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Serialize Custom Instruction Data

Lesson Objectives

By the end of this lesson, you will be able to:

  • Explain the contents of a transaction
  • Explain transaction instructions
  • Explain the basics of Solana's runtime optimizations
  • Explain Borsh
  • Use Borsh to serialize custom instruction data

TL;DR

  • Transactions are made up of an array of instructions, a single transaction can have any number of instructions in it, each targeting its own program. When a transaction is submitted, the Solana runtime will process its instructions in order and atomically, meaning that if any of the instructions fail for any reason, the entire transaction will fail to be processed.
  • Every instruction is made up of 3 components: the intended program's ID, an array of all account’s involved, and a byte buffer of instruction data.
  • Every transaction contains: an array of all accounts it intends to read from or write to, one or more instructions, a recent blockhash, and one or more signatures.
  • In order to pass instruction data from a client, it must be serialized into a byte buffer. To facilitate this process of serialization, we will be using Borsh.
  • Transactions can fail to be processed by the blockchain for any number of reasons, we’ll discuss some of the most common ones here.

Overview

Transactions

Transactions are how we send information to the blockchain in order to be processed. So far, we’ve learned how to create very basic transactions with limited functionality. But transactions, and the programs they are sent to, can be designed to be far more flexible and handle far more complexity than we’ve dealt with up to now.

Transaction Contents

Every transaction contains:

  • An array that includes every account it intends to read from or write to
  • One or more instructions
  • A recent blockhash
  • One or more signatures

@solana/web3.js simplifies this process for you so that all you really need to focus on is adding instructions and signatures. The library builds the array of accounts based on that information and handles the logic for including a recent blockhash.

Instructions

Every instruction contains:

  • The program ID (public key) of the intended program
  • An array listing every account that will be read from or written to during execution
  • A byte buffer of instruction data

Identifying the program by its public key ensures that the instruction is carried out by the correct program.

Including an array of every account that will be read from or written to allows the network to perform a number of optimizations which allow for high transaction load and quicker execution.

The byte buffer lets you pass external data to a program.

You can include multiple instructions in a single transaction. The Solana runtime will process these instructions in order and atomically. In other words, if every instruction succeeds then the transaction as a whole will be successful, but if a single instruction fails then the entire transaction will fail immediately with no side-effects.

A note on the account array and optimization:

It is not just an array of the accounts’ public keys. Each object in the array includes the account’s public key, whether or not it is a signer on the transaction, and whether or not it is writable. Including whether or not an account is writable during the execution of an instruction allows the runtime to facilitate parallel processing of smart contracts. Because you must define which accounts are read-only and which you will write to, the runtime can determine which transactions are non-overlapping or read-only and allow them to execute concurrently. To learn more about the Solana’s runtime, check out this blog post.

Instruction Data

The ability to add arbitrary data to an instruction ensures that programs can be dynamic and flexible enough for broad use cases in the same way that the body of an HTTP request lets you build dynamic and flexible REST APIs.

Just as the structure of the body of an HTTP request is dependent on the endpoint you intend to call, the structure of the byte buffer used as instruction data is entirely dependent on the recipient program. If you’re building a full-stack dApp on your own, then you’ll need to copy the same structure that you used when building the program over to the client-side code. If you’re working with another developer who is handling the program development, you can coordinate to ensure matching buffer layouts.

Let’s think about a concrete example. Imagine working on a Web3 game and being responsible for writing client-side code that interacts with a player inventory program. The program was designed to allow the client to:

  • Add inventory based on a player’s game-play results
  • Transfer inventory from one player to another
  • Equip a player with selected inventory items

This program would have been structured such that each of these is encapsulated in its own function.

Each program, however, only has one entry point. You would instruct the program on which of these functions to run through the instruction data.

You would also include in the instruction data any information the function needs in order to execute properly, e.g. an inventory item’s ID, a player to transfer inventory to, etc.

Exactly how this data would be structured would depend on how the program was written, but it’s common to have the first field in instruction data be a number that the program can map to a function, after which additional fields act as function arguments.

Serialization

In addition to knowing what information to include in an instruction data buffer, you also need to serialize it properly. The most common serializer used in Solana is Borsh. Per the website:

Borsh stands for Binary Object Representation Serializer for Hashing. It is meant to be used in security-critical projects as it prioritizes consistency, safety, speed; and comes with a strict specification.

