Transaction Structure

Summary

A transaction has signatures + a message. The message contains a header, account addresses, recent blockhash, and compiled instructions. Max serialized size: 1,232 bytes.

A Transaction has two top-level fields:

signatures: An array of signatures
message: Transaction information, including the list of instructions to be processed

Transaction

pub struct Transaction {
    pub signatures: Vec<Signature>,
    pub message: Message,
}

Diagram showing the two parts of a transaction

The total serialized size of a transaction must not exceed PACKET_DATA_SIZE (1,232 bytes). This limit equals 1,280 bytes (the IPv6 minimum MTU) minus 48 bytes for network headers (40 bytes IPv6 + 8 bytes fragment header). The 1,232 bytes include both the signatures array and the message struct.

Diagram showing the transaction format and size limits

Signatures

The signatures field is a compact-encoded array of Signature values. Each Signature is a 64-byte Ed25519 signature of the serialized Message, signed with the signer account's private key. One signature is required for every signer account referenced by the transaction's instructions.

The first signature in the array belongs to the fee payer, the account that pays the transaction base fee and prioritization fee. This first signature also serves as the transaction ID, used to look up the transaction on the network. The transaction ID is commonly referred to as the transaction signature.

Fee payer requirements:

Must be the first account in the message (index 0) and a signer.
Must be a System Program-owned account or a nonce account (validated by validate_fee_payer).
Must hold enough lamports to cover rent_exempt_minimum + total_fee; otherwise the transaction fails with InsufficientFundsForFee.

Message

The message field is a Message struct containing the transaction's payload:

header: The message header
account_keys: An array of account addresses required by the transaction's instructions
recent_blockhash: A blockhash that acts as a timestamp for the transaction
instructions: An array of instructions

Message

pub struct Message {
    /// The message header, identifying signed and read-only `account_keys`.
    pub header: MessageHeader,

    /// All the account keys used by this transaction.
    #[serde(with = "short_vec")]
    pub account_keys: Vec<Pubkey>,

    /// The id of a recent ledger entry.
    pub recent_blockhash: Hash,

    /// Programs that will be executed in sequence and committed in
    /// one atomic transaction if all succeed.
    #[serde(with = "short_vec")]
    pub instructions: Vec<CompiledInstruction>,
}

The header field is a MessageHeader struct with three u8 fields that partition the account_keys array into permission groups:

num_required_signatures: Total number of signatures required by the transaction.
num_readonly_signed_accounts: Number of signed accounts that are read-only.
num_readonly_unsigned_accounts: Number of unsigned accounts that are read-only.

MessageHeader

pub struct MessageHeader {
    /// The number of signatures required for this message to be considered
    /// valid. The signers of those signatures must match the first
    /// `num_required_signatures` of [`Message::account_keys`].
    pub num_required_signatures: u8,

    /// The last `num_readonly_signed_accounts` of the signed keys are read-only
    /// accounts.
    pub num_readonly_signed_accounts: u8,

    /// The last `num_readonly_unsigned_accounts` of the unsigned keys are
    /// read-only accounts.
    pub num_readonly_unsigned_accounts: u8,
}

Diagram showing the three parts of the message header

Account addresses

The account_keys field is a compact-encoded array of public keys. Each entry identifies an account used by at least one of the transaction's instructions. The array must include every account and must follow this strict ordering:

Signer + Writable
Signer + Read-only
Non-signer + Writable
Non-signer + Read-only

This strict ordering allows the account_keys array to be combined with the three counts in the message's header to determine the permissions for each account without storing per-account metadata flags. The header counts partition the array into the four permission groups listed above.

Diagram showing the order of the account addresses array

Recent blockhash

The recent_blockhash field is a 32-byte hash that serves two purposes:

Timestamp: proves the transaction was created recently.
Deduplication: prevents the same transaction from being processed twice.

A blockhash expires after 150 slots. If the blockhash is no longer valid when the transaction arrives, it is rejected with BlockhashNotFound, unless it is a valid durable nonce transaction.

The getLatestBlockhash RPC method allows you to get the current blockhash and last block height at which the blockhash will be valid.

