consumer_scope.hpp

#pragma once
 
#include <functional>
 
#include <userver/kafka/message.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace kafka {
 
namespace impl {
 
class Consumer;
 
}  // namespace impl
 
// clang-format off
 
/// @ingroup userver_clients
///
/// @brief RAII class that used as interface for Apache Kafka Consumer interaction
/// and proper lifetime management.
///
/// Its main purpose is to stop the message polling in user
/// component (that holds ConsumerScope) destructor, because ConsumerScope::Callback
/// often captures `this` pointer on user component.
///
/// Common usage:
///
/// @snippet samples/kafka_service/src/consumer_handler.cpp  Kafka service sample - consumer usage
///
/// ## Important implementation details
///
/// ConsumerScope holds reference to kafka::impl::Consumer that actually
/// represents the Apache Kafka Balanced Consumer.
///
/// It exposes the API for asynchronous message batches processing that is
/// polled from the subscribed topics partitions.
///
/// Consumer periodically polls the message batches from the
/// subscribed topics partitions and invokes processing callback on each batch.
///
/// Also, Consumer maintains per topic statistics including the broker
/// connection errors.
///
/// @note Each ConsumerScope instance is not thread-safe. To speed up the topic
/// messages processing, create more consumers with the same `group_id`.
///
/// @see https://docs.confluent.io/platform/current/clients/consumer.html for
/// basic consumer concepts
/// @see
/// https://docs.confluent.io/platform/current/clients/librdkafka/html/md_INTRODUCTION.html#autotoc_md62
/// for understanding of balanced consumer groups
///
/// @warning ConsumerScope::Start and ConsumerScope::Stop maybe called multiple
/// times, but only in "start-stop" order and **NOT** concurrently.
///
/// @note Must be placed as one of the last fields in the consumer component.
/// Make sure to add a comment before the field:
///
/// @code
/// // Subscription must be the last field! Add new fields above this comment.
/// @endcode
 
// clang-format on
 
class ConsumerScope final {
public:
    /// @brief Callback that is invoked on each polled message batch.
    /// @warning If callback throws, it called over and over again with the batch
    /// with the same messages, until successful invocation.
    /// Though, user should consider idempotent message processing mechanism.
    using Callback = std::function<void(MessageBatchView)>;
 
    /// @brief Stops the consumer (if not yet stopped).
    ~ConsumerScope();
 
    ConsumerScope(ConsumerScope&&) noexcept = delete;
    ConsumerScope& operator=(ConsumerScope&&) noexcept = delete;
 
    /// @brief Subscribes for configured topics and starts the consumer polling
    /// process.
    /// @note If `callback` throws an exception, entire message batch (also
    /// with successfully processed messages) come again, until callback succeeds
    /// @warning Each callback duration must not exceed the
    /// `max_callback_duration` time. Otherwise, consumer may stop consuming the
    /// message for unpredictable amount of time.
    void Start(Callback callback);
 
    /// @brief Revokes all topic partition consumer was subscribed on. Also closes
    /// the consumer, leaving the consumer balanced group.
    ///
    /// Called in the destructor of ConsumerScope automatically.
    ///
    /// Can be called in the beginning of your destructor if some other
    /// actions in that destructor prevent the callback from functioning
    /// correctly.
    ///
    /// After ConsumerScope::Stop call, subscribed topics partitions are
    /// distributed between other consumers with the same `group_id`.
    ///
    /// @warning Blocks until all kafka::Message destroyed (e.g. consumer cannot
    /// be stopped until user-callback is executing).
    void Stop() noexcept;
 
    /// @brief Schedules the current assignment offsets commitment task.
    /// Intended to be called after each message batch processing cycle (but not
    /// necessarily).
    ///
    /// @warning Commit does not ensure that messages do not come again --
    /// they do not come again also without the commit within the same process.
    /// Commit, indeed, restricts other consumers in consumers group from reading
    /// messages already processed (committed) by the current consumer if current
    /// has stopped and leaved the group
    void AsyncCommit();
 
private:
    friend class impl::Consumer;
 
    explicit ConsumerScope(impl::Consumer& consumer) noexcept;
 
    impl::Consumer& consumer_;
};
 
}  // namespace kafka
 
USERVER_NAMESPACE_END