MXS-1777: Initial version of routing based on query response time.

The main piece of code, slave selection (backend_cmp_response_time), uses the available
method of pair-wise comparison of slaves. This will be changed to selection using all
available slaves, along with removal of hard coded values.

server/modules/routing/readwritesplit/response_stat.cc

@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2022-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+#include "response_stat.hh"
+
+#include <algorithm>
+
+namespace maxscale
+{
+ResponseStat::ResponseStat(int ignore_first_n, int num_filter_samples,
+                           maxbase::Duration sync_duration) :
+    m_ignore_first_n{ignore_first_n},
+    m_num_filter_samples {num_filter_samples},
+    m_sync_duration{sync_duration},
+    m_sample_count{0},
+    m_samples(num_filter_samples),
+    m_last_start{maxbase::TimePoint()},
+    m_next_sync{maxbase::Clock::now() + sync_duration}
+{}
+
+
+void ResponseStat::query_started()
+{
+    if (m_ignore_first_n)
+    {
+        --m_ignore_first_n;
+        return;
+    }
+    m_last_start = maxbase::Clock::now();
+}
+
+void ResponseStat::query_ended()
+{
+    if (m_last_start == maxbase::TimePoint())
+    {
+        // m_last_start is defaulted. Ignore, avoids extra logic in call sites.
+        return;
+    }
+    m_samples[m_sample_count] = maxbase::Clock::now() - m_last_start;
+
+    if (++m_sample_count == m_num_filter_samples)
+    {
+        std::sort(begin(m_samples), end(m_samples));
+        maxbase::Duration new_sample = m_samples[m_num_filter_samples / 2];
+        m_average.add(std::chrono::duration<double>(new_sample).count());
+        m_sample_count = 0;
+    }
+    m_last_start = maxbase::TimePoint();
+}
+
+bool ResponseStat::is_valid() const
+{
+    return m_average.num_samples();
+}
+
+int ResponseStat::num_samples() const
+{
+    return m_average.num_samples();
+}
+
+maxbase::Duration ResponseStat::average() const
+{
+    return maxbase::Duration(m_average.average());
+}
+
+bool ResponseStat::sync_time_reached(int num_synch_medians)
+{
+    auto now = maxbase::Clock::now();
+    bool reached = m_next_sync < now || m_average.num_samples() >= num_synch_medians;
+    if (reached)
+    {
+        m_next_sync = now + m_sync_duration;
+    }
+    return reached;
+}
+
+void ResponseStat::reset()
+{
+    m_sample_count = 0;
+    m_average.reset();
+    m_next_sync = maxbase::Clock::now() + m_sync_duration;
+}
+
+}
+