Retrieves a document from MongoDB twice, updates it twice, and writes it twice. Points out the danger with doing this in parallel.

RetrieveAndUpdateEntireState.ts

// Receive notification of update of property 'b' for system_id: 123
const notification_b = {
  system_id: 123,
  b: 4
}

// Retrieve state of system_id: 123 from MongoDB
const current_state_before_b_change = {
  system_id: 123,
  a: 1,
  b: 2,
  c: 3
}

// Begin to modify state object with new value for 'b' from notification
const state_in_memory_after_b_change = {
  system_id: 123,
  a: 1,
  b: 4,
  c: 3
}

// Receive notification of update of property 'a' for system_id: 123
const notification_a = {
  system_id: 123,
  a: 5
}

// Retrieve state of system_id: 123 from MongoDB
const current_state_before_a_change = {
 system_id: 123,
  a: 1,
  b: 2,
  c: 3 
}

// Write new object with new value of 'b' to MongoDB
const state_in_mongodb_after_b_change = {
  system_id: 123,
  a: 1,
  b: 4,
  c: 3
}

// Begin to modify state object with new value for 'a' from notification
const state_in_memory_after_a_change = {
  system_id: 123,
  a: 5,
  b: 2,
  c: 3
}

// Write new object with new value of 'a' to MongoDB
const state_in_mongodb_after_a_and_b_change = {
  system_id: 123,
  a: 5,
  b: 2,
  c: 3
}

/** 
 b's new value has been overwritten because the 
 'current state' to handle a's change was retrieved 
 before b's new state was written to MongoDB!
*/