Solana Cookbook

Account Maps

Maps are data structures we frequently use in programming to associate a key with a value of some kind. The key and value could be any arbitrary type and the key acts as an identifier for a given value that is being saved. It then, given its key, allows us to efficiently insert, retrieve and update these values efficiently.

Solana's Account model, as we know, requires program data and its relevant state data to be stored in different accounts. These accounts have an address associated with them. This, in itself, acts as a map! Learn more about Solana's Account mode hereopen in new window.

So, it would make sense to store your values in separate accounts, with its address being the key required to retrieve the value. But this brings up a few issues, such as,

  • The addresses mentioned above are most probably not going to be ideal keys, which you could remember and retrieve the required value.

  • The addresses mentioned above, referred to public keys of different Keypairs, where each public key (or address) would have a private key associated with it as well. This private key would be required to sign different instructions if and when needed, requiring us to store the private key somewhere, which is most definitely not recommended!

This presents a problem many Solana developers face, which is implementing a Map-like logic into their programs. Let's look at a couple of way how we would go about this problem,

Deriving PDAs

PDA stands for Program Derived Addressopen in new window, and are in short, addresses derived from a set of seeds, and a program id (or address).

The unique thing about PDAs is that, these addresses are not associated with any private key. This is because these addresses do not lie on the ED25519 curve. Hence, only the program, from which this address was derived, can sign an instruction with the key, provided the seeds as well. Learn more about this hereopen in new window.

Now that we have an idea about what PDAs are, let's use them to map some accounts! We'll take an example of a Blog program to demonstrate how this would be implemented.

In this Blog program, we would like each User to have a single Blog. This blog could have any number of Posts. That would mean that we are mapping each user to a blog, and each post is mapped to a certain blog.

In short, there is a 1:1 mapping between a user and his/her blog, whereas a 1:N mapping between a blog and its posts.

For the 1:1 mapping, we would want a blog's address to be derived only from its user, which would allow us to retrieve a blog, given its authority (or user). Hence, the seeds for a blog would consist of its authority's key, and possibly a prefix of "blog", to act as a type identifier.

For the 1:N mapping, we would want each post's address to be derived not only from the blog which it is associated with, but also another identifier, allowing us to differentiate between the N number of posts in the blog. In the example below, each post's address is derived from the blog's key, a slug to identify each post, and a prefix of "post", to act as a type identifier.

The code is as shown below,

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use anchor_lang::prelude::*;

declare_id!("2vD2HBhLnkcYcKxnxLjFYXokHdcsgJnyEXGnSpAX376e");

#[program]
pub mod mapping_pda {
    use super::*;
    pub fn initialize_blog(ctx: Context<InitializeBlog>, _blog_account_bump: u8, blog: Blog) -> ProgramResult {
        ctx.accounts.blog_account.set_inner(blog);
        Ok(())
    }

    pub fn create_post(ctx: Context<CreatePost>, _post_account_bump: u8, post: Post) -> ProgramResult {
        if (post.title.len() > 20) || (post.content.len() > 50) {
            return Err(ErrorCode::InvalidContentOrTitle.into());
        }

        ctx.accounts.post_account.set_inner(post);
        ctx.accounts.blog_account.post_count += 1;

        Ok(())
    }
}

#[derive(Accounts)]
#[instruction(blog_account_bump: u8)]
pub struct InitializeBlog<'info> {
    #[account(
        init,
        seeds = [
            b"blog".as_ref(),
            authority.key().as_ref()
        ],
        bump = blog_account_bump,
        payer = authority,
        space = Blog::LEN
    )]
    pub blog_account: Account<'info, Blog>,

    #[account(mut)]
    pub authority: Signer<'info>,

    pub system_program: Program<'info, System>
}

#[derive(Accounts)]
#[instruction(post_account_bump: u8, post: Post)]
pub struct CreatePost<'info> {
    #[account(mut, has_one = authority)]
    pub blog_account: Account<'info, Blog>,

    #[account(
        init,
        seeds = [
            b"post".as_ref(),
            blog_account.key().as_ref(),
            post.slug.as_ref(),
        ],
        bump = post_account_bump,
        payer = authority,
        space = Post::LEN
    )]
    pub post_account: Account<'info, Post>,

