Canary¶
Canary, borrowed from the miner’s canary once used to detect dangerous gas underground, has two distinct meanings in software engineering:
A canary release, or canary deployment, is a rollout strategy where a new version is deployed to a small subset of servers or users first. Its behavior and metrics are monitored before the rollout is extended to everyone, which limits the blast radius of a regression. It is a close relative of the feature flag and of blue-green deployment.
A stack canary, or stack cookie, is a known value placed on the call stack, right before a function’s return address. If a buffer overflow overwrites the return address, it also overwrites the canary first, so the corruption is detected before the corrupted address is ever used. It is a memory-safety technique specific to compiled, memory-unsafe languages such as C and C++.
PHP scripts do not manage the stack directly, so stack canaries are not a PHP-level concern; the Zend Engine that executes PHP, however, is itself written in C, and may benefit from stack canaries at the compiler level. Canary releases, on the other hand, are a common practice for deploying PHP applications.
<?php
// a simple canary release, gated by a user's id
if ($user->id % 100 < 5) { // only 5% of users see the new checkout flow
return new CheckoutFlowV2();
}
return new CheckoutFlowV1();
?>
See also Canary Release and Stack canary.
Related : Feature Flag, Release, Circuit Breaker, Sentinel
