*This is an alternative to `tut17`.*

This example is an *echo server* to which you can send a `ping` message, and you will get a `pong` message in return.

In this example session we start two Erlang nodes (doesn't have to be on the same machine), one named `es` for "echo
server" and another one named `node2`.

Start and compile/load the code on both nodes:

```
$ erl -sname es
(es@biwia)1> c(echo_server).
```

```
$ erl -sname node2
(node2@biwia)1> c(echo_server).
```

Start the server:

```
(es@biwia)2> echo_server:start().
Starting server, pid=<0.95.0>
true
(es@biwia)3> 
```

The server uses `spawn/3` to run in the background. With the PID that `spawn` returns, it registers an `echo` name
that the client will use. We can look at this process with `regs()`:

```
(es@biwia)3> regs().

** Registered procs on node es@biwia **
Name                  Pid          Initial Call                      Reds Msgs
application_controlle <0.44.0>     erlang:apply/2                     784    0
auth                  <0.60.0>     auth:init/1                        784    0
code_server           <0.50.0>     erlang:apply/2                  129042    0
echo                  <0.95.0>     echo_server:loop/1                   5    0
...
```

On the client side we can now send a ping message with `echo_client:send_ping(es@biwia)`. The `es@biwia` atom names
the *node* on the specific *host*. A host can run many nodes so the node name needs to be included.

```
(node2@biwia)2> echo_client:send_ping(es@biwia).
Sending echo to es@biwia, self=<0.88.0>
got pong from <14715.95.0>
ok
```

On the server you will notice this output:

```
(es@biwia)4> 
<0.95.0>: Got ping req from: <14771.88.0>
(es@biwia)4> 
```

Both the server and client uses `self/0` to find their own PID and you see that the server has the PID `0.95.0` and the
client has `0.85.0`. When the server receives the message the client pid is slightly different: `14771.88.0`, and the
same for the client's reply from the server.

After the server receives 10 messages, it will terminate itself:

```
<0.95.0>: Got ping req from: <14771.88.0>
<0.95.0>: Got ping req from: <14771.88.0>
<0.95.0>: Got ping req from: <14771.88.0>
<0.95.0>: Got ping req from: <14771.88.0>
<0.95.0>: Got ping req from: <14771.88.0>
<0.95.0>: Got ping req from: <14771.88.0>
<0.95.0>: Got ping req from: <14771.88.0>
<0.95.0>: Got ping req from: <14771.88.0>
<0.95.0>: Got ping req from: <14771.88.0>
<0.95.0>: Got ping req from: <14771.88.0>
ping finished
(es@biwia)4> 
```

If the client sends another message, it will time out:

```
(node2@biwia)14> echo_client:send_ping(es@biwia).
Sending echo to es@biwia, self=<0.88.0>
timeout
ok
```