Borsh maintains a JS library that handles serializing common types into a buffer. There are also other packages built on top of borsh that try to make this process even easier. We’ll be using the @project-serum/borsh library which can be installed using npm.

Building off of the previous game inventory example, let’s look at a hypothetical scenario where we are instructing the program to equip a player with a given item. Assume the program is designed to accept a buffer that represents a struct with the following properties:

  1. variant as an unsigned, 8-bit integer that instructs the program which instruction, or function, to execute.
  2. playerId as an unsigned, 16-bit integer that represents the player ID of the player who is to be equipped with the given item.
  3. itemId as an unsigned, 256-bit integer that represents the item ID of the item that will be equipped to the given player.

All of this will be passed as a byte buffer that will be read in order, so ensuring proper buffer layout order is crucial. You would create the buffer layout schema or template for the above as follows:

import * as borsh from '@project-serum/borsh'

const equipPlayerSchema = borsh.struct([
	borsh.u8('variant'),
	borsh.u16('playerId'),
	borsh.u256('itemId')
])

You can then encode data using this schema with the encode method. This method accepts as arguments an object representing the data to be serialized and a buffer. In the below example, we allocate a new buffer that’s much larger than needed, then encode the data into that buffer and slice it down into a new buffer that’s only as large as needed.

import * as borsh from '@project-serum/borsh'

const equipPlayerSchema = borsh.struct([
	borsh.u8('variant'),
	borsh.u16('playerId'),
	borsh.u256('itemId')
])

const buffer = Buffer.alloc(1000)
equipPlayerSchema.encode({ variant: 2, playerId: 1435, itemId: 737498 }, buffer)

const instructionBuffer = buffer.slice(0, equipPlayerSchema.getSpan(buffer))

Once a buffer is properly created and the data serialized, all that’s left is building the transaction. This is similar to what you’ve done in previous lessons. The example below assumes that:

  • player, playerInfoAccount, and PROGRAM_ID are already defined somewhere outside the code snippet
  • player is a user’s public key
  • playerInfoAccount is the public key of the account where inventory changes will be written
  • SystemProgram will be used in the process of executing the instruction.
import * as borsh from '@project-serum/borsh'
import * as web3 from '@solana/web3.js'

const equipPlayerSchema = borsh.struct([
	borsh.u8('variant'),
	borsh.u16('playerId'),
	borsh.u256('itemId')
])

const buffer = Buffer.alloc(1000)
equipPlayerSchema.encode({ variant: 2, playerId: 1435, itemId: 737498 }, buffer)

const instructionBuffer = buffer.slice(0, equipPlayerSchema.getSpan(buffer))

const endpoint = web3.clusterApiUrl('devnet')
const connection = new web3.Connection(endpoint)

const transaction = new web3.Transaction()
const instruction = new web3.TransactionInstruction({
	keys: [
		{
      pubkey: player.publicKey,
      isSigner: true,
      isWritable: false,
    },
    {
      pubkey: playerInfoAccount,
      isSigner: false,
      isWritable: true,
    },
    {
      pubkey: web3.SystemProgram.programId,
      isSigner: false,
      isWritable: false,
    }
	],
	data: instructionBuffer,
	programId: PROGRAM_ID
})

transaction.add(instruction)

web3.sendAndConfirmTransaction(connection, transaction, [player]).then((txid) => {
	console.log(`Transaction submitted: https://explorer.solana.com/tx/${txid}?cluster=devnet`)
})

Demo

Let’s practice this together by building a Movie Review app that lets users submit a movie review and have it stored on Solana’s network. We’ll build this app a little bit at a time over the next few lessons, adding new functionality each lesson.

Screenshot of movie review frontend

The public key of the Solana program we’ll use for this application is CenYq6bDRB7p73EjsPEpiYN7uveyPUTdXkDkgUduboaN.

1. Download the starter code

Before we get started, go ahead and download the starter code.

The project is a fairly simple Next.js application. It includes the WalletContextProvider we created in the Wallets lesson, a Card component for displaying a movie review, a MovieList component that displays reviews in a list, a Form component for submitting a new review, and a Movie.ts file that contains a class definition for a Movie object.

