scylladb/read_context.hh

/*
 * Copyright (C) 2017 ScyllaDB
 */

/*
 * This file is part of Scylla.
 *
 * Scylla is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * Scylla is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with Scylla.  If not, see <http://www.gnu.org/licenses/>.
 */

#pragma once

#include "schema.hh"
#include "query-request.hh"
#include "mutation_fragment.hh"
#include "partition_version.hh"
#include "tracing/tracing.hh"
#include "row_cache.hh"

namespace cache {

/*
* Represent a flat reader to the underlying source.
* This reader automatically makes sure that it's up to date with all cache updates
*/
class autoupdating_underlying_reader final {
    row_cache& _cache;
    read_context& _read_context;
    std::optional<flat_mutation_reader> _reader;
    utils::phased_barrier::phase_type _reader_creation_phase;
    dht::partition_range _range = { };
    std::optional<dht::decorated_key> _last_key;
    std::optional<dht::decorated_key> _new_last_key;
public:
    autoupdating_underlying_reader(row_cache& cache, read_context& context)
        : _cache(cache)
        , _read_context(context)
    { }
    future<mutation_fragment_opt> move_to_next_partition(db::timeout_clock::time_point timeout) {
        _last_key = std::move(_new_last_key);
        auto start = population_range_start();
        auto phase = _cache.phase_of(start);
        if (!_reader || _reader_creation_phase != phase) {
            if (_last_key) {
                auto cmp = dht::ring_position_comparator(*_cache._schema);
                auto&& new_range = _range.split_after(*_last_key, cmp);
                if (!new_range) {
                    _reader = {};
                    return make_ready_future<mutation_fragment_opt>();
                }
                _range = std::move(*new_range);
                _last_key = {};
            }
            if (_reader) {
                ++_cache._tracker._stats.underlying_recreations;
            }
            auto& snap = _cache.snapshot_for_phase(phase);
            _reader = {}; // See issue #2644
            _reader = _cache.create_underlying_reader(_read_context, snap, _range);
            _reader_creation_phase = phase;
        }
        _reader->next_partition();

        if (_reader->is_end_of_stream() && _reader->is_buffer_empty()) {
            return make_ready_future<mutation_fragment_opt>();
        }
        return (*_reader)(timeout).then([this] (auto&& mfopt) {
            if (mfopt) {
                assert(mfopt->is_partition_start());
                _new_last_key = mfopt->as_partition_start().key();
            }
            return std::move(mfopt);
        });
    }
    future<> fast_forward_to(dht::partition_range&& range, db::timeout_clock::time_point timeout) {
        auto snapshot_and_phase = _cache.snapshot_of(dht::ring_position_view::for_range_start(_range));
        return fast_forward_to(std::move(range), snapshot_and_phase.snapshot, snapshot_and_phase.phase, timeout);
    }
    future<> fast_forward_to(dht::partition_range&& range, mutation_source& snapshot, row_cache::phase_type phase, db::timeout_clock::time_point timeout) {
        _range = std::move(range);
        _last_key = { };
        _new_last_key = { };
        if (_reader) {
            if (_reader_creation_phase == phase) {
                ++_cache._tracker._stats.underlying_partition_skips;
                return _reader->fast_forward_to(_range, timeout);
            } else {
                ++_cache._tracker._stats.underlying_recreations;
                _reader = {}; // See issue #2644
            }
        }
        _reader = _cache.create_underlying_reader(_read_context, snapshot, _range);
        _reader_creation_phase = phase;
        return make_ready_future<>();
    }
    utils::phased_barrier::phase_type creation_phase() const {
        assert(_reader);
        return _reader_creation_phase;
    }
    const dht::partition_range& range() const {
        return _range;
    }
    flat_mutation_reader& underlying() { return *_reader; }
    dht::ring_position_view population_range_start() const {
        return _last_key ? dht::ring_position_view::for_after_key(*_last_key)
                         : dht::ring_position_view::for_range_start(_range);
    }
};

