The word mining originates in the context of the gold analogy for crypto currencies. Gold or precious metals are scarce, so are digital tokens, and the only way to increase the total volume is through mining. This is appropriate to the extent that in Expanse too, the only mode of issuance post launch is via mining. Unlike these examples however, mining is also the way to secure the network by creating, verifying, publishing and propagating blocks in the blockchain.
- Mining Expanse = Securing the Network = Verifying Computation
What is mining?
Expanse, like all blockchain technologies, uses an incentive-driven model of security. Consensus is based on choosing the block with the highest total difficulty. Miners produce blocks which the others check for validity. Among other well-formedness criteria, a block is only valid if it contains proof of work (PoW) of a given difficulty. Note that in the Expanse Serenity milestone, this is likely going to be replaced by a (see proof of stake model ).
The Expanse blockchain is in many ways similar to the Bitcoin blockchain, although it does have some differences. The main difference between Expanse and Bitcoin with regard to the blockchain architecture is that, unlike Bitcoin, Expanse blocks contain a copy of both the transaction list and the most recent state (the root hash of the merkle patricia trie encoding the state to be more precise). Aside from that, two other values, the block number and the difficulty, are also stored in the block.
The proof of work algorithm used is called Ethash (a modified version of the Dagger-Hashimoto algorithm) and involves finding a nonce input to the algorithm so that the result is below a certain difficulty threshold. The point in PoW algorithms is that there is no better strategy to find such a nonce than enumerating the possibilities, while verification of a solution is trivial and cheap. Since outputs have a uniform distribution (as they are the result of the application of a hash function), we can guarantee that, on average, the time needed to find such a nonce depends on the difficulty threshold. This makes it possible to control the time of finding a new block just by manipulating the difficulty.
As dictated by the protocol, the difficulty dynamically adjusts in such a way that on average one block is produced by the entire network every 15 seconds. We say that the network produces a blockchain with a 15 second block time. This “heartbeat” basically punctuates the synchronisation of system state and guarantees that maintaining a fork (to allow double spend) or rewriting history by malicious actors are impossible unless the attacker possesses more than half of the network mining power (this is the so called 51% attack).
Any node participating in the network can be a miner and their expected revenue from mining will be directly proportional to their (relative) mining power or hashrate, i.e., the number of nonces tried per second normalised by the total hashrate of the network.
Ethash PoW is memory hard, making it ASIC resistant. Memory hardness is achieved with a proof of work algorithm that requires choosing subsets of a fixed resource dependent on the nonce and block header. This resource (a few gigabyte size data) is called a DAG. The DAG is totally different every 30000 blocks, a 125-hour window called epoch (roughly 5.2 days) and takes a while to generate. Since the DAG only depends on block height, it can be pregenerated but if its not, the client needs to wait until the end of this process to produce a block. If clients do not pregenerate and cache DAGs ahead of time the network may experience massive block delay on each epoch transition. Note that the DAG does not need to be generated for verifying the PoW essentially allowing for verification with both low CPU and small memory.
As a special case, when you start up your node from scratch, mining will only start once the DAG is built for the current epoch.
The successful PoW miner of the winning block receives:
- a static block reward for the ‘winning’ block, consisting of exactly 8.0 Expanse
- cost of the gas expended within the block – an amount of expanse that depends on the current gas price
- an extra reward for including uncles as part of the block, in the form of an extra 1/32 per uncle included
All the gas consumed by the execution of all the transactions in the block submitted by the winning miner is paid by the senders of each transaction. The gas cost incurred is credited to the miner’s account as part of the consensus protocol. Over time, it is expected these will dwarf the static block reward.
Uncles are stale blocks i.e. with parents that are ancestors (max 6 blocks back) of the including block. Valid uncles are rewarded in order to neutralize the effect of network lag on the dispersion of mining rewards, thereby increasing security (this is called the GHOST protocol). Uncles included in a block formed by the successful PoW miner receive 7/8 of the static block reward (=4.375 expanse). A maximum of 2 uncles are allowed per block.
Mining success depends on the set block difficulty. Block difficulty dynamically adjusts each block in order to regulate the network hashing power to produce a 12 second blocktime. Your chances of finding a block therefore follows from your hashrate relative to difficulty.
Ethash uses a DAG (directed acyclic graph) for the proof of work algorithm, this is generated for each epoch, i.e., every 30000 blocks (125 hours, ca. 5.2 days). The DAG takes a long time to generate. If clients only generate it on demand, you may see a long wait at each epoch transition before the first block of the new epoch is found. However, the DAG only depends on the block number, so it can and should be calculated in advance to avoid long wait times at each epoch transition. Both gexp and ethminer implement automatic DAG generation and maintains two DAGs at a time for smooth epoch transitions. Automatic DAG generation is turned on and off when mining is controlled from the console. It is also turned on by default if gexp is launched with the –mine option. Note that clients share a DAG resource, so if you are running multiple instances of any client, make sure automatic dag generation is switched off in all but one instance.
To generate the DAG for an arbitrary epoch:
gexp makedag <block number> <outputdir>
For instance gexp makedag 360000 ~/.ethash. Note that ethash uses ~/.ethash (Mac/Linux) or ~/AppData/Ethash (Windows) for the DAG so that it can shared between different client implementations as well as multiple running instances.