Page, Order, and Filter Program Data

Summary #

This lesson delves into some functionality of the RPC calls that we used in the deserializing account data lesson
To save on computing time, you can fetch a large number of accounts without their data by filtering them to return just an array of public keys
Once you have a filtered list of public keys, you can order them and fetch the account data they belong to

Lesson #

You may have noticed in the last lesson that while we could fetch and display a list of account data, we didn't have any granular control over how many accounts to fetch or their order. In this lesson, we'll learn about some configuration options for the getProgramAccounts function that will enable things like paging, ordering accounts, and filtering.

Use `dataSlice` to only fetch the data you need #

Imagine the Movie Review app we worked on in past lessons having four million movie reviews and the average review is 500 bytes. That would make the total download for all review accounts over 2GB. Not something you want to have your frontend download every time the page refreshes.

Fortunately, the getProgramAccounts function that you use to get all of the accounts takes a configuration object as an argument. One of the configuration options is dataSlice which lets you provide two things:

offset - the offset from the beginning of the data buffer to start slicing
length - the number of bytes to return, starting from the provided offset

When you include a dataSlice in the configuration object, the function will only return the subset of the data buffer that you specified.

Paging Accounts #

One area where this becomes helpful is with paging. If you want to have a list that displays all accounts but there are so many accounts that you don't want to pull all the data at once, you can fetch all of the accounts but not fetch their data by using a dataSlice of { offset: 0, length: 0 }. You can then map the result to a list of account keys whose data you can fetch only when needed.

const accountsWithoutData = await connection.getProgramAccounts(programId, {
  dataSlice: { offset: 0, length: 0 },
});
 
const accountKeys = accountsWithoutData.map(account => account.pubkey);

With this list of keys, you can then fetch account data in “pages” using the getMultipleAccountsInfo method:

const paginatedKeys = accountKeys.slice(0, 10);
const accountInfos = await connection.getMultipleAccountsInfo(paginatedKeys);
const deserializedObjects = accountInfos.map(accountInfo => {
  // put logic to deserialize accountInfo.data here
});

Ordering Accounts #

The dataSlice option is also helpful when you need to order a list of accounts while paging. You still don't want to fetch all the data at once, but you do need all of the keys and a way to order them upfront. In this case, you need to understand the layout of the account data and configure the data slice to only be the data you need to use for ordering.

For example, you might have an account that stores contact information like so:

initialized as a boolean
phoneNumber as an unsigned, 64-bit integer
firstName as a string
secondName as a string

If you want to order all of the account keys alphabetically based on the user's first name, you need to find out the offset where the name starts. The first field, initialized, takes the first byte, then phoneNumber takes another 8, so the firstName field starts at offset 1 + 8 = 9. However, dynamic data fields in borsh use the first 4 bytes to record the length of the data, so we can skip an additional 4 bytes, making the offset 13.

You then need to determine the length to make the data slice. Since the length is variable, we can't know for sure before fetching the data. But you can choose a length that is large enough to cover most cases and short enough to not be too much of a burden to fetch. 15 bytes is plenty for most first names but would result in a small enough download even with a million users.

Once you've fetched accounts with the given data slice, you can use the sort method to sort the array before mapping it to an array of public keys.

// account type as returned by getProgramAccounts()
type ProgramAccount = {
  pubkey: PublicKey;
  account: AccountInfo<Buffer>;
};
 
// 4 bytes are used to store the current length of a string
const STRING_LENGTH_SPACE = 4;
 
// 9 bytes are reserved for some metadata related to the account structure.
const ACCOUNT_METADATA_SPACE = 9;
 
// The offset where the actual data begins.
const DATA_OFFSET = STRING_LENGTH_SPACE + ACCOUNT_METADATA_SPACE;
 
// Length of data we need to retrieve (15 bytes in this case, based on the expected size of the relevant data slice).
const DATA_LENGTH = 15;
 
// Get readonly accounts response
const readOnlyAccounts = await connection.getProgramAccounts(programId, {
  dataSlice: { offset: DATA_OFFSET, length: DATA_LENGTH },
});
 
