Per my comment to the (wrongly, imho) accepted answer...
I used to produce/supply/document bus ticket analysis software. Most on-bus electronic ticket recording systems support a ticket cancellation transaction which is usually paired up with an earlier ticket issue. I always referred to such pairs as cancellor / cancellee because -or/-ee is far more dedicated to the agent / thing acted upon distinction than -er / -ed.
As RegDwight points out in this answer to an earlier (possibly duplicate) question, there are many words where both forms are perfectly common. You could say Brits still favour adviser and convener over the -or versions more than Americans, but you've only to look at this chart to see we're all moving strongly towards -or.
Therefore I would unhesitatingly recommend cancellor in OP's context, if he uses cancel in other documentation / variable names relating to the specific operation / computer code involved. Only in the (unlikely) event the documentation refers to the possibility of terminating (as opposed to cancelling) the operation should he opt for terminator.