CassandraBackend.hpp

//------------------------------------------------------------------------------
/*
    This file is part of clio: https://github.com/XRPLF/clio
    Copyright (c) 2023, the clio developers.
 
    Permission to use, copy, modify, and distribute this software for any
    purpose with or without fee is hereby granted, provided that the above
    copyright notice and this permission notice appear in all copies.
 
    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
//==============================================================================
 
#pragma once
 
#include "data/BackendInterface.hpp"
#include "data/DBHelpers.hpp"
#include "data/Types.hpp"
#include "data/cassandra/Concepts.hpp"
#include "data/cassandra/Handle.hpp"
#include "data/cassandra/Schema.hpp"
#include "data/cassandra/SettingsProvider.hpp"
#include "data/cassandra/Types.hpp"
#include "data/cassandra/impl/ExecutionStrategy.hpp"
#include "util/Assert.hpp"
#include "util/LedgerUtils.hpp"
#include "util/Profiler.hpp"
#include "util/log/Logger.hpp"
 
#include <boost/asio/spawn.hpp>
#include <boost/json/object.hpp>
#include <cassandra.h>
#include <xrpl/basics/Blob.h>
#include <xrpl/basics/base_uint.h>
#include <xrpl/basics/strHex.h>
#include <xrpl/protocol/AccountID.h>
#include <xrpl/protocol/Indexes.h>
#include <xrpl/protocol/LedgerHeader.h>
#include <xrpl/protocol/nft.h>
 
#include <atomic>
#include <chrono>
#include <cstddef>
#include <cstdint>
#include <iterator>
#include <limits>
#include <optional>
#include <stdexcept>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
 
namespace data::cassandra {
 
template <SomeSettingsProvider SettingsProviderType, SomeExecutionStrategy ExecutionStrategyType>
class BasicCassandraBackend : public BackendInterface {
    util::Logger log_{"Backend"};
 
    SettingsProviderType settingsProvider_;
    Schema<SettingsProviderType> schema_;
 
    std::atomic_uint32_t ledgerSequence_ = 0u;
 
protected:
    Handle handle_;
 
    // have to be mutable because BackendInterface constness :(
    mutable ExecutionStrategyType executor_;
 
public:
    BasicCassandraBackend(SettingsProviderType settingsProvider, bool readOnly)
        : settingsProvider_{std::move(settingsProvider)}
        , schema_{settingsProvider_}
        , handle_{settingsProvider_.getSettings()}
        , executor_{settingsProvider_.getSettings(), handle_}
    {
        if (auto const res = handle_.connect(); not res)
            throw std::runtime_error("Could not connect to database: " + res.error());
 
        if (not readOnly) {
            if (auto const res = handle_.execute(schema_.createKeyspace); not res) {
                // on datastax, creation of keyspaces can be configured to only be done thru the admin
                // interface. this does not mean that the keyspace does not already exist tho.
                if (res.error().code() != CASS_ERROR_SERVER_UNAUTHORIZED)
                    throw std::runtime_error("Could not create keyspace: " + res.error());
            }
 
            if (auto const res = handle_.executeEach(schema_.createSchema); not res)
                throw std::runtime_error("Could not create schema: " + res.error());
        }
 
        try {
            schema_.prepareStatements(handle_);
        } catch (std::runtime_error const& ex) {
            LOG(log_.error()) << "Failed to prepare the statements: " << ex.what() << "; readOnly: " << readOnly;
            throw;
        }
 
        LOG(log_.info()) << "Created (revamped) CassandraBackend";
    }
 
    TransactionsAndCursor
    fetchAccountTransactions(
        ripple::AccountID const& account,
        std::uint32_t const limit,
        bool forward,
        std::optional<TransactionsCursor> const& cursorIn,
        boost::asio::yield_context yield
    ) const override
    {
        auto rng = fetchLedgerRange();
        if (!rng)
            return {.txns = {}, .cursor = {}};
 
        Statement const statement = [this, forward, &account]() {
            if (forward)
                return schema_->selectAccountTxForward.bind(account);
 
            return schema_->selectAccountTx.bind(account);
        }();
 
        auto cursor = cursorIn;
        if (cursor) {
            statement.bindAt(1, cursor->asTuple());
            LOG(log_.debug()) << "account = " << ripple::strHex(account) << " tuple = " << cursor->ledgerSequence
                              << cursor->transactionIndex;
        } else {
            auto const seq = forward ? rng->minSequence : rng->maxSequence;
            auto const placeHolder = forward ? 0u : std::numeric_limits<std::uint32_t>::max();
 
            statement.bindAt(1, std::make_tuple(placeHolder, placeHolder));
            LOG(log_.debug()) << "account = " << ripple::strHex(account) << " idx = " << seq
                              << " tuple = " << placeHolder;
        }
 
