Five layer cactus farm

ilmango's cactus farm, rebuilt by Adil Gamez. Each layer is an 81 cactus grid on sand inside a glass border, with fences and fence gates beside every plant so the cactus snaps off the moment it grows, and a water sweep across the floor carries everything to six hoppers and four chests. Layers stack on slabs above one another and the build in the video is five of them. The fence gates are the reason this design beats the cheaper cactus farms, since a thinner obstacle destroys less of what it harvests.

Cactus flowers break this on the newest versions. discord_Seaky says a flower can only appear on a cactus that is not fully surrounded, so ring every cactus with fences, and WallaWallaBingBang-X_X reports the farm broken by them outright.
Place the fences before the cacti. SnipyKage's order stops the cacti destroying the fence items while you are still building.
Dead spots in the water flow are the single most common build problem. NC_JordanBlack breaks every block the water misses plus the ones nearest them so the flow re-forms, then replaces them, and mr_jeef6837 avoids it by placing the water before the sand.
The border has to be glass. dibludas3323 explains that fences connect to a solid block, which stops the cactus breaking against them.
Cacti do not need light. Two commenters insist on light level 8 or 9 and both are wrong, which is why there is no light source anywhere in the material list.
Build it as tall as you want. Several people run 8 to 11 layers and BenzALys says the world height is the only real ceiling, with each layer adding the same output.