Transaction
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Transaction in a blockchain is a signed instruction that modifies the network's state — typically moving funds from one address to another, calling a smart contract, or creating a token. Each transaction is authorized by a cryptographic signature from the sender's private key.
Basic components
- Inputs / sender — who's authorizing the transaction
- Outputs / recipient — where value is going
- Amount — how much is being moved
- Fee — paid to miners/validators for inclusion
- Signature — cryptographic proof the sender authorized it
- Nonce — a counter that prevents replay attacks
Two transaction models
UTXO (Bitcoin, Litecoin, Tari, Cardano)
The ledger is a set of Unspent Transaction Outputs (UTXOs) — indivisible coin chunks, each attached to a lock script (usually requiring a specific signature). A transaction consumes one or more UTXOs as inputs and creates new UTXOs as outputs. If inputs exceed the send amount + fee, the difference goes back as "change".
Account-based (Ethereum, Solana, BNB Chain)
The ledger is a set of accounts, each with a running balance. A transaction directly debits the sender's account and credits the recipient's. Simpler to reason about but requires a nonce counter to prevent double-spends.
From signing to confirmation
- Sender builds and signs a transaction with their private key
- The transaction is broadcast to nodes and enters the mempool — a pool of pending transactions
- A miner or validator selects it for inclusion in a block
- Once the block is mined, the transaction has 1 confirmation
- Each additional block on top adds another confirmation
The more confirmations, the harder it becomes to reverse the transaction. Bitcoin exchanges typically wait for 3–6 confirmations; Ethereum treats finality differently under proof of stake.
Mempool
The mempool (memory pool) holds valid unconfirmed transactions. Miners typically pick the highest-fee-per-byte transactions first. When the network is congested, low-fee transactions can wait hours or days — or be dropped entirely.
