<p>Hi Vinal,</p>
<p>the process that starts the server is linked to it. In your example that is the shell process. The exception due to the pattern mismatch kills your shell and therefore also your linked server.</p>
<p>The "real" way to run a server is in a supervision tree. There it us the supervision process that starts your server and can react to it crashing. (Or if you don't care about error propagation, you could start your server without linking.)</p>
<p>If you need to handle a termination of your server, for example to do some cleanup, then your server needs to "trap exits".</p>
<p>I realise that all of this can be a bit overwhelming when first starting out on the OTP behaviours. Reading and re-reading the behaviour docs ( <a href="http://www.erlang.org/doc/design_principles/users_guide.html">http://www.erlang.org/doc/design_principles/users_guide.html</a>) and playing around with small systems like your example will get you there.</p>
<p>Hth,<br>
Robby</p>
<div class="gmail_quote">On Sep 27, 2015 9:30 AM, "Vimal Kumar" <<a href="mailto:vimal7370@gmail.com">vimal7370@gmail.com</a>> wrote:<br type="attribution"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><font face="arial, helvetica, sans-serif">Hi all,</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">Erlang newbie here. This is my first attempt to write an OTP module myself - a simple bank server. API functions allow users to register, login, deposit/withdraw money to/from their own account. </font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">The bank_server:login/2 function returns a secret key (sort of unique session id, lets assume) which is required for further meaningful transactions (like bank_server:deposit/3 or bank_server:withdraw/3).</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">1> bank_server:start_link().</font></div><div><font face="arial, helvetica, sans-serif">{ok,<0.549.0>}</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">2> whereis(bank_server).</font></div><div><font face="arial, helvetica, sans-serif"><0.549.0></font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">3> bank_server:new_user(user1, "mysecretpass1").</font></div><div><font face="arial, helvetica, sans-serif">{ok}</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">4> {_, SecretKey1} = bank_server:login(user1, "mysecretpass1").</font></div><div><font face="arial, helvetica, sans-serif">{ok,"lYWrBFIi7CoeYN8KSK3QgA=="}</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">5> bank_server:deposit(user1, SecretKey1, 10.00). </font></div><div><font face="arial, helvetica, sans-serif">{ok,success}</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">6> bank_server:check_balance(user1, SecretKey1). </font></div><div><font face="arial, helvetica, sans-serif">{ok,10.0}</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">7> bank_server:new_user(user2, "mysecretpass2"). </font></div><div><font face="arial, helvetica, sans-serif">{ok}</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">8> {_, SecretKey1} = bank_server:login(user2, "mysecretpass2").</font></div><div><font face="arial, helvetica, sans-serif">** exception error: no match of right hand side value {ok, "vYaWQbNQvROqXybVf424lQ=="}</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">9> whereis(bank_server).</font></div><div><font face="arial, helvetica, sans-serif">undefined</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">In line 8, I understand that I should have used a different variable, since SecretKey1 already has a value. But I am curious why a simple mistake *** from client side *** crashed the server itself? Is there any way to avoid it?</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">I am yet to learn OTP supervisors, but I believe it might have restarted the server without any delay in such a crash. </font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">The complete code for bank_server.erl is available at <a href="http://pastebin.com/4bMWjNG7" target="_blank">http://pastebin.com/4bMWjNG7</a> to check.</font></div><div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">Thank you!</font></div></div>
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