        // FIXME: Limit is a hack to support uint32_t properly for the time
        // being. Should be removed later and schema updated to use proper
        // types.
        statement.bindAt(2, Limit{limit});
        auto const res = executor_.read(yield, statement);
        auto const& results = res.value();
        if (not results.hasRows()) {
            LOG(log_.debug()) << "No rows returned";
            return {};
        }
 
        std::vector<ripple::uint256> hashes = {};
        auto numRows = results.numRows();
        LOG(log_.info()) << "num_rows = " << numRows;
 
        for (auto [hash, data] : extract<ripple::uint256, std::tuple<uint32_t, uint32_t>>(results)) {
            hashes.push_back(hash);
            if (--numRows == 0) {
                LOG(log_.debug()) << "Setting cursor";
                cursor = data;
            }
        }
 
        auto const txns = fetchTransactions(hashes, yield);
        LOG(log_.debug()) << "Txns = " << txns.size();
 
        if (txns.size() == limit) {
            LOG(log_.debug()) << "Returning cursor";
            return {txns, cursor};
        }
 
        return {txns, {}};
    }
 
    void
    waitForWritesToFinish() override
    {
        executor_.sync();
    }
 
    bool
    doFinishWrites() override
    {
        waitForWritesToFinish();
 
        if (!range_) {
            executor_.writeSync(schema_->updateLedgerRange, ledgerSequence_, false, ledgerSequence_);
        }
 
        if (not executeSyncUpdate(schema_->updateLedgerRange.bind(ledgerSequence_, true, ledgerSequence_ - 1))) {
            LOG(log_.warn()) << "Update failed for ledger " << ledgerSequence_;
            return false;
        }
 
        LOG(log_.info()) << "Committed ledger " << ledgerSequence_;
        return true;
    }
 
    void
    writeLedger(ripple::LedgerHeader const& ledgerHeader, std::string&& blob) override
    {
        executor_.write(schema_->insertLedgerHeader, ledgerHeader.seq, std::move(blob));
 
        executor_.write(schema_->insertLedgerHash, ledgerHeader.hash, ledgerHeader.seq);
 
        ledgerSequence_ = ledgerHeader.seq;
    }
 
    std::optional<std::uint32_t>
    fetchLatestLedgerSequence(boost::asio::yield_context yield) const override
    {
        if (auto const res = executor_.read(yield, schema_->selectLatestLedger); res) {
            if (auto const& result = res.value(); result) {
                if (auto const maybeValue = result.template get<uint32_t>(); maybeValue)
                    return maybeValue;
 
                LOG(log_.error()) << "Could not fetch latest ledger - no rows";
                return std::nullopt;
            }
 
            LOG(log_.error()) << "Could not fetch latest ledger - no result";
        } else {
            LOG(log_.error()) << "Could not fetch latest ledger: " << res.error();
        }
 
        return std::nullopt;
    }
 
    std::optional<ripple::LedgerHeader>
    fetchLedgerBySequence(std::uint32_t const sequence, boost::asio::yield_context yield) const override
    {
        auto const res = executor_.read(yield, schema_->selectLedgerBySeq, sequence);
        if (res) {
            if (auto const& result = res.value(); result) {
                if (auto const maybeValue = result.template get<std::vector<unsigned char>>(); maybeValue) {
                    return util::deserializeHeader(ripple::makeSlice(*maybeValue));
                }
 
                LOG(log_.error()) << "Could not fetch ledger by sequence - no rows";
                return std::nullopt;
            }
 
            LOG(log_.error()) << "Could not fetch ledger by sequence - no result";
        } else {
            LOG(log_.error()) << "Could not fetch ledger by sequence: " << res.error();
        }
 
        return std::nullopt;
    }
 
    std::optional<ripple::LedgerHeader>
    fetchLedgerByHash(ripple::uint256 const& hash, boost::asio::yield_context yield) const override
    {
        if (auto const res = executor_.read(yield, schema_->selectLedgerByHash, hash); res) {
            if (auto const& result = res.value(); result) {
                if (auto const maybeValue = result.template get<uint32_t>(); maybeValue)
                    return fetchLedgerBySequence(*maybeValue, yield);
 