Instructions

The instructions field is a compact-encoded array of CompiledInstruction structs. Each CompiledInstruction references accounts by index into the account_keys array rather than by full public key. It contains:

program_id_index: Index into account_keys identifying the program to invoke.
accounts: Array of indices into account_keys specifying the accounts to pass to the program.
data: Byte array containing the instruction discriminator and serialized arguments.

CompiledInstruction

pub struct CompiledInstruction {
    /// Index into the transaction keys array indicating the program account that executes this instruction.
    pub program_id_index: u8,
    /// Ordered indices into the transaction keys array indicating which accounts to pass to the program.
    #[serde(with = "short_vec")]
    pub accounts: Vec<u8>,
    /// The program input data.
    #[serde(with = "short_vec")]
    pub data: Vec<u8>,
}

Compact array of Instructions

Transaction binary format

Transactions are serialized using a compact encoding scheme. All variable-length arrays (signatures, account keys, instructions) are prefixed with a compact-u16 length encoding. This format uses 1 byte for values 0-127 and 2-3 bytes for larger values.

Legacy transaction layout (on the wire):

Field	Size	Description
`num_signatures`	1-3 bytes (compact-u16)	Number of signatures
`signatures`	`num_signatures` x 64 bytes	Ed25519 signatures
`num_required_signatures`	1 byte	`MessageHeader` field 1
`num_readonly_signed`	1 byte	`MessageHeader` field 2
`num_readonly_unsigned`	1 byte	`MessageHeader` field 3
`num_account_keys`	1-3 bytes (compact-u16)	Number of static account keys
`account_keys`	`num_account_keys` x 32 bytes	Public keys
`recent_blockhash`	32 bytes	Blockhash
`num_instructions`	1-3 bytes (compact-u16)	Number of instructions
`instructions`	variable	Array of compiled instructions

Each compiled instruction is serialized as:

Field	Size	Description
`program_id_index`	1 byte	Index into account keys
`num_accounts`	1-3 bytes (compact-u16)	Number of account indices
`account_indices`	`num_accounts` x 1 byte	Account key indices
`data_len`	1-3 bytes (compact-u16)	Length of instruction data
`data`	`data_len` bytes	Opaque instruction data

Size calculation

Given PACKET_DATA_SIZE = 1,232 bytes, the available space can be calculated:

Total = 1232 bytes
 - compact-u16(num_sigs)          # 1 byte
 - num_sigs * 64                  # signature bytes
 - 3                              # message header
 - compact-u16(num_keys)          # 1 byte
 - num_keys * 32                  # account key bytes
 - 32                             # recent blockhash
 - compact-u16(num_ixs)           # 1 byte
 - sum(instruction_sizes)         # per-instruction overhead + data

Example: SOL transfer transaction

The diagram below shows how transactions and instructions work together to allow users to interact with the network. In this example, SOL is transferred from one account to another.

The sender account's metadata indicates that it must sign for the transaction. This allows the System Program to deduct lamports. Both the sender and recipient accounts must be writable, in order for their lamport balance to change. To execute this instruction, the sender's wallet sends the transaction containing its signature and the message containing the SOL transfer instruction.

SOL transfer diagram

After the transaction is sent, the System Program processes the transfer instruction and updates the lamport balance of both accounts.

SOL transfer process diagram

The example below shows the code relevant to the above diagrams. See the System Program's transfer function.

import {
  airdropFactory,
  appendTransactionMessageInstructions,
  createSolanaRpc,
  createSolanaRpcSubscriptions,
  createTransactionMessage,
  generateKeyPairSigner,
  getSignatureFromTransaction,
  lamports,
  pipe,
  sendAndConfirmTransactionFactory,
  setTransactionMessageFeePayerSigner,
  setTransactionMessageLifetimeUsingBlockhash,
  signTransactionMessageWithSigners
} from "@solana/kit";
import { getTransferSolInstruction } from "@solana-program/system";

// Create a connection to cluster
const rpc = createSolanaRpc("http://localhost:8899");
const rpcSubscriptions = createSolanaRpcSubscriptions("ws://localhost:8900");

