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Readme.html
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<!DOCTYPE html>
<html>
<head>
<title>Readme.md</title>
<meta http-equiv="Content-type" content="text/html;charset=UTF-8">
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<div class="content-wrapper">
<nav class="table-of-contents"></nav><p><img src="image/Readme/1664886254246.png" alt="1664886254246"></p>
<p>In this tutorial i will show you how to build an Erlang Web Api using Nova Framework and Redis as a data store.</p>
<p><img src="image/Readme/1664883753188.png" alt="1664883753188"></p>
<p>This will be a simple web api supporting the following operations over a group of users.</p>
<ul>
<li>Create user</li>
<li>Update user</li>
<li>Delete user</li>
<li>Get user by id</li>
<li>Get all users</li>
</ul>
<p><strong>The repository containing the code as well as the tutorial can be found</strong> <a href="https://github.com/sanzor/Nova_Api">here</a></p>
<hr>
<p>For those of you that have already installed the prerequisites which are : Erlang , Rebar,Nova **you can skip this part:</p>
<p><strong>## Setup</strong></p>
<ol>
<li>Install Redis on your computer and start the redis server using the command <code>redis-server</code></li>
<li>Installing Erlang: <a href="https://adoptingerlang.org/docs/development/setup/">Setup | Adopting Erlang</a></li>
<li>Installing Rebar3 <a href="http://rebar3.org/docs/getting-started/">Getting Started | Rebar3</a></li>
<li>Install Nova Framework using this script:<pre class='hljs'><code><div>sh -c <span class="hljs-string">"<span class="hljs-subst">$(sh -c <span class="hljs-string">"<span class="hljs-subst">$(curl -fsSL https://raw.githubusercontent.com/novaframework/
rebar3_nova/master/install.sh)</span>"</span>)</span>"</span>
</div></code></pre>
</li>
</ol>
<p><strong>Create a new project in the terminal :</strong></p>
<p>Run the following command:</p>
<pre class='hljs'><code><div>rebar3 new nova fcourse
</div></code></pre>
<p>This tells rebar to create a new project named <code>fcourse</code> using the <code>nova</code> template.</p>
<p>First thing we are going to edit is the <code>rebar.config</code> file by adding the redis client library dependency like below:</p>
<pre class='hljs'><code><div><span class="hljs-comment"># rebar.config</span>
{deps, [
nova,
{flatlog, <span class="hljs-string">"0.1.2"</span>},
{eredis,{git,<span class="hljs-string">"https://github.com/wooga/eredis.git"</span>,{branch,<span class="hljs-string">"master"</span>}}}
]}.
</div></code></pre>
<p>Next</p>
<p>Add the <code>eredis</code> depedency in the <code>src/fcourse.app.src</code> file :</p>
<pre class='hljs'><code><div>{application, fcourse,
[{description, "fcourse managed by Nova"},
{vsn, "0.1.0"},
{registered, []},
{mod, { fcourse_app, []}},
{included_applications, []},
{applications,
[
kernel,
stdlib,
nova,
eredis
]},
{env,[]},
{modules, []},
{maintainers, []},
{licenses, ["Apache 2.0"]},
{links, []}
]}.
</div></code></pre>
<p><strong>dev_sys.config.src</strong></p>
<p>Edit your <code>config/dev_sys.config.src</code> file in the <code>plugins:pre_request </code>so that you enable the below options:</p>
<p><img src="image/Readme/1665083264209.png" alt="1665083264209"></p>
<p>In production nova will use the <code>prod_sys.config.src </code>so you will need to edit that file too like above.</p>
<hr>
<p>Defining the routes:</p>
<p>In the router module the routes are defined as follows: <code>{Path, {Controller_Name,Controller_Method}, #{methods =>[options, HTTPMethod]}</code></p>
<p>In the <code>fcourse_router.erl</code> which contains the routing logic add the specific CRUD methods like below:</p>
<pre class='hljs'><code><div><span class="hljs-comment"># fcourse_router.erl</span>
-module(fcourse_router).
-behaviour(nova_router).
-<span class="hljs-built_in">export</span>([
routes/1
]).
%% The Environment-variable is defined <span class="hljs-keyword">in</span> your sys.config <span class="hljs-keyword">in</span> {nova, [{environment, Value}]}
routes(_Environment) ->
[<span class="hljs-comment">#{prefix => "/users", # you cand add a prefix to your routes</span>
security => <span class="hljs-literal">false</span>,
routes => [
{<span class="hljs-string">"/"</span>, { fcourse_main_controller, index}, <span class="hljs-comment">#{methods => [options,get]}},</span>
{<span class="hljs-string">"/add"</span>,{fcourse_main_controller,add},<span class="hljs-comment">#{methods=>[post]}},</span>
{<span class="hljs-string">"/update"</span>,{fcourse_main_controller,add},<span class="hljs-comment">#{methods=>[update]}},</span>
{<span class="hljs-string">"/delete"</span>,{fcourse_main_controller,delete},<span class="hljs-comment">#{methods=>[delete]}},</span>
{<span class="hljs-string">"/get"</span>,{fcourse_main_controller,get},<span class="hljs-comment">#{methods=>[get]}},</span>
{<span class="hljs-string">"/getall"</span>,{fcourse_main_controller,getall},<span class="hljs-comment">#{methods=>[get]}},</span>
{<span class="hljs-string">"/assets/[...]"</span>, <span class="hljs-string">"assets"</span>}
]
}].