Note that for now, the movies displayed on the page when you run npm run dev are mocks. In this lesson, we’ll focus on adding a new review but we won’t actually be able to see that review displayed. Next lesson, we’ll focus on deserializing custom data from on-chain accounts.

2. Create the buffer layout

Remember that to properly interact with a Solana program, you need to know how it expects data to be structured. Our Movie Review program is expecting instruction data to contain:

  1. variant as an unsigned, 8-bit integer representing which instruction should be executed (in other words which function on the program should be called).
  2. title as a string representing the title of the movie that you are reviewing.
  3. rating as an unsigned, 8-bit integer representing the rating out of 5 that you are giving to the movie you are reviewing.
  4. description as a string representing the written portion of the review you are leaving for the movie.

Let’s configure a borsh layout in the Movie class. Start by importing @project-serum/borsh. Next, create a borshInstructionSchema property and set it to the appropriate borsh struct containing the properties listed above.

import * as borsh from '@project-serum/borsh'

export class Movie {
	title: string;
	rating: number;
	description: string;

	...

	borshInstructionSchema = borsh.struct([
		borsh.u8('variant'),
		borsh.str('title'),
		borsh.u8('rating'),
		borsh.str('description'),
	])
}

Keep in mind that order matters. If the order of properties here differs from how the program is structured, the transaction will fail.

3. Create a method to serialize data

Now that we have the buffer layout set up, let’s create a method in Movie called serialize() that will return a Buffer with a Movie object’s properties encoded into the appropriate layout.

import * as borsh from '@project-serum/borsh'

export class Movie {
	title: string;
	rating: number;
	description: string;

	...

	borshInstructionSchema = borsh.struct([
		borsh.u8('variant'),
		borsh.str('title'),
		borsh.u8('rating'),
		borsh.str('description'),
	])

	serialize(): Buffer {
		const buffer = Buffer.alloc(1000)
		this.borshInstructionSchema.encode({ ...this, variant: 0 }, buffer)
		return buffer.slice(0, this.borshInstructionSchema.getSpan(buffer))
	}
}

The method shown above first creates a large enough buffer for our object, then encodes { ...this, variant: 0 } into the buffer. Because the Movie class definition contains 3 of the 4 properties required by the buffer layout and uses the same naming, we can use it directly with the spread operator and just add the variant property. Finally, the method returns a new buffer that leaves off the unused portion of the original.

4. Send transaction when user submits form

Now that we have the building blocks for the instruction data, we can create and send the transaction when a user submits the form. Open Form.tsx and locate the handleTransactionSubmit function. This gets called by handleSubmit each time a user submits the Movie Review form.

Inside this function, we’ll be creating and sending the transaction that contains the data submitted through the form.

Start by importing @solana/web3.js and importing useConnection and useWallet from @solana/wallet-adapter-react.

import { FC } from 'react'
import { Movie } from '../models/Movie'
import { useState } from 'react'
import { Box, Button, FormControl, FormLabel, Input, NumberDecrementStepper, NumberIncrementStepper, NumberInput, NumberInputField, NumberInputStepper, Textarea } from '@chakra-ui/react'
import * as web3 from '@solana/web3.js'
import { useConnection, useWallet } from '@solana/wallet-adapter-react'

Next, before the handleSubmit function, call useConnection() to get a connection object and call useWallet() to get publicKey and sendTransaction.

import { FC } from 'react'
import { Movie } from '../models/Movie'
import { useState } from 'react'
import { Box, Button, FormControl, FormLabel, Input, NumberDecrementStepper, NumberIncrementStepper, NumberInput, NumberInputField, NumberInputStepper, Textarea } from '@chakra-ui/react'
import * as web3 from '@solana/web3.js'
import { useConnection, useWallet } from '@solana/wallet-adapter-react'

const MOVIE_REVIEW_PROGRAM_ID = 'CenYq6bDRB7p73EjsPEpiYN7uveyPUTdXkDkgUduboaN'

export const Form: FC = () => {
	const [title, setTitle] = useState('')
	const [rating, setRating] = useState(0)
	const [message, setMessage] = useState('')

	const { connection } = useConnection();
	const { publicKey, sendTransaction } = useWallet();

	const handleSubmit = (event: any) => {
		event.preventDefault()
		const movie = new Movie(title, rating, description)
		handleTransactionSubmit(movie)
	}

