def parse_binary_search(self, response, minimum=None, maximum=None):
offset = int(get_parameter_value(response.request.url, 'offset'))
first_offset = response.request.meta['first']
if minimum and maximum:
self.logger.info(f'Starting binary search for {first_offset:,} within [{minimum:,}, {maximum:,}]')
elif self.is_http_success(response):
minimum = response.request.meta['minimum']
maximum = offset
else:
minimum = offset + 1
maximum = response.request.meta['maximum']
# If the search succeeded, parse the response as usual. We use a threshold, because getting the exact
# millisecond requires 27 requests.
if minimum + THRESHOLD >= maximum:
self.logger.info(f'New offset found after {first_offset:,} at {maximum:,}!')
if offset == maximum:
# If the last request used the offset, we can reuse its response.
yield from self.parse(response)
else:
url = replace_parameters(response.request.url, offset=maximum)
yield self._build_request(url, self.parse, {})
else:
url = replace_parameters(response.request.url, offset=(minimum + maximum) // 2)
yield self._build_request(url, self.parse_binary_search, {'minimum': minimum, 'maximum': maximum,
'first': first_offset})
def parse_list(self, response, **kwargs):
hrefs = response.xpath('//a/@href').getall()
for href in hrefs:
if '.json' in href:
# the href looks like
# http://www.google.com/url?q=http%3A%2F%2F116.202.173.47%3A8080%2Fmd_covid_2020-11-06.json&sa=D&sntz=1
url = get_parameter_value(href, 'q')
if not url:
url = href
yield self.build_request(url, formatter=components(-1))
def parse(self, response):
# If the request was successful, parse the response as usual.
if self.is_http_success(response):
yield self.build_file_from_response(response, data_type=self.data_type)
# Use `dont_filter` in case the search for a successful timestamp used the same offset. Use `dont_retry`
# since errors are expected.
yield self.next_link(response, dont_filter=True, meta={'dont_retry': True})
# Otherwise, parse the response as usual, then (1) pick a date range and (2) do a binary search within it.
# This approach assumes that, if two offsets error, then intervening offsets error, too.
else:
yield self.build_file_error_from_response(response)
# If the error occurs on the first request, we have no starting offset.
if get_parameter_value(response.request.url, 'offset'):
yield from self.parse_date_range(response)
def parse_date_range(self, response):
offset = int(get_parameter_value(response.request.url, 'offset'))
# Scrapy uses `datetime.datetime.utcnow()`, so we don't need to worry about time zones.
start_time = int(self.crawler.stats.get_value('start_time').timestamp() * 1000)
# We use the first offset to calculate the new offset, and in log lessages.
first_offset = response.request.meta.get('first', offset)
# The exponent for the exponential search.
exponent = response.request.meta.get('exponent', -1) + 1
# If this offset succeeded, do a binary search from the previous offset to this offset.
if self.is_http_success(response):
yield from self.parse_binary_search(response, response.request.meta['prev'], offset)
# If this offset failed and reached a limit, stop.
elif offset >= start_time or exponent > EXPONENT_LIMIT:
self.logger.info(f'No offset found after {first_offset:,} within {2 ** EXPONENT_LIMIT} days.')
yield self.build_file_error_from_response(response)
# Otherwise, continue.
else:
new_offset = min(first_offset + MILLISECONDS_PER_DAY * 2 ** exponent, start_time)
url = replace_parameters(response.request.url, offset=new_offset)
yield self._build_request(url, self.parse_date_range, {'prev': offset, 'exponent': exponent,
'first': first_offset})
def test_get_parameter_value(url, expected):
assert get_parameter_value(url, 'page') == expected
