iOS chat app design

Question

I'm building a simple chat app on iOS for fun (and to have projects to gain experience from), using socketsIO and a node backend. I am trying to figure out the best design for messages. I was planning to use a mongoDB database where each conversation would have its message data stored. Whenever the client sends a new message to the server, the server adds it to the appropriate conversation in the database.

I was also hoping to create a user Sign Up/Log In system which would add you to the database. However, I've googled around quite a bit and I am really not sure if creating a database made up of conversations (that get updated whenever a sentMessage event is triggered) and user data is the right way to go. Additionally, I've seen some people talk about saving the chats on the actual devices themselves, not in a database? What is the common design pattern for a chat app like this?

Answer 1

for the design I would use socket.io for emitting messages as well. It has a great community behind it, I woul also use MongoDb because everything is using JSON format and it's integrated so well with Node due to it using JavaScript.

Now the part you are interested about, is REDIS. Redis is a database that sits in RAM on the web and should be used with mongodb if you're going to be having higher traffic / need quick speed / less hanging and waiting.

REDIS would be your temporary save for the chat with a session because doing disk write/read/querying is a lot on the machine (looking at you MongoDB), If you plan on saving the chat with every message. Doing so MongoDb would just not scale all the well in the long run and is not as fast as REDIS. Mind you REDIS database will only hold the temporary chat log of let's say the last 1 million chat session or some limit (it's all in RAM so the size is limited can't have Terabytes or hundreds of Gigabytes of RAM on 1 server).

so the data flow would look something like

1. user sends message
2. server receives messsage via HTTP(S) post/put - Ajax/Observable
3. Server will use socket.io to emit the message to the designated user while saving the message to REDIS with a specific key/session/message.
4. designated user get's the update on their screen via io event.

-- inbetween there should be a check on the REDIS db of whether it is getting full. if it's full remove the last 10,000 inactive messages (could be from 1 year ago if the server hasn't gotten full yet) to make some space.

Saving the chat on the phone is an okay idea as it would save the users data/bandwidth and they could potentially look at their message while offline. a solution is using SQL Lite which is a lightweight library that will sit inside your app acting as a database which you can perform queries on if your familiar with RDBMS you will have no problem implementing it. But now you gotta find a good way to manage saving data to REDIS/SQL-LITE/MongoDb.