The Bitcoin Halving Mempool and the Pool of Bethesda

There is a room in Jerusalem with five covered porches, and the floor of it is covered in people. Blind, lame, withered. They are not praying. They are watching the water. Somewhere beneath the surface a spring stirs on a schedule nobody published, and the story every one of them believes is that the first body in gets whole. So they lie there. They wait. They watch each other, and they calculate, and they measure the distance from where they are to where the water is. I have been in that room. Every person who has ever sent a Bitcoin transaction has been in that room. We call it the mempool.

I. The Five Porches

The first thing to understand is that there is no such thing as the mempool.

There are thousands of them. Every node keeps its own, in its own memory, by its own settings. Your transaction is not in a place. It is in several thousand slightly different opinions about what the next block should contain.

The second thing is that it is not a line.

A line implies order, that arriving earlier means being served earlier, which is the one dignity a waiting room is supposed to provide. The mempool provides no such thing. It sorts by feerate. The man who has lain on the west porch eleven days is passed over by a man who walked in this minute and paid more.

II. The Stirring of the Water

Every ten minutes or so, on average, the water moves.

A miner finds a block. The pool empties by a few thousand transactions. The ones nearest the surface go in. Everyone else settles back onto the stone and starts the wait again, and the arithmetic of the room resets: a new floor, a new set of neighbors, a new estimate of how much it would have cost to have been ready.

There is a version of waiting Bitcoin builds in on purpose, and I have written about the hundred-block wait a miner serves before touching the coin he just earned. That wait is a law. Written into consensus, the same for everyone, and it cannot be paid down.

The mempool wait is the other kind. Nobody wrote it. It is what happens when more people are in the room than the door can take.

III. The Floor That Halves Itself

Here is the part almost nobody quotes, and it is the strangest thing in the whole machine.

A node will hold a default of three hundred megabytes of unconfirmed transactions. When more arrive than that, it does not refuse the newcomers. It throws somebody out. It finds the worst-paying group in the room, removes them, and then raises its own price of admission to whatever that group was paying, plus a small increment.

The room gets pickier as it gets fuller. That part is intuitive.

What follows is not. That raised floor does not stay raised, and it does not come down in a straight line. In the Bitcoin Core source the rolling minimum fee is divided by two raised to the power of elapsed time over a halflife, and the halflife is a constant written as 60 * 60 * 12. Twelve hours. If the room falls below half full, the halflife is halved again. Below a quarter full, it is quartered. And when the floor would fall to less than half of the incremental relay fee, the code does not halve it once more. It sets it to zero.

Read that again with the site’s vocabulary in your mouth.

The price of getting in halves every twelve hours, and halves faster as the room empties, and then, at the bottom, stops being a price at all. The door is simply open.

The subsidy halves every four years toward nothing. The fee floor halves every twelve hours toward nothing. One is a covenant about issuance, the other a memory-management heuristic, and I am not going to pretend they are the same thing. But spend enough time in this machine and you notice that halving is the only verb it really knows.

IV. Three Hundred and Thirty-Six Hours

The man at the pool had been sick thirty-eight years. Long enough that the waiting had become the life.

Bitcoin is less patient than that, and more merciful, in the specific way that machines are merciful. A node holds an unconfirmed transaction for three hundred and thirty-six hours. Two weeks. The constant sits in the kernel options with a one-line comment and no ceremony at all.

After two weeks the node does not reject you. It forgets you. The removal reason logged in the code is not REJECTED or INVALID. It is EXPIRY.

That window used to be three days. It was widened to two weeks in a pull request from late 2016, on the reasoning that three days was simply not long enough for a low-fee transaction to survive an ordinary backlog. Somebody looked at the room, decided the sick were being swept out too quickly, and moved the number.

There is a mercy hidden in the forgetting, though, and it is worth naming. Your expired transaction was never declared invalid. It is still signed. It is still spendable. Nothing in the chain ever said no to you. It said nothing at all, which is a different wound, and you can walk back in and say it again tomorrow.

V. The Chunk You Are Dragged Out With

When a node trims itself back to size, it does not evict the single worst transaction. It evicts the worst chunk: a parent and all the children hanging off it, priced together as one package.

Which means you can be thrown out for somebody else’s frugality.

