Say you have a program. A popular program (at least in some circles) that other people want to write add-ons for. Your program does some job and in some processing steps you want to be able to execute some external code and then proceed according to the result returned by that code. Similar to the way CGI code is executed when a HTTP server processes a HTTP request.
The other day I was trying to enumerate all possible ways to do that. I found following solutions:
- Running external program. Data can be passed in program’s standard input and read from its standard output – just like when CGI program is launched from the HTTP server. Simple, stable (it is simple to protect against malfunctioning add-ons), but quite slow.
- Third-party DLL. Relatively simple, very fast, but can seriously destabilize the whole product. Complicated to upgrade the DLL (must shut down main application to upgrade add-on).
- [D]COM[+]. Not my bag of Swedish … sorry, wrong movie. Definitely not the way I’d like to pursue. Unstable. Leads to problems that nobody seems to be able to troubleshoot.
- Windows messages. Messy. Plus the main program runs as a service while add-on maybe wouldn’t.
- TCP. Implement add-on as a text-processing TCP/IP service (another HTTP server, if we continue the CGI analogy). Interesting idea, but not very simple to implement. Fast when both are running on the same machine. Flexible – each can be shutdown and upgraded independently; processing can be distributed over several computers. Complicated to configure when multiple add-ons are installed (each must be configured to a different port). Firewall and antivirus software may cause problems.
- Drop folder. Main app drops a file into some folder and waits for the result file. Clumsy, possibly faster than the external program solution (add-on can be always running), simple to implement and very stable.
- Message queues (as in the MSMQ). Interesting but possibly too complicated for most customers to install and manage.
And now to the main point of my ramblings. What did I miss? Are there more possibilities? If you have any idea how to approach my problem from a different direction, leave me a comment. Don’t mention SOAP, BTW, it is implicitly included in the “add-on as a TCP server” solution.