Hello Shreya. 

You have to sign a bond amounting to Rs. 27 lakhs(28 from this year) when you are through medicals. It's a 7 years service liability bond which includes 1 year of internship and upto 3 years of field posting but they don't allow you to do your post-graduation in case you choose to take short service commission (seven years of service).

Unless you choose to break the bond, it's compulsory to join one of the tri-services for minimum seven years.

Thank You