CGI vs. Servlet: Key Differences, Performance & When to Use
CGI is the original gateway protocol that spawns a new OS process for every web request; a Servlet is a Java class loaded once by the web container that handles each request inside a persistent JVM thread.
People confuse them because both “sit in front” of server resources, but CGI feels like a quick shell script while Servlets feel like heavyweight Java. That gut reaction hides the real cost: CGI starts a brand-new process every click—imagine rebooting your calculator after every sum.
Key Differences
CGI launches a separate process per HTTP request; Servlets reuse a single JVM process and thread pool. CGI scripts can be written in any language; Servlets must be Java classes. Memory footprint spikes with CGI, stays flat with Servlets. CGI scales linearly with CPU cores; Servlets scale via thread pooling and connection reuse.
Which One Should You Choose?
Use CGI for quick prototypes, legacy C/C++ libraries, or infrequent admin scripts. Choose Servlets when you need high concurrency, session state, or integration with Java frameworks like Spring. If your traffic is light and simplicity trumps speed, CGI still works; otherwise, Servlets win on throughput and resource efficiency.
Examples and Daily Life
A weather site polling a Fortran climate model via CGI spawns 10,000 processes at noon and melts the server. The same site rewritten as a Servlet fetches model data in one JVM, reusing connections, and serves 10,000 users with 20 threads and 200 MB RAM instead of 10 GB.
Can I mix CGI and Servlets in one project?
Yes; map legacy CGI scripts to /cgi-bin and route new features to Servlet URLs. Most containers allow both.
Does a Servlet always beat CGI in speed?
For high load, yes. For a single, heavy computational task written in optimized C, CGI can finish faster once than JVM warm-up.