People have been comparing software components to LEGO blocks for a couple decades. We should be able to assemble applications by snapping together modular components, just like LEGOs. There has been progress, but for the most part we haven’t realized the promise LEGO-style software development.
Integrating two software systems is usually more like performing a heart transplant than snapping together LEGO blocks. It can be done — if there’s a close enough match and the people doing it have enough skill — but the pieces don’t fit together trivially. And failure may not be immediately obvious; it may take a while to see signs of rejection.
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