I’m building a multi-threaded service application in Delphi XE2. Each thread serves its own purpose apart from the other ones. The main service thread is only responsible for keeping the other threads going and saving a log file, etc. Each of these threads reports back to the main service thread through synchronized event triggers. These threads are created when the service starts and destroyed when the service ends.
I’d like to introduce a separate thread as a centralized database connection to avoid having to create many instances of TADOConnection. My service code can call standard functions such as UserListDataSet := DBThread.GetUserList(SomeUserListDataSet); or it would also be nice if I could send direct SQL statements like SomeDataSet := DBThread.Get(MySqlText);. I’d also like to avoid too many occasions of CoInitialize() etc.
The job threads will need to use this db thread. I need to figure out how to “ask” it for certain data, “wait” for a response, and “acquire” that response back in the thread which requested it. I’m sure there are many approaches to this, but I need to know which one is best suited for my scenario. Windows messages? Events? Should I have some sort of queue? Should it send data sets or something else? Is there already something that can do this? I need to figure out how to structure this DB thread in a way that it can be re-used from other threads.
The structure looks like this:
+ SvcThread
+ DBThread
+ TADOConnection
+ Thread1
+ Thread2
+ Thread3
I need threads 1 2 and 3 to send requests to the DBThread. When a thread sends any request to it, it needs to wait until it gets a response. Once there’s a response, the DB Thread needs to notify the asking thread. Each of the threads might send a request to this DB Thread at the same time too.
A good tutorial on how to accomplish this would be perfect – it just needs to be a suitable fit for my scenario. I don’t need to know just “how to make two threads talk together” but rather “how to make many threads talk to a centralized database thread”. These job threads are created as children of the main service thread, and are not owned by the db thread. The db thread has no knowledge of the job threads.
Normally, you’d have a request queue where all the requests are stored. Your database thread reads a request from the queue, handles it, then invokes a callback routine specified by the requester to handle the result. Not sure how this maps to Delphi paradigms, but the basics should be the same.