如何避免在 MongoDB 中多次读取相同的数据？

Question

I develop a service (in .net) for sending a lot of emails

The service can be executed on multiple nodes

I have a collection in MongoDB with all the data of the emails, how i can read a data on a node without the others nodes read it too ?

I want to avoid sending the same mail multiple times

Answer 1

Basically, you'd need to signal the other nodes that a document is already being processed. Therefore, you could add a marker property to your document, eg:

public class MyDocument 
{
  // ... other members

  // I suppose there is some kind of status property on the document
  public bool Done { get; set; }
  
  public DateTime? StartedProcession { get; set; }
}

Why is this a DateTime? property? Because this way you can handle errors that are encountered by a node when sending the mail and updating the status flag.

You can use MongoDB's FindAndModify method to identify the next mail a node should send. This method finds exactly one document and performs an atomic update on this. The conditions would be:

• Done is false
• StartedProcession is null or more than an hour (or a value that better suits your needs) in the past

The update should set the StartedProcession property to the current time. This way, other nodes will not try to send the same mail.

In the node, the mail is sent. If all is well and good, you update the Done flag on the document; if not, you could reset the StartedProcession property to null to allow for a retry (in addition, you could also store the error details in the document for later analysis).

In case of an error that is so catastrophic that the node does not reset the StartedProcession property, the filter above asserts that another node will retry the transmission after an hour.

Please note that in some rare cases the mail might still be sent twice, eg if the mail is sent successfully, but the update of the Done flag fails. However, this should be very rare as the FindAndModify as the first step went well and some milliseconds after the update is performed.