caching_component_base.hpp

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#pragma once
 
/// @file userver/cache/caching_component_base.hpp
/// @brief @copybrief components::CachingComponentBase
 
#include <memory>
#include <string>
#include <utility>
 
#include <fmt/format.h>
 
#include <userver/cache/cache_update_trait.hpp>
#include <userver/cache/exceptions.hpp>
#include <userver/compiler/demangle.hpp>
#include <userver/components/component_base.hpp>
#include <userver/components/component_fwd.hpp>
#include <userver/concurrent/async_event_channel.hpp>
#include <userver/dump/helpers.hpp>
#include <userver/dump/meta.hpp>
#include <userver/dump/operations.hpp>
#include <userver/engine/async.hpp>
#include <userver/rcu/rcu.hpp>
#include <userver/utils/assert.hpp>
#include <userver/utils/impl/wait_token_storage.hpp>
#include <userver/utils/meta.hpp>
#include <userver/utils/shared_readable_ptr.hpp>
#include <userver/yaml_config/schema.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace components {
 
// clang-format off
 
/// @ingroup userver_components userver_base_classes
///
/// @brief Base class for caching components
///
/// Provides facilities for creating periodically updated caches.
/// You need to override cache::CacheUpdateTrait::Update
/// then call cache::CacheUpdateTrait::StartPeriodicUpdates after setup
/// and cache::CacheUpdateTrait::StopPeriodicUpdates before teardown.
/// You can also override cache::CachingComponentBase::PreAssignCheck and set
/// has-pre-assign-check: true in the static config to enable check.
///
/// Caching components must be configured in service config (see options below)
/// and may be reconfigured dynamically via components::DynamicConfig.
///
/// @ref scripts/docs/en/userver/caches.md provide a more detailed introduction.
///
/// ## Dynamic config
/// * @ref USERVER_CACHES
/// * @ref USERVER_DUMPS
///
/// ## Static options:
///
/// Name | Description | Default value
/// ---- | ----------- | -------------
/// update-types | specifies whether incremental and/or full updates will be used | see below
/// update-interval | (*required*) interval between Update invocations | --
/// update-jitter | max. amount of time by which update-interval may be adjusted for requests dispersal | update_interval / 10
/// full-update-interval | interval between full updates | --
/// full-update-jitter | max. amount of time by which full-update-interval may be adjusted for requests dispersal | full-update-interval / 10
/// updates-enabled | if false, cache updates are disabled (except for the first one if !first-update-fail-ok) | true
/// first-update-fail-ok | whether first update failure is non-fatal; see also @ref MayReturnNull | false
/// task-processor | the name of the TaskProcessor for running DoWork | main-task-processor
/// config-settings | enables dynamic reconfiguration with CacheConfigSet | true
/// exception-interval | Used instead of `update-interval` in case of exception | update_interval
/// additional-cleanup-interval | how often to run background RCU garbage collector | 10 seconds
/// is-strong-period | whether to include Update execution time in update-interval | false
/// testsuite-force-periodic-update | override testsuite-periodic-update-enabled in TestsuiteSupport component config | --
/// failed-updates-before-expiration | the number of consecutive failed updates for data expiration | --
/// has-pre-assign-check | enables the check before changing the value in the cache, by default it is the check that the new value is not empty | false
/// alert-on-failing-to-update-times | fire an alert if the cache update failed specified amount of times in a row. If zero - alerts are disabled. Value from dynamic config takes priority over static | 0
/// safe-data-lifetime | enables awaiting data destructors in the component's destructor. Can be set to `false` if the stored data does not refer to the component and its dependencies. | true
/// dump.* | Manages cache behavior after dump load | -
/// dump.first-update-mode | Behavior of update after successful load from dump. See info on modes below | skip
/// dump.first-update-type | Update type after successful load from dump (`full`, `incremental` or `incremental-then-async-full`) | full
///
/// ### Update types
///  * `full-and-incremental`: both `update-interval` and `full-update-interval`
///    must be specified. Updates with UpdateType::kIncremental will be triggered
///    each `update-interval` (adjusted by jitter) unless `full-update-interval`
///    has passed and UpdateType::kFull is triggered.
///  * `only-full`: only `update-interval` must be specified. UpdateType::kFull
///    will be triggered each `update-interval` (adjusted by jitter).
///  * `only-incremental`: only `update-interval` must be specified. UpdateType::kFull is triggered
///    on the first update, afterwards UpdateType::kIncremental will be triggered
///    each `update-interval` (adjusted by jitter).
///
/// ### Avoiding memory leaks
///
/// If you don't implement the deletion of objects that are deleted from the data source and don't use full updates,
/// you may get an effective memory leak, because garbage objects will pile up in the cached data.
///
/// Calculation example:
/// * size of database: 1000 objects
/// * removal rate: 30 objects per minute (0.5 objects per second)
///
/// Let's say we allow 20% extra garbage objects in cache in addition to the actual objects from the database. In this case we need:
///
/// full-update-interval = (size-of-database * 20% / removal-rate) = 400s
///
/// ### Dealing with nullptr data in CachingComponentBase
///
/// The cache can become `nullptr` through multiple ways:
///
/// * If the first cache update fails, and `first-update-fail-ok` config
///   option is set to `true` (otherwise the service shutdown at start)
/// * Through manually calling @ref Set with `nullptr` in @ref Update
/// * If `failed-updates-before-expiration` is set, and that many periodic
///   updates fail in a row
///
/// By default, the cache's user can expect that the pointer returned
/// from @ref Get will never be `nullptr`. If the cache for some reason is
/// in `nullptr` state, then @ref Get will throw. This is the safe default
/// behavior for most cases.
///
/// If all systems of a service are expected to work with a cache in `nullptr`
/// state, then such a cache should override `MayReturnNull` to return `true`.
/// It will also serve self-documentation purposes: if a cache defines
/// @ref MayReturnNull, then pointers returned from @ref Get should be checked
/// for `nullptr` before usage.
///
/// ### `first-update-mode` modes
///
/// Further customizes the behavior of @ref dump::Dumper "cache dumps".
///
/// Mode          | Description
/// ------------- | -----------
/// `skip`        | after successful load from dump, do nothing
/// `required`    | make a synchronous update of type `first-update-type`, stop the service on failure
/// `best-effort` | make a synchronous update of type `first-update-type`, keep working and use data from dump on failure
///
/// ### testsuite-force-periodic-update
///  use it to enable periodic cache update for a component in testsuite environment
///  where testsuite-periodic-update-enabled from TestsuiteSupport config is false
///
/// By default, update types are guessed based on update intervals presence.
/// If both `update-interval` and `full-update-interval` are present,
/// `full-and-incremental` types is assumed. Otherwise `only-full` is used.
///
/// @see `dump::Dumper` for more info on persistent cache dumps and
/// corresponding config options.
///
/// @see @ref scripts/docs/en/userver/caches.md. pytest_userver.client.Client.invalidate_caches()
/// for a function to force cache update from testsuite.
 
