Instantsend

Dash Core's InstantSend feature provides a way to lock transaction inputs and enable secure, instantaneous transactions. The network automatically attempts to upgrade any qualifying transaction to InstantSend without a need for the sending wallet to explicitly request it.

• To qualify for InstantSend, each transaction input must meet at least one of the following criteria:
  • Be locked by InstantSend
  • Be in a block that has a ChainLock
  • Have at least the number confirmations (block depth) indicated by the table below

Network	Confirmations Required
Mainnet	6 Blocks (normal transactions) 100 Blocks (mining/masternode rewards)
Testnet / Regtest / Devnet	2 Blocks

The introduction of the Long-Living Masternode Quorum feature in Dash Core 0.14 provided a foundation to scale InstantSend. The transaction input locking process (and resulting network traffic) now occurs only within the quorum. This minimizes network congestion since only the islock message produced by the locking process is relayed to the entire Dash network. This message contains all the information necessary to verify a successful transaction lock.

Sporks 2 (SPORK_2_INSTANTSEND_ENABLED) and 20 (SPORK_20_INSTANTSEND_LLMQ_BASED) are used to manage InstantSend. Spork 2 enables or disables the entire InstantSend feature. Spork 20 was used to support the transition to LLMQ-based InstantSend and is currently retained for backward compatibility. It will be deprecated in a future release.

Note: A transaction will not be included in the block template (from the getblocktemplate RPC) unless it:

1. Has been locked, or
2. Has been in the mempool for >=10 minutes (WAIT_FOR_ISLOCK_TIMEOUT)

A miner may still include any transaction, but blocks containing only locked transactions (or ones older than the timeout) should achieve a ChainLock faster. This is desirable to miners since it prevents any reorgs that might orphan their block.

InstantSend Data Flow

InstantSend Client	Direction	Peers	Description
tx message	→		Client sends InstantSend transaction
LLMQ Signing Sessions			Quorums internally process locking
			Quorum(s) responsible for the transaction's inputs lock the inputs via LLMQ signing sessions
			Once all inputs are locked, the quorum responsible for the overall transaction creates the transaction lock (islock) via an LLMQ signing session
LLMQ Results			Quorum results broadcast to the network
	←	inv message (islock)	Quorum responds with lock inventory
getdata message (islock)	→		Client requests lock message
	←	islock message	Quorum responds with lock message

Once a transaction lock is approved, the instantlock field of various RPCs is set to true (e.g. the getmempoolentry RPC).