// Make a mutable copy of the readonly array
const accounts: Array<ProgramAccount> = Array.from(readonlyAccounts);
 
accounts.sort((a, b) => {
  try {
    // Check if buffers are long enough to avoid out-of-bounds access
    const lengthA = a.account.data.readUInt32LE(0);
    const lengthB = b.account.data.readUInt32LE(0);
 
    if (
      a.account.data.length < STRING_LENGTH_SPACE + lengthA ||
      b.account.data.length < STRING_LENGTH_SPACE + lengthB
    ) {
      throw new Error("Buffer length is insufficient");
    }
 
    const dataA = a.account.data.subarray(
      STRING_LENGTH_SPACE,
      STRING_LENGTH_SPACE + lengthA,
    );
    const dataB = b.account.data.subarray(
      STRING_LENGTH_SPACE,
      STRING_LENGTH_SPACE + lengthB,
    );
 
    return dataA.compare(dataB);
  } catch (error) {
    console.error("Error sorting accounts: ", error);
    return 0; // Default sort order in case of error
  }
});
 
const accountKeys = accounts.map(account => account.pubkey);

Note that in the snippet above we don't compare the data as given. This is because for dynamically sized types like strings, Borsh places an unsigned, 32-bit (4 byte) integer at the start to indicate the length of the data representing that field. So to compare the first names directly, we need to get the length for each, then create a data slice with a 4 byte offset and the proper length.

Use `filters` to only retrieve specific accounts #

Limiting the data received per account is great, but what if you only want to return accounts that match a specific criteria rather than all of them? That's where the filters configuration option comes in. This option is an array that can have objects matching the following:

memcmp - compares a provided series of bytes with program account data at a particular offset. Fields:
- offset - the number to offset into program account data before comparing data
- bytes - a base-58 encoded string representing the data to match; limited to less than 129 bytes
dataSize - compares the program account data length with the provided data size

These let you filter based on matching data and/or total data size.

For example, you could search through a list of contacts by including a memcmp filter:

type ProgramAccount = {
  pubkey: PublicKey;
  account: AccountInfo<Buffer>;
};
 
const DATA_OFFSET = 2; // Skip the first 2 bytes, which store versioning information for the data schema of the account. This versioning ensures that changes to the account's structure can be tracked and managed over time.
const DATA_LENGTH = 18; // Retrieve 18 bytes of data, including the part of the account's data that stores the user's public key for comparison.
 
async function fetchMatchingContactAccounts(
  connection: Connection,
  search: string,
): Promise<Array<AccountInfo<Buffer> | null>> {
  let filters: Array<{ memcmp: { offset: number; bytes: string } }> = [];
 
  if (search.length > 0) {
    filters = [
      {
        memcmp: {
          offset: DATA_OFFSET,
          bytes: bs58.encode(Buffer.from(search)), // Convert the search string to Base58 for comparison with the on-chain data.
        },
      },
    ];
  }
 
  // Get readonly accounts response
  const readonlyAccounts = await connection.getProgramAccounts(
    new PublicKey(MOVIE_REVIEW_PROGRAM_ID),
    {
      dataSlice: { offset: DATA_OFFSET, length: DATA_LENGTH }, // Only retrieve the portion of data relevant to the search.
      filters,
    },
  );
 
  // Make a mutable copy of the readonly array
  const accounts: Array<ProgramAccount> = Array.from(readonlyAccounts);
 
  return accounts.map(account => account.account); // Return the account data.
}

Two things to note in the example above:

We're setting the offset to 13 because we determined previously that the offset for firstName in the data layout is 9 and we want to additionally skip the first 4 bytes indicating the length of the string.
We're using a third-party library bs58`` to perform base-58 encoding on the search term. You can install it using npm install bs58`.

Lab #

Remember that Movie Review app we worked on in the last two lessons? We're going to spice it up a little by paging the review list, ordering the reviews so they aren't so random, and adding some basic search functionality. No worries if you're just jumping into this lesson without having looked at the previous ones - as long as you have the prerequisite knowledge, you should be able to follow the lab without having worked in this specific project yet.

movie review frontend movie review frontend

1. Download the starter code #

If you didn't complete the lab from the last lesson or just want to make sure that you didn't miss anything, you can 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.

