In the previous études, the client and server have been running in the same shell. In this étude, you will make the server available to clients running in other shells.

To make a node available to other nodes, you need to name the node by using the -name option when starting erl. It looks like this:

michele@localhost $ erl -name serverNode
Erlang R15B02 (erts-5.9.2) [source] [smp:2:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.2  (abort with ^G)
([email protected])1>

This is a long name. You can also set up a node with a short name by using the -sname option:

michele@localhost $ erl -sname serverNode
Erlang R15B02 (erts-5.9.2) [source] [smp:2:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.2  (abort with ^G)
(serverNode@localhost)1>

To connect to a node, use the net_adm:ping/1 function, and give it the name of the server you want to connect to as its argument. If you connect succesfully, the function will return the atom pong; otherwise, it will return pang.

Here is an example. First, start a shell with a (very bad) secret cookie:

michele@localhost $ erl -sname serverNode -setcookie chocolateChip
Erlang R15B02 (erts-5.9.2) [source] [smp:2:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.2  (abort with ^G)
(serverNode@localhost)1>

Now, open another terminal window, start a shell with a different cookie, and try to connect to the server node. I have purposely used a different user name to show that this works too.

steve@localhost $ erl -sname clientNode -setcookie oatmealRaisin
Erlang R15B02 (erts-5.9.2) [source] [smp:2:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.2  (abort with ^G)
(clientNode@localhost)1> net_adm:ping(serverNode@localhost).
pang

The server node will detect this attempt and let you know about it:

=ERROR REPORT==== 28-Feb-2013::22:41:38 ===
** Connection attempt from disallowed node clientNode@localhost **

Quit the client shell, and restart it with a matching cookie, and all will be well.

steve@localhost erltest $ erl -sname clientNode -setcookie chocolateChip
Erlang R15B02 (erts-5.9.2) [source] [smp:2:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.2  (abort with ^G)
(clientNode@localhost)1> net_adm:ping(serverNode@localhost).
pong

To make your weather report server available to other nodes, you need to do these things:

Here is what it looks like when one user starts the server in a shell.

michele@localhost $ erl -sname serverNode -setcookie meteorology
Erlang R15B02 (erts-5.9.2) [source] [smp:2:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.2  (abort with ^G)
(serverNode@localhost)1> weather:start_link().
{ok,<0.39.0>}

And here’s another user in a different shell, calling upon the server.

steve@localhost $ erl -sname clientNode -setcookie meteorology
Erlang R15B02 (erts-5.9.2) [source] [smp:2:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.9.2  (abort with ^G)
(clientNode@localhost)1> weather:connect(serverNode@localhost).
Connected to server.
ok
(clientNode@localhost)2> weather:report("KSJC").
{ok,[{location,"San Jose International Airport, CA"},
     {observation_time_rfc822,"Thu, 28 Feb 2013 21:53:00 -0800"},
     {weather,"Fair"},
     {temperature_string,"52.0 F (11.1 C)"}]}
(clientNode@localhost)3> weather:report("KITH").
{ok,[{location,"Ithaca / Tompkins County, NY"},
     {observation_time_rfc822,"Fri, 01 Mar 2013 00:56:00 -0500"},
     {weather,"Light Snow"},
     {temperature_string,"31.0 F (-0.5 C)"}]}
(clientNode@localhost)4> weather:recent().
ok

Whoa! What happened to the output from that last call? The problem is that the weather:recent/0 call does an io:format/3 call; that output will go to the server shell, since the server is running that code, not the client. Bonus points if you fix this problem by changing weather:recent/0 from using gen_server:cast/2 to use gen_server:call/2 instead to return the recently reported weather stations as its reply.

There’s one more question that went through my mind after I implemented my solution: how did I know that the client was calling the weather code running on the server and not the weather code in its own shell? It was easy to find out: I stopped the server.

(serverNode@localhost)2>
User switch command
 --> q
michele@localhost $

Then I had the client try to get a weather report.

(clientNode@localhost)5> weather:report("KSJC").
** exception exit: {noproc,{gen_server,call,[{global,weather},"KSJC"]}}
     in function  gen_server:call/2 (gen_server.erl, line 180)

The fact that it failed told me that yes, indeed, the client was getting its information from the server.

See a suggested solution in Appendix A.