I am having a community portal on one end and a mobile app on second to send and receive feeds(and its comments) on specific cases. So if a user post a feed from portal, how can I get the feed on my mobile app in real time(like as we chat on whatsapp or hangout)
Streaming API I can't use as it doesn't work for community user.
Other than streaming API I think you can consider the option of event based notifications called platform events. You can publish the platform events into enterprise event bus from salesforce object triggers and subscribe to the same from external applications or from mobile apps. Check for salesforce platform events documentation and try to figure out if that will suit your scenario.
