What to write yourself.
WHAT TO ALMOST ALWAYS BUY
Payment processing Email sending Error tracking Analytics Authentication for enterprise sign-on Search, at scale
WHY
They are solved, specialised, and building them produces nothing customers value.
WHAT TO BUILD YOURSELF
Whatever is your actual product.
WHAT TO ASK OF ANY COMPONENT
Would a customer choose us because of this?
WHAT THE ANSWER USUALLY IS
No, for infrastructure.
WHAT TO CONSIDER BEFORE ADOPTING A SERVICE
Cost at your scale What happens if it fails What happens if they raise prices How difficult replacement would be Whether it touches customer data
WHY THAT LAST POINT
It becomes a sub-processor with obligations attached.
WHAT TO AVOID
Dependence on a service with no alternative Building your product's core on someone else's product Adopting services faster than you can review them
WHAT TO ISOLATE
Third-party services behind your own interface.
WHY
It makes replacement possible without touching everything.
WHAT TO MAINTAIN
A list of every dependency, what it does, and what it costs.
WHAT TO REVIEW
That list, periodically, against value.
