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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
#pragma once
#include "const_iterator.h"
#include <vespa/document/bucket/bucketid.h>
#include <functional>
#include <memory>
#include <vector>
namespace storage::bucketdb {
/*
* Read guard for accessing the bucket tree of an underlying bucket database
* in a thread-safe, read-only manner.
*
* Important: If the underlying database is _not_ backed by a B-tree, the
* read guard does _not_ provide a stable view of the bucket key set when
* iterating, as that is not possible without locking the entire DB.
*
* If the guard is created by a B-tree DB, the following properties hold:
* - The set of bucket keys that can be iterated over is stable for the lifetime
* of the read guard.
* - The bucket _values_ may change during the lifetime of the read guard,
* but the reader will always observe a fully consistent value as if it were
* written atomically.
*
* Do not hold read guards for longer than absolutely necessary, as they cause
* memory to be retained by the backing DB until released.
*/
template <typename ValueT, typename ConstRefT = const ValueT&>
class ReadGuard {
public:
ReadGuard() = default;
virtual ~ReadGuard() = default;
ReadGuard(ReadGuard&&) = delete;
ReadGuard& operator=(ReadGuard&&) = delete;
ReadGuard(const ReadGuard&) = delete;
ReadGuard& operator=(const ReadGuard&) = delete;
virtual std::vector<ValueT> find_parents_and_self(const document::BucketId& bucket) const = 0;
virtual std::vector<ValueT> find_parents_self_and_children(const document::BucketId& bucket) const = 0;
virtual void for_each(std::function<void(uint64_t, const ValueT&)> func) const = 0;
virtual std::unique_ptr<ConstIterator<ConstRefT>> create_iterator() const = 0;
// If the underlying guard represents a snapshot, returns its monotonically
// increasing generation. Otherwise returns 0.
[[nodiscard]] virtual uint64_t generation() const noexcept = 0;
};
}
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