    #[account(mut)]
    pub authority: Signer<'info>,
    
    pub system_program: Program<'info, System>
}

#[account]
pub struct Blog {
    pub authority: Pubkey,
    pub bump: u8,
    pub post_count: u8,
}

#[account]
pub struct Post {
    pub author: Pubkey,
    pub slug: String, // 10 characters max
    pub title: String, // 20 characters max
    pub content: String // 50 characters max
}

impl Blog {
    const LEN: usize = 8 + 32 + 1 + (4 + (10 * 32));
}

impl Post {
    const LEN: usize = 8 + 32 + 32 + (4 + 10) + (4 + 20) + (4 + 50); 
}

#[error]
pub enum ErrorCode {
    #[msg("Invalid Content or Title.")]
    InvalidContentOrTitle,
}

use std::convert::TryInto;
use borsh::{BorshDeserialize, BorshSerialize};
use solana_program::{
    sysvar::{rent::Rent, Sysvar},
    borsh::try_from_slice_unchecked,
    account_info::{AccountInfo, next_account_info},
    entrypoint,
    entrypoint::ProgramResult, 
    pubkey::Pubkey, 
    msg,
    program_error::ProgramError, system_instruction, program::invoke_signed,
};
use thiserror::Error;


entrypoint!(process_instruction);
fn process_instruction(
    program_id: &Pubkey,
    accounts: &[AccountInfo],
    instruction_data: &[u8],
) -> ProgramResult {
    Processor::process(program_id, accounts, instruction_data)
}

pub enum BlogInstruction {

    /// Accounts expected:
    /// 
    /// 0. `[signer]` User account who is creating the blog
    /// 1. `[writable]` Blog account derived from PDA
    /// 2. `[]` The System Program
    InitBlog {},

    /// Accounts expected:
    /// 
    /// 0. `[signer]` User account who is creating the post
    /// 1. `[writable]` Blog account for which post is being created
    /// 2. `[writable]` Post account derived from PDA
    /// 3. `[]` System Program
    CreatePost {
        slug: String,
        title: String,
        content: String,
    }
}

pub struct Processor;
impl Processor {
    pub fn process(program_id: &Pubkey, accounts: &[AccountInfo], instruction_data: &[u8]) -> ProgramResult {
        
        let instruction = BlogInstruction::unpack(instruction_data)?;

        match instruction {
            BlogInstruction::InitBlog {} => {
                msg!("Instruction: InitBlog");
                Self::process_init_blog(accounts, program_id)
            },
            BlogInstruction::CreatePost { slug, title, content} => {
                msg!("Instruction: CreatePost");
                Self::process_create_post(accounts, slug, title, content, program_id)
            }
        }
    }

    fn process_create_post(
        accounts: &[AccountInfo],
        slug: String,
        title: String,
        content: String,
        program_id: &Pubkey
    ) -> ProgramResult {
        if slug.len() > 10 || content.len() > 20 || title.len() > 50 {
            return Err(BlogError::InvalidPostData.into())
        }

        let account_info_iter = &mut accounts.iter();

        let authority_account = next_account_info(account_info_iter)?;
        let blog_account = next_account_info(account_info_iter)?;
        let post_account = next_account_info(account_info_iter)?;
        let system_program = next_account_info(account_info_iter)?;

        if !authority_account.is_signer {
            return Err(ProgramError::MissingRequiredSignature);
        }

        let (blog_pda, _blog_bump) = Pubkey::find_program_address(
            &[b"blog".as_ref(), authority_account.key.as_ref()],
            program_id
        );
        if blog_pda != *blog_account.key || !blog_account.is_writable || blog_account.data_is_empty() {
            return Err(BlogError::InvalidBlogAccount.into())
        }

        let (post_pda, post_bump) = Pubkey::find_program_address(
            &[b"post".as_ref(), slug.as_ref(), authority_account.key.as_ref()],
            program_id
        );
        if post_pda != *post_account.key {
            return Err(BlogError::InvalidPostAccount.into())
        }

        let post_len: usize = 32 + 32 + 1 + (4 + slug.len()) + (4 + title.len()) + (4 + content.len());

