Method isDiskCacheHit

src/tsd/GraphHandler.java:451–512

Checks whether or not it's possible to re-serve this query from disk. @param query The query to serve. @param end_time The end time on the query (32-bit unsigned int, seconds). @param max_age The maximum time (in seconds) we wanna allow clients to cache the result in case of a cache hit. @param base

(final HttpQuery query,
                                 final long end_time,
                                 final int max_age,
                                 final String basepath)

   * the query needs to be processed).
   */
  private boolean isDiskCacheHit(final HttpQuery query,
                                 final long end_time,
                                 final int max_age,
                                 final String basepath) throws IOException {
    final String cachepath = basepath + (query.hasQueryStringParam("ascii")
                                         ? ".txt" : ".png");
    final File cachedfile = new File(cachepath);
    if (cachedfile.exists()) {
      final long bytes = cachedfile.length();
      if (bytes < 21) {  // Minimum possible size for a PNG: 21 bytes.
                         // For .txt files, <21 bytes is almost impossible.
        logWarn(query, "Cached " + cachepath + " is too small ("
                + bytes + " bytes) to be valid.  Ignoring it.");
        return false;
      }
      if (staleCacheFile(query, end_time, max_age, cachedfile)) {
        return false;
      }
      if (query.hasQueryStringParam("json")) {
        HashMap<String, Object> map = loadCachedJson(query, end_time,
            max_age, basepath);
        if (map == null) {
          map = new HashMap<String, Object>();
        }
        map.put("timing", query.processingTimeMillis());
        map.put("cachehit", "disk");
        query.sendReply(JSON.serializeToBytes(map));
      } else if (query.hasQueryStringParam("png")
                 || query.hasQueryStringParam("ascii")) {
        query.sendFile(cachepath, max_age);
      } else {
        query.sendReply(HttpQuery.makePage("TSDB Query", "Your graph is ready",
            "<img src=\"" + query.request().getUri() + "&amp;png\"/><br/>"
            + "<small>(served from disk cache)</small>"));
      }
      graphs_diskcache_hit.incrementAndGet();
      return true;
    }
    // We didn't find an image.  Do a negative cache check.  If we've seen
    // this query before but there was no result, we at least wrote the JSON.
    final HashMap<String, Object> map = loadCachedJson(query, end_time,
        max_age, basepath);
    // If we don't have a JSON file it's a complete cache miss.  If we have
    // one, and it says 0 data points were plotted, it's a negative cache hit.
    if (map == null || !map.containsKey("plotted") ||
        ((Integer)map.get("plotted")) == 0) {
      return false;
    }
    if (query.hasQueryStringParam("json")) {
      map.put("timing", query.processingTimeMillis());
      map.put("cachehit", "disk");
      query.sendReply(JSON.serializeToBytes(map));
    } else if (query.hasQueryStringParam("png")) {
      query.sendReply(" ");  // Send back an empty response...
    } else {
        query.sendReply(HttpQuery.makePage("TSDB Query", "No results",
            "Sorry, your query didn't return anything.<br/>"
            + "<small>(served from disk cache)</small>"));