PHP
Zero-downtime deployment
Learn zero-downtime deployment with symlink swapping and atomic deploys.
Zero-downtime deployment
Zero-downtime deploy activates the new release without stopping the old one. Users never see a 502 or a timeout.
The common pattern: ship each release into its own folder like releases/2024-10-25, then atomically swap the current symlink to the new folder.
In PHP symlink($target, $link) creates a symbolic link; it is atomic like rename: all or nothing.
The health check endpoint
Before activating the new release a HealthCheck endpoint is called: is the database, cache and opcache ready?
If /health returns 200 OK with {"status":"ok"} the link swaps; otherwise the deploy halts and rollback kicks in.
This check lets the automated pipeline confirm nothing is broken; no human intervention needed.
Exercise Write HealthCheck and deploy
Define a HealthCheck class with a status() method that returns "ok". Then write a deploy() function: print deploy: started, instantiate HealthCheck and print its status() result prefixed with health: , then print deploy: done. Add the deploy() call.
Show example solution
string { return "ok"; } }. Then write function deploy(): void { echo "deploy: started\n"; $hc = new HealthCheck(); echo "health: " . $hc->status() . "\n"; echo "deploy: done\n"; }. Finish with deploy();.
Example solution:
<?php
// HealthCheck class and deploy function
class HealthCheck {
public function status(): string {
return "ok";
}
}
function deploy(): void {
echo "deploy: started\n";
$hc = new HealthCheck();
echo "health: " . $hc->status() . "\n";
echo "deploy: done\n";
}
deploy();
Test yourself
-
Which PHP function creates a symbolic link?
Show answer
symlink — Correct. symlink($target, $link) creates a symbolic link.
-
Complete the symlink call
<?php
// Switch to new release
____('releases/2024-10-25', 'current');
print_r(["link" => "current"]);
Show answer
<?php
// Switch to new release
symlink('releases/2024-10-25', 'current');
print_r(["link" => "current"]);
-
Match the deployment concepts
- symlink
- Creates a symbolic link
- rollback
- Returns to previous release
- atomic
- Indivisible operation
- health check
- Liveness probe
Next lesson: Summary - Deployment