                LOG(log_.error()) << "Could not fetch ledger by hash - no rows";
                return std::nullopt;
            }
 
            LOG(log_.error()) << "Could not fetch ledger by hash - no result";
        } else {
            LOG(log_.error()) << "Could not fetch ledger by hash: " << res.error();
        }
 
        return std::nullopt;
    }
 
    std::optional<LedgerRange>
    hardFetchLedgerRange(boost::asio::yield_context yield) const override
    {
        auto const res = executor_.read(yield, schema_->selectLedgerRange);
        if (res) {
            auto const& results = res.value();
            if (not results.hasRows()) {
                LOG(log_.debug()) << "Could not fetch ledger range - no rows";
                return std::nullopt;
            }
 
            // TODO: this is probably a good place to use user type in
            // cassandra instead of having two rows with bool flag. or maybe at
            // least use tuple<int, int>?
            LedgerRange range;
            std::size_t idx = 0;
            for (auto [seq] : extract<uint32_t>(results)) {
                if (idx == 0) {
                    range.maxSequence = range.minSequence = seq;
                } else if (idx == 1) {
                    range.maxSequence = seq;
                }
 
                ++idx;
            }
 
            if (range.minSequence > range.maxSequence)
                std::swap(range.minSequence, range.maxSequence);
 
            LOG(log_.debug()) << "After hardFetchLedgerRange range is " << range.minSequence << ":"
                              << range.maxSequence;
            return range;
        }
        LOG(log_.error()) << "Could not fetch ledger range: " << res.error();
 
        return std::nullopt;
    }
 
    std::vector<TransactionAndMetadata>
    fetchAllTransactionsInLedger(std::uint32_t const ledgerSequence, boost::asio::yield_context yield) const override
    {
        auto hashes = fetchAllTransactionHashesInLedger(ledgerSequence, yield);
        return fetchTransactions(hashes, yield);
    }
 
    std::vector<ripple::uint256>
    fetchAllTransactionHashesInLedger(std::uint32_t const ledgerSequence, boost::asio::yield_context yield)
        const override
    {
        auto start = std::chrono::system_clock::now();
        auto const res = executor_.read(yield, schema_->selectAllTransactionHashesInLedger, ledgerSequence);
 
        if (not res) {
            LOG(log_.error()) << "Could not fetch all transaction hashes: " << res.error();
            return {};
        }
 
        auto const& result = res.value();
        if (not result.hasRows()) {
            LOG(log_.warn()) << "Could not fetch all transaction hashes - no rows; ledger = "
                             << std::to_string(ledgerSequence);
            return {};
        }
 
        std::vector<ripple::uint256> hashes;
        for (auto [hash] : extract<ripple::uint256>(result))
            hashes.push_back(std::move(hash));
 
        auto end = std::chrono::system_clock::now();
        LOG(log_.debug()) << "Fetched " << hashes.size() << " transaction hashes from database in "
                          << std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count()
                          << " milliseconds";
 
        return hashes;
    }
 
    std::optional<NFT>
    fetchNFT(ripple::uint256 const& tokenID, std::uint32_t const ledgerSequence, boost::asio::yield_context yield)
        const override
    {
        auto const res = executor_.read(yield, schema_->selectNFT, tokenID, ledgerSequence);
        if (not res)
            return std::nullopt;
 
        if (auto const maybeRow = res->template get<uint32_t, ripple::AccountID, bool>(); maybeRow) {
            auto [seq, owner, isBurned] = *maybeRow;
            auto result = std::make_optional<NFT>(tokenID, seq, owner, isBurned);
 
            // now fetch URI. Usually we will have the URI even for burned NFTs,
            // but if the first ledger on this clio included NFTokenBurn
            // transactions we will not have the URIs for any of those tokens.
            // In any other case not having the URI indicates something went
            // wrong with our data.
            //
            // TODO - in the future would be great for any handlers that use
            // this could inject a warning in this case (the case of not having
            // a URI because it was burned in the first ledger) to indicate that
            // even though we are returning a blank URI, the NFT might have had
            // one.
            auto uriRes = executor_.read(yield, schema_->selectNFTURI, tokenID, ledgerSequence);
            if (uriRes) {
                if (auto const maybeUri = uriRes->template get<ripple::Blob>(); maybeUri)
                    result->uri = *maybeUri;
            }
 
            return result;
        }
 
        LOG(log_.error()) << "Could not fetch NFT - no rows";
        return std::nullopt;
    }
 