</div></code></pre>
<p><strong># Implementing the CRUD endpoints:</strong></p>
<p>We will write the logic for the CRUD endpoints in the controller which is <code>controllers/fcourse_main_controller.erl</code> file.</p>
<p>Adding the endpoint methods definition:</p>
<pre class='hljs'><code><div>-export([get/1,
<span class="hljs-code"> getall/1,
add/1,
delete/1,
update/1]).
</span>
</div></code></pre>
<p>We start by exporting the CRUD methods in our controller module. Each method has an arity of <code>1</code> that means it receives only one argument.</p>
<p>We then start implementing all the above methods:</p>
<p><strong>ENDPOINT: Add</strong></p>
<pre class='hljs'><code><div>add(<span class="hljs-comment">#{json := #{<<"id">> := Id , <<"age">> := Age}})-></span>
try
{ok,Port}=eredis:start_link(),
{ok,Result}=eredis:q(Port,[<span class="hljs-string">"hset"</span>,<span class="hljs-string">"users"</span>|[Id,Age]]),
{json,200,<span class="hljs-comment">#{},#{<<"result">> => Result}} </span>
catch
Error:Cause -> {json,500,<span class="hljs-comment">#{<<"Content-Type">> => <<"json">>},#{<<"error">> =>Error , <<"cause">> => Cause}}</span>
end.
</div></code></pre>
<p>The argument is a map that holds a key <code>json</code>. The <code>json</code> key contains a json like the one below:</p>
<pre class='hljs'><code><div>{
<span class="hljs-string">"id"</span>: SomeValue,
<span class="hljs-string">"age"</span>: SomeOtherValue
}
</div></code></pre>
<p>We are deconstructing the input argument and binding the values of the json like <code>id</code> and the <code>age</code> to variables (<code>Id</code> and <code>Age</code>) . We then use the bound variables in our logic:</p>
<pre class='hljs'><code><div> {ok,Port}=eredis:start_link(),
{ok,Result}=eredis:q(Port,[<span class="hljs-string">"hset"</span>,<span class="hljs-string">"users"</span>|[Id,Age]]),
{json,200,<span class="hljs-comment">#{},#{<<"result">> => Result}}</span>
</div></code></pre>
<ul>
<li>We are starting a connection to redis that will be stored in the <code>Port</code> variable.</li>
<li>We then issue the redis <code>HSET</code> command , using the <code>Port</code> as the connection , <code>users</code> as the hash and <code>[Id,Age]</code> as the Key-Value.</li>
<li>We then return a json , a status code <code>200</code> , and the json of the form:</li>
</ul>
<pre class='hljs'><code><div>{
<span class="hljs-string">"result"</span>:Result
}
</div></code></pre>
<p>So this is how we add items !</p>
<hr>
<p><strong>ENDPOINT: Get by Id</strong></p>
<p>In the same file which is <code>fcourse_main_controller</code> define a new endpoint for fetching users by id</p>
<pre class='hljs'><code><div>get(<span class="hljs-comment">#{ bindings := #{<<"user">> := UserId}})-></span>
try
{ok,Port}=eredis:start_link(),
<span class="hljs-keyword">case</span> eredis:q(Port,[<span class="hljs-string">"hget"</span>,<span class="hljs-string">"users"</span>,UserId]) of
{ok,Result} ->{json,200,<span class="hljs-comment">#{},#{<<"UserId">> => list_to_binary(UserId) , <<"value">> => Result}};</span>
_ -> {status,404}
end
catch
Error:Cause -> {json,500,<span class="hljs-comment">#{},#{<<"error">> =>Error , <<"cause">> => Cause}}</span>
end.
