Tubal ligations and an Ovary-Sparing Spay are two ways to leave your doggy girl able and willing to breed.
Tubal ligation: Blocks / cuts access to, the fallopian tubes, preventing sperm from reaching the egg. Rendering the bitch unable to breed, but her uterus and full vaginal tract will remain intact.
Ovary-Sparing Spay (OSS): Involves removing the uterus and cervix, while leaving one or both of the ovaries intact.
Both surgeries have their positives and negatives.
A tubal ligation will almost guarentee that the dog will be able to breed. But it doesn't help with the risk of Pyometra. In fact, it may even increase the risk, since breeding your dog and allowing it to have pups helps prevent it. Pyometra is fatal if left untreated, and in many cases requires a spay to be done since it is an infection in the uterus.
An OSS will almost eliminate the risk of Pyometra, some cases of "stump pyo" may occur, but it's usually very treatable with medication and it is rare. The downside is that some vets will cut the vaginal tract too short. If they do this, it'll be harder to actually breed with your girl. She'll still have heat cycles, but if a large male dog ends up trying her, it may cause vaginal tearing. If they leave plenty of vaginal tract, you should be fine. But you can never know if said vet did so until after the surgery has been performed.