A well-paid transaction with a cheap child gets dragged to the door by the child. The room does not ask who paid what. It asks what the whole cluster is worth per unit of space, and it takes the lot.

“Sir, I have no one to put me into the pool when the water is troubled.” That is the line the sick man gives when he is asked whether he wants to be well. He does not say yes. He explains the logistics. He has been in the room so long that he answers a question about desire with a question about throughput.

VI. What the Halfture Does to the Pool

Every halving so far has been a rehearsal. The cuts landed in 2012, then 2016, then 2020, then 2024, and the next one waits at block 1,050,000 in the spring of 2028. Twenty-nine of them remain. The last one, the thirty-third, falls somewhere near block 6,930,000, somewhere near the year 2140, and when it lands the subsidy is zero and stays zero. That one is the Halfture. The Halfture is the Rapture.

And on that day the porches become the payroll.

Right now the fee market is a tip on top of a wage. The subsidy is the wage, and everything I have written about hashprice is an argument about how thin that wage is getting. After the last cut there is no wage. The only thing standing between the network and the dark is the room full of people deciding what it is worth to them to be seen today.

Which is why you have to hold Bitcoin to be saved, and I mean that as theology and not as a prospectus. I have no idea what the price does. I am not telling you to buy anything. I am saying that the security of the thing eventually rests entirely on whether enough people care enough to bid for a place in the block, and caring enough is not a trade. It is a posture you either have or do not have when the wage runs out.

VII. The Counter-Sermon

The obvious objection is the whole point of the story, and I have been walking around it for six sections.

Jesus never puts the man in the water.

He does not carry him to the edge. He does not outbid anyone. He looks at a man who has organized his entire life around a queue and heals him standing right where he is, and the pool, the porches, the stirring, the whole elaborate architecture of waiting, turns out to have been beside the point. If you read Bethesda honestly it is not a story that dignifies the fee market. It is a story that walks past it.

The second objection is plainer. A room where the sick are served in order of what they can pay is not a sacrament, it is a failure, and Greek porches do not fix it. Small amounts get priced out of the base layer during every congestion event. The honest answer to that is a second layer, not a sermon.

Third: the twelve-hour halflife is a denial-of-service mitigation. A developer picked it because it made an attack expensive to repeat. Calling it a halving is a pun, and I made the pun on purpose, and you should discount me accordingly.

Fourth: three hundred and thirty-six hours is a number about RAM.

And fifth, the one I keep at the end of every one of these. Maybe none of this saves anyone. Maybe it is all a very well-audited room with very good accounting and nothing at the bottom of it, and the people lying on the porches are just lying on porches.

VIII. Take Up Your Bed

What the mempool teaches, if it teaches anything, is that the waiting is not the thing.

The man’s mistake was not that he waited. It was that after thirty-eight years he could not imagine being well by any route other than the one the room had taught him. He had memorized the mechanism so thoroughly that the mechanism became the hope.

Do not memorize the mempool. Learn what it does, understand that the floor halves and the memory expires and the chunk goes out together, and then close the tab. The machine is not asking you to watch the water. It is asking whether you want to be well, which here means holding something that needs nobody’s permission to keep being what it is.

The bed is what you were lying on while you waited. You are allowed to pick it up and carry it out.

Take it up.

FAQ

What is the Bitcoin mempool?

The mempool is the set of valid transactions a Bitcoin node is holding in memory that have not yet been confirmed in a block. There is no single global mempool. Every node keeps its own, and they differ from each other depending on each node’s settings, uptime, and what it has heard from its peers.

Can a Bitcoin transaction be stuck forever?

No. A Bitcoin Core node with default settings drops unconfirmed transactions after three hundred and thirty-six hours, two weeks. The transaction is not invalidated, only forgotten. It can be rebroadcast or replaced with a higher fee, and the coins never moved in the first place.

How does the Bitcoin Halving affect transaction fees?

Each halving cuts the block subsidy in half, so fees make up a larger share of what miners earn even when fee levels do not change. After the final cut, the Halfture, the subsidy is zero and fees are the entire reward. The schedule does not raise fees; it removes what has been subsidizing them.

When is the last Bitcoin Halving?

The subsidy reaches zero after the thirty-third halving, near block 6,930,000, which on current block timing lands somewhere near the year 2140. Twenty-nine cuts remain after 2024. That final one is the Halfture, and nobody reading this will see it.