        let rent = Rent::get()?;
        let rent_lamports = rent.minimum_balance(post_len);

        let create_post_pda_ix = &system_instruction::create_account(
            authority_account.key,
            post_account.key,
            rent_lamports,
            post_len.try_into().unwrap(),
            program_id
        );
        msg!("Creating post account!");
        invoke_signed(
            create_post_pda_ix, 
            &[
                authority_account.clone(),
                post_account.clone(),
                system_program.clone()
            ],
            &[&[
                b"post".as_ref(),
                slug.as_ref(),
                authority_account.key.as_ref(),
                &[post_bump]
            ]]
        )?;

        let mut post_account_state = try_from_slice_unchecked::<Post>(&post_account.data.borrow()).unwrap();
        post_account_state.author = *authority_account.key;
        post_account_state.blog = *blog_account.key;
        post_account_state.bump = post_bump;
        post_account_state.slug = slug;
        post_account_state.title = title;
        post_account_state.content = content;

        msg!("Serializing Post data");
        post_account_state.serialize(&mut &mut post_account.data.borrow_mut()[..])?;


        let mut blog_account_state = Blog::try_from_slice(&blog_account.data.borrow())?;
        blog_account_state.post_count += 1;

        msg!("Serializing Blog data");
        blog_account_state.serialize(&mut &mut blog_account.data.borrow_mut()[..])?;

        Ok(())
    }

    fn process_init_blog(
        accounts: &[AccountInfo],
        program_id: &Pubkey
    ) -> ProgramResult {
        let account_info_iter = &mut accounts.iter();
        
        let authority_account = next_account_info(account_info_iter)?;
        let blog_account = next_account_info(account_info_iter)?;
        let system_program = next_account_info(account_info_iter)?;

        if !authority_account.is_signer {
            return Err(ProgramError::MissingRequiredSignature);
        }

        let (blog_pda, blog_bump) = Pubkey::find_program_address(
            &[b"blog".as_ref(), authority_account.key.as_ref()],
            program_id 
        );
        if blog_pda != *blog_account.key {
            return Err(BlogError::InvalidBlogAccount.into())
        }

        let rent = Rent::get()?;
        let rent_lamports = rent.minimum_balance(Blog::LEN);
        
        let create_blog_pda_ix = &system_instruction::create_account(
            authority_account.key,
            blog_account.key,
            rent_lamports,
            Blog::LEN.try_into().unwrap(),
            program_id
        );
        msg!("Creating blog account!");
        invoke_signed(
            create_blog_pda_ix, 
            &[
                authority_account.clone(),
                blog_account.clone(),
                system_program.clone()
            ],
            &[&[
                b"blog".as_ref(),
                authority_account.key.as_ref(),
                &[blog_bump]
            ]]
        )?;

        let mut blog_account_state = Blog::try_from_slice(&blog_account.data.borrow())?;
        blog_account_state.authority = *authority_account.key;
        blog_account_state.bump = blog_bump;
        blog_account_state.post_count = 0;
        blog_account_state.serialize(&mut &mut blog_account.data.borrow_mut()[..])?;
        

        Ok(())
    }
}



#[derive(BorshDeserialize, Debug)]
struct PostIxPayload {
    slug: String,
    title: String,
    content: String
}


impl BlogInstruction {
    pub fn unpack(input: &[u8]) -> Result<Self, ProgramError> {
        let (variant, rest) = input.split_first().ok_or(BlogError::InvalidInstruction)?;
        let payload = PostIxPayload::try_from_slice(rest).unwrap();

        Ok(match variant {
            0 => Self::InitBlog {},
            1 => Self::CreatePost {
                slug: payload.slug,
                title: payload.title,
                content: payload.content
            },
            _ => return Err(BlogError::InvalidInstruction.into()),
        })
    }
}

#[derive(BorshSerialize, BorshDeserialize, Debug, Clone)]
pub struct Blog {
    pub authority: Pubkey,
    pub bump: u8,
    pub post_count: u8 // 10 posts max
}

