从多个javascript线程访问IndexedDB [英] Accessing IndexedDB from multiple javascript threads

问题描述

概述：
我试图通过从网页和网络工作者访问IndexedDB来避免竞争条件。

Overview: I am trying to avoid a race condition with accessing an IndexedDB from both a webpage and a web-worker.

设置：
当用户使用网站时，将项目保存到本地IndexedDB的网页。每当用户将数据保存到本地数据库时，记录就会被标记为未发送。

Setup: Webpage that is saving items to the local IndexedDB as the user works with the site. Whenever a user saves data to the local DB the record is marked as "Unsent".

从IndexedDB中提取数据的Web工作者后台线程，将其发送到服务器，一旦服务器收到它，将IndexedDB中的数据标记为已发送。

Web-worker background thread that is pulling data from the IndexedDB, sending it to the server and once the server receives it, marking the data in the IndexedDB as "Sent".

问题：
由于访问IndexedDB是异步的，我不能保证用户不会在web-worker将其发送到服务器的同时更新记录。时间表如下所示：

Problem: Since access to the IndexedDB is asynchronous, I can not be guaranteed that the user won't update a record at the same time the web-worker is sending it to the server. The timeline is shown below:

1. 网络工作者从数据库获取数据并将其发送到服务器


2. 当传输发生时，用户更新数据将其保存到数据库。


3. Web工作者从服务器获取响应，然后将数据库更新为已发送


4. 现在DB中的数据尚未发送到服务器但标记为已发送

1. Web-worker gets data from DB and sends it to the server
2. While the transfer is happening, the user updates the data saving it to the DB.
3. The web-worker gets the response from the server and then updates the DB to "Sent"
4. There is now data in DB that hasn't been sent to the server but marked as "Sent"

解决方案失败：
从服务器获得响应后，我可以重新检查行以查看是否有任何更改。但是我仍然留有一个小窗口，可以将数据写入数据库并且它永远不会被发送到服务器。

Failed Solution: After getting the response from the server, I can recheck to row to see if anything has been changed. However I am still left with a small window where data can be written to the DB and it will never be sent to the server.

示例：
服务器之后说数据已保存，然后：

Example: After server says data is saved, then:

IndexedDB.HasDataChanged(
    function(changed) { 
        // Since this is async, this changed boolean could be lying.
        // The data might have been updated after I checked and before I was called.
        if (!changed){ 
          IndexedDB.UpdateToSent() }
    });

其他注意事项：
有一个同步api根据W3规范，但还没有人实现它，因此无法使用（http://www.w3.org/TR/IndexedDB/#sync-database）。 sync api旨在供网络工作者使用，以避免我假设的这种情况。

Other notes: There is a sync api according to the W3 spec, but no one has implemented it yet so it can not be used (http://www.w3.org/TR/IndexedDB/#sync-database). The sync api was designed to be used by web-workers, to avoid this exact situation I would assume.

对此的任何想法都将非常感激。已经工作了大约一个星期，并且无法提出任何可行的方法。

Any thoughts on this would be greatly appreciated. Have been working on it for about a week and haven't been able to come up with anything that will work.

推荐答案

我我觉得我现在找到了解决这个问题的方法。并不像我想的那样干净，但似乎是线程安全的。

I think I found a work around for this for now. Not really as clean as I would like, but it seems to be thread safe.

每当我更新数据时，我都会将datetime存储到LastEdit字段中。
来自网络工作者，我正在向浏览器发帖子。

I start by storing the datetime into a LastEdit field, whenever I update the data. From the web-worker, I am posting a message to the browser.

self.postMessage('UpdateDataSent#' + data.ID + '#' + data.LastEdit);

然后在浏览器中我更新我发送的标志，只要上次编辑日期没有改变。

Then in the browser I am updating my sent flag, as long as the last edit date hasn't changed.

// Get the data from the DB in a transaction
if (data.LastEdit == lastEdit)
{
    data.Sent = true;
    var saveStore = trans.objectStore("Data");
    var saveRequest = saveStore.put(data);
    console.log('Data updated to Sent');
}

由于这一切都是在浏览器端的事务中完成的，所以似乎工作得很好。一旦浏览器支持Sync API，我就可以把它扔掉。

Since this is all done in a transaction in the browser side, it seems to work fine. Once the browsers support the Sync API I can throw it all away anyway.

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