Topic: How does a block complete when there are lots of transactions? (Read 2028 times)

Bitcoin maintains a memory pool of all valid transactions that are waiting for a block. This contains all transactions that are valid and at least meet the minimum anti-spam fee rules. It also includes "orphan" transactions, which are not yet funded by a payment in the blockchain. These are relayed to other Bitcoin nodes when first seen.

When mining, Bitcoin assembles a sub-set of the memory pool transactions into a block (there may be some or many memory pool transactions that won't fit in the structure of the current block with the miner's rules), and keeps mining the same block data until there are new transactions and it's been over 60 seconds, if 4 billion hashes are done, or if a new network block is announced.

Main.cpp

Only when these conditions happen is a new Merkle tree built and a new block assembled with another set of transactions (if the block is full, transactions can be bumped out of the temporary block by newer ones with higher priority or more fees). A flood of transactions won't create a CPU denial-of-service attack (for OP here).

The transactions (and other block data) that would be included in a block by your Bitcoin is available for your viewing with the Bitcoin RPC command getblocktemplate.

When mining for a pool, their Bitcoin assembles blocks the same way (although they may have tweaked the rules). The only difference is pools have an additional layer of software that assigns work to specific miners and keeps track of what miners are working on, and will reject unassigned work submissions.

Now I hope I understand it well enough to explain it to about 120 engineers tomorrow at noon.

Just to clarify, since we're apparently going to get nit-picky here...

For the individual miner (pool), there is no such thing as a tie. The first valid block received is the one the miner is (supposed to) work on, regardless of whether a subsequent valid block for the same spot in the chain has more transactions or a higher difficulty (lower hash).

What happens in the case of multiple valid blocks for the same spot is that the chain is forked

and the miners will work on whichever block they got first.

When the NEXT block is solved (next spot in chain), that chain is now longer (and has highest "proof-of-work", as Trongersoll pointed out) and everyone switches to that chain. The valid block in the other fork is orphaned (its transactions not part of the blockchain

and the miners get nothing for their work).

Where "longest blockchain" actually means the blockchain with the highest total amount of "proof-of-work".

In the case of a tie. there is a fork and the longest blockchain is accepted.

So the basic concept of Bitcoin mining is that there’s a little chunk of each block that contains meaningless random data, and Bitcoin miners take all the data in the current block, shuffle up that random chunk and calculate the hash of the whole thing.  Remember that while hashes are easy to reproduce, they’re impossible to predict and they seem to function completely randomly, so the miner has no way of predicting what chunk of random data will produce what hash, he just has to keep changing that chunk until he gets the result he’s looking for. Eventually, some miner somewhere finds a chunk of random data that produces a hash that is smaller than a certain value, as determined by the difficulty, and submits this block and its accompanying hash to the Bitcoin network. Once the network confirms the solution is correct, the miner is rewarded with a number of Bitcoins.