MXS-2068: Move RWBackend into mysqlcommon

This cleanly allows multiple modules to use it.

include/maxscale/response_stat.hh

@@ -0,0 +1,62 @@
+/*
+ * 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/ccdefs.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 num_filter_samples - collect num samples, use median
+     * @param num_synch_medians  - this many medians before the average should be synced, or
+     * @param sync_duration      - this much time between syncs.
+     */
+    ResponseStat(int num_filter_samples = 5,
+                 int num_synch_medians = 500,
+                 maxbase::Duration sync_duration = std::chrono::seconds(5));
+
+    void              query_started();
+    void              query_ended();// ok to call without a query_started
+    bool              make_valid(); // make valid even if there are too few filter_samples
+    bool              is_valid() const;
+    int               num_samples() const;
+    maxbase::Duration average() const;
+    bool              sync_time_reached();  // is it time to apply the average?
+    void              reset();
+private:
+    const int                      m_num_filter_samples;
+    const int                      m_num_synch_medians;
+    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;
+};
+}