How to maintain state with Cloud Functions and Cloud FIrestore - firebase

Question

How do you maintain the correct state when using Cloud Functions? They are not guaranteed to fire in the same order that they are called.
Here is a sequence of events:
A document is updated currentState: state1
A document is updated currentState: state2
The Cloud Function triggers the state2 update.
The Cloud Function triggers the state1 update.
If your application requires the carrying out of functions in the correct order of states, there's a problem.

Answer 1

Cloud Functions are not guaranteed to fire in order or only once. Therefore, you must make them idempotent.
You can resolve this in the following way:
Always use transactions to update the state, so that 2 clients don't try and change the state at the same time.
Create a state table which manages the state and runs functions based on the current state vs. the previous state.
Clients must not change the state to a value less than that which exists currently.
states.json
[
{"currentState": "state1", "action": "state2", "newStates": ["state2"]},
{"currentState": "state1", "action": "state3", "newStates": ["state2", "state3"]},
{"currentState": "state1", "action": "state4", "newStates": ["state2", "state3", "state4"]},
{"currentState": "state1", "action": "state5", "newStates": ["state2", "state3", "state4", "state5"]},
{"currentState": "state2", "action": "state3", "newStates": ["state3"]},
{"currentState": "state2", "action": "state4", "newStates": ["state3", "state4"]},
{"currentState": "state2", "action": "state5", "newStates": ["state3", "state4", "state5"]},
{"currentState": "state3", "action": "state4", "newStates": ["state4"]},
{"currentState": "state3", "action": "state5", "newStates": ["state4", "state5"]},
{"currentState": "state4", "action": "state5", "newStates": ["state5"]}
]
app.js
function processStates (beforeState, afterState) {
const states = require('../states');
let newStates;
// Check the states and set the new state
try {
newStates = states.filter(function(e) {return e.currentState == beforeState && e.action == afterState;})[0].newStates;
}
catch (err) {
newStates = null;
}
console.log(`newStates: ${newStates}`);
if (newStates) {
newStates.forEach(newState) {
// Process state change here
switch (newState) {
case 'state1': {
// Process state1 change
break;
}
case 'state2': {
// Process state2 change
break;
}
default: {
}
}
}
}
}
Once you have an array of states, you can iterate through the using something like forEach or map to process the required commands.