optimizations.

Use Cases

1. ISP Edge Caching – Reduce upstream bandwidth by caching popular content for subscribers.
2. Enterprise Content Delivery – Serve internal web applications with reduced latency and load on origin servers.
3. CDN Edge Nodes – Deploy lightweight caches in geographically distributed data centers to accelerate global delivery.
4. Security Gateways – Combine with ssl_bump and custom ACLs to inspect traffic while caching.

Advantages

• Performance: Mature, highly tuned codebase that can handle millions of requests per second with minimal CPU overhead.
• Flexibility: Extensive configuration options and plugin architecture allow tailoring to niche requirements (e.g., custom authentication, complex routing).
• Licensing: GPLv2 guarantees no vendor lock‑in; developers can modify internals to fit proprietary systems.
• Maturity & Stability: Decades of production use in large ISPs and data centers provide confidence in reliability.

In summary, Squid offers a robust, extensible proxy solution that empowers developers to build scalable, high‑throughput caching infrastructures while retaining full control over configuration and integration pathways.