    TransactionsAndCursor
    fetchNFTTransactions(
        ripple::uint256 const& tokenID,
        std::uint32_t const limit,
        bool const forward,
        std::optional<TransactionsCursor> const& cursorIn,
        boost::asio::yield_context yield
    ) const override
    {
        auto rng = fetchLedgerRange();
        if (!rng)
            return {.txns = {}, .cursor = {}};
 
        Statement const statement = [this, forward, &tokenID]() {
            if (forward)
                return schema_->selectNFTTxForward.bind(tokenID);
 
            return schema_->selectNFTTx.bind(tokenID);
        }();
 
        auto cursor = cursorIn;
        if (cursor) {
            statement.bindAt(1, cursor->asTuple());
            LOG(log_.debug()) << "token_id = " << ripple::strHex(tokenID) << " tuple = " << cursor->ledgerSequence
                              << cursor->transactionIndex;
        } else {
            auto const seq = forward ? rng->minSequence : rng->maxSequence;
            auto const placeHolder = forward ? 0 : std::numeric_limits<std::uint32_t>::max();
 
            statement.bindAt(1, std::make_tuple(placeHolder, placeHolder));
            LOG(log_.debug()) << "token_id = " << ripple::strHex(tokenID) << " idx = " << seq
                              << " tuple = " << placeHolder;
        }
 
        statement.bindAt(2, Limit{limit});
 
        auto const res = executor_.read(yield, statement);
        auto const& results = res.value();
        if (not results.hasRows()) {
            LOG(log_.debug()) << "No rows returned";
            return {};
        }
 
        std::vector<ripple::uint256> hashes = {};
        auto numRows = results.numRows();
        LOG(log_.info()) << "num_rows = " << numRows;
 
        for (auto [hash, data] : extract<ripple::uint256, std::tuple<uint32_t, uint32_t>>(results)) {
            hashes.push_back(hash);
            if (--numRows == 0) {
                LOG(log_.debug()) << "Setting cursor";
                cursor = data;
 
                // forward queries by ledger/tx sequence `>=`
                // so we have to advance the index by one
                if (forward)
                    ++cursor->transactionIndex;
            }
        }
 
        auto const txns = fetchTransactions(hashes, yield);
        LOG(log_.debug()) << "NFT Txns = " << txns.size();
 
        if (txns.size() == limit) {
            LOG(log_.debug()) << "Returning cursor";
            return {txns, cursor};
        }
 
        return {txns, {}};
    }
 
    NFTsAndCursor
    fetchNFTsByIssuer(
        ripple::AccountID const& issuer,
        std::optional<std::uint32_t> const& taxon,
        std::uint32_t const ledgerSequence,
        std::uint32_t const limit,
        std::optional<ripple::uint256> const& cursorIn,
        boost::asio::yield_context yield
    ) const override
    {
        NFTsAndCursor ret;
 
        Statement const idQueryStatement = [&taxon, &issuer, &cursorIn, &limit, this]() {
            if (taxon.has_value()) {
                auto r = schema_->selectNFTIDsByIssuerTaxon.bind(issuer);
                r.bindAt(1, *taxon);
                r.bindAt(2, cursorIn.value_or(ripple::uint256(0)));
                r.bindAt(3, Limit{limit});
                return r;
            }
 
            auto r = schema_->selectNFTIDsByIssuer.bind(issuer);
            r.bindAt(
                1,
                std::make_tuple(
                    cursorIn.has_value() ? ripple::nft::toUInt32(ripple::nft::getTaxon(*cursorIn)) : 0,
                    cursorIn.value_or(ripple::uint256(0))
                )
            );
            r.bindAt(2, Limit{limit});
            return r;
        }();
 
        // Query for all the NFTs issued by the account, potentially filtered by the taxon
        auto const res = executor_.read(yield, idQueryStatement);
 
        auto const& idQueryResults = res.value();
        if (not idQueryResults.hasRows()) {
            LOG(log_.debug()) << "No rows returned";
            return {};
        }
 
        std::vector<ripple::uint256> nftIDs;
        for (auto const [nftID] : extract<ripple::uint256>(idQueryResults))
            nftIDs.push_back(nftID);
 
        if (nftIDs.empty())
            return ret;
 
        if (nftIDs.size() == limit)
            ret.cursor = nftIDs.back();
 
        std::vector<Statement> selectNFTStatements;
        selectNFTStatements.reserve(nftIDs.size());
 
