Maintain consistency in concurrent read operations in MongoDB - node.js

Question

I have an API in Node.js say /create/appointment which does 2 operations
STEP 1: Check for active appointment in DB.
STEP 2: Create an appointment if not exist.
I have 3 collections
doctors
patients
appointments
appointments collection has 3 main fields
doctor_id: MongoDb ObjectID
patient_id: MongoDB ObjectID
is_active: Boolean
STEP 1 contains one DB read operation.
STEP 2 contains one DB write operation.
When the API is fired multiple times simultaneously,
STEP 1 of second cycle is executed before STEP 2 of first cycle completes. Since STEP 2 of first cycle is not completed, STEP 1 of second cycle does not return the active appointment entry created in first cycle.
I cannot use compound unique index on:
{doctor_id, patient_id}, since appointment collection can contain historical data.
{doctor_id, patient_id, is_active}, then appointment collection can contain only one inactive entry.
Implementation:
// All functions below return promise object.
doctorManager
.getDoctor(doctor_id)
.then(doctor => {
// throw error if doctor does not exist
return patientManager.getPatient(patient_id);
})
.then(patient => {
// throw error if patient does not exist
return getActiveAppointment(doctor_id, patient_id)
})
.then(activeAppointment => {
// throw error if active appointment exist
return appointmentManager.createAppointment(doctor_id, patient_id)
})
.then(() => {
// return API response
})
.catch(error => {
// handel error
});
Is there a way to lock appointment collection when any sort of operation is going on, or any other better solution.
I cannot shard my DB nor setup replication.

Answer 1

Here's something it will look when you use async await functionality. Its not problem solution but just an example of handling your problem properly.
doctor = await doctorManager.getDoctor(doctor_id);
patient = await patientManager.getPatient(patient_id);
appointment = await doAppointmentExists(doctor_id, patient_id);
if(isEmpty(appointment)) {
create appointment = await appointmentManager.createAppointment(doctor_id, patient_id)
} else {
throw new Error("Duplicate Appointment");
}