Provide example on how to throttle requests to live in rate limits

[nvkelso]

Ian's done a bit of that for vector tiles in his Tilepacks work here.

def fetch_tile(format_args):
    sleep_time = 0.5 * random.uniform(1.0, 1.7)
    response_info = []
    while True:
        url = 'https://tile.mapzen.com/mapzen/vector/v1/{layer}/{zoom}/{x}/{y}.{fmt}?api_key={api_key}'.format(**format_args)
        try:
            start = time.time()

            resp = sess.get(url, timeout=(6.1, 30))

            data = resp.content
            finish = time.time()

            response_info.append({
                "url": url,
                "response status": resp.status_code,
                "server": resp.headers.get('Server'),
                "time to headers millis": int(resp.elapsed.total_seconds() * 1000),
                "time for content millis": int((finish - start) * 1000),
                "response length bytes": len(data),
            })

            resp.raise_for_status()
            return (format_args, response_info, data)
        except requests.exceptions.RequestException as e:
            if isinstance(e, requests.exceptions.HTTPError):
                if e.response.status_code == 404:
                    print("HTTP 404 -- {} while retrieving {}. Not trying again.".format(
                        e.response.status_code, e.response.text, url)
                    )
                    return (format_args, response_info, None)
                else:
                    print("HTTP error {} -- {} while retrieving {}, retrying after {:0.2f} sec".format(
                        e.response.status_code, e.response.text, url, sleep_time)
                    )
            else:
                print("{} while retrieving {}, retrying after {:0.2f} sec".format(
                    type(e), url, sleep_time)
                )
            time.sleep(sleep_time)
            sleep_time = min(sleep_time * 2.0, 30.0) * random.uniform(1.0, 1.7)
        except Exception as e:
            print("Ran into an unexpected exception: {}".format(e))
            traceback.print_tb()
            raise