? 503 Service Unavailable

? Quick Overview

The 503 Service Unavailable HTTP status code indicates that the server is currently unable to handle the request. This condition is usually temporary and often caused by server overload, maintenance, or upstream service failures.

? Key Concepts

• Server is reachable but cannot process requests
• Typically a temporary issue
• Common in high-traffic or microservice systems
• Often paired with a Retry-After header

? Syntax / Theory

HTTP status code 503 belongs to the 5xx Server Error category. It means the server understands the request but is unable to fulfill it at the moment.

• Server overload (too many requests)
• Server under maintenance
• Database or dependent service down
• Load balancer failing to reach backend

? Code Example(s)

// Example of sending a 503 status in Node.js (Express)
res.status(503).send("Service temporarily unavailable");

// Apache .htaccess custom 503 error page
ErrorDocument 503 /maintenance.html

? Live Output / Explanation

? Interactive Example / Visualization

Think of a restaurant kitchen:

• Customers can enter (server is reachable)
• Kitchen is overloaded or closed (cannot serve requests)
• Customers are asked to wait or return later

// Simple fetch handling for a 503 response
fetch("/api/data")
.then(res => {
if (res.status === 503) {
console.log("Try again later");
}
});

? Use Cases

• Planned server maintenance
• Traffic spikes during sales or launches
• Microservices dependency failure
• Rate-limiting and graceful degradation