        std::transform(
            std::cbegin(nftIDs),
            std::cend(nftIDs),
            std::back_inserter(selectNFTStatements),
            [&](auto const& nftID) { return schema_->selectNFT.bind(nftID, ledgerSequence); }
        );
 
        auto const nftInfos = executor_.readEach(yield, selectNFTStatements);
 
        std::vector<Statement> selectNFTURIStatements;
        selectNFTURIStatements.reserve(nftIDs.size());
 
        std::transform(
            std::cbegin(nftIDs),
            std::cend(nftIDs),
            std::back_inserter(selectNFTURIStatements),
            [&](auto const& nftID) { return schema_->selectNFTURI.bind(nftID, ledgerSequence); }
        );
 
        auto const nftUris = executor_.readEach(yield, selectNFTURIStatements);
 
        for (auto i = 0u; i < nftIDs.size(); i++) {
            if (auto const maybeRow = nftInfos[i].template get<uint32_t, ripple::AccountID, bool>(); maybeRow) {
                auto [seq, owner, isBurned] = *maybeRow;
                NFT nft(nftIDs[i], seq, owner, isBurned);
                if (auto const maybeUri = nftUris[i].template get<ripple::Blob>(); maybeUri)
                    nft.uri = *maybeUri;
                ret.nfts.push_back(nft);
            }
        }
        return ret;
    }
 
    MPTHoldersAndCursor
    fetchMPTHolders(
        ripple::uint192 const& mptID,
        std::uint32_t const limit,
        std::optional<ripple::AccountID> const& cursorIn,
        std::uint32_t const ledgerSequence,
        boost::asio::yield_context yield
    ) const override
    {
        auto const holderEntries = executor_.read(
            yield, schema_->selectMPTHolders, mptID, cursorIn.value_or(ripple::AccountID(0)), Limit{limit}
        );
 
        auto const& holderResults = holderEntries.value();
        if (not holderResults.hasRows()) {
            LOG(log_.debug()) << "No rows returned";
            return {};
        }
 
        std::vector<ripple::uint256> mptKeys;
        std::optional<ripple::AccountID> cursor;
        for (auto const [holder] : extract<ripple::AccountID>(holderResults)) {
            mptKeys.push_back(ripple::keylet::mptoken(mptID, holder).key);
            cursor = holder;
        }
 
        auto mptObjects = doFetchLedgerObjects(mptKeys, ledgerSequence, yield);
 
        auto it = std::remove_if(mptObjects.begin(), mptObjects.end(), [](Blob const& mpt) { return mpt.empty(); });
 
        mptObjects.erase(it, mptObjects.end());
 
        ASSERT(mptKeys.size() <= limit, "Number of keys can't exceed the limit");
        if (mptKeys.size() == limit)
            return {mptObjects, cursor};
 
        return {mptObjects, {}};
    }
 
    std::optional<Blob>
    doFetchLedgerObject(ripple::uint256 const& key, std::uint32_t const sequence, boost::asio::yield_context yield)
        const override
    {
        LOG(log_.debug()) << "Fetching ledger object for seq " << sequence << ", key = " << ripple::to_string(key);
        if (auto const res = executor_.read(yield, schema_->selectObject, key, sequence); res) {
            if (auto const result = res->template get<Blob>(); result) {
                if (result->size())
                    return result;
            } else {
                LOG(log_.debug()) << "Could not fetch ledger object - no rows";
            }
        } else {
            LOG(log_.error()) << "Could not fetch ledger object: " << res.error();
        }
 
        return std::nullopt;
    }
 
    std::optional<std::uint32_t>
    doFetchLedgerObjectSeq(ripple::uint256 const& key, std::uint32_t const sequence, boost::asio::yield_context yield)
        const override
    {
        LOG(log_.debug()) << "Fetching ledger object for seq " << sequence << ", key = " << ripple::to_string(key);
        if (auto const res = executor_.read(yield, schema_->selectObject, key, sequence); res) {
            if (auto const result = res->template get<Blob, std::uint32_t>(); result) {
                auto [_, seq] = result.value();
                return seq;
            }
            LOG(log_.debug()) << "Could not fetch ledger object sequence - no rows";
        } else {
            LOG(log_.error()) << "Could not fetch ledger object sequence: " << res.error();
        }
 
        return std::nullopt;
    }
 
    std::optional<TransactionAndMetadata>
    fetchTransaction(ripple::uint256 const& hash, boost::asio::yield_context yield) const override
    {
        if (auto const res = executor_.read(yield, schema_->selectTransaction, hash); res) {
            if (auto const maybeValue = res->template get<Blob, Blob, uint32_t, uint32_t>(); maybeValue) {
                auto [transaction, meta, seq, date] = *maybeValue;
                return std::make_optional<TransactionAndMetadata>(transaction, meta, seq, date);
            }
 
