Friday, November 27, 2015

Project time estimates and Murphy's law

As we saw in the last few iterations, Murphy's law: "Anything that can go wrong, will go wrong" tend to be 
very strong resident in our midst.

Let's take a client-server short-project as an example. When you ask the client developer how much time will it take he answers 10 hours of uninterrupted work.
Usually a developer will interact with the world and his team mates (eat-lunch, do code-review, attend a meeting) , so let's count this as 2 days.

Now let's go to Murphy

    1. Complexity in library-code:  You thought the default 1 row of code will do the work for you but it turns out it does not support your use-case, or t fails unexpectadly only in your input/
    2. Complexity in code: changing one element caused an unexpected bug in another unrelated component.
    3. Infrastructure failures:  your machine/network/server- can be down for few hours.
    4. computer/network/coruppted-int server can take 0-5 hours to overrcome
    5. Product requirements may slightly change when they see your finished feature. They don't ask for a new feature, they just explaining you what they ment before better.
UI related code:
    1. UI/Art integration may not be flawless, depending on your workflow
    2. Sometimes, you can't use the Art "as is" and it is not apparent until you actually try.
client-server potential issues:
    1. Miss-understanding of the API usage, can take some time to debug and understand.
    2. Sometimes it is not a miss-understanding, but a plain bug in the server-code.
    3. Sometimes it's not a bug,but a miss-configuration / not latest-version of code etc

So, there are at least 10 potential risks. What should be the estimate in this case?

One approach is to look at the optimistic scenario and say 2 days. It will sometimes work, but a lot of the time will fail poorly.

Second, opposite approach is to look at the worst case scenario and assume all the problems will happen simultaneously. This approach will say let's estimate 8 days.  One might say "better safe than sorry" and if it will be faster, the developer will say he is done.
A third approach is to aim to be correct most of the time (80-90%) but assuming that some problems might happen , but not all of them at the same week. 
Someone, on the upper-management has to fully understand this, and to manage the risk accordingly.
If you actually want numbers.  Let's say this is an example of (a-low) risk-matrix for the 2 days task. No real human think of this in this manner, but you do have "hunches" about it. In this case, I include a 0.1 chance fo r 2 extra-days due to bad-library behavior. 0.2 extra-day if sever-code does not work etc.


The probability of failure for this risks is:
46% - 0 late
71% - up to 1d late
83% - up to 2d late
90% - up to 3d late
94% - up to 4d late
97% - up to 5d late
98.7% up to 6d late
99.1% up to 7d late
99.8% up to 8d late (remember it's on a 2d task....)

Now let's talk about who takes the buffer.  
Let's say a developer gives the matrix to his manager, and ask the manager to tell him how to estimate the task.  What should the manager answer?
Due to Parkinsons's law "work expands so as to fill the time available for its completion", a reasonable manager should do three things.: 
  • Ask the developer to be 80% right, and in this case to add 2d to the estimate : 4d. Tell him that is is ok and fully expected to finish a day early, and they if all hell break loose, the manager got his back with few extra days.
  • Keep a mangment buffer for the next 17% (3 more days) which can be used by the developer, and will be totally ok to use, if the developer explained that the risk actually happened.
    One buffer pool can sometimes be shared amongst multiple team members.
  • Inform his managment about the chances.  If it's a live-or-die scenario, they may have their own buffer on the extra 3% risk.

Sunday, March 8, 2015

Just few random notes on Parse and Mobile FileUpload

FileUpload on mobile

 <input type="file" accept="image/*">
Android (Chrome and 4.4 native) provides a reasonable, but not-pretty, interface.

If you want to use camera, add the capture camera option. Note that it will not allow other images. allows to upload the file and query it by URL. It`s very simple, but note that it means an image access counts as an API call, and as images usually come in packs, this can cost your quota.

Detect change on textarea

Use $('#textAreaId#).on('change',function() { do something here });
change will be called when the focus is changed and any of the inside-text change.