// clang-format on
 
template <typename T>
// NOLINTNEXTLINE(fuchsia-multiple-inheritance)
class CachingComponentBase : public ComponentBase,
                             protected cache::CacheUpdateTrait {
 public:
  CachingComponentBase(const ComponentConfig& config, const ComponentContext&);
  ~CachingComponentBase() override;
 
  using cache::CacheUpdateTrait::Name;
 
  using DataType = T;
 
  /// @return cache contents. May be `nullptr` if and only if @ref MayReturnNull
  /// returns `true`.
  /// @throws cache::EmptyCacheError if the contents are `nullptr`, and
  /// @ref MayReturnNull returns `false` (which is the default behavior).
  utils::SharedReadablePtr<T> Get() const;
 
  /// @return cache contents. May be nullptr regardless of MayReturnNull().
  utils::SharedReadablePtr<T> GetUnsafe() const;
 
  /// Subscribes to cache updates using a member function. Also immediately
  /// invokes the function with the current cache contents.
  template <class Class>
  concurrent::AsyncEventSubscriberScope UpdateAndListen(
      Class* obj, std::string name,
      void (Class::*func)(const std::shared_ptr<const T>&));
 
  concurrent::AsyncEventChannel<const std::shared_ptr<const T>&>&
  GetEventChannel();
 
  static yaml_config::Schema GetStaticConfigSchema();
 
 protected:
  /// Sets the new value of cache. As a result the Get() member function starts
  /// returning the value passed into this function after the Update() finishes.
  ///
  /// @warning Do not forget to update cache::UpdateStatisticsScope, otherwise
  /// the behavior is undefined.
  void Set(std::unique_ptr<const T> value_ptr);
 
