+\f
+/* "Instant" stats.
+ *
+ * Some information in the database must be kept as up-to-date as possible to
+ * allow controllers to respond rapidly to network outages. We call these
+ * statistics "instant" stats.
+ *
+ * We wish to update these statistics every INSTANT_INTERVAL_MSEC milliseconds,
+ * assuming that they've changed. The only means we have to determine whether
+ * they have changed are:
+ *
+ * - Try to commit changes to the database. If nothing changed, then
+ * ovsdb_idl_txn_commit() returns TXN_UNCHANGED, otherwise some other
+ * value.
+ *
+ * - instant_stats_run() is called late in the run loop, after anything that
+ * might change any of the instant stats.
+ *
+ * We use these two facts together to avoid waking the process up every
+ * INSTANT_INTERVAL_MSEC whether there is any change or not.
+ */
+
+/* Minimum interval between writing updates to the instant stats to the
+ * database. */
+#define INSTANT_INTERVAL_MSEC 100
+
+/* Current instant stats database transaction, NULL if there is no ongoing
+ * transaction. */
+static struct ovsdb_idl_txn *instant_txn;
+
+/* Next time (in msec on monotonic clock) at which we will update the instant
+ * stats. */
+static long long int instant_next_txn = LLONG_MIN;
+
+/* True if the run loop has run since we last saw that the instant stats were
+ * unchanged, that is, this is true if we need to wake up at 'instant_next_txn'
+ * to refresh the instant stats. */
+static bool instant_stats_could_have_changed;