#[derive(BorshSerialize, BorshDeserialize, Debug, Clone)]
pub struct Post {
    pub author: Pubkey,
    pub blog: Pubkey,
    pub bump: u8,
    pub slug: String, // 10 chars max
    pub title: String, // 20 chars max
    pub content: String, // 50 chars max
}

impl Blog {
    pub const LEN: usize = 32 + 1 + 1;
}

#[derive(Error, Debug, Copy, Clone)]
pub enum BlogError {
    #[error("Invalid Instruction")]
    InvalidInstruction,

    #[error("Invalid Blog Account")]
    InvalidBlogAccount,

    #[error("Invalid Post Account")]
    InvalidPostAccount,

    #[error("Invalid Post Data")]
    InvalidPostData,

    #[error("Account not Writable")]
    AccountNotWritable,
}

impl From<BlogError> for ProgramError {
    fn from(e: BlogError) -> Self {
        return ProgramError::Custom(e as u32);
    }
}

On the client-side, you can use PublicKey.findProgramAddress() to obtain the required Blog and Post account address, which you can pass into connection.getAccountInfo() to fetch the account data. An example is shown below,

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import * as borsh from "@project-OpenBook/borsh";
import { PublicKey } from "@solana/web3.js";

export const BLOG_ACCOUNT_DATA_LAYOUT = borsh.struct([
  borsh.publicKey("authorityPubkey"),
  borsh.u8("bump"),
  borsh.u8("postCount"),
]);

export const POST_ACCOUNT_DATA_LAYOUT = borsh.struct([
  borsh.publicKey("author"),
  borsh.publicKey("blog"),
  borsh.u8("bump"),
  borsh.str("slug"),
  borsh.str("title"),
  borsh.str("content"),
]);

async () => {
  const connection = new Connection("http://localhost:8899", "confirmed");

  const [blogAccount] = await PublicKey.findProgramAddress(
    [Buffer.from("blog"), user.publicKey.toBuffer()],
    MY_PROGRAM_ID
  );

  const [postAccount] = await PublicKey.findProgramAddress(
    [Buffer.from("post"), Buffer.from("slug-1"), user.publicKey.toBuffer()],
    MY_PROGRAM_ID
  );

  const blogAccountInfo = await connection.getAccountInfo(blogAccount);
  const blogAccountState = BLOG_ACCOUNT_DATA_LAYOUT.decode(
    blogAccountInfo.data
  );
  console.log("Blog account state: ", blogAccountState);

  const postAccountInfo = await connection.getAccountInfo(postAccount);
  const postAccountState = POST_ACCOUNT_DATA_LAYOUT.decode(
    postAccountInfo.data
  );
  console.log("Post account state: ", postAccountState);
};

Single Map Account

Another way to implement mapping would be to have a BTreeMap data structure explicitly stored in a single account. This account's address itself could be a PDA, or the public key of a generated Keypair.

This method of mapping accounts is not ideal because of the following reasons,

  • You will have to first initialize the account storing the BTreeMap, before you can insert the necessary key-value pairs to it. Then, you will also have to store the address of this account somewhere, so as to update it every time.

  • There are memory limitations to an account, where an account can have a maximum size of 10 megabytes, which restricts the BTreeMap from storing a large number of key-value pairs.

Hence, after considering your use-case, you can implement this method as shown below,

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use std::{collections::BTreeMap};
use thiserror::Error;
use borsh::{BorshSerialize, BorshDeserialize};
use num_traits::FromPrimitive;
use solana_program::{sysvar::{rent::Rent, Sysvar}, entrypoint, entrypoint::ProgramResult, pubkey::Pubkey, account_info::{AccountInfo, next_account_info}, program_error::ProgramError, system_instruction, msg, program::{invoke_signed}, borsh::try_from_slice_unchecked};

entrypoint!(process_instruction);

fn process_instruction(
    program_id: &Pubkey,
    accounts: &[AccountInfo],
    instruction_data: &[u8],
) -> ProgramResult {
    msg!("instruction_data: {:?}", instruction_data);
    Processor::process(program_id, accounts, instruction_data)
}

pub struct Processor;

impl Processor {
    pub fn process(
        program_id: &Pubkey,
        accounts: &[AccountInfo],
        instruction_data: &[u8]
    ) -> ProgramResult {
        let instruction = FromPrimitive::from_u8(instruction_data[0]).ok_or(ProgramError::InvalidInstructionData)?;

