Firebase实时数据库事务处理程序大多数时候被调用两次 [英] Firebase realtime database transaction handler gets called twice most of the time
问题描述
我正在使用事务在Node.js应用中使用Firebase管理API写入Firebase实时数据库中的特定位置.我观察到，即使没有其他客户端使用数据库，事务处理程序也会被调用两次.
I am using transactions to write to a specific location in firebase realtime database with firebase admin api in my nodejs app. I observed that the transaction handler gets called twice, even when there are no other clients using the database.
以下是显示此行为的最少代码.
Following is a minimal code which displays this behavior.
firebaseAdmin.database().ref('some/path').transaction(currentData => {
console.log('transaction handler got called');
return {'abc': 'def'};
}, null, false).then(value => {
console.log('transaction complete')
}).catch(reason => {
console.log('transaction failed. ' + reason);
});
我可以观察到，每次执行上述代码，transaction handler got called都会被记录两次.
I can observe that transaction handler got called gets logged twice for each execution of above code.
我了解，如果读取其他currentData窗口中的db路径中的其他客户端写入事务，并且尝试将新数据提交到db路径，则可以多次调用该处理程序.但是，在我的情况下，没有其他客户端，因此我无法理解为什么需要两次调用事务处理程序.
I understand that the handler can get called multiple times if some other client writes to the db path in the window between currentData is read for a transaction and the new data is attempted to be committed to the db path. But, there are no other clients in my case, so I cannot understand why the transaction handler needs to get called twice.
有人知道这是什么原因吗?
Does anyone know what is the reason for this?
推荐答案
这是预期的行为.当您运行事务时，Firebase客户端会以其对当前值some/path的最佳猜测立即调用您的事务处理程序.第一次运行它时，最好的猜测通常是null.如果some/path已经存在，那总是错误的，并且一旦客户端具有正确的当前值，将始终导致对事务处理程序的第二次调用.
This is expected behavior. When you run a transaction, the Firebase client immediately calls your transaction handler with its best guess of the current value of some/path. The first time you run it, this best guess is typically null. If some/path already exists that is always wrong, and will always lead to a second call to your transaction handler once the client has the correct current value.
在流程图中看起来像这样
In a flow chart it looks something like this
app code client server
+ +
transaction() | |
|+--+ |
| |current == null |
| v |
| |new = 0 |
|<--+ |
| |
| current==null, new=0 |
|+----------------------->|
| |+--+
| | |current != null
| | v
| | |current = 0
| |<--+
| NACK, current=0 |
|<-----------------------+|
| |
|+--+ |
| |curent==0 |
| v |
| |new=1 |
|<--+ |
| |
| current==0, new=1 |
|+----------------------->|
| |+--+
| | |current == 0
| | v
| | |current = 1
| |<--+
| ACK, current=1 |
|<-----------------------+|
| |
+ +
另请参阅以下有关交易方式的说明:
Also see these explanations of how transactions work:
- Firebase runTransaction not working - MutableData is null
- Strange behaviour of firebase transaction
- Android Firebase database transaction
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