gistfile1.txt

  var getAttachmentQueue = {};
  var getAttachmentActive = {};
  var getAttachmentCount = 0;
  var getAttachmentConcurrency = 10;

  function processQueue() {
    console.log('processQueue', getAttachmentCount, Object.keys(getAttachmentActive).length, Object.keys(getAttachmentQueue).length);
    while (Object.keys(getAttachmentActive).length < getAttachmentConcurrency && Object.keys(getAttachmentQueue).length > 0) {
      var key = Object.keys(getAttachmentQueue)[0];
      var args = getAttachmentActive[key] = getAttachmentQueue[key];
      delete getAttachmentQueue[key];

      var opts = args[0], url = args[1], callback = args[2];

      ajax$$(opts, {
        method: 'GET',
        url: url,
        binary: true
      }, function() {
        delete getAttachmentActive[key];
        processQueue();
        callback.apply(this, arguments);
      });
    }
  }

  // Get the attachment
  api.getAttachment =
    adapterFun$$('getAttachment', function (docId, attachmentId, opts,
                                                callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }

    var params = opts.rev ? ('?rev=' + opts.rev) : '';
    var url = genDBUrl(host, encodeDocId(docId)) + '/' +
      encodeAttachmentId(attachmentId) + params;

    getAttachmentQueue["key"+(getAttachmentCount++)] = [opts, url, callback];
    processQueue();
  });