// Generate sender and recipient keypairs
const sender = await generateKeyPairSigner();
const recipient = await generateKeyPairSigner();

const LAMPORTS_PER_SOL = 1_000_000_000n;
const transferAmount = lamports(LAMPORTS_PER_SOL / 100n); // 0.01 SOL

// Fund sender with airdrop
await airdropFactory({ rpc, rpcSubscriptions })({
  recipientAddress: sender.address,
  lamports: lamports(LAMPORTS_PER_SOL), // 1 SOL
  commitment: "confirmed"
});

// Check balance before transfer
const { value: preBalance1 } = await rpc.getBalance(sender.address).send();
const { value: preBalance2 } = await rpc.getBalance(recipient.address).send();

// Create a transfer instruction for transferring SOL from sender to recipient
const transferInstruction = getTransferSolInstruction({
  source: sender,
  destination: recipient.address,
  amount: transferAmount // 0.01 SOL in lamports
});

// Add the transfer instruction to a new transaction
const { value: latestBlockhash } = await rpc.getLatestBlockhash().send();
const transactionMessage = pipe(
  createTransactionMessage({ version: 0 }),
  (tx) => setTransactionMessageFeePayerSigner(sender, tx),
  (tx) => setTransactionMessageLifetimeUsingBlockhash(latestBlockhash, tx),
  (tx) => appendTransactionMessageInstructions([transferInstruction], tx)
);

// Send the transaction to the network
const signedTransaction =
  await signTransactionMessageWithSigners(transactionMessage);
await sendAndConfirmTransactionFactory({ rpc, rpcSubscriptions })(
  signedTransaction,
  { commitment: "confirmed" }
);
const transactionSignature = getSignatureFromTransaction(signedTransaction);

// Check balance after transfer
const { value: postBalance1 } = await rpc.getBalance(sender.address).send();
const { value: postBalance2 } = await rpc.getBalance(recipient.address).send();

console.log(
  "Sender prebalance:",
  Number(preBalance1) / Number(LAMPORTS_PER_SOL)
);
console.log(
  "Recipient prebalance:",
  Number(preBalance2) / Number(LAMPORTS_PER_SOL)
);
console.log(
  "Sender postbalance:",
  Number(postBalance1) / Number(LAMPORTS_PER_SOL)
);
console.log(
  "Recipient postbalance:",
  Number(postBalance2) / Number(LAMPORTS_PER_SOL)
);
console.log("Transaction Signature:", transactionSignature);

Console

Click to execute the code.

import {
  airdropFactory,
  appendTransactionMessageInstructions,
  createSolanaRpc,
  createSolanaRpcSubscriptions,
  createTransactionMessage,
  generateKeyPairSigner,
  getSignatureFromTransaction,
  lamports,
  pipe,
  sendAndConfirmTransactionFactory,
  setTransactionMessageFeePayerSigner,
  setTransactionMessageLifetimeUsingBlockhash,
  signTransactionMessageWithSigners
} from "@solana/kit";
import { getTransferSolInstruction } from "@solana-program/system";

// Create a connection to cluster
const rpc = createSolanaRpc("http://localhost:8899");
const rpcSubscriptions = createSolanaRpcSubscriptions("ws://localhost:8900");

// Generate sender and recipient keypairs
const sender = await generateKeyPairSigner();
const recipient = await generateKeyPairSigner();

const LAMPORTS_PER_SOL = 1_000_000_000n;
const transferAmount = lamports(LAMPORTS_PER_SOL / 100n); // 0.01 SOL

// Fund sender with airdrop
await airdropFactory({ rpc, rpcSubscriptions })({
  recipientAddress: sender.address,
  lamports: lamports(LAMPORTS_PER_SOL), // 1 SOL
  commitment: "confirmed"
});

// Check balance before transfer
const { value: preBalance1 } = await rpc.getBalance(sender.address).send();
const { value: preBalance2 } = await rpc.getBalance(recipient.address).send();

// Create a transfer instruction for transferring SOL from sender to recipient
const transferInstruction = getTransferSolInstruction({
  source: sender,
  destination: recipient.address,
  amount: transferAmount // 0.01 SOL in lamports
});

