getminingcandidate or GMC is a Remote Procedure Call (RPC) available in the Bitcoin SV node software.

Its purpose is to provide mining software/pools with a suitable block candidate for hashing in order to mine a block. In addition it is greatly decoupled from block size unlike its predecessor GetBlockTemplate, meaning it can scale and support very large block sizes.

The way it supports blocks of very large sizes is by only providing a minimal set of data required for mining hardware to hash and generate a possible block solution. If a blocksolution is found it can be returned to the node software via the sibling RPC call submitminingsolution.

GMCs predecessor getblocktemplate gave the calling mining software more control, but in order to do this it had to provide a full and complete dataset of the current block and pending transactions in the mempool. A miner trying to mine large blocks must put a lot more strain on both the node software and RPC interface to transmit a record of every transaction that is in the mempool, often resulting in lack of memory and/or performance issues.

GMC removes this issue by only providing the mining software with a merkleproof of included transactions rather than a complete listing, this lowers the amount of data to be produced and transmitted to log2(blocksize).

When the node software receives a GMC request it will respond with a JSON representation of the current block candidate. A description of this response body is shown below.

Response body of getminingcandidate

Variable Description
[uuid] id The assigned id from Bitcoind. This must be provided with submitting a potential solution
[hex string] prevHash Big Endian hash of the previous block
[hex string] Coinbase (optional) Suggested coinbase transaction is provided. Miner is free to supply their own or alter the supplied one. Altering will require parsing and splitting the coinbase in order to splice in/out data as required. Requires Wallet to be enabled. Only returned when provide_coinbase_tx argument is set to true. Returns error if Wallet is not enabled
[int32_t] version The block version
[int64_t] coinbaseValue Total funds available for the coinbase tx (in Satoshis)
[uint32_t] nBits Compressed hexadecimal difficulty
[uint32_t] time Current block time
[int] height The candidate block height
[array] merkleProof Merkle branch/path for the block, used to calculate the Merkle Root of the block candidate. This is a list of Little-Endian hex strings ordered from top to bottom

For a more technical breakdown of GetMiningCandidate and its implementation see the protocol spec here