2. Add paging to the reviews #

First things first, let's create a space to encapsulate the code for fetching account data. Create a new file MovieCoordinator.ts and declare a MovieCoordinator class. Then let's move the MOVIE_REVIEW_PROGRAM_ID constant from MovieList into this new file since we'll be moving all references to it

const MOVIE_REVIEW_PROGRAM_ID = "CenYq6bDRB7p73EjsPEpiYN7uveyPUTdXkDkgUduboaN";
 
export class MovieCoordinator {}

Now we can use MovieCoordinator to create a paging implementation. A quick note before we dive in: this will be as simple a paging implementation as possible so that we can focus on the complex part of interacting with Solana accounts. You can, and should, do better for a production application.

With that out of the way, let's create a static property accounts of type web3.PublicKey[], a static function prefetchAccounts(connection: web3.Connection), and a static function fetchPage(connection: web3.Connection, page: number, perPage: number): Promise<Array<Movie>>. You'll also need to import @solana/web3.js and Movie.

import { Connection, PublicKey, AccountInfo } from "@solana/web3.js";
import { Movie } from "../models/Movie";
 
const MOVIE_REVIEW_PROGRAM_ID = "CenYq6bDRB7p73EjsPEpiYN7uveyPUTdXkDkgUduboaN";
 
export class MovieCoordinator {
  static accounts: PublicKey[] = [];
 
  static async prefetchAccounts(connection: Connection) {}
 
  static async fetchPage(
    connection: Connection,
    page: number,
    perPage: number,
  ): Promise<Array<Movie>> {}
}

The key to paging is to prefetch all the accounts without data. Let's fill in the body of prefetchAccounts to do this and set the retrieved public keys to the static accounts property.

static async prefetchAccounts(connection: Connection) {
  try {
    const accounts = await connection.getProgramAccounts(
      new PublicKey(MOVIE_REVIEW_PROGRAM_ID),
      {
        dataSlice: { offset: 0, length: 0 },
      }
    );
 
    this.accounts = accounts.map((account) => account.pubkey);
  } catch (error) {
    console.error("Error prefetching accounts:", error);
  }
}

Now, let's fill in the fetchPage method. First, if the accounts haven't been prefetched yet, we'll need to do that. Then, we can get the account public keys that correspond to the requested page and call connection.getMultipleAccountsInfo. Finally, we deserialize the account data and return the corresponding Movie objects.

static async fetchPage(
  connection: Connection,
  page: number,
  perPage: number,
  reload = false
): Promise<Array<Movie>> {
  if (this.accounts.length === 0 || reload) {
    await this.prefetchAccounts(connection);
  }
 
  const paginatedPublicKeys = this.accounts.slice(
    (page - 1) * perPage,
    page * perPage
  );
 
    if (!paginatedPublicKeys.length) {
      return [];
    }
 
    const accounts = await connection.getMultipleAccountsInfo(
      paginatedPublicKeys
    );
 
    const movies = accounts.reduce((accumulator: <Array<Movie>>, account) => {
      try {
        const movie = Movie.deserialize(account?.data);
        if (movie) {
          accumulator.push(movie);
        }
      } catch (error) {
        console.error("Error deserializing movie data: ", error);
      }
      return accumulator;
    }, []);
 
    return movies;
  }

With that done, we can reconfigure MovieList to use these methods. In MovieList.tsx, add const [page, setPage] = useState(1) near the existing useState calls. Then, update useEffect to call MovieCoordinator.fetchPage instead of fetching the accounts inline.

const connection = new Connection(clusterApiUrl("devnet"));
const [movies, setMovies] = useState<Array<Movie>>([]);
const [page, setPage] = useState(1);
 
useEffect(() => {
  const fetchMovies = async () => {
    try {
      const movies = await MovieCoordinator.fetchPage(connection, page, 10);
      setMovies(movies);
    } catch (error) {
      console.error("Failed to fetch movies:", error);
    }
  };
 
  fetchMovies();
}, [page, connection]);

Lastly, we need to add buttons to the bottom of the list for navigating to different pages:

return (
  <div className="py-5 flex flex-col w-fullitems-center justify-center">
    {movies.map((movie, i) => (
      <Card key={i} movie={movie} />
    ))}
 
    <div className="flex justify-between mt-4">
      {page > 1 && (
        <button
          onClick={() => setPage(page - 1)}
          className="px-6 py-2 bg-gray-300 text-black font-semibold rounded"
        >
          Previous
        </button>
      )}
      {movies.length === 5 && (
        <button
          onClick={() => setPage(page + 1)}
          className="px-6 py-2 bg-gray-300 text-black font-semibold rounded"
        >
          Next
        </button>
      )}
    </div>
  </div>
);

At this point, you should be able to run the project and click between pages!