Parse model update

On many views, you will want to first refresh the view from a saved DB element, and then on edit update(re-save) the element and refresh the view again.
For this pattern, always keep a cached parse object which was returned from the DB and is connected to parse via the row-id.
On refresh, update the cached-element, if found in the DB.
On edit either re-save (on the cached one) if it already exists, or save a new one and create cache.

You can save multiple objects via Parse.Object.saveAll (array)

Parse queries
When querying a pointer, sadly, you can't just pass an instance of a previously returned item (item itself != item pointer). You have to create a pointer object. 
var pointerToUser = {  __type: "Pointer",  className: "_User",       objectId: };

You can combine multiple queries using Query.or(q1, q2)

Parse weird save/query results

  • Parse will not fail if you query a db class which do not exist at all. Beware of Parse.Object.extend("SpellingMistakeWillNotBeNoticed");
  • It will also allow you to save db columns which do not exist at all. Beware of (item.set('spellingMistake',666).

Tuesday, February 3, 2015

Tinder-clone in javascript

Tinder-like moving images:

Have a look at the end result here.
And now let`s go step by step:

[Step 1]
  • Check if it actually doing anything: Use requeststAnimationFrame (+big function to make it better)
  • The actual moving of the element is done in js using: = 'translate3d('+newX+'px,'+ newY+'px,5px)';
  • To return it to first position, add style{transition:all 0.3s;}
  • To make sure the image is not dragged by itself:   user-drag: none;  -moz-user-select: none;  -webkit-user-drag: none;
[Step 2]
Lets use a card-style (see here) and move the entire card.

[Step 3]
Add tinder buttons  (V,X) below, add z-index to the .card ( z-index:1;    position: relative; ) so that it will move above them.
Also note that when we move the card down, it causes scrolling (solve by adding overflow:hidden to the main content) , and that the end of the content ends with the end of the buttons instead to extend to the full view. This is solved by a small js, to change main content size according to full size- header height.
Last change is addition of rotation to the transform  rotate( startX-newX)*0.05 + deg

[Step 4]
check the release location on hammer "panend" event, and a check of ev.isFinal and ev.deltaX ev.deltaY.  If it is above a threshold, move it far away.
Also , let`s use multiple cards, so we will use multiple Hammer instances, one for each card. Luckily hammer supports it and hammer events are separated per card.

[Final notes]
On mobile, panning may cause scrolling the window down. To avoid this, add:
        $(document).bind('touchmove', function(e) {

Saturday, December 6, 2014

Searching for close matches to my choices


Every user is asked a serious of binary questions:
What do you prefer, cats or dogs
Do prefer winter or summer
Do you consider computer-science gradatees as scientinst. Yes/No

There will be lots of users (1M), but not more than 300 questions.
Find the 10 closest matches for a certain user.


This problem is a similar to search nearest-neighbors of  Hamming distance. It is simplified as there are only binary(yes/no questions), hamming works with more options, but nearest-neighbors is extremely difficult problem....

1. For a query for unkown, new user, this stackoverflow question says that it`s a tricky problem.  For short distances (4 from 32 bit integers) BK-tree and VP-tree can speed up queries up to 10 times, or even 1000 times for very short distance of 1.   For medium ranges and above, no good data structure exists, and brute-force is needed.

If we can assume that our users always have 10 close matches with short distance, we can use one of the suggested data-structure, otherwise, just use blunt-force each time.

2. For a big calculation of all the matches, there might be a better solution. Didn`t find one.

Tuesday, November 4, 2014

Page pagination in JqueryMobile


  Infinite number of different, similar, pages like Tinder app, where each page contains images,labels and text.
One ajax request to the server contains the data for the next 10 pages, so we won`t do the "simple" solution of reading a new ajax page each time.