            LOG(log_.debug()) << "Could not fetch transaction - no rows";
        } else {
            LOG(log_.error()) << "Could not fetch transaction: " << res.error();
        }
 
        return std::nullopt;
    }
 
    std::optional<ripple::uint256>
    doFetchSuccessorKey(ripple::uint256 key, std::uint32_t const ledgerSequence, boost::asio::yield_context yield)
        const override
    {
        if (auto const res = executor_.read(yield, schema_->selectSuccessor, key, ledgerSequence); res) {
            if (auto const result = res->template get<ripple::uint256>(); result) {
                if (*result == kLAST_KEY)
                    return std::nullopt;
                return result;
            }
 
            LOG(log_.debug()) << "Could not fetch successor - no rows";
        } else {
            LOG(log_.error()) << "Could not fetch successor: " << res.error();
        }
 
        return std::nullopt;
    }
 
    std::vector<TransactionAndMetadata>
    fetchTransactions(std::vector<ripple::uint256> const& hashes, boost::asio::yield_context yield) const override
    {
        if (hashes.empty())
            return {};
 
        auto const numHashes = hashes.size();
        std::vector<TransactionAndMetadata> results;
        results.reserve(numHashes);
 
        std::vector<Statement> statements;
        statements.reserve(numHashes);
 
        auto const timeDiff = util::timed([this, yield, &results, &hashes, &statements]() {
            // TODO: seems like a job for "hash IN (list of hashes)" instead?
            std::transform(
                std::cbegin(hashes),
                std::cend(hashes),
                std::back_inserter(statements),
                [this](auto const& hash) { return schema_->selectTransaction.bind(hash); }
            );
 
            auto const entries = executor_.readEach(yield, statements);
            std::transform(
                std::cbegin(entries),
                std::cend(entries),
                std::back_inserter(results),
                [](auto const& res) -> TransactionAndMetadata {
                    if (auto const maybeRow = res.template get<Blob, Blob, uint32_t, uint32_t>(); maybeRow)
                        return *maybeRow;
 
                    return {};
                }
            );
        });
 
        ASSERT(numHashes == results.size(), "Number of hashes and results must match");
        LOG(log_.debug()) << "Fetched " << numHashes << " transactions from database in " << timeDiff
                          << " milliseconds";
        return results;
    }
 
    std::vector<Blob>
    doFetchLedgerObjects(
        std::vector<ripple::uint256> const& keys,
        std::uint32_t const sequence,
        boost::asio::yield_context yield
    ) const override
    {
        if (keys.empty())
            return {};
 
        auto const numKeys = keys.size();
        LOG(log_.trace()) << "Fetching " << numKeys << " objects";
 
        std::vector<Blob> results;
        results.reserve(numKeys);
 
        std::vector<Statement> statements;
        statements.reserve(numKeys);
 
        // TODO: seems like a job for "key IN (list of keys)" instead?
        std::transform(
            std::cbegin(keys),
            std::cend(keys),
            std::back_inserter(statements),
            [this, &sequence](auto const& key) { return schema_->selectObject.bind(key, sequence); }
        );
 
        auto const entries = executor_.readEach(yield, statements);
        std::transform(
            std::cbegin(entries),
            std::cend(entries),
            std::back_inserter(results),
            [](auto const& res) -> Blob {
                if (auto const maybeValue = res.template get<Blob>(); maybeValue)
                    return *maybeValue;
 
                return {};
            }
        );
 
        LOG(log_.trace()) << "Fetched " << numKeys << " objects";
        return results;
    }
 
    std::vector<ripple::uint256>
    fetchAccountRoots(std::uint32_t number, std::uint32_t pageSize, std::uint32_t seq, boost::asio::yield_context yield)
        const override
    {
        std::vector<ripple::uint256> liveAccounts;
        std::optional<ripple::AccountID> lastItem;
 
        while (liveAccounts.size() < number) {
            Statement const statement = lastItem ? schema_->selectAccountFromToken.bind(*lastItem, Limit{pageSize})
                                                 : schema_->selectAccountFromBegining.bind(Limit{pageSize});
 
