Retrieving messages in chat application-CodePudding

I am working on an application for chatting. I wondering how to design the API for getting messages from a server.

When the application loads the chat window, I display the last 20 messages from the server using the endpoint:

/messages?user1={user1Id}&user2={user2Id}&page=0

Secondly, I allow user to load and display previous messages when the users scrolls to the top using the same endpoint but with different page (1)

/messages?user1={user1Id}&user2={user2Id}&page=1

But this design doesn't work correctly when users start to send messages to each others. The reason is that the endpoint returns the messages using descending order. Invoking the endpoint will give different result sets before sending/receiving a message and after.

My goal is to get always 20 previous messages when a user scrolls through conversation history.

How would you implement this in a clean way (including the REST API design)? I can imagine some solutions, but they seem dirty to me.

CodePudding user response：

Generally speaking, for "system design" questions, there isn't any one correct answer. Multiple approaches can be considered, based on their pros and cons. You've mentioned you want a clean way to retrieve messages, so your expectation might be different from mine.

Here is a simple approach/idea to get you started:

Have a field in your database that can be used to order messages. It could be a timestamp, a message_id, or something else that you want. Let's call this m_id for now.
On your client, have a field that tracks the m_id of the earliest message currently available locally on the client. This key (let's call it earliest_m_id) will be used to query the database, because you can simply fetch x number of messages before the earliest_m_id.
This would solve your issue of incorrect messages being fetched. Using paging will not work, since pages will continuously change with the exchange of messages.

CodePudding user response：

REST does not scale well for real time applications. It takes too many resources to open the HTTP connection again and again. Better to use websockets for it.

As of the upper problem if you start pagination with the latest message, not with the first message, then I would not wonder that it changes. To keep the pages after it you need to send the message id you started with. So for example: GET ...&count=20&latest=1, after that you get a 20 list of messages, you get the message id of the latest one and do the pagination with GET ...&count=20&basePoint={messageId}&page=0 to always get the exact same page no matter that you got new messages. After that you continue with GET ...&count=20&basePoint={messageId}&page=1 and so on... You will have negative pages GET ...&count=20&basePoint={messageId}&page=-1 if there are newer messages. Though I don't know any chatting application which uses pagination this way.

What I would do is GET ...&count=20&latest=1 and get the 20th message and do GET ...&count=20&before={messageId_20th} after that get the last message again from that list which is the 40th and do GET ...&count=20&before={messageId_40th} and so on. To get the new messages you can do GET ...&count=20&latest=1 again or GET ...&count=20&after={messageId_1st}.

None of the above is really good if you are using a caching mechanism in the client. I would add something like key frames in videos, so key messages. For example every 20th message can be a key message. I would do caching based on the key message ids so the responses could be cacheable, something like GET ...&count=20&latest=1 would return messages and one in the middle of the list would have a property of keyMessage=true. I would start the next request from the last key message something like GET ...&count=20&before={messageId_lastKey}, so the response will be cacheable.

Another way of doing this is starting pagination from the very first message. So when you do GET ...&count=20&latest=1 it will write the index of the message, something like 1234th message. You can do caching based on every 20th message just like in the key message solution, but instead of the message ids, you can do GET ...&count=20&to=1220 and merge the pages on the client. Or if you really want to spare with data, then GET ...&from=1201&to=1220&before=1215 or GET ...&from=1201&to=1220&last=1214 or GET ...&from=1201&to=1214, etc... And continue normal pagination with GET ...&count=20&to=1200 or GET ...&from=1181&to=1200.

What is good with the upper fixed pages approach, that you can use range headers too. For example GET .../latest would return the last 20 message with the header of Content-Range: messages 1215-1234/1234. After that you can do GET .../ Range: messages=1201-1214 and GET .../ Range: messages=1181-1200 and so on... When the total message count is updated in the response Content-Range: messages 1181-1200/1245, then you can automagically download the new message with GET .../ Range: messages=1235-1245 or with GET .../ Range: messages=1235- if you expect the 1245 to change meanwhile. You can do the same thing with the URI too, but if you have a standard solution like range headers it is better to use it instead of reinventing the wheel.

As of the &user1={user1Id}&user2={user2Id} part I would order it based on alphabet, so always user1 would be earlier in alphabetic order than user2, something like &user1=B&user2=A -> &user1=A&user2=B. So that will be properly cacheable too, not necassarily on the client, but you can add a server side cache too, so if the two participating users try to get the same conversation recently it won't be queried from the database again. Another way of doing this is adding a conversation id, which can be random unique or generated from the two user ids, but it must be always the same for the two users e.g. /conversations/A B or /conversations/x23542asda. I would do the latter, because if you start support conversations with multiple users it is better to have a dedicated conversation identifier. Another thing you can support is having multiple topic related conversations between the two users, so you won't need to do /conversations/A B/{topic} just use a unique id and search for conversations before creating a new one, so either /conversations?participants[]=A&participants[]=B&topic={topic} will give you an empty list or a list with a single conversation. This could be used to list coversations of a single user /conversations?participants[]=A or list conversations of a single user in a certain topic /conversations?participants[]=A&topic={topic}.