</div></code></pre>
<p>Instead of a <code>json</code> we receive a <code>query string</code> , for example <code>/users/get?user=13</code></p>
<p>We could add other variables in the query string separated by comma e.g:</p>
<p><code>#{ bindings := #{<<"user">> := UserId , <<"age">> := Age}}</code></p>
<p>This would translate to : <code>users/get?user=UserId&age=Age</code></p>
<ul>
<li>we start a connection to redis</li>
<li>we use the redis command <code>HGET</code>, which fetches the key <code>UserId</code> from the hash <code>users</code> and treat its result with a <code>case</code> clause specific to erlang</li>
</ul>
<pre class='hljs'><code><div><span class="hljs-keyword">case</span> eredis:q(Port,[<span class="hljs-string">"hget"</span>,<span class="hljs-string">"users"</span>,UserId]) of
{ok,Result} ->{json,200,<span class="hljs-comment">#{},#{<<"UserId">> => list_to_binary(UserId) , <<"value">> => Result}};</span>
_ -> {status,404}
end
</div></code></pre>
<ul>
<li>If result of eredis <code>HGET</code> command is of the form <code>{ok,Result}</code> we return a json with the statuscode <code>200</code> and the json <code>{ "UserId": UserId , "value" : Result }</code> , and we also use <code>list_to_binary </code> to transform the value in a binary .</li>
<li>If the result is anything else (<code>_</code> means wildcard , we don't care) ,we return a status code of <code>404</code></li>
</ul>
<p>Everything is in a <code>try-catch </code> clause in case connection to redis fails in which case we can pattern match on the <code>Error:Cause</code> and return a json with the status code <code>500</code> and the said <code>Error,Cause</code></p>
<hr>
<p><strong>ENDPOINT: Get All</strong></p>
<pre class='hljs'><code><div>getall(_Request)->
try
{ok,Port}=eredis:start_link(),
{ok,Result}=eredis:q(Port,["hgetall","users"]),
io:format("List: ~p",[Result]),
TupleList=split(Result),
io:format("Formatted : ~p",[TupleList]),
{json,200,#{},#{<<"users">> => TupleList}}
catch
Error:Cause -> {json,500,#{},#{ <<"error">> => Error , <<"cause">> => Cause}}
end.
</div></code></pre>
<p>We run the redis command <code>HGETALL</code> on the <code>users</code> key, which is a hashset and we receive a result of the form:</p>
<pre class='hljs'><code><div> [Key1,Value1,Key2,Value2.....]
</div></code></pre>
<p>We want to return a list of key values so we will use these two helper methods:</p>
<p>Helper methods:</p>
<pre class='hljs'><code><div>
<span class="hljs-comment"># first method</span>
<span class="hljs-comment"># checks if argument is list and then if the list is odd , nr of keys has to be equal to those of values </span>
split(List) when is_list(List) ->
<span class="hljs-keyword">case</span> length(List) rem 2 of
0 -> split(List,[]); <span class="hljs-comment">#calls the second method</span>
1 -> throw(odd_list)
end.
<span class="hljs-comment">#second method</span>
split([],Accu)->Accu;
split([Key,Value|Rest],Accu)->split(Rest,[<span class="hljs-comment">#{Key=> Value}|Accu]).</span>
</div></code></pre>
<p>The second method is a tailrecursive one.</p>
<p><code>split([],Accu)->Accu;</code></p>
<p>The first clause is the stop condition, when the first argument is the <code>[]</code> which means an empty list,therefore, we return the second argument , the accumulator (<code>Accu</code>).</p>
<p><code>split([Key,Value|Rest],Accu)->split(Rest,[#{Key=> Value}|Accu]).</code></p>
<p>The second clause decomposes the first argument in <code>[Key,Value | Rest]</code> basically extracting 2 elements at a time from the original list , and calling itself again with <code>Rest</code> as the new starting list , and the map <code>#{Key => Value}</code> appended on top of the <code>Accumulator</code>.</p>
<hr>
<p><strong>ENDPOINT: Delete</strong></p>
<pre class='hljs'><code><div>delete(#{bindings := #{<<"id">> :=Id}})->
try
{ok,Port}=eredis:start_link(),
{ok,_}=eredis:q(Port,["hdel","users",Id]),
{status,200}
catch
Error:Clause ->{json,500,#{},#{<<"error">> => Error ,<<"cause">> => Clause}}
end.
</div></code></pre>
<p>Nothing special here , we again use <code>bindings</code> , meaning we get a query string and we want to extract the <code>id</code> of the record <code>Id</code> which we want to delete, eg: <code>/users/delete?id=33</code></p>
<p>We start connection to redis , and then issue the redis command <code>HDEL</code> which deletes the key <code>Id</code> from the hash <code>users</code>.</p>
<p>When exception we return status <code>500</code> and the json <code>{ "error": Error ,"cause": Cause}</code></p>
<hr>
<p><strong>ENDPOINT: Update</strong></p>
<p>In this endpoint we just want to update the <code>age</code> of the user.</p>
<pre class='hljs'><code><div>update(#{json := #{<<"user">> := User , <<"new<span class="hljs-emphasis">_age">> := NewAge}})->
try
{ok,Port}=eredis:start_</span>link(),
<span class="hljs-code"> case eredis:q(Port,["hget","users",User]) of
{ok,OldAge} -> {ok,_}=eredis:q(Port,["hset","users"|[User,NewAge]]),
{status,200};
_ ->{status,404}
end
catch
Error:Clause -> {status,500,#{},#{<<"error">> => Error ,<<"clause">> =>Clause}}
end.
