I feel like maybe this is too nit-picky, but at 20 lines it might be better to bust out a stringstream here. It'd look like this:

```c++
stringstream msg;
msg << "{"
    "\"method\": \"update_backup\", "
    "\"args\": { " 
         "\"instance_id\": \"" << backup_info.get_instance_id << "\", "
         "\"backup_id\": \"" << backup_info.get_id() << "\", "
         "\"updated\": \"" << now.c_str() << "\", "
         "\"checksum\": \"" << backup_info.get_checksum() << "\", "
         "\"backup_type\": \"" << backup_info.get_backup_type() << "\", "
         "\"location\": \"" << backup_info.get_location() << "\", "
         "\"state\": \"" << state << "\""
    "}"
"}";
sender->send(msg.str().c_str());