            auto const res = executor_.read(yield, statement);
            if (res) {
                auto const& results = res.value();
                if (not results.hasRows()) {
                    LOG(log_.debug()) << "No rows returned";
                    break;
                }
                // The results should not contain duplicates, we just filter out deleted accounts
                std::vector<ripple::uint256> fullAccounts;
                for (auto [account] : extract<ripple::AccountID>(results)) {
                    fullAccounts.push_back(ripple::keylet::account(account).key);
                    lastItem = account;
                }
                auto const objs = doFetchLedgerObjects(fullAccounts, seq, yield);
 
                for (auto i = 0u; i < fullAccounts.size(); i++) {
                    if (not objs[i].empty()) {
                        if (liveAccounts.size() < number) {
                            liveAccounts.push_back(fullAccounts[i]);
                        } else {
                            break;
                        }
                    }
                }
            } else {
                LOG(log_.error()) << "Could not fetch account from account_tx: " << res.error();
                break;
            }
        }
 
        return liveAccounts;
    }
 
    std::vector<LedgerObject>
    fetchLedgerDiff(std::uint32_t const ledgerSequence, boost::asio::yield_context yield) const override
    {
        auto const [keys, timeDiff] = util::timed([this, &ledgerSequence, yield]() -> std::vector<ripple::uint256> {
            auto const res = executor_.read(yield, schema_->selectDiff, ledgerSequence);
            if (not res) {
                LOG(log_.error()) << "Could not fetch ledger diff: " << res.error() << "; ledger = " << ledgerSequence;
                return {};
            }
 
            auto const& results = res.value();
            if (not results) {
                LOG(log_.error()) << "Could not fetch ledger diff - no rows; ledger = " << ledgerSequence;
                return {};
            }
 
            std::vector<ripple::uint256> resultKeys;
            for (auto [key] : extract<ripple::uint256>(results))
                resultKeys.push_back(key);
 
            return resultKeys;
        });
 
        // one of the above errors must have happened
        if (keys.empty())
            return {};
 
        LOG(log_.debug()) << "Fetched " << keys.size() << " diff hashes from database in " << timeDiff
                          << " milliseconds";
 
        auto const objs = fetchLedgerObjects(keys, ledgerSequence, yield);
        std::vector<LedgerObject> results;
        results.reserve(keys.size());
 
        std::transform(
            std::cbegin(keys),
            std::cend(keys),
            std::cbegin(objs),
            std::back_inserter(results),
            [](auto const& key, auto const& obj) { return LedgerObject{key, obj}; }
        );
 
        return results;
    }
 
    std::optional<std::string>
    fetchMigratorStatus(std::string const& migratorName, boost::asio::yield_context yield) const override
    {
        auto const res = executor_.read(yield, schema_->selectMigratorStatus, Text(migratorName));
        if (not res) {
            LOG(log_.error()) << "Could not fetch migrator status: " << res.error();
            return {};
        }
 
        auto const& results = res.value();
        if (not results) {
            return {};
        }
 
        for (auto [statusString] : extract<std::string>(results))
            return statusString;
 
        return {};
    }
 
    void
    doWriteLedgerObject(std::string&& key, std::uint32_t const seq, std::string&& blob) override
    {
        LOG(log_.trace()) << " Writing ledger object " << key.size() << ":" << seq << " [" << blob.size() << " bytes]";
 
        if (range_)
            executor_.write(schema_->insertDiff, seq, key);
 
        executor_.write(schema_->insertObject, std::move(key), seq, std::move(blob));
    }
 
    void
    writeSuccessor(std::string&& key, std::uint32_t const seq, std::string&& successor) override
    {
        LOG(log_.trace()) << "Writing successor. key = " << key.size() << " bytes. "
                          << " seq = " << std::to_string(seq) << " successor = " << successor.size() << " bytes.";
        ASSERT(!key.empty(), "Key must not be empty");
        ASSERT(!successor.empty(), "Successor must not be empty");
 
        executor_.write(schema_->insertSuccessor, std::move(key), seq, std::move(successor));
    }
 
    void
    writeAccountTransactions(std::vector<AccountTransactionsData> data) override
    {
        std::vector<Statement> statements;
        statements.reserve(data.size() * 10);  // assume 10 transactions avg
 
        for (auto& record : data) {
            std::transform(
                std::begin(record.accounts),
                std::end(record.accounts),
                std::back_inserter(statements),
                [this, &record](auto&& account) {
                    return schema_->insertAccountTx.bind(
                        std::forward<decltype(account)>(account),
                        std::make_tuple(record.ledgerSequence, record.transactionIndex),
                        record.txHash
                    );
                }
            );
        }
 
