xmpp messages are lost when client connection lost suddently

Question

I am using ejabberd server and ios xmppframework. there are two clients, A and B.

1. When A and B are online, A can send message to B successfu

Answer 1

I guess the reason is that B lost connection suddenly and the server still think B is online. Thus the offline message does work under this condition

Yes you are absolutely correct,this is well known limitation of TCP connections.

There are two approaches to your problem

1 Server side

As I can see you are using ejabbed as XMPP server you can implement mod_ping , Enabling this module will enables server side heartbeat[ping] ,in case of broken connection to server[ejabbed] will try to send heartbeat to connection and will detect connection is lost between server and client. Use of this approach has one drawback,module mod_ping has property called ping_interval which states how often to send heartbeat to connected clients, here lower limit is 32 seconds any value below 32 is ignored by ejabbed,means you have 32 seconds black window in which messages can be lost if user is sowing as online

2 Client side

From client side you can implement Message Delivery Receipts mechanism .With each Chat message send a receipt to receiver user of as soon as receiver user receives message send back this receipt id. This way you can detect that your message is actually delivered to receiver. If you don't receive such acknowledgement between certain time interval you can show user as offline locally(on mobile phone),store any further messages to this user as offline message locally[in SQLLight database ],and wait for offline presence stanza for that user ,as soon as you receive offline presence stanza it means that server has finally detected connection to that user is lost and makes user status as offline ,now you can send all messages to that user ,which will be again stored as offline messages on server.This is best approach to avoid black-window.

Conclusion You can either use Approach 2 and design you client such way ,you can also use Approach 1 along with approach 2 to minimize server broken connection detraction time.