screen recording in mac using QuickTime player

WiFi 101

wifi 101

• The WiFi radio waves are very similar to the radios used for cell phones and other devices.
  They can convert 1s and 0s into radio waves and convert the radio waves back into 1s and 0s.
• Wifi can transmit from 2.4 GHz or 5 GHz
• 5G wifi (802.11ac) is the newest standard as of early 2013.

wifi radio waves RSSI

/System/Library/PrivateFrameworks/Apple*.framework/Versions/Current/Resources/airport -I
     agrCtlRSSI: -44
     agrExtRSSI: 0
    agrCtlNoise: -92
    agrExtNoise: 0
          state: running
        op mode: station 
     lastTxRate: 1170
        maxRate: 1300
lastAssocStatus: 0
    802.11 auth: open
      link auth: wpa2-psk
          BSSID: _:_:_:_:_:_
           SSID: progessive_energy_5g
            MCS: 9
        channel: 153,80

Closer I move to router better signal strength I receive.

jshell> var signalToNoiseRatio = -44 - (-92)
signalToNoiseRatio ==> 48

Higher SNR margin values mean clearer signals.

RSSI table

RSSI	desc
-30 dBm	Maximum signal strength (NEAR)
-50 dBm	excellent signal strength
-60 dBm	reliable signal strength
-67 dBm	reliable signal strength
-70 dBm	Not a strong signal. Light browsing and email.
-80 dBm	Unreliable signal strength, will not suffice for most services. Connecting to the network.
-90 dBm	The chances of even connecting are very low at this level.

hashmap datastructure with php REPL

php > $user_cache=array("uid1"  => "1", "uid2" => "2");

php > echo $user_cache["uid1"];
1

php > if(!$user_cache["uid3"]) { echo "cache miss";}
PHP Notice:  Undefined index: uid3 in php shell code on line 1

Notice: Undefined index: uid3 in php shell code on line 1
cache miss

java async task

.map

jshell> var t1 = CompletableFuture.completedFuture(100)
t1 ==> java.util.concurrent.CompletableFuture@25f38edc[Completed normally]

jshell> var t2 = t1.thenApply(x -> x * 2)
t2 ==> java.util.concurrent.CompletableFuture@1a86f2f1[Completed normally]

jshell> t2.get()
$14 ==> 200

.fmap

jshell> var t1 = CompletableFuture.completedFuture(100)
t1 ==> java.util.concurrent.CompletableFuture@506c589e[Completed normally]

jshell> var t2 = CompletableFuture.completedFuture(200)
t2 ==> java.util.concurrent.CompletableFuture@69d0a921[Completed normally]

jshell> var result = t1.thenCompose($ -> t2)
result ==> java.util.concurrent.CompletableFuture@7aec35a[Completed normally]

jshell> result.get()
$19 ==> 200

javascript equivalent

> var t1 = Promise.resolve(100)
undefined
> var t2 = t1.then(x => x * 2)
undefined
> t2
Promise {
  200,
  domain: 
   Domain {
     domain: null,
     _events: { error: [Function: debugDomainError] },
     _eventsCount: 1,
     _maxListeners: undefined,
     members: [] } }

Extracting URL components in Java

URL, Uniform Resource Locator is composed of
1) protocol
2) domain
3) path
4) query string
4) reference/ anchor

example:

[https]://[www.introverse.com]/[abc/def]?[id=1&id=2]#[index1]

scala> import java.net.URL
scala> val url = new URL("https://www.introverse.com/abc/def?id=1&id=2#index1")
url: java.net.URL = https://www.introverse.com/abc/def?id=1&id=2#index1

scala> url.getProtocol
res1: String = https

scala> url.getHost
res2: String = www.introverse.com

scala> url.getPath
res3: String = /abc/def

scala> url.getQuery
res4: String = id=1&id=2

scala> url.getRef
res5: String = index1

oracle create Entity Relation Diagram(ERD) from existing SQL DDL

I have a existing DDL for my application but I wanted to see the ERD for that. Oracle SQL developer provides Data modeler feature to generate ERD from existing DDL as shown in following screenshot
(import as Data Dictionary)

nodejs - call REST api with url encoded parameters

I have a REST API that expects url encoded request params as POST request body. So, API schema looks like

POST /login HTTP/1.1
Host: intro-api.com
Accept: */*
Content-Type: application/x-www-form-urlencoded
Content-Length: xxx

And, If you use curl request would look like,

curl -v --request POST -H "Content-Type: application/x-www-form-urlencoded" intro-api.com/login -d 'username=admin' -d 'password=admin'

Also, since content-type is urlencoded type, request params are in query string format. So -d "username=admin&password=admin" is valid as well.

Now,

I have a frontend(html/js) served by nodejs server which would have to validate user login by calling above REST api. This is what I'm talking in this article.

So, first of all I tried standard nodejs http library as a http client. But could not make it work for url encoded params. I was getting 408 error, which means it never sent the params?

function getUserSession(username, password) {

  const postData = qs.stringify({
    'username': username,
    'password': password
  });

  var options = {
    protocol: 'https:',
    host: 'intro.com',
    port: 443,
    path: '/login',
    method: 'POST',
    headers: {
      'Content-Type'  : 'application/x-www-form-urlencoded',
      'Content-Length': Buffer.byteLength(postData)
    }
  };

  https.request(options, function(resp){
    var response = '';
    resp.on('data', function(chunk){
      response += chunk;
      console.log(response)
    });
  }).on("error", function(e){
    console.log("Got error: " + e.message);
  });

}

So I ended up using request library. which needs to added to package.json (can also be done with npm install request, for more see official documentation).

Next, I will create auth.js with with a function that expects username and password and call the REST api, and respond the cookie value.

var request = require('request');

function getUserSessionId(username, password) {
  console.log("making login request")

  request.post({
    url: 'https://intro.com/login',
    form: {
      username: username,
      password: password
    }
  }).on("response", function(httpResponse) {
        var session = httpResponse.headers['set-cookie']
        console.log("session: " + session)
        return session
  });

}

var token = getUserSessionId('admin', 'admin')

Now, lets run it on terminal using node runtime (without having run it on http)

node auth.js

That is it to call "a REST api with url encoded params" using nodejs.