  /// @overload
  void Set(T&& value);
 
  /// @overload Set()
  template <typename... Args>
  void Emplace(Args&&... args);
 
  /// Clears the content of the cache by string a default constructed T.
  void Clear();
 
  /// Whether @ref Get is expected to return `nullptr`.
  virtual bool MayReturnNull() const;
 
  /// @{
  /// Override to use custom serialization for cache dumps
  virtual void WriteContents(dump::Writer& writer, const T& contents) const;
 
  virtual std::unique_ptr<const T> ReadContents(dump::Reader& reader) const;
  /// @}
 
  /// @brief If the option has-pre-assign-check is set true in static config,
  /// this function is called before assigning the new value to the cache
  /// @note old_value_ptr and new_value_ptr can be nullptr.
  virtual void PreAssignCheck(const T* old_value_ptr,
                              const T* new_value_ptr) const;
 
 private:
  void OnAllComponentsLoaded() final;
 
  void Cleanup() final;
 
  void MarkAsExpired() final;
 
  void GetAndWrite(dump::Writer& writer) const final;
  void ReadAndSet(dump::Reader& reader) final;
 
  std::shared_ptr<const T> TransformNewValue(
      std::unique_ptr<const T> new_value);
 
  rcu::Variable<std::shared_ptr<const T>> cache_;
  concurrent::AsyncEventChannel<const std::shared_ptr<const T>&> event_channel_;
  utils::impl::WaitTokenStorage wait_token_storage_;
};
 
template <typename T>
CachingComponentBase<T>::CachingComponentBase(const ComponentConfig& config,
                                              const ComponentContext& context)
    : ComponentBase(config, context),
      cache::CacheUpdateTrait(config, context),
      event_channel_(components::GetCurrentComponentName(config),
                     [this](auto& function) {
                       const auto ptr = cache_.ReadCopy();
                       if (ptr) function(ptr);
                     }) {
  const auto initial_config = GetConfig();
}
 
template <typename T>
CachingComponentBase<T>::~CachingComponentBase() {
  // Avoid a deadlock in WaitForAllTokens
  cache_.Assign(nullptr);
  // We must wait for destruction of all instances of T to finish, otherwise
  // it's UB if T's destructor accesses dependent components
  wait_token_storage_.WaitForAllTokens();
}
 
template <typename T>
utils::SharedReadablePtr<T> CachingComponentBase<T>::Get() const {
  auto ptr = GetUnsafe();
  if (!ptr && !MayReturnNull()) {
    throw cache::EmptyCacheError(Name());
  }
  return ptr;
}
 
template <typename T>
template <typename Class>
concurrent::AsyncEventSubscriberScope CachingComponentBase<T>::UpdateAndListen(
    Class* obj, std::string name,
    void (Class::*func)(const std::shared_ptr<const T>&)) {
  return event_channel_.DoUpdateAndListen(obj, std::move(name), func, [&] {
    auto ptr = Get();  // TODO: extra ref
    (obj->*func)(ptr);
  });
}
 
template <typename T>
concurrent::AsyncEventChannel<const std::shared_ptr<const T>&>&
CachingComponentBase<T>::GetEventChannel() {
  return event_channel_;
}
 
template <typename T>
utils::SharedReadablePtr<T> CachingComponentBase<T>::GetUnsafe() const {
  return utils::SharedReadablePtr<T>(cache_.ReadCopy());
}
 
template <typename T>
void CachingComponentBase<T>::Set(std::unique_ptr<const T> value_ptr) {
  const std::shared_ptr<const T> new_value =
      TransformNewValue(std::move(value_ptr));
 
  if (HasPreAssignCheck()) {
    auto old_value = cache_.Read();
    PreAssignCheck(old_value->get(), new_value.get());
  }
 
  cache_.Assign(new_value);
  event_channel_.SendEvent(new_value);
  OnCacheModified();
}
 
template <typename T>
void CachingComponentBase<T>::Set(T&& value) {
  Emplace(std::move(value));
}
 
template <typename T>
template <typename... Args>
void CachingComponentBase<T>::Emplace(Args&&... args) {
  Set(std::make_unique<T>(std::forward<Args>(args)...));
}
 
