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How to programatically send SMS notifications to 1 million users using queues?

What we have?
An API build in Node.js (using Moleculer.js for micro-services and PostgreSQL for storing data) which has the functionality of keeping track of users and user groups. We have in average 3k users per group, and a user can be part of multiple groups.
What we want to achieve?
We want to create a special service which will send text messages. The admins will select multiple groups, the code will remove the duplicated users and send them an SMS.
After a selection we can have around 1 million users. How can we send them text messages in an efficient way?
What have we tried?
Paginate the users and for each page send a request to the SMS service.
const users = db.getPage(1); // [{ id: 1, phone: '+123456789' }, ...]
smsClient.sendBulk(users);
PROBLEM: The user list in the database can change in the process and can affect the pagination by giving us duplicates or skipping some users.
Load all the results in the memory and send all the users to the SMS service.
const users = db.getAll(); // [..., { id: 988123, phone: '+987654321' }]
smsClient.sendBulk(users);
PROBLEM: We think it's a bad idea, resource wise, to make this kind of queries to the database and keep them in the memory. In the same time, we don't want to send 1 million entities through an HTTP request to the SMS service.
How can we select a 1 million users and send them an SMS message without worry about duplicates, skipped data or any other alteration to the admin's selection? We were thinking about queues as a necessary step but after we find a solution for the cases mentioned above. Or, is the queue part of the solution?

How can we select a 1 million users and send them an SMS message without worry about duplicates, skipped data, or any other alteration to the admin's selection?
For managing duplicates You could use an additional DB to save a Hash Table for the users that been handled already. This is a bit more expensive because you will need to check the user before each SMS send.
Managing not skipping is a bit tricky because you will need to add more recipients to an ongoing SMS transaction. You will need the ability to detect (hook) when a user is added to a group and add it as recipients to the ongoing transactions accordingly.
You will need to find a fast DB and save that user in a HashSet for a fast set and get (O(1))
We were thinking about queues as a necessary step but after we find a solution for the cases mentioned above. Or, is the queue part of the solution?
Defenently. Queue is the correct way to go for this scenario (queueing many small tasks). Some queues come with a re-queue features that will re-queue any task that didn't get acknowledgment.

you need to check out RabbitMQ.message-driven microservices

Have you considered creating an indirect state between the user and sent SMS? Something like SmsRequest / SmsTask / however you'd call it.
It'd consist of necessary user-data, message content, status of the request (to-send, sending, sent, failed, ...) and some additional metadata depending on your needs.
Then the first step you'd do is to prepare these request and store them in db, effectively making a queue out of a table. You can add some constraints on user and message type that'd prevent any duplicates and then start second asynchronous process that simply fetches requests in to-send state, sets the state to sending and then saves the outcome.
This also gives you the benefit of audit + you can batch the outgoing messages and.
Of course it'd increase your data volume significantly but I guess it's cheap nowadays anyway.

How to remove particular messages in rabbitmq before publishing new messages?

I have a subscriber which pushes data into queues. Now the messages looks this
{
"Content": {
"_id" ""5ceya67bbsbag3",
"dataset": {
"upper": {},
"lower": {}
}
}
Now a new message can be pushed with same content id but data will be different. So in that i want to delete the old message with same id or replaece the message those id is same & retain only latest message.
I have not found a direct solution for this in rabbitmq. Please guide me how we can do this ?
I have already gone through some posts.
Post 1
Post 2

What you are trying to achieve cannot be trivially solved with RabbitMQ (or rather the AMQP protocol).
RabbitMQ queues are simple FIFO stacks and don't offer any mean of access to the elements beyond publishing at their top and consuming from their bottom.
Therefore, the only way to "update" an already existing message without relying on an another service would be to fetch all the messages until you find the one you are interested in, discard it, and publish the new one with the other messages you fetched together with it.
Overall, the recommendation when using RabbitMQ in regards of message duplication is to make their consumption idempotent. In other words, the consumption of 2 messages deemed to be the same should lead to the same outcome.
One way to achieve idempotency is to rely on a secondary cache where you store the message identifiers and their validity. Once a consumer fetches a new message from RabbitMQ, it would check the cache to see if it's a valid message or not and act accordingly.

