<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _init_cache(self):
 """Initialise the depth cache calling REST endpoint :return: """

        self._last_update_id = None
        self._depth_message_buffer = []

        res = self._client.get_order_book(symbol=self._symbol, limit=self._limit)

        # process bid and asks from the order book
        for bid in res['bids']:
            self._depth_cache.add_bid(bid)
        for ask in res['asks']:
            self._depth_cache.add_ask(ask)

        # set first update id
        self._last_update_id = res['lastUpdateId']

        # set a time to refresh the depth cache
        if self._refresh_interval:
            self._refresh_time = int(time.time()) + self._refresh_interval

        # Apply any updates from the websocket
        for msg in self._depth_message_buffer:
            self._process_depth_message(msg, buffer=True)

        # clear the depth buffer
        del self._depth_message_buffer