        executor_.write(std::move(statements));
    }
 
    void
    writeNFTTransactions(std::vector<NFTTransactionsData> const& data) override
    {
        std::vector<Statement> statements;
        statements.reserve(data.size());
 
        std::transform(std::cbegin(data), std::cend(data), std::back_inserter(statements), [this](auto const& record) {
            return schema_->insertNFTTx.bind(
                record.tokenID, std::make_tuple(record.ledgerSequence, record.transactionIndex), record.txHash
            );
        });
 
        executor_.write(std::move(statements));
    }
 
    void
    writeTransaction(
        std::string&& hash,
        std::uint32_t const seq,
        std::uint32_t const date,
        std::string&& transaction,
        std::string&& metadata
    ) override
    {
        LOG(log_.trace()) << "Writing txn to database";
 
        executor_.write(schema_->insertLedgerTransaction, seq, hash);
        executor_.write(
            schema_->insertTransaction, std::move(hash), seq, date, std::move(transaction), std::move(metadata)
        );
    }
 
    void
    writeNFTs(std::vector<NFTsData> const& data) override
    {
        std::vector<Statement> statements;
        statements.reserve(data.size() * 3);
 
        for (NFTsData const& record : data) {
            if (!record.onlyUriChanged) {
                statements.push_back(
                    schema_->insertNFT.bind(record.tokenID, record.ledgerSequence, record.owner, record.isBurned)
                );
 
                // If `uri` is set (and it can be set to an empty uri), we know this
                // is a net-new NFT. That is, this NFT has not been seen before by
                // us _OR_ it is in the extreme edge case of a re-minted NFT ID with
                // the same NFT ID as an already-burned token. In this case, we need
                // to record the URI and link to the issuer_nf_tokens table.
                if (record.uri) {
                    statements.push_back(schema_->insertIssuerNFT.bind(
                        ripple::nft::getIssuer(record.tokenID),
                        static_cast<uint32_t>(ripple::nft::getTaxon(record.tokenID)),
                        record.tokenID
                    ));
                    statements.push_back(
                        schema_->insertNFTURI.bind(record.tokenID, record.ledgerSequence, record.uri.value())
                    );
                }
            } else {
                // only uri changed, we update the uri table only
                statements.push_back(
                    schema_->insertNFTURI.bind(record.tokenID, record.ledgerSequence, record.uri.value())
                );
            }
        }
 
        executor_.writeEach(std::move(statements));
    }
 
    void
    writeMPTHolders(std::vector<MPTHolderData> const& data) override
    {
        std::vector<Statement> statements;
        statements.reserve(data.size());
        for (auto [mptId, holder] : data)
            statements.push_back(schema_->insertMPTHolder.bind(std::move(mptId), std::move(holder)));
 
        executor_.write(std::move(statements));
    }
 
    void
    startWrites() const override
    {
        // Note: no-op in original implementation too.
        // probably was used in PG to start a transaction or smth.
    }
 
    void
    writeMigratorStatus(std::string const& migratorName, std::string const& status) override
    {
        executor_.writeSync(
            schema_->insertMigratorStatus, data::cassandra::Text{migratorName}, data::cassandra::Text(status)
        );
    }
 
    bool
    isTooBusy() const override
    {
        return executor_.isTooBusy();
    }
 
    boost::json::object
    stats() const override
    {
        return executor_.stats();
    }
 
private:
    bool
    executeSyncUpdate(Statement statement)
    {
        auto const res = executor_.writeSync(statement);
        auto maybeSuccess = res->template get<bool>();
        if (not maybeSuccess) {
            LOG(log_.error()) << "executeSyncUpdate - error getting result - no row";
            return false;
        }
 
        if (not maybeSuccess.value()) {
            LOG(log_.warn()) << "Update failed. Checking if DB state is what we expect";
 
            // error may indicate that another writer wrote something.
            // in this case let's just compare the current state of things
            // against what we were trying to write in the first place and
            // use that as the source of truth for the result.
            auto rng = hardFetchLedgerRangeNoThrow();
            return rng && rng->maxSequence == ledgerSequence_;
        }
 
        return true;
    }
};
 
using CassandraBackend = BasicCassandraBackend<SettingsProvider, impl::DefaultExecutionStrategy<>>;
 
}  // namespace data::cassandra