Add the salt and baking soda to a large pot of water and bring to a boil. Add the spaghetti and cook until al dente, about 6 minutes. Drain.
While spaghetti is cooking, make the broth. Heat the oil in a soup pot or Dutch oven over medium-high heat. Add the garlic, ginger, the white part of the green onion, and carrots. Cook until the vegetables are softened, about 5 minutes. Add the chicken broth, dried mushrooms, sake, soy sauce, and sesame oil. Cook until the broth is hot and the mushrooms have softened, about 10 minutes. Add the cooked spaghetti to the broth and heat through. Serve in big bowls topped with a peeled soft boiled egg, some of the green parts of the green onions, some bean sprouts, and a few slices of pepper. Add sriracha to taste.