We will have 3 pages  (match1, match2, match3) each one as a "next" button pointing to the next one.
We will save the data of the next 10 pages at any given point, let`s call it the model.
On each page pagebeforeshow event, we will change the data on the non visible page.
Once the model data is empty (all the 10 pages were seen), we will use an ajax call to fill the model.

model state= 50,51,52,53,...,59
[page1] [page2] [page3]

on the first time (when all pages are empty) and later, when one page becomes obsolete, we will fill the obsolete pages
  50            51         52
[page1] [page2] [page3]

  53            51         52
[page1] [page2] [page3]


 59            57         58
[page1] [page2] [page3]

when we get to this state, once 58 becomes active, we will want to replace page2 with 60, so we will do an ajax call and update the model with it,
delicate points:  
  • If ajax call is on it`s way, there is no reason to make a new call.
  • We should also clean the previous page to null data (default image and text), in case the user will be faster than our ajax call. We may even consider creating a "loading" progress bar, if the ajax is very slow.


Thursday, September 11, 2014

Facebook user authentication

On the client side, you can call the facebook api to login a user using their popup window. The user will get an very temporary (few hours) access-token which will be saved in a cookie (by the facebook api automatically) and can be used to create API calls.

In this post, we will discuss user-authentication which not quite the same. We want that our server will know that the user is legit.

How it worked before:

Signup: You had the signup with user&password pair (and usually an email for lost password...). The server will save this pair in the database.
Sign-in: Each time you attemp to login, you send the pair to the server and the server replay with good or bad.
Sign-in cookie: As you don`t want the user to re-login in every page, or every day, you can decide to put a cookie which bypass this. An (unscure) cookie can be the user&password, but in this case, if someone browse the cookies on the user-machine, he can see them. It can also be a temporary access-token, for example a unique number which is good for one-day and saved with the user/password pair. Tomorrow, it will be deleted and the user will have to relogin.


Signup: The user already signed-up for facebook years ago. The user will have to permit your application to see his details, on the first login.
: The user will see facebook login.
Sign-in cookie: facebook, behind-the-scenes, saves a temporary access-token so you will not have to, so if you call the user login again after a short time, the user will not see the login dialog.

The user code for using it is quite short, as facebook sdk is doing most of the work, and can be seen at facebook api login samples.

Combine the two: But how can my server know that the user is a legit one?

On the client side, if login was correct, we can send the server the facebook user-id and use it as a token.  This is only half-secure, as anyone with some programming skills, can get the facebook user id and modify our client code to impersonate someone else.
But, it will stop non-programmers/hackers , and can be used for prototyping ONLY stage.

A proper authentication is to send the server the facebook-id and the access-token (which is like user/password pair). The server will ask facebook to verify it (instead of looking into the local database) and then send a temporary cookie, which is the same as the user/password cookie. This will used so the user will not have to login again and again, and that our server will not have to verify the facebook-id/access-token.

And to the implementation

  1. Do it yourself means server code for asking facebook verification and a db table with facebook-user-id to session.
  2. On node.js you can use passport library to do part of the work for you.
  3. Using facebook parse clould-server framework, they will do all of it, including storing the db for you (but it is limiting as you won't have direct access)

Tuesday, September 9, 2014

PhoneGap build

Phonegap build is a great tool for quick testing of your mobile-app as a deployed app. Copy all the relevant client file locally  (including /js /img /css folders). Make sure you have index.html file. create a simple config.xml file.

<?xml version="1.0" encoding="UTF-8"?>
<widget xmlns=""  xmlns:gap="" id=""  versionCode="10" version="1.1.0">
  <!-- versionCode is optional and Android only -->
    MyName application
  <author href="http:/" email="">
    Developer name


Facebook login depends on a domain which you configure at facebook. If your local app is deployed on the phone and the page is not served from the server, it will fail to work.

TODO: solution: load your facebook-login page from the server.

beauty:  to have integration with facebook native app, you will need to work harder and use a plugin, lets ignore for now.