        match instruction {
            0 => {
                msg!("Initializing map!");
                Self::process_init_map(accounts, program_id)?;
            },
            1 => {
                msg!("Inserting entry!");
                Self::process_insert_entry(accounts, program_id)?;
            },
            _ => {
                return Err(ProgramError::InvalidInstructionData)
            }
        }
        Ok(())
    }

    fn process_init_map(accounts: &[AccountInfo], program_id: &Pubkey) -> ProgramResult {
        let account_info_iter = &mut accounts.iter();

        let authority_account = next_account_info(account_info_iter)?;
        let map_account = next_account_info(account_info_iter)?;
        let system_program = next_account_info(account_info_iter)?;

        if !authority_account.is_signer {
            return Err(ProgramError::MissingRequiredSignature)
        }

        let (map_pda, map_bump) = Pubkey::find_program_address(
            &[b"map".as_ref()],
            program_id
        );

        if map_pda != *map_account.key || !map_account.is_writable || !map_account.data_is_empty() {
            return Err(BlogError::InvalidMapAccount.into())
        }

        let rent = Rent::get()?;
        let rent_lamports = rent.minimum_balance(MapAccount::LEN);

        let create_map_ix = &system_instruction::create_account(
            authority_account.key, 
            map_account.key, 
            rent_lamports, 
            MapAccount::LEN.try_into().unwrap(), 
            program_id
        );

        msg!("Creating MapAccount account");
        invoke_signed(
            create_map_ix, 
            &[
                authority_account.clone(),
                map_account.clone(),
                system_program.clone()
            ],
            &[&[
                b"map".as_ref(),
                &[map_bump]
            ]]
        )?;

        msg!("Deserializing MapAccount account");
        let mut map_state = try_from_slice_unchecked::<MapAccount>(&map_account.data.borrow()).unwrap();
        let empty_map: BTreeMap<Pubkey, Pubkey> = BTreeMap::new();

        map_state.is_initialized = 1;
        map_state.map = empty_map;

        msg!("Serializing MapAccount account");
        map_state.serialize(&mut &mut map_account.data.borrow_mut()[..])?;

        Ok(())
    }

    fn process_insert_entry(accounts: &[AccountInfo], program_id: &Pubkey) -> ProgramResult {
        
        let account_info_iter = &mut accounts.iter();

        let a_account = next_account_info(account_info_iter)?;
        let b_account = next_account_info(account_info_iter)?;
        let map_account = next_account_info(account_info_iter)?;

        if !a_account.is_signer {
            return Err(ProgramError::MissingRequiredSignature)
        }

        if map_account.data.borrow()[0] == 0 || *map_account.owner != *program_id {
            return Err(BlogError::InvalidMapAccount.into())
        }

        msg!("Deserializing MapAccount account");
        let mut map_state = try_from_slice_unchecked::<MapAccount>(&map_account.data.borrow())?;

        if map_state.map.contains_key(a_account.key) {
            return Err(BlogError::AccountAlreadyHasEntry.into())
        }

        map_state.map.insert(*a_account.key, *b_account.key);
        
        msg!("Serializing MapAccount account");
        map_state.serialize(&mut &mut map_account.data.borrow_mut()[..])?;

        Ok(())
    }
}

#[derive(BorshSerialize, BorshDeserialize, Clone, Debug)]
pub struct MapAccount {
    pub is_initialized: u8,
    pub map: BTreeMap<Pubkey, Pubkey> // 100
}

impl MapAccount {
    const LEN: usize = 1 + (4 + (10 * 64)); // 10 user -> blog
}

#[derive(Error, Debug, Copy, Clone)]
pub enum BlogError {
    #[error("Invalid MapAccount account")]
    InvalidMapAccount,

    #[error("Invalid Blog account")]
    InvalidBlogAccount,

    #[error("Account already has entry in Map")]
    AccountAlreadyHasEntry,
}

impl From<BlogError> for ProgramError {
    fn from(e: BlogError) -> Self {
        return ProgramError::Custom(e as u32);
    }
}

The client-side code to test the above program would look like something as shown below,

