added test case for polling strategy refactored the strategy class

include/cppkafka/utils/roundrobin_poll_strategy.h

@@ -0,0 +1,139 @@
+/*
+ * Copyright (c) 2017, Matias Fontanini
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ * * Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following disclaimer
+ *   in the documentation and/or other materials provided with the
+ *   distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#ifndef CPPKAFKA_ROUNDROBIN_POLL_STRATEGY_H
+#define CPPKAFKA_ROUNDROBIN_POLL_STRATEGY_H
+
+#include <map>
+#include <string>
+#include "../exceptions.h"
+#include "../consumer.h"
+#include "../queue.h"
+#include "poll_strategy_base.h"
+
+namespace cppkafka {
+
+/**
+ * \brief This adapter changes the default polling strategy of the Consumer into a fair round-robin
+ *        polling mechanism.
+ *
+ * The default librdkafka (and cppkafka) poll() and poll_batch() behavior is to consume batches of
+ * messages from each partition in turn. For performance reasons, librdkafka pre-fetches batches
+ * of messages from the kafka broker (one batch from each partition), and stores them locally in
+ * partition queues. Since all the internal partition queues are forwarded by default unto the
+ * group consumer queue (one per consumer), these batches end up being polled and consumed in the
+ * same sequence order.
+ * This adapter allows fair round-robin polling of all assigned partitions, one message at a time
+ * (or one batch at a time if poll_batch() is used). Note that poll_batch() has nothing to do with
+ * the internal batching mechanism of librdkafka.
+ *
+ * Example code on how to use this:
+ *
+ * \code
+ * // Create a consumer
+ * Consumer consumer(...);
+ * consumer.subscribe({ "my_topic" });
+ *
+ * // Optionally set the callbacks. This must be done *BEFORE* creating the strategy adapter
+ * consumer.set_assignment_callback(...);
+ * consumer.set_revocation_callback(...);
+ * consumer.set_rebalance_error_callback(...);
+ *
+ * // Create the adapter and use it for polling
+ * RoundRobinPollStrategy poll_strategy(consumer);
+ *
+ * while (true) {
+ *     // Poll each partition in turn
+ *     Message msg = poll_strategy.poll();
+ *     if (msg) {
+ *         // process valid message
+ *         }
+ *     }
+ * }
+ * \endcode
+ *
+ * \warning Calling directly poll() or poll_batch() on the Consumer object while using this adapter will
+ * lead to undesired results since the RoundRobinPollStrategy modifies the internal queuing mechanism of
+ * the Consumer instance it owns.
+ */
+
+class RoundRobinPollStrategy : public PollStrategyBase
+{
+public:
+    RoundRobinPollStrategy(Consumer& consumer);
+    
+    ~RoundRobinPollStrategy();
+    
+    /**
+     * \sa PollInterface::poll
+     */
+    Message poll() override;
+    
+    /**
+     * \sa PollInterface::poll
+     */
+    Message poll(std::chrono::milliseconds timeout) override;
+
+    /**
+     * \sa PollInterface::poll_batch
+     */
+    MessageList poll_batch(size_t max_batch_size) override;
+
+    /**
+     * \sa PollInterface::poll_batch
+     */
+    MessageList poll_batch(size_t max_batch_size,
+                           std::chrono::milliseconds timeout) override;
+ 
+protected:
+    /**
+     * \sa PollStrategyBase::get_next_queue
+     */
+    QueueData& get_next_queue(void* opaque = nullptr) final;
+    
+    /**
+     * \sa PollStrategyBase::reset_state
+     */
+    void reset_state() final;
+    
+private:
+    void consume_batch(Queue& queue,
+                       MessageList& messages,
+                       ssize_t& count,
+                       std::chrono::milliseconds timeout);
+    
+    void restore_forwarding();
+    
+    // Members
+    QueueMap::iterator  queue_iter_;
+};
+
+} //cppkafka
+
+#endif //CPPKAFKA_ROUNDROBIN_POLL_STRATEGY_H