I think this is a slightly wrong way to use rabbitMQ.
only immutable (not intended to change) tasks should be put into queues which a worker shall consume.
An alternative way to implement your particular task is
just push immutable data into queue { "content" : { "_id" : "5ceya67bbsbag3"} .. }
store mutable data in db (mongo) or in-mem db (something like redis is suggested here).
whenever update needed, update in db
let your worker fetch required data using your "_id" ref from the db

I am not sure if removing a message is a good idea. If your requirement is to update the data as it comes so that always latest data is maintained for same Id.
Other thing is as messages are getting consumed always the last message data will get updated. So I don't see a issue here in Rabbit MQ.

Firebase Auth Built-in identity

I am building a web platform to connect learners with tutors using Firebase Auth and Firestore. I am storing the learners' user info in a Firestore collection, and the tutor's info in another. I am building a function that can detect if a client is a learner or a tutor upon login.
One solution I was thinking is to query in which collection the user is found using UID. Although, this is a potential solution. But it sounds like it requires a lot of transaction, and may have a long run time if collections get big. Is there something else I can do for that? I am wondering if I can use authentication to build an embedded identity to each user to differentiate between "Tutor" and "Learner"? So each time the user logs in, we know the identity immediately?
I know that in the decoded authentication token, there is info such as this:
{ iss: 'https://securetoken.google.com/xxxx',
aud: 'xxxx',
auth_time: 1569886929,
user_id: 'aB03',
sub: 'aB03',
iat: 1570582588,
exp: 1570586188,
email: 'aa#gmail.com',
email_verified: false,
firebase:
{ identities: { email: [Array] }, sign_in_provider: 'password' },
uid: 'aa3' }
Can I leverage one of these to build what I want?
Really appreciate your time and help.

Since you mention "collection", I assume you're looking to use Firestore. In that case: the size of the collection has no effect on the query performance. No matter if there are 1,000 user documents in the collection, 1,000,000 or even 1,000,000,000, the time it takes to query for a single user document will always be the same.
Aside from that you can also store the user's type in the ID token (which is what you've shown in your question). This is known as a "custom claim", since you're adding a custom property for your app to the information. You'll want to do this from a trusted environment (your development machine, a server you control, or Cloud Functions), as I'd assume users should not be able to change their own role. See the documentation on controlling access with custom claims for more information on this.

Couchdb / Pouchdb Relation between multiple users and multiple documents

I have a problematic here:
I'm builing a mobile app with ionic frmaework who needs to be able to work offline.
I'd like to adopt the CouchDB / PouchDB solution. But to do that i need to know how to put my data in the noSQl datatbase (MySQL user before ...). So, nosql is new to me but it seems interesting.
So my app has a connection part so a user database. And each user has documents who are attached to him. But many users can have many documents (sharing documents). And I want to replicate the data of one user (so his information + his documents on the mobile app).
What I thought is this:
One database per. One database for all Document with a server filtering to send only the documents that belongs to the user.
And on the client side I'd juste have to call :
var localDB = new PouchDB("myuser");
var remoteDB = new PouchDB("http://128.199.48.178:5984/myuser");
localDB.sync(remoteDB, {
live: true
});
And like that on the client side I'd have something like that :
{
username: "myuser",
birthday : "Date",
documents : [{
"_id": "2",
"szObject": "My Document",
},
{
"_id": "85",
"szObject": "My Document",
}]
}
Do you think something like that is possible using Couchdb and pouchdb, and if yes, am I thinking about it the right way?
I read it's not a problem to have one database per document, but I don't know if the replication will work like I imagine it

