通过REST API进行Firebase交易

)是处理并发的一种很酷的方式,但似乎它们只能从客户端完成.

我们使用Firebase的方式主要是从我们的服务器写入数据并在客户端上观察它们.有没有办法在通过REST API编写数据时实现乐观并发模型？

谢谢！

您可以使用更新计数器使写操作以与事务类似的方式工作. (我将在下面使用一些伪代码;对不起,但我不想写一个完整的REST API作为示例.)

例如,如果我有这样的对象：

{
   total: 100,
   update_counter: 0
}

这样的写规则：

{
   ".write": "newData.hasChild('update_counter')",
   "update_counter": {
      ".validate": "newData.val() === data.val()+1"
   }
}

我现在可以通过简单地在每个操作中传入update_counter来防止并发修改.例如：

var url = 'https://<INSTANCE>.firebaseio.com/path/to/data.json';
addToTotal(url, 25, function(data) {
   console.log('new total is '+data.total);
});

function addToTotal(url, amount, next) {
   getCurrentValue(url, function(in) {
      var data = { total: in.total+amount, update_counter: in.update_counter+1 };
      setCurrentValue(ref, data, next, addToTotal.bind(null, ref, amount, next));
   });
}

function getCurrentValue(url, next) {
   // var data = (results of GET request to the URL) 
   next( data );
}

function setCurrentValue(url, data, next, retryMethod) {
   // set the data with a PUT request to the URL
   // if the PUT fails with 403 (permission denied) then
   // we assume there was a concurrent edit and we need
   // to try our pseudo-transaction again
   // we have to make some assumptions that permission_denied does not
   // occur for any other reasons, so we might want some extra checking, fallbacks,
   // or a max number of retries here
   // var statusCode = (server's response code to PUT request)
   if( statusCode === 403 ) {
       retryMethod();
   }
   else {
       next(data);
   }
}