// Add the transfer instruction to a new transaction
const { value: latestBlockhash } = await rpc.getLatestBlockhash().send();
const transactionMessage = pipe(
  createTransactionMessage({ version: 0 }),
  (tx) => setTransactionMessageFeePayerSigner(sender, tx),
  (tx) => setTransactionMessageLifetimeUsingBlockhash(latestBlockhash, tx),
  (tx) => appendTransactionMessageInstructions([transferInstruction], tx)
);

// Send the transaction to the network
const signedTransaction =
  await signTransactionMessageWithSigners(transactionMessage);
await sendAndConfirmTransactionFactory({ rpc, rpcSubscriptions })(
  signedTransaction,
  { commitment: "confirmed" }
);
const transactionSignature = getSignatureFromTransaction(signedTransaction);

// Check balance after transfer
const { value: postBalance1 } = await rpc.getBalance(sender.address).send();
const { value: postBalance2 } = await rpc.getBalance(recipient.address).send();

console.log(
  "Sender prebalance:",
  Number(preBalance1) / Number(LAMPORTS_PER_SOL)
);
console.log(
  "Recipient prebalance:",
  Number(preBalance2) / Number(LAMPORTS_PER_SOL)
);
console.log(
  "Sender postbalance:",
  Number(postBalance1) / Number(LAMPORTS_PER_SOL)
);
console.log(
  "Recipient postbalance:",
  Number(postBalance2) / Number(LAMPORTS_PER_SOL)
);
console.log("Transaction Signature:", transactionSignature);

Console

Click to execute the code.

The following example shows the structure of a transaction that contains a single SOL transfer instruction.

import {
  createSolanaRpc,
  generateKeyPairSigner,
  lamports,
  createTransactionMessage,
  setTransactionMessageFeePayerSigner,
  setTransactionMessageLifetimeUsingBlockhash,
  appendTransactionMessageInstructions,
  pipe,
  signTransactionMessageWithSigners,
  getCompiledTransactionMessageDecoder
} from "@solana/kit";
import { getTransferSolInstruction } from "@solana-program/system";

const rpc = createSolanaRpc("http://localhost:8899");
const { value: latestBlockhash } = await rpc.getLatestBlockhash().send();

// Generate sender and recipient keypairs
const sender = await generateKeyPairSigner();
const recipient = await generateKeyPairSigner();

// Define the amount to transfer
const LAMPORTS_PER_SOL = 1_000_000_000n;
const transferAmount = lamports(LAMPORTS_PER_SOL / 100n); // 0.01 SOL

// Create a transfer instruction for transferring SOL from sender to recipient
const transferInstruction = getTransferSolInstruction({
  source: sender,
  destination: recipient.address,
  amount: transferAmount
});

// Create transaction message
const transactionMessage = pipe(
  createTransactionMessage({ version: 0 }),
  (tx) => setTransactionMessageFeePayerSigner(sender, tx),
  (tx) => setTransactionMessageLifetimeUsingBlockhash(latestBlockhash, tx),
  (tx) => appendTransactionMessageInstructions([transferInstruction], tx)
);

const signedTransaction =
  await signTransactionMessageWithSigners(transactionMessage);

// Decode the messageBytes
const compiledTransactionMessage =
  getCompiledTransactionMessageDecoder().decode(signedTransaction.messageBytes);

console.log(JSON.stringify(compiledTransactionMessage, null, 2));

Console

Click to execute the code.

import {
  createSolanaRpc,
  generateKeyPairSigner,
  lamports,
  createTransactionMessage,
  setTransactionMessageFeePayerSigner,
  setTransactionMessageLifetimeUsingBlockhash,
  appendTransactionMessageInstructions,
  pipe,
  signTransactionMessageWithSigners,
  getCompiledTransactionMessageDecoder
} from "@solana/kit";
import { getTransferSolInstruction } from "@solana-program/system";

const rpc = createSolanaRpc("http://localhost:8899");
const { value: latestBlockhash } = await rpc.getLatestBlockhash().send();

// Generate sender and recipient keypairs
const sender = await generateKeyPairSigner();
const recipient = await generateKeyPairSigner();

