使用Java实现电子邮件队列

Question

I have a DB whose columns are:

| from_email | to_email | subject | body | processed(Y/N) |

Now I want to implement a system in Java that will access this DB for not processed, send the mail (I'm using fakeSMTP for testing) and set it as processed.

It works fine enough, although I have yet to add any threading.

But the problem is to speed up the procedure it might be deployed over more than one server, in that case how can I ensure no mail gets sent twice?

How can it be made faster?

Answer 1

The following should be the solution to your problem:

1. Each server should have a unique id

2. Add the server_id with default value of null to your mails table

3. When a server intends to send a mail, run a query, like:

   update mails set server_id = <your server_id> where (server_id is null) and (<your other criteria>)

4. After you processed the update , select the row and if server_id has the expected value (no concurrency issue occurred), then process it

5. If a record's server_id is not null , then its processing has been already started. If its server_id is not null and its processed is Y , then it has been processed.