3. Order reviews alphabetically by title #

If you look at the reviews, you might notice they aren't in any specific order. We can fix this by adding back just enough data into our data slice to help us do some sorting. The various properties in the movie review data buffer are laid out as follows

initialized - unsigned 8-bit integer; 1 byte
rating - unsigned 8-bit integer; 1 byte
title - string; unknown number of bytes
description - string; unknown number of bytes

Based on this, the offset we need to provide to the data slice to access title is 2. The length, however, is indeterminate, so we can just provide what seems to be a reasonable length. I'll stick with 18 as that will cover the length of most titles without fetching too much data every time.

Once we've modified the data slice in getProgramAccounts, we then need to actually sort the returned array. To do this, we need to compare the part of the data buffer that actually corresponds to title. The first 4 bytes of a dynamic field in Borsh are used to store the length of the field in bytes. So in any given buffer data that is sliced the way we discussed above, the string portion is data.slice(4, 4 + data[0]).

Now that we've thought through this, let's modify the implementation of prefetchAccounts in MovieCoordinator:

// account type as returned by getProgramAccounts()
type ProgramAccount = {
  pubkey: PublicKey;
  account: AccountInfo<Buffer>;
};
 
const DATA_OFFSET = 2; // Skip the first 2 bytes, which store versioning information for the data schema of the account. This versioning ensures that changes to the account's structure can be tracked and managed over time.
const DATA_LENGTH = 18; // Retrieve 18 bytes of data, including the part of the account's data that stores the user's public key for comparison.
// Define a constant for the size of the header in each account buffer
const HEADER_SIZE = 4; // 4 bytes for length header
 
static async prefetchAccounts(connection: Connection) {
  // Get readonly accounts response
  const readonlyAccounts = (await connection.getProgramAccounts(
    new PublicKey(MOVIE_REVIEW_PROGRAM_ID),
    {
      dataSlice:{ offset: DATA_OFFSET, length: DATA_LENGTH },
    }
  ))
 
  const accounts: Array<ProgramAccount> = Array.from(readonlyAccounts);   // Make a mutable copy of the readonly array
 
  accounts.sort((a, b) => {
    try {
      // Check if buffers are long enough to avoid out-of-bounds access
      const lengthA = a.account.data.readUInt32LE(0); // Reads the first 4 bytes for length
      const lengthB = b.account.data.readUInt32LE(0);
 
      if (
        a.account.data.length < HEADER_SIZE + lengthA ||
        b.account.data.length < HEADER_SIZE + lengthB
      ) {
        throw new Error('Buffer length is insufficient');
      }
 
      const dataA = a.account.data.subarray(HEADER_SIZE, HEADER_SIZE + lengthA);
      const dataB = b.account.data.subarray(HEADER_SIZE, HEADER_SIZE + lengthB);
 
      return dataA.compare(dataB);
    } catch (error) {
      console.error('Error sorting accounts: ', error);
      return 0; // Default sort order in case of error
    }
  });
 
    this.accounts = accounts.map(account => account.pubkey)
 
    } catch (error) {
    console.error("Error prefetching accounts:", error);
  }
}

And just like that, you should be able to run the app and see the list of movie reviews ordered alphabetically.

4. Add search #

The last thing we'll do to improve this app is to add some basic search capability. Let's add a search parameter to prefetchAccounts and reconfigure the body of the function to use it.

