Can I edit messages on mqtt server?
Building an instant chat app (native IOS and web). Exploring whether to use XMPP or MQTT as application protocols. Seemingly I can't have users editing old messages on XMPP. Can messages be edited on MQTT?
Example: I want to implement "Edit Message" like Slack offers, but upon clicking "(edited)" to allow the user to see the different versions of the message and their timestamps (like the edit history for comments you find in Facebook), enabling an "audit trail" of the conversation.
Follow-up: As it seems this can only be achieved through a "hack", would it be better to get the hack done on XMPP or MQTT or some other protocol/websockets/JSON, etc?
Once a MQTT message is published to the broker the publishing client has no more control over that message at all.
Most brokers will not allow you to edit the message either as they will just forward the message instantly to all clients subscribed to the relevant topics and queue the message for any offline clients with persistent subscriptions.
The only exception may be the mosca broker that has a call back for when messages reach the broker, but this would not allow a user to edit a message, only the system to possibly update the payload in the instant before it was forwarded to the subscribed clients.
Hardlib's advice is correct, editing messages in this way is not supported by most MQTT implementations and to implement it would break the loose coupling between publisher and subscriber that is the virtue of MQTT. In other words this should be implemented at a higher level or through other means.
That said, if I understand editing to mean the ability to change what the broker forwards to clients that were not online during the initial publication, you could implement this with retained messages. Consider this:
Client A is subscribed to topic clientb/# and Client B is subscribed to topic clienta/#.
Client A publishes a message to clienta/(unique message id) while Client B is not actively connected. The broker retains the message.
Client A decides to edit the message so (through some interface you devise) they publish an amended message to clienta/(unique message id) which replaces the message and, from a subscribers perspective, edits what is there.
Client B receives the amended message when they come online and (as long as there isn't a persistent session or something like that) has no knowledge of the change.
From this example you can probably tell why this is a bad idea as the server would retain every single message in a different topic and would likely need regular pruning... not mention that it would make a mess out of timestamps and require all sorts of other work arounds. However, if there is some reason that you have to implement it this way you could hack something usable together.
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