Issue #3498193 by catch: Optimize last_write_timestamp writes in ChainedFastBackend

   */

  protected function markAsOutdated() {

    // Clocks on a single server can drift. Multiple servers may have slightly

    // different opinions about the current time. Given that, do not assume

    // 'now' on this server is always later than our stored timestamp.

    // Also add 1 millisecond, to ensure that caches written earlier in the same

    // millisecond are invalidated. It is possible that caches will be later in

    // the same millisecond and are then incorrectly invalidated, but that only

    // costs one additional roundtrip to the persistent cache.

    $now = round(microtime(TRUE) + .001, 3);

    if ($now > $this->getLastWriteTimestamp()) {

    // differing opinions about the current time. Given that, do not assume

    // 'now' on this server is always later than our stored timestamp. Add 50ms

    // to the current time each time we write it to the persistent cache, and

    // make sure it is always at least 1ms ahead of the current time. This

    // somewhat protects against clock drift, while also reducing the number of

    // persistent cache writes to one every 50ms if this method is called

    // multiple times during a request.

    $compare = round(microtime(TRUE) + .001, 3);

    if ($compare > $this->getLastWriteTimestamp()) {

      $now = round(microtime(TRUE) + .050, 3);

      $this->lastWriteTimestamp = $now;

      $this->consistentBackend->set(self::LAST_WRITE_TIMESTAMP_PREFIX . $this->bin, $this->lastWriteTimestamp);

    }