Method SelectFromAlternates

src/iocore/cache/HttpTransactCache.cc:177–274

Given a set of alternates, select the best match. The current school of thought: quality 1st, freshness 2nd. Loop through alternates and find the one with the highest quality factor. Then determine if it is fresh enough. If not, find the next best match. In keeping with "quality is job 1", subsequent matches will only be considered if their quality is equal to the quality of the first

*/
int
HttpTransactCache::SelectFromAlternates(CacheHTTPInfoVector *cache_vector, HTTPHdr *client_request,
                                        const HttpConfigAccessor *http_config_params)
{
  time_t current_age, best_age = CacheHighAgeWatermark;
  time_t t_now          = 0;
  int    best_index     = -1;
  float  best_Q         = -1.0;
  float  unacceptable_Q = 0.0;

  int alt_count = cache_vector->count();
  if (alt_count == 0) {
    return -1;
  }

  Dbg(dbg_ctl_http_match, "[SelectFromAlternates] # alternates = %d", alt_count);
  Dbg(dbg_ctl_http_seq, "[SelectFromAlternates] %d alternates for this cached doc", alt_count);
  if (dbg_ctl_http_alts.on()) {
    fprintf(stderr, "[alts] There are %d alternates for this request header.\n", alt_count);
  }

  // so that plugins can make cache reads for http
  // docs to check if the doc exists in the cache
  if (!client_request->valid()) {
    return 0;
  }

  for (int i = 0; i < alt_count; i++) {
    float          Q;
    CacheHTTPInfo *obj             = cache_vector->get(i);
    HTTPHdr       *cached_request  = obj->request_get();
    HTTPHdr       *cached_response = obj->response_get();

    if (!(obj->object_key_get().is_zero())) {
      ink_assert(cached_request->valid());
      ink_assert(cached_response->valid());

      Q = calculate_quality_of_match(http_config_params, client_request, cached_request, cached_response);

      if (alt_count > 1) {
        if (t_now == 0) {
          t_now = ink_hrtime_to_sec(ink_get_hrtime());
        }
        current_age = HttpTransactCache::calculate_document_age(obj->request_sent_time_get(), obj->response_received_time_get(),
                                                                cached_response, cached_response->get_date(), t_now);
        // Overflow?
        if (current_age < 0) {
          current_age = CacheHighAgeWatermark;
        }
      } else {
        current_age = static_cast<time_t>(0);
      }

      if (dbg_ctl_http_alts.on()) {
        fprintf(stderr, "[alts] ---- alternate #%d (Q = %g) has these request/response hdrs:\n", i + 1, Q);
        char b[4096];
        int  used, tmp, offset;
        int  done;