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.hh

@@ -0,0 +1,61 @@
+/*
+ * 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.
+ */
+#pragma once
+
+#include <maxscale/cppdefs.hh>
+
+#include <maxbase/stopwatch.hh>
+#include <maxbase/average.hh>
+
+/** This could arguably be a utility, but is written specifically for rwsplit
+ *  so it stays here at least for now.
+ */
+namespace maxscale
+{
+
+/**
+ * Query response statistics. Uses median of N samples to filter noise, then
+ * uses those medians to calculate the average response time.
+ * The class makes an average of the durations between calls to query_started()
+ * and query_ended(). Once the stats are good, sync_time_reached(int max) returns true,
+ * based on the average containing at least max samples (or medians), or the time
+ * sync_duration (constructor arg) has passed since the last reset().
+ */
+class ResponseStat
+{
+public:
+    /* @param ignore_first_n - the first few queries tend to have more overhead
+     * @param n_filter_samples - collect num samples, use median
+     * @param num_synch_samples - this many medians before the average should be synced, or
+     * @param sync_duration     - this much time between syncs.
+     */
+    ResponseStat(int ignore_first_n = 5, int num_filter_samples = 3,
+                 maxbase::Duration sync_duration = std::chrono::seconds(5));
+    void query_started();
+    void query_ended();  // ok to call without a query_started
+    bool is_valid() const;
+    int  num_samples() const;
+    maxbase::Duration average() const;
+    bool sync_time_reached(int num_synch_medians);  // is it time to apply the average?
+    void reset();
+private:
+    int m_ignore_first_n;
+    const int m_num_filter_samples;
+    const maxbase::Duration m_sync_duration;
+    int m_sample_count;
+    std::vector<maxbase::Duration> m_samples; // N sampels from which median is used
+    maxbase::CumulativeAverage m_average;
+    maxbase::TimePoint m_last_start;
+    maxbase::TimePoint m_next_sync;
+};
+}