	...
}

Before we implement handleTransactionSubmit, let’s talk about what needs to be done. We need to:

  1. Check that publicKey exists to ensure that the user has connected their wallet.
  2. Call serialize() on movie to get a buffer representing the instruction data.
  3. Create a new Transaction object.
  4. Get all of the accounts that the transaction will read from or write to.
  5. Create a new Instruction object that includes all of these accounts in the keys argument, includes the buffer in the data argument, and includes the program’s public key in the programId argument.
  6. Add the instruction from the last step to the transaction.
  7. Call sendTransaction, passing in the assembled transaction.

That’s quite a lot to process! But don’t worry, it gets easier the more you do it. Let’s start with the first 3 steps from above:

const handleTransactionSubmit = async (movie: Movie) => {
	if (!publicKey) {
		alert('Please connect your wallet!')
		return
	}

	const buffer = movie.serialize()
	const transaction = new web3.Transaction()
}

The next step is to get all of the accounts that the transaction will read from or write to. In past lessons, the account where data will be stored has been given to you. This time, the account’s address is more dynamic, so it needs to be computed. We’ll cover this in depth in the next lesson, but for now you can use the following, where pda is the address to the account where data will be stored:

const [pda] = await web3.PublicKey.findProgramAddress(
	[publicKey.toBuffer(), Buffer.from(movie.title)],
	new web3.PublicKey(MOVIE_REVIEW_PROGRAM_ID)
)

In addition to this account, the program will also need to read from SystemProgram, so our array needs to include web3.SystemProgram.programId as well.

With that, we can finish the remaining steps:

const handleTransactionSubmit = async (movie: Movie) => {
	if (!publicKey) {
		alert('Please connect your wallet!')
		return
	}

	const buffer = movie.serialize()
	const transaction = new web3.Transaction()

	const [pda] = await web3.PublicKey.findProgramAddress(
		[publicKey.toBuffer(), new TextEncoder().encode(movie.title)],
		new web3.PublicKey(MOVIE_REVIEW_PROGRAM_ID)
	)

	const instruction = new web3.TransactionInstruction({
		keys: [
			{
				pubkey: publicKey,
				isSigner: true,
				isWritable: false,
			},
			{
				pubkey: pda,
				isSigner: false,
				isWritable: true
			},
			{
				pubkey: web3.SystemProgram.programId,
				isSigner: false,
				isWritable: false
			}
		],
		data: buffer,
		programId: new web3.PublicKey(MOVIE_REVIEW_PROGRAM_ID)
	})

	transaction.add(instruction)

	try {
		let txid = await sendTransaction(transaction, connection)
		console.log(`Transaction submitted: https://explorer.solana.com/tx/${txid}?cluster=devnet`)
	} catch (e) {
		alert(JSON.stringify(e))
	}
}

And that’s it! You should now be able to use the form on the site to submit a movie review. While you won’t see the UI update to reflect the new review, you can look at the transaction’s program logs on Solana Explorer to see that it was successful.

If you need a bit more time with this project to feel comfortable, have a look at the complete solution code.

Challenge

Now it’s your turn to build something independently. Create an application that lets students of this course introduce themselves! The Solana program that supports this is at HdE95RSVsdb315jfJtaykXhXY478h53X6okDupVfY9yf.

Screenshot of Student Intros frontend

  1. You can build this from scratch or you can download the starter code here.
  2. Create the instruction buffer layout in StudentIntro.ts. The program expects instruction data to contain:
    1. variant as an unsigned, 8-bit integer representing the instruction to run (should be 0).
    2. name as a string representing the student's name.
    3. message as a string representing the message the student is sharing about their Solana journey.
  3. Create a method in StudentIntro.ts that will use the buffer layout to serialize a StudentIntro object.
  4. In the Form component, implement the handleTransactionSubmit function so that it serializes a StudentIntro, builds the appropriate transaction and transaction instructions, and submits the transaction to the user's wallet.
  5. You should now be able to submit introductions and have the information stored on chain! Be sure to log the transaction ID and look at it in Solana Explorer to verify that it worked.

If you get really stumped, you can check out the solution code here.

Feel free to get creative with these challenges and take them even further. The instructions aren't here to hold you back!

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