class read_context final : public enable_lw_shared_from_this<read_context> {
    row_cache& _cache;
    schema_ptr _schema;
    const dht::partition_range& _range;
    const query::partition_slice& _slice;
    const io_priority_class& _pc;
    tracing::trace_state_ptr _trace_state;
    mutation_reader::forwarding _fwd_mr;
    bool _range_query;
    // When reader enters a partition, it must be set up for reading that
    // partition from the underlying mutation source (_underlying) in one of two ways:
    //
    //  1) either _underlying is already in that partition
    //
    //  2) _underlying is before the partition, then _underlying_snapshot and _key
    //     are set so that _underlying_flat can be fast forwarded to the right partition.
    //
    autoupdating_underlying_reader _underlying;
    uint64_t _underlying_created = 0;

    mutation_source_opt _underlying_snapshot;
    dht::partition_range _sm_range;
    std::optional<dht::decorated_key> _key;
    row_cache::phase_type _phase;
public:
    read_context(row_cache& cache,
            schema_ptr schema,
            const dht::partition_range& range,
            const query::partition_slice& slice,
            const io_priority_class& pc,
            tracing::trace_state_ptr trace_state,
            mutation_reader::forwarding fwd_mr)
        : _cache(cache)
        , _schema(std::move(schema))
        , _range(range)
        , _slice(slice)
        , _pc(pc)
        , _trace_state(std::move(trace_state))
        , _fwd_mr(fwd_mr)
        , _range_query(!range.is_singular() || !range.start()->value().has_key())
        , _underlying(_cache, *this)
    {
        ++_cache._tracker._stats.reads;
        if (range.is_singular() && range.start()->value().has_key()) {
            _key = range.start()->value().as_decorated_key();
        }
    }
    ~read_context() {
        ++_cache._tracker._stats.reads_done;
        if (_underlying_created) {
            _cache._stats.reads_with_misses.mark();
            ++_cache._tracker._stats.reads_with_misses;
        } else {
            _cache._stats.reads_with_no_misses.mark();
        }
    }
    read_context(const read_context&) = delete;
    row_cache& cache() { return _cache; }
    const schema_ptr& schema() const { return _schema; }
    const dht::partition_range& range() const { return _range; }
    const query::partition_slice& slice() const { return _slice; }
    const io_priority_class& pc() const { return _pc; }
    tracing::trace_state_ptr trace_state() const { return _trace_state; }
    mutation_reader::forwarding fwd_mr() const { return _fwd_mr; }
    bool is_range_query() const { return _range_query; }
    autoupdating_underlying_reader& underlying() { return _underlying; }
    row_cache::phase_type phase() const { return _phase; }
    const dht::decorated_key& key() const { return *_key; }
    void on_underlying_created() { ++_underlying_created; }
    bool digest_requested() const { return _slice.options.contains<query::partition_slice::option::with_digest>(); }
private:
    future<> ensure_underlying(db::timeout_clock::time_point timeout) {
        if (_underlying_snapshot) {
            return create_underlying(true, timeout);
        }
        return make_ready_future<>();
    }
public:
    future<> create_underlying(bool skip_first_fragment, db::timeout_clock::time_point timeout);
    void enter_partition(const dht::decorated_key& dk, mutation_source& snapshot, row_cache::phase_type phase) {
        _phase = phase;
        _underlying_snapshot = snapshot;
        _key = dk;
    }
    void enter_partition(const dht::decorated_key& dk, row_cache::phase_type phase) {
        _phase = phase;
        _underlying_snapshot = {};
        _key = dk;
    }
    // Fast forwards the underlying streamed_mutation to given range.
    future<> fast_forward_to(position_range range, db::timeout_clock::time_point timeout) {
        return ensure_underlying(timeout).then([this, range = std::move(range), timeout] {
            return _underlying.underlying().fast_forward_to(std::move(range), timeout);
        });
    }
    // Gets the next fragment from the underlying reader
    future<mutation_fragment_opt> get_next_fragment(db::timeout_clock::time_point timeout) {
        return ensure_underlying(timeout).then([this, timeout] {
            return _underlying.underlying()(timeout);
        });
    }
};

}