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import {
  Connection,
  Keypair,
  LAMPORTS_PER_SOL,
  PublicKey,
  SystemProgram,
  Transaction,
  TransactionInstruction,
} from "@solana/web3.js";

import * as borsh from "@project-OpenBook/borsh";

const MY_PROGRAM_ID = new PublicKey(
  "FwcG3yKuAkCfX68q9GPykNWDaaPjdZFaR1Tgr8qSxaEa"
);

const MAP_DATA_LAYOUT = borsh.struct([
  borsh.u8("is_initialized"),
  borsh.map(borsh.publicKey("user_a"), borsh.publicKey("user_b"), "blogs"),
]);

async () => {
  const connection = new Connection("http://localhost:8899", "confirmed");

  const userA = Keypair.generate();
  const userB = Keypair.generate();
  const userC = Keypair.generate();

  const [mapKey] = await PublicKey.findProgramAddress(
    [Buffer.from("map")],
    MY_PROGRAM_ID
  );

  const airdropASig = await connection.requestAirdrop(
    userA.publicKey,
    5 * LAMPORTS_PER_SOL
  );
  const airdropBSig = await connection.requestAirdrop(
    userB.publicKey,
    5 * LAMPORTS_PER_SOL
  );
  const airdropCSig = await connection.requestAirdrop(
    userC.publicKey,
    5 * LAMPORTS_PER_SOL
  );
  const promiseA = connection.confirmTransaction(airdropASig);
  const promiseB = connection.confirmTransaction(airdropBSig);
  const promiseC = connection.confirmTransaction(airdropCSig);

  await Promise.all([promiseA, promiseB, promiseC]);

  const initMapIx = new TransactionInstruction({
    programId: MY_PROGRAM_ID,
    keys: [
      {
        pubkey: userA.publicKey,
        isSigner: true,
        isWritable: true,
      },
      {
        pubkey: mapKey,
        isSigner: false,
        isWritable: true,
      },
      {
        pubkey: SystemProgram.programId,
        isSigner: false,
        isWritable: false,
      },
    ],
    data: Buffer.from(Uint8Array.of(0)),
  });

  const insertABIx = new TransactionInstruction({
    programId: MY_PROGRAM_ID,
    keys: [
      {
        pubkey: userA.publicKey,
        isSigner: true,
        isWritable: true,
      },
      {
        pubkey: userB.publicKey,
        isSigner: false,
        isWritable: false,
      },
      {
        pubkey: mapKey,
        isSigner: false,
        isWritable: true,
      },
    ],
    data: Buffer.from(Uint8Array.of(1)),
  });

  const insertBCIx = new TransactionInstruction({
    programId: MY_PROGRAM_ID,
    keys: [
      {
        pubkey: userB.publicKey,
        isSigner: true,
        isWritable: true,
      },
      {
        pubkey: userC.publicKey,
        isSigner: false,
        isWritable: false,
      },
      {
        pubkey: mapKey,
        isSigner: false,
        isWritable: true,
      },
    ],
    data: Buffer.from(Uint8Array.of(1)),
  });

  const insertCAIx = new TransactionInstruction({
    programId: MY_PROGRAM_ID,
    keys: [
      {
        pubkey: userC.publicKey,
        isSigner: true,
        isWritable: true,
      },
      {
        pubkey: userA.publicKey,
        isSigner: false,
        isWritable: false,
      },
      {
        pubkey: mapKey,
        isSigner: false,
        isWritable: true,
      },
    ],
    data: Buffer.from(Uint8Array.of(1)),
  });

  const tx = new Transaction();
  tx.add(initMapIx);
  tx.add(insertABIx);
  tx.add(insertBCIx);
  tx.add(insertCAIx);

  const sig = await connection.sendTransaction(tx, [userA, userB, userC], {
    skipPreflight: false,
    preflightCommitment: "confirmed",
  });
  await connection.confirmTransaction(sig);

  const mapAccount = await connection.getAccountInfo(mapKey);
  const mapData = MAP_DATA_LAYOUT.decode(mapAccount.data);
  console.log("MapData: ", mapData);
};
Last Updated:
Contributors: Sayantan Karmakar, Todsaporn Banjerdkit