We can use the filters property of the config parameter of getProgramAccounts to filter accounts by specific data. The offset to the title fields is 2, but the first 4 bytes are the length of the title so the actual offset to the string itself is 6. Remember that the bytes need to be base 58 encoded, so let's install and import bs58.

import bs58 from 'bs58'
 
...
 
static async prefetchAccounts(connection: Connection, search: string) {
  const readonlyAccounts = (await connection.getProgramAccounts(
      new PublicKey(MOVIE_REVIEW_PROGRAM_ID),
      {
        dataSlice: { offset: DATA_OFFSET, length: DATA_LENGTH },
        filters:
          search === ""
            ? []
            : [
                {
                  memcmp: {
                    offset: 6,
                    bytes: bs58.encode(Buffer.from(search)),
                  },
                },
              ],
      }
  ));
 
  const accounts: Array<ProgramAccount> = Array.from(readonlyAccounts);  // Make a mutable copy of the readonly array
 
  accounts.sort((a, b) => {
      try {
        const lengthA = a.account.data.readUInt32LE(0);
        const lengthB = b.account.data.readUInt32LE(0);
 
      if (
        a.account.data.length < HEADER_SIZE + lengthA ||
        b.account.data.length < HEADER_SIZE + lengthB
      ) {
        throw new Error('Buffer length is insufficient');
      }
 
      const dataA = a.account.data.subarray(HEADER_SIZE, HEADER_SIZE + lengthA);
      const dataB = b.account.data.subarray(HEADER_SIZE, HEADER_SIZE + lengthB);
 
        return dataA.compare(dataB);
      } catch (error) {
        console.error("Error sorting accounts: ", error);
        return 0;
      }
    });
 
    this.accounts = accounts.map((account) => account.pubkey);
}

Now, add a search parameter to fetchPage and update its call to prefetchAccounts to pass it along. We'll also need to add a reload boolean parameter to fetchPage so that we can force a refresh of the account prefetching every time the search value changes.

static async fetchPage(
  connection: Connection,
  page: number,
  perPage: number,
  search: string,
  reload = false
): Promise<Array<Movie>> {
  if (this.accounts.length === 0 || reload) {
    await this.prefetchAccounts(connection, search);
  }
 
  const paginatedPublicKeys = this.accounts.slice(
    (page - 1) * perPage,
    page * perPage
  );
 
  if (paginatedPublicKeys.length === 0) {
    return [];
  }
 
  const accounts = await connection.getMultipleAccountsInfo(
    paginatedPublicKeys
  );
 
  const movies = accounts.reduce((accumulator: <Array<Movie>>, account) => {
    try {
      const movie = Movie.deserialize(account?.data);
      if (movie) {
        accumulator.push(movie);
      }
    } catch (error) {
        console.error('Error deserializing movie data: ', error);
      }
    return accumulator;
  }, []);
 
  return movies;
  }

With that in place, let's update the code in MovieList to call this properly.

First, add const [search, setSearch] = useState('') near the other useState calls. Then update the call to MovieCoordinator.fetchPage in the useEffect to pass the search parameter and to reload when search !== ''.

const connection = new Connection(clusterApiUrl("devnet"));
const [movies, setMovies] = useState<Array<Movie>>([]);
const [page, setPage] = useState(1);
const [search, setSearch] = useState("");
 
useEffect(() => {
  const fetchMovies = async () => {
    try {
      const movies = await MovieCoordinator.fetchPage(
        connection,
        page,
        5,
        search,
        search !== "",
      );
      setMovies(movies);
    } catch (error) {
      console.error("Failed to fetch movies:", error);
    }
  };
 
  fetchMovies();
}, [connection, page, search]);

Finally, add a search bar that will set the value of search:

return (
  <div className="py-5 flex flex-col w-fullitems-center justify-center">
    <input
      id="search"
      className="w-[300px] p-2 mb-4 bg-gray-700 border border-gray-600 rounded text-gray-400"
      onChange={e => setSearch(e.target.value)}
      placeholder="Search"
    />
    ...
  </div>
);

And that's it! The app now has ordered reviews, paging, and search.

That was a lot to digest, but you made it through. If you need to spend some more time with the concepts, feel free to reread the sections that were most challenging for you and/or have a look at the solution code.

Challenge #

Now it's your turn to try and do this on your own. Using the Student Intros app from last lesson, add paging, ordering alphabetically by name, and searching by name.

Student Intros frontend Student Intros frontend

You can build this from scratch or you can download the starter code
Add paging to the project by prefetching accounts without data, then only fetching the account data for each account when it's needed.
Order the accounts displayed in the app alphabetically by name.
Add the ability to search through introductions by a student's name.

This is challenging. If you get stuck, feel free to reference the solution code.

As always, get creative with these challenges and take them beyond the instructions if you want!

Completed the lab?

Push your code to GitHub and tell us what you thought of this lesson!