We can use REST API for chat messaging, but IMHO, XMPP is a better alternative. Let's consider what XMPP has to offer.
XMPP beside supporting TCP transport also provides HTTP (via polling and binding) and websocket transports. Read XMPP via HTTP and WebSocket transports
It would be interesting to understand pros and cons of each transport from XMPP perspective.
XMPP could use HTTP in two ways: polling[18] and binding.
XMPP over HTTP Polling
The polling
method, now deprecated, essentially implies messages stored on a
server-side database are being fetched (and posted) regularly by an
XMPP client by way of HTTP 'GET' and 'POST' requests.
XMPP over HTTP Binding (BOSH)
The binding method is considered more efficient than the regular HTTP 'GET' and 'POST' requests in Polling method because it reduces latency and bandwidth consumption over other HTTP polling techniques
However, this also poses a disadvantage that sockets remain open for an extended length of time, awaiting the client's next request
The binding method, implemented using Bidirectional-streams Over Synchronous HTTP
(BOSH),[19] allows servers to push messages to clients as soon as they
are sent. This push model of notification is more efficient than
polling, where many of the polls return no new data.
It would be good if we understand how the BOSH technique works.
The technique employed by BOSH, which is sometimes called "HTTP long
polling", reduces latency and bandwidth consumption over other HTTP
polling techniques. When the client sends a request, the connection
manager does not immediately send a response; instead it holds the
request open until it has data to actually send to the client (or an
agreed-to length of inactivity has elapsed). The client then
immediately sends a new request to the connection manager, continuing
the long polling loop.
If the connection manager does not have any data to send to the client
after some agreed-to length of time [12], it sends a response with an
empty . This serves a similar purpose to whitespace keep-alives
or XMPP Ping (XEP-0199) [13]; it helps keep a socket connection active
which prevents some intermediaries (firewalls, proxies, etc) from
silently dropping it, and helps to detect breaks in a reasonable
amount of time.
XMPP over WebSocket binding
XMPP supports WebSocket binding which is a more efficient transport
A perhaps more efficient transport for real-time messaging is
WebSocket, a web technology providing for bi-directional, full-duplex
communications channels over a single TCP connection. XMPP over
WebSocket binding is defined in the IETF proposed standard RFC 7395.
Speaking of the learning curve, yes, you might be tempted to use the REST API, but now there are several resources to learn about Android and XMPP, and XMPP server softwares that you can use to run your own XMPP service, either over the Internet or on a local area network. It would be worth spending this effort before you decide your architecture.