// Define the amount to transfer
const LAMPORTS_PER_SOL = 1_000_000_000n;
const transferAmount = lamports(LAMPORTS_PER_SOL / 100n); // 0.01 SOL

// Create a transfer instruction for transferring SOL from sender to recipient
const transferInstruction = getTransferSolInstruction({
  source: sender,
  destination: recipient.address,
  amount: transferAmount
});

// Create transaction message
const transactionMessage = pipe(
  createTransactionMessage({ version: 0 }),
  (tx) => setTransactionMessageFeePayerSigner(sender, tx),
  (tx) => setTransactionMessageLifetimeUsingBlockhash(latestBlockhash, tx),
  (tx) => appendTransactionMessageInstructions([transferInstruction], tx)
);

const signedTransaction =
  await signTransactionMessageWithSigners(transactionMessage);

// Decode the messageBytes
const compiledTransactionMessage =
  getCompiledTransactionMessageDecoder().decode(signedTransaction.messageBytes);

console.log(JSON.stringify(compiledTransactionMessage, null, 2));

Console

Click to execute the code.

The code below shows the output from the previous code snippets. The format differs between SDKs, but notice that each instruction contains the same required information.

{
  "version": 0,
  "header": {
    "numSignerAccounts": 1,
    "numReadonlySignerAccounts": 0,
    "numReadonlyNonSignerAccounts": 1
  },
  "staticAccounts": [
    "HoCy8p5xxDDYTYWEbQZasEjVNM5rxvidx8AfyqA4ywBa",
    "5T388jBjovy7d8mQ3emHxMDTbUF8b7nWvAnSiP3EAdFL",
    "11111111111111111111111111111111"
  ],
  "lifetimeToken": "EGCWPUEXhqHJWYBfDirq3mHZb4qDpATmYqBZMBy9TBC1",
  "instructions": [
    {
      "programAddressIndex": 2,
      "accountIndices": [0, 1],
      "data": {
        "0": 2,
        "1": 0,
        "2": 0,
        "3": 0,
        "4": 128,
        "5": 150,
        "6": 152,
        "7": 0,
        "8": 0,
        "9": 0,
        "10": 0,
        "11": 0
      }
    }
  ]
}

Fetching transaction details

After submission, retrieve transaction details using the transaction signature and the getTransaction RPC method.

You can also find the transaction using Solana Explorer.

Transaction Data

{
  "blockTime": 1745196488,
  "meta": {
    "computeUnitsConsumed": 150,
    "err": null,
    "fee": 5000,
    "innerInstructions": [],
    "loadedAddresses": {
      "readonly": [],
      "writable": []
    },
    "logMessages": [
      "Program 11111111111111111111111111111111 invoke [1]",
      "Program 11111111111111111111111111111111 success"
    ],
    "postBalances": [989995000, 10000000, 1],
    "postTokenBalances": [],
    "preBalances": [1000000000, 0, 1],
    "preTokenBalances": [],
    "rewards": [],
    "status": {
      "Ok": null
    }
  },
  "slot": 13049,
  "transaction": {
    "message": {
      "header": {
        "numReadonlySignedAccounts": 0,
        "numReadonlyUnsignedAccounts": 1,
        "numRequiredSignatures": 1
      },
      "accountKeys": [
        "8PLdpLxkuv9Nt8w3XcGXvNa663LXDjSrSNon4EK7QSjQ",
        "7GLg7bqgLBv1HVWXKgWAm6YoPf1LoWnyWGABbgk487Ma",
        "11111111111111111111111111111111"
      ],
      "recentBlockhash": "7ZCxc2SDhzV2bYgEQqdxTpweYJkpwshVSDtXuY7uPtjf",
      "instructions": [
        {
          "accounts": [0, 1],
          "data": "3Bxs4NN8M2Yn4TLb",
          "programIdIndex": 2,
          "stackHeight": null
        }
      ],
      "indexToProgramIds": {}
    },
    "signatures": [
      "3jUKrQp1UGq5ih6FTDUUt2kkqUfoG2o4kY5T1DoVHK2tXXDLdxJSXzuJGY4JPoRivgbi45U2bc7LZfMa6C4R3szX"
    ]
  },
  "version": "legacy"
}

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