How to call VB6 DLL from another machine (DLL as a service)

Question

I have a vb6-mysql client-server desktop application which is distributed as a setup file.

It uses DLL all logical operations as well as database operations. The EXE and the DLL are installed in the server as well as the client machines. When I say server which only means the database resides in that machine no other difference in EXE or DLLs.

As all the database operations are done in the DLL when connected from a client machine, performance would be less. It is not possible now to change all the logic into database.

• Is it possible to store the DLL in the server machine only and use the same DLL by the client machine also so that database connection will always be from the server itself?

• Is converting the DLL to a windows service the possible solution for this?

• How can I to convert it to a service?
• And finally, if it is possible to make the DLL act as a service, what would the connection issues be?

Answer 1

You appear to be trying to rediscover n-tier application development.

The usual way this would be done using VB6 within a LAN would be to create an ActiveX EXE instead of a DLL so you can use DCOM. However DCOM isn't something you'd want to expose over the Internet.

For such cases it is more typical to use a commonly-open-port protocol such as HTTP or HTTPS. Almost everyone has firewall settings permitting outbound HTTP and HTTPS connections and most of the major Web servers undergo regular hardening to make them safer to expose to the Internet.

The classic way to do this with VB6 was to use IIS to host the Remote Data Service, which uses a form of Web Service "under the covers" where your program doesn't deal with the gory details. However this is a deprecated approach, and today configuring IIS and the RDS components can be a chore since they are locked down hard by default.

This leaves you with such things as the deprecated SOAP Toolkit or 3rd party tools such as those in the PocketSOAP suite... or you can roll your own.

Doing this from scratch can be a bit of work but is more flexible, allowing REST instead of SOAP which can have advantages in itself. You could use whatever Web server you choose that can work with VB6 (via CGI, etc.).

The hardest approach to justify might seem the simplest on the surface: create your own protocol over TCP and write a Windows Service. This can be the most flexible of all but it can be more work than other options and you are on your own as far as making it and keeping it secure. You'll probably also face firewall issues depending on where your clients are and what the local firewall policies are there.

When we could rely on DCOM the issues were relatively small aside from security configuration headaches. With the Internet in the picture it is an entirely different story.

This really isn't something you undertake casually. Even assuming that your database is safe exposed to the Internet is naive and should be rethought.