</span>
</div></code></pre>
<p>We the <code>HGET</code> redis command like we did in the get-by-id endpoint.</p>
<p>If redis returns us the result <code>{ok,OldAge} </code>we then set the value of the key <code>User </code>within the <code>users </code>hash to value <code>Age </code>, and return status code <code>200</code>.</p>
<p>Otherwise we return the status code <code>500</code> with the <code>{ "error": Error ,"cause": Cause}</code> json.</p>
<hr>
<p><strong>Putting it all togeter in the controller module:</strong></p>
<p><strong>fcourse_main_controller</strong></p>
<pre class='hljs'><code><div>-module(fcourse<span class="hljs-emphasis">_main_</span>controller).
-export([get/1,
<span class="hljs-code"> getall/1,
add/1,
delete/1,
update/1]).
</span>
split(List) when is<span class="hljs-emphasis">_list(List) ->
case length(List) rem 2 of
0 -> split(List,[]);
1 -> throw(odd_</span>list)
end.
split([],Accu)->Accu;
split([Key,Value|Rest],Accu)->split(Rest,[#{Key => Value}|Accu]).
get(#{ bindings := #{<<"user">> := UserId}})->
<span class="hljs-code"> try
{ok,Port}=eredis:start_link(),
case eredis:q(Port,["hget","users",UserId]) of
{ok,Result} ->{json,200,#{},#{<<"UserId">> => list_to_binary(UserId) , <<"value">> => Result}};
_ -> {status,404}
end
catch
Error:Cause -> {json,500,#{<<"Authorization">> => <<"Basic 1212121">>, <<"Content-Type">> => <<"json">>},#{<<"error">> =>Error , <<"cause">> => Cause}}
end.
</span>
getall(<span class="hljs-emphasis">_Request)->
try
{ok,Port}=eredis:start_</span>link(),
<span class="hljs-code"> {ok,Result}=eredis:q(Port,["hgetall","users"]),
io:format("List: ~p",[Result]),
TupleList=split(Result),
io:format("Formatted : ~p",[TupleList]),
{json,200,#{},#{<<"users">> => TupleList}}
catch
Error:Cause -> {json,500,#{},#{ <<"error">> => Error , <<"cause">> => Cause}}
end.
add(#{json := #{<<"id">> := Id , <<"age">> := Age}})->
try
{ok,Port}=eredis:start_link(),
{ok,Result}=eredis:q(Port,["hset","users"|[Id,Age]]),
{json,200,#{},#{<<"result">> => Result}}
catch
Error:Cause -> {json,500,#{<<"Content-Type">> => <<"json">>},#{<<"error">> =>Error , <<"cause">> => Cause}}
end.
</span>
delete(#{bindings := #{<<"id">> :=Id}})->
<span class="hljs-code"> try
{ok,Port}=eredis:start_link(),
{ok,_}=eredis:q(Port,["hdel","users",Id]),
{status,200}
catch
Error:Clause ->{json,500,#{},#{<<"error">> => Error ,<<"cause">> => Clause}}
end.
</span>
update(#{json := #{<<"user">> := User , <<"new<span class="hljs-emphasis">_age">> := NewAge}})->
try
{ok,Port}=eredis:start_</span>link(),
<span class="hljs-code"> case eredis:q(Port,["hget","users",User]) of
{ok,OldAge} -> {ok,_}=eredis:q(Port,["hset","users"|[User,NewAge]]),
{status,200};
_ ->{status,404}
end
catch
Error:Clause -> {status,500,#{},#{<<"error">> => Error ,<<"clause">> =>Clause}}
end.
</span>
</div></code></pre>
<p><strong>Testing it:</strong></p>
<p>In order for this to work you need to have <code>redis-server</code> installed on your computer and run the command <code>redis-server</code> in order to start the server to accept commands from our Nova API.</p>
<p><strong>Run the application</strong></p>
<p>From the root folder of the application run the command : <code>rebar3 nova serve</code> , wait till all applications are booted ( wait till you get the blow output in the terminal ) :</p>
<p><img src="image/Readme/1664566537315.png" alt=""><img src="image/Readme/1664565644564.png" alt=""></p>
<p>Once the application is built , we have finished ! Voila !</p>
<p>The application can be tested via postman like below:</p>
<p><strong>Adding a user</strong></p>
<p><img src="image/Readme/1665003590129.png" alt="1665003590129"></p>
<p><strong>Fetching a user</strong></p>
<p><img src="image/Readme/1665003790077.png" alt="1665003790077"></p>
<p>A step by step video implementation will also follow soon, stay tuned !</p>
</div>
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