Legendary four ring gold and XP farm

Zombified pigman gold and XP farm on the nether roof, ilmango's design. Four stacked spawning rings feed kill chambers where minecarts pack the pigmen tight enough to die of entity cramming, and because they die angry they drop XP as well as gold. Twelve name tagged pigmen sit as alarms, so one snowball thrown from the AFK spot turns the whole farm hostile and keeps it that way while you stand still. Hopper minecarts under the chambers pick the gold up. Java only, entity cramming does not exist on Bedrock. The design is from 2017 and the comments have modernised most of it: from 1.16 the rings are magma blocks in a nether wastes biome, which removes the need for the torches and the iron golems entirely.

• From 1.14 use grindstones or cobblestone walls instead of the 56 anvils. Same hitbox, far cheaper, and with walls the pigmen stop glitching through.
• From 1.16 build the rings out of magma blocks and put the whole farm inside nether wastes. Nothing else can spawn then, so the torches and the iron golems are not needed.
• If the pigmen stop spawning, spawn proof under the bedrock. Everything within 128 blocks of the AFK spot at y242 counts, so check down to y114.
• If they lose aggro after a few minutes, raise the render distance to 16, which is ilmango's own answer, or move the named pigmen 2 to 3 blocks further out, because their follow range varies between 30 and 40 blocks.
• On spigot, paper or bukkit set exp-merging merge-radius to 0 in spigot.yml, or the XP stacks into a single orb and never reaches you.
• Give each collection hopper its own storage. Running them all into one hopper chain overflows and causes heavy lag.