Address Lookup Tables

Address Lookup Tables, commonly referred to as "lookup tables" or "ALTs" for short, allow developers to create a collection of related addresses to efficiently load more addresses in a single transaction.

Since each transaction on the Nexis blockchain requires a listing of every address that is interacted with as part of the transaction, this listing would effectively be capped at 32 addresses per transaction. With the help of Address Lookup Tables, a transaction would now be able to raise that limit to 256 addresses per transaction.

Compressing on chain addresses #

After all the desired addresses have been stored on chain in an Address Lookup Table, each address can be referenced inside a transaction by its 1-byte index within the table (instead of their full 32-byte address). This lookup method effectively "compresses" a 32-byte address into a 1-byte index value.

This "compression" enables storing up to 256 addresses in a single lookup table for use inside any given transaction.

Versioned Transactions #

To utilize an Address Lookup Table inside a transaction, developers must use v0 transactions that were introduced with the new Versioned Transaction format.

How to create an address lookup table #

Creating a new lookup table with the @solana/web3.js library is similar to the older legacy transactions, but with some differences.

Using the @solana/web3.js library, you can use the createLookupTable function to construct the instruction needed to create a new lookup table, as well as determine its address:

const web3 = require("@solana/web3.js");
 
// connect to a cluster and get the current `slot`
const connection = new web3.Connection(web3.clusterApiUrl("devnet"));
const slot = await connection.getSlot();
 
// Assumption:
// `payer` is a valid `Keypair` with enough NZT to pay for the execution
 
const [lookupTableInst, lookupTableAddress] =
  web3.AddressLookupTableProgram.createLookupTable({
    authority: payer.publicKey,
    payer: payer.publicKey,
    recentSlot: slot,
  });
 
console.log("lookup table address:", lookupTableAddress.toBase58());
 
// To create the Address Lookup Table on chain:
// send the `lookupTableInst` instruction in a transaction

Info

NOTE: Address lookup tables can be created with either a v0 transaction or a legacy transaction. But the Nexis runtime can only retrieve and handle the additional addresses within a lookup table while using v0 Versioned Transactions.

Add addresses to a lookup table #

Adding addresses to a lookup table is known as "extending". Using the @solana/web3.js library, you can create a new extend instruction using the extendLookupTable method:

// add addresses to the `lookupTableAddress` table via an `extend` instruction
const extendInstruction = web3.AddressLookupTableProgram.extendLookupTable({
  payer: payer.publicKey,
  authority: payer.publicKey,
  lookupTable: lookupTableAddress,
  addresses: [
    payer.publicKey,
    web3.SystemProgram.programId,
    // list more `publicKey` addresses here
  ],
});
 
// Send this `extendInstruction` in a transaction to the cluster
// to insert the listing of `addresses` into your lookup table with address `lookupTableAddress`

Info

NOTE: Due to the same memory limits of legacy transactions, any transaction used to extend an Address Lookup Table is also limited in how many addresses can be added at a time. Because of this, you will need to use multiple transactions to extend any table with more addresses (~20) that can fit within a single transaction's memory limits.

Once these addresses have been inserted into the table, and stored on chain, you will be able to utilize the Address Lookup Table in future transactions. Enabling up to 256 addresses in those future transactions.

Fetch an Address Lookup Table #

Similar to requesting another account (or PDA) from the cluster, you can fetch a complete Address Lookup Table with the getAddressLookupTable method:

// define the `PublicKey` of the lookup table to fetch
const lookupTableAddress = new web3.PublicKey("");
 
// get the table from the cluster
const lookupTableAccount = (
  await connection.getAddressLookupTable(lookupTableAddress)
).value;
 
// `lookupTableAccount` will now be a `AddressLookupTableAccount` object
 
console.log("Table address from cluster:", lookupTableAccount.key.toBase58());

Our lookupTableAccount variable will now be a AddressLookupTableAccount object which we can parse to read the listing of all the addresses stored on chain in the lookup table:

// loop through and parse all the addresses stored in the table
for (let i = 0; i < lookupTableAccount.state.addresses.length; i++) {
  const address = lookupTableAccount.state.addresses[i];
  console.log(i, address.toBase58());
}

How to use an address lookup table in a transaction #

After you have created your lookup table, and stored your needed address on chain (via extending the lookup table), you can create a v0 transaction to utilize the on chain lookup capabilities.

Just like older legacy transactions, you can create all the instructions your transaction will execute on chain. You can then provide an array of these instructions to the Message used in the `v0 transaction.

Info

NOTE: The instructions used inside a v0 transaction can be constructed using the same methods and functions used to create the instructions in the past. There is no required change to the instructions used involving an Address Lookup Table.

// Assumptions:
// - `arrayOfInstructions` has been created as an `array` of `TransactionInstruction`
// - we are using the `lookupTableAccount` obtained above
 
// construct a v0 compatible transaction `Message`
const messageV0 = new web3.TransactionMessage({
  payerKey: payer.publicKey,
  recentBlockhash: blockhash,
  instructions: arrayOfInstructions, // note this is an array of instructions
}).compileToV0Message([lookupTableAccount]);
 
// create a v0 transaction from the v0 message
const transactionV0 = new web3.VersionedTransaction(messageV0);
 
// sign the v0 transaction using the file system wallet we created named `payer`
transactionV0.sign([payer]);
 
// send and confirm the transaction
// (NOTE: There is NOT an array of Signers here; see the note below...)
const txid = await web3.sendAndConfirmTransaction(connection, transactionV0);
 
console.log(
  `Transaction: https://nexscan.io/tx/${txid}?cluster=devnet`,
);

Info

NOTE: When sending a VersionedTransaction to the cluster, it must be signed BEFORE calling the sendAndConfirmTransaction method. If you pass an array of Signer (like with legacy transactions) the method will trigger an error!

More Resources #

• Read the proposal for Address Lookup Tables and Versioned transactions

• Example Rust program using Address Lookup Tables