add missing file

lib/central-config.js

@@ -0,0 +1,35 @@
+'use strict'
+
+// Central config-related utilities for the APM http client.
+
+const INTERVAL_DEFAULT_S = 300 // 5 min
+const INTERVAL_MIN_S = 5
+const INTERVAL_MAX_S = 86400 // 1d
+
+/**
+ * Determine an appropriate delay until the next fetch of Central Config.
+ * Default to 5 minutes, minimum 5s, max 1d.
+ *
+ * The maximum of 1d ensures we don't get surprised by an overflow value to
+ * `setTimeout` per https://developer.mozilla.org/en-US/docs/Web/API/setTimeout#maximum_delay_value
+ *
+ * @param {Number|undefined} seconds - A number of seconds, typically pulled
+ *    from a `Cache-Control: max-age=${seconds}` header on a previous central
+ *    config request.
+ * @returns {Number}
+ */
+function getCentralConfigIntervalS(seconds) {
+  if (typeof seconds !== 'number' || seconds <= 0) {
+    return INTERVAL_DEFAULT_S
+  }
+  return Math.min(Math.max(seconds, INTERVAL_MIN_S), INTERVAL_MAX_S)
+}
+
+module.exports = {
+  getCentralConfigIntervalS,
+
+  // These are exported for testing.
+  INTERVAL_DEFAULT_S,
+  INTERVAL_MIN_S,
+  INTERVAL_MAX_S
+}