Plain CouchDB doesn't have any per-document access options, but these could be your solutions:
A. Create a View, then sync Pouch-To-Couch with a filter. But although this will only sync the documents that the user is supposed to see, anyone with enough knowledge could alter the code and view someone else's documents or just do anything with the database actually (probably not what you're looking for).
B. Create a master DB with all documents, then a database for each user, and a filtered replication between the master & per-user-dbs. Probably the simplest and most proper way to handle this.
C. Unfortunately there isn't a validate_doc_read (as there is a validate_doc_update) but perhaps you could make a simple HTTP proxy, which would parse out incoming JSON, check if a particular user can view it and if not, throw a 403 Forbidden. Well you'd also have to catch any views that query with include_docs=true.
(late reply, I hope it's still useful - or if not, that you found a good solution for your problem)

Adding a user to PFRelation using Parse Cloud Code

I am using Parse.com with my iPhone app.
I ran into a problem earlier where I was trying to add the currently logged in user to another user's PFRelation key/column called "friendsRelation" which is basically the friends list.
The only problem, is that you are not allowed to save changes to any other users besides the one that is currently logged in.
I then learned, that there is a workaround you can use, using the "master key" with Parse Cloud Code.
I ended up adding the code here to my Parse Cloud Code: https://stackoverflow.com/a/18651564/3344977
This works great and I can successfully test this and add an NSString to a string column/key in the Parse database.
However, I do not know how to modify the Parse Cloud Code to let me add a user to another user's PFRelation column/key.
I have been trying everything for the past 2 hours with the above Parse Cloud Code I linked to and could not get anything to work, and then I realized that my problem is with the actual cloud code, not with how I'm trying to use it in xcode, because like I said I can get it to successfully add an NSString object for testing purposes.
My problem is that I do not know javascript and don't understand the syntax, so I don't know how to change the Cloud Code which is written in javascript.
I need to edit the Parse Cloud Code that I linked to above, which I will also paste below at the end of this question, so that I can add the currently logged in PFUser object to another user's PFRelation key/column.
The code that I would use to do this in objective-c would be:
[friendsRelation addObject:user];
So I am pretty sure it is the same as just adding an object to an array, but like I said I don't know how to modify the Parse Cloud Code because it's in javascript.
Here is the Parse Cloud Code:
Parse.Cloud.define('editUser', function(request, response) {
var userId = request.params.userId,
newColText = request.params.newColText;
var User = Parse.Object.extend('_User'),
user = new User({ objectId: userId });
user.set('new_col', newColText);
Parse.Cloud.useMasterKey();
user.save().then(function(user) {
response.success(user);
}, function(error) {
response.error(error)
});
});
And then here is how I would use it in xcode using objective-c:
[PFCloud callFunction:#"editUser" withParameters:#{
#"userId": #"someuseridhere",
#"newColText": #"new text!"
}];
Now it just needs to be modified for adding the current PFUser to another user's PFRelation column/key, which I am pretty sure is technically just adding an object to an array.
This should be fairly simple for someone familiar with javascript, so I really appreciate the help.
Thank you.

I would recommend that you rethink your data model, and extract the followings out of the user table. When you plan a data model, especially for a NoSQL database, you should think about your queries first and plan your structure around that. This is especially true for mobile applications, as server connections are costly and often introduces latency issues if your app performs lots of connections.
Storing followings in the user class makes it easy to find who a person is following. But how would you solve the task of finding all users who follow YOU? You would have to check all users if you are in their followings relation. That would not be an efficient query, and it does not scale well.
When planning a social application, you should build for scalabilty. I don't know what kind of social app you are building, but imagine if the app went ballistic and became a rapidly growing success. If you didn't build for scalability, it would quickly fall apart, and you stood the chance of losing everything because the app suddenly became sluggish and therefore unusable (people have almost zero tolerance for waiting on mobile apps).
Forget all previous prioities about consistency and normalization, and design for scalability.
For storing followings and followers, use a separate "table" (Parse class) for each of those two. For each user, store an array of all usernames (or their objectId) they follow. Do the same for followers. This means that when YOU choose to follow someone, TWO tables need to be updated: you add the other user's username to the array of who you follow (in the followings table), and you also add YOUR username to the array of the other user's followers table.
Using this method, getting a list of followers and followings is extremely fast.
Have a look at this example implementation of Twitter for the Cassandra NoSQL database:
https://github.com/twissandra/twissandra