> For the complete documentation index, see [llms.txt](https://calnix.gitbook.io/aave-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://calnix.gitbook.io/aave-book/functions/supply/on-check-effects-interactions-pattern.md).

# On check-effects-interactions pattern

You might have noticed that supply breaks the check-effects-interactions pattern, by making an external `safeTransferFrom` call before the aToken collateral is issued to the user.

The motivation behind adhering to check-effects-interactions pattern is 

* it cannot be avoided to hand over control flow to an external entity.
* you want to guard your functions against re-entrancy attacks.

Adherence to this pattern would require that all the state-changing internal accounting occur first, then the external `safeTransferFrom` to occur at the end of supply.

**However, breaking it is safer**

* You don’t want to be giving collateral to user before the underlying capital is taken in
* On re-entry, instead of the protocol receiving capital, the user would have free collateral to make malicious moves.