template <typename T>
void CachingComponentBase<T>::Clear() {
  cache_.Assign(std::make_unique<const T>());
}
 
template <typename T>
bool CachingComponentBase<T>::MayReturnNull() const {
  return false;
}
 
template <typename T>
void CachingComponentBase<T>::GetAndWrite(dump::Writer& writer) const {
  const auto contents = GetUnsafe();
  if (!contents) throw cache::EmptyCacheError(Name());
  WriteContents(writer, *contents);
}
 
template <typename T>
void CachingComponentBase<T>::ReadAndSet(dump::Reader& reader) {
  auto data = ReadContents(reader);
  if constexpr (meta::kIsSizable<T>) {
    if (data) {
      SetDataSizeStatistic(std::size(*data));
    }
  }
  Set(std::move(data));
}
 
template <typename T>
void CachingComponentBase<T>::WriteContents(dump::Writer& writer,
                                            const T& contents) const {
  if constexpr (dump::kIsDumpable<T>) {
    writer.Write(contents);
  } else {
    dump::ThrowDumpUnimplemented(Name());
  }
}
 
template <typename T>
std::unique_ptr<const T> CachingComponentBase<T>::ReadContents(
    dump::Reader& reader) const {
  if constexpr (dump::kIsDumpable<T>) {
    // To avoid an extra move and avoid including common_containers.hpp
    return std::unique_ptr<const T>{new T(reader.Read<T>())};
  } else {
    dump::ThrowDumpUnimplemented(Name());
  }
}
 
template <typename T>
void CachingComponentBase<T>::OnAllComponentsLoaded() {
  AssertPeriodicUpdateStarted();
}
 
template <typename T>
void CachingComponentBase<T>::Cleanup() {
  cache_.Cleanup();
}
 
template <typename T>
void CachingComponentBase<T>::MarkAsExpired() {
  Set(std::unique_ptr<const T>{});
}
 
namespace impl {
 
yaml_config::Schema GetCachingComponentBaseSchema();
 
template <typename T, typename Deleter>
auto MakeAsyncDeleter(engine::TaskProcessor& task_processor, Deleter deleter) {
  return [&task_processor,
          deleter = std::move(deleter)](const T* raw_ptr) mutable {
    std::unique_ptr<const T, Deleter> ptr(raw_ptr, std::move(deleter));
 
    engine::CriticalAsyncNoSpan(task_processor, [ptr =
                                                     std::move(ptr)]() mutable {
    }).Detach();
  };
}
 
}  // namespace impl
 
template <typename T>
yaml_config::Schema CachingComponentBase<T>::GetStaticConfigSchema() {
  return impl::GetCachingComponentBaseSchema();
}
 
template <typename T>
void CachingComponentBase<T>::PreAssignCheck(
    const T*, [[maybe_unused]] const T* new_value_ptr) const {
  UINVARIANT(
      meta::kIsSizable<T>,
      fmt::format("{} type does not support std::size(), add implementation of "
                  "the method size() for this type or "
                  "override cache::CachingComponentBase::PreAssignCheck.",
                  compiler::GetTypeName<T>()));
 
  if constexpr (meta::kIsSizable<T>) {
    if (!new_value_ptr || std::size(*new_value_ptr) == 0) {
      throw cache::EmptyDataError(Name());
    }
  }
}
 
template <typename T>
std::shared_ptr<const T> CachingComponentBase<T>::TransformNewValue(
    std::unique_ptr<const T> new_value) {
  // Kill garbage asynchronously as T::~T() might be very slow
  if (IsSafeDataLifetime()) {
    // Use token only if `safe-data-lifetime` is true
    auto deleter_with_token =
        [token = wait_token_storage_.GetToken()](const T* raw_ptr) {
          // Make sure *raw_ptr is deleted before token is destroyed
          std::default_delete<const T>{}(raw_ptr);
        };
    return std::shared_ptr<const T>(
        new_value.release(),
        impl::MakeAsyncDeleter<T>(GetCacheTaskProcessor(),
                                  std::move(deleter_with_token)));
  } else {
    return std::shared_ptr<const T>(
        new_value.release(),
        impl::MakeAsyncDeleter<T>(GetCacheTaskProcessor(),
                                  std::default_delete<const T>{}));
  }
}
 
}  // namespace components
 
USERVER_NAMESPACE_END