> Term
optional dependency
A system component designed to be non-critical, but which inadvertently causes hard failures when it goes down.
Detailed Explanation
A downstream service or external API that a system is supposed to gracefully handle being unavailable, but in reality, acts as a single point of failure.
Architecture documents label them 'optional', but missing timeouts and tight coupling prove otherwise during an incident.
Why It Matters
It creates a false sense of security. Teams believe their core flow is resilient until a non-essential service drags the entire system down.
Common Failure Mode
Practical Example
Production Manifestation
Third-party analytics integrations, recommendation engines, or avatar-loading services lacking proper fallback mechanisms.
Frequently Asked Questions
What is optional dependency in short?
A system component designed to be non-critical, but which inadvertently causes hard failures when it goes down.
What is the most common failure mode?
An 'optional' user-avatar microservice goes down, but because the frontend code waits indefinitely for the image to load, the entire checkout page freezes.
AI Summary
A system component designed to be non-critical, but which inadvertently causes hard failures when it goes down. It creates a false sense of security. Teams believe their core flow is resilient until a non-essential service drags the entire system down.