On chat message, you want to get chat message, session details and visitor details which you can use to keep a repository of chat at your server.
{
"id":"104111",
"thread_id":"-1",
"account_id":"10111",
"session_id":"61111",
"room_id":"9395ad5a....",
"visitor_id":"61111",
"message":"{
"type":"message",
"msg":"sadsdd",
"user":{
"id":15111,
"name":"John",
"type":"agent",
"photo":null,
"state":"online"
}
}",
"recording":"[]",
"chat_wait_time":"null",
"chat_duration":"null",
"translate_to":"en",
"account_translate_language":"",
"translate_form_show":"false",
"date_created":"2018-07-23 05:17:40",
"date_updated":"2018-07-23 05:17:40",
"date_picked":"2018-07-23 05:17:42",
"event":"CHAT_ON_MESSAGE"
}