Tuesday, September 30, 2014

Overview of Netflix architecture

In order to observe the basic service behavior, Lets create a new user account, login into the Netflix website and play a movie. Start monitoring the traffic during all of this activity and record the host names of the servers involved in the process and then perform DNS resolutions to collect the canonical names (CNAMEs) and IP addresses of all the server names that the browser have contacted. We also perform WHOIS lookups for the IP addresses to find out their owners. Table
I summarizes the most relevant hostnames and their owners. Fig. shows the basic architecture for Netflix video streaming platform.


It consists of four key components: Netflix data center, Amazon cloud, CDNs and players for Netflix

Data Centers. Our analysis reveals that Netflix uses its own IP address space for the hostname
www.netflix.com. This server primarily handles two key functions: (a) registration of new user accounts and capture of payment information (credit card or Paypal account), and (b) redirect users to movies.netflix.com or signup.netflix.com based on whether the user is logged in or not respectively. This server does not interact with the client during the movie playback, which is consistent with the
recent presentation from Netflix team [9]

Amazon Cloud. Except for www.netflix.com which is hosted by Netflix, most of the other Netflix
servers such as agmoviecontrol.netflix.com and movies.netflix.com are served off the Amazon
cloud [10]. [9] indicates that Netflix uses various Amazon cloud services, ranging from EC2 and S3, to SDB and VPC. Key functions, such as content ingestion, log recording/analysis, DRM, CDN routing, user sign-in, and mobile device support, are all done in Amazon cloud.

Content Distribution Networks (CDNs). Netflix employs multiple CDNs to deliver the video content to end users. The encoded and DRM protected videos are sourced in Amazon
cloud and copied to CDNs. Netflix employs three CDNs: Akamai, LimeLight, and Level-3. For the same video with the  same quality level, the same encoded content is delivered from all three CDNs. In Section II-D we study the Netflix strategy used to select these CDNs to serve videos.

Netflix Players. Netflix uses Silverlight to download, decode and play Netflix movies on desktop web browsers. The run-time environment for Silverlight is available as a plug-in for most web browsers. There are also players for mobile phones and other devices such as Wii, Roku, etc.

Netflix uses the DASH (Dynamic Streaming over HTTP) protocol for streaming. In DASH, each video is encoded at several different quality levels, and is divided into small ‘chunks’- video segments of no more than a few seconds in length. The client requests one video chunk at a time via HTTP. With each download, it measures the received bandwidth and runs a rate
determination algorithm to determine the quality of the next chunk to request. DASH allows the player to freely switch between different quality levels at the chunk boundaries.

Ref: Unreeling Netflix: Understanding and Improving Multi-CDN Movie Delivery IEEE paper

Wednesday, September 17, 2014

libde265/HEVC Supported features

P slicesyesyesyes
B slicesyesyesyes
weighted pred.yesyesyes
adaptive quant.incompleteyesyes
multiple slicesnoincompleteyes
dependent slicesnoincompleteyes
scaling listsnoyesyes
long-term MCnoincompleteyes
ref.pic.list modificationnoyesyes
chroma 4:2:2nonono
10 bitnonono
parallel WPPyesyesyes
parallel tilesyesyesyes
parallel framesnonono
SSE accelerationyesyesyes
ARM accelerationnonono
frame-dropping APInonoincomplete(1)
non-conformant speed hacksnonoyes(2)

incomplete = may work for some streams
(1) - streams with multiple temporal sub-streams only
(2) - deblocking and sao can be switched off to increase decoding speed

Friday, September 5, 2014

Writing Great Unit Tests: Why Bother?

Writing Great Unit Tests: Why Bother?
Unit testing is the practice of testing the components of a program automatically, using a test program to provide inputs to each component and check the outputs. The tests are usually written by the same programmers as the software being tested, either before or at the same time as the rest of the software. What’s the difference between a good unit test and a bad one? How do you learn how to write good unit tests? It’s far from obvious. Even if you’re a brilliant coder with decades of experience, your existing knowledge and habits won’t automatically lead you to write good unit tests, because it’s a different kind of coding and most people start with unhelpful false assumptions about what unit tests are supposed to achieve. It’s overwhelmingly easy to write bad unit tests that add very little value to a project while inflating the cost of code changes astronomically. Does that sound agile to you?
Unit testing is not about finding Defects
Unit tests, by definition, examine each unit of your code separately. But when your application is run for real, all those units have to work together, and the whole is more complex and subtle than the sum of its independently-tested parts. At a high-level, unit testing refers to the practice of testing certain functions and areas – or units – of our code. This gives us the ability to verify that our functions work as expected. That is to say that for any function and given a set of inputs, we can determine if the function is returning the proper values and will gracefully handle failures during the course of execution should invalid input be provided.
Ultimately, this helps us to identify failures in our algorithms and/or logic to help improve the quality of the code that composes a certain function. As you begin to write more and more tests, you end up creating a suite of tests that you can run at any time during development to continually verify the quality of your work.
A second advantage to approaching development from a unit testing perspective is that you'll likely be writing code that is easy to test. Since unit testing requires that your code be easily testable, it means that your code must support this particular type of evaluation. As such, you're more likely to have a higher number of smaller, more focused functions that provide a single operation on a set of data rather than large functions performing a number of different operations.
A third advantage for writing solid unit tests and well-tested code is that you can prevent future changes from breaking functionality. Since you're testing your code as you introduce your functionality, you're going to begin developing a suite of test cases that can be run each time you work on your logic. When a failure happens, you know that you have something to address.
Well then, if unit testing isn’t about finding bugs, what is it about?
I bet you’ve heard the answer a hundred times already, but since the testing misconception stubbornly hangs on in developers’ minds, per principle. As Test plan development forum keep saying, “TDD is a design process, not a testing process”. Let me elaborate: TDD is a robust way of designing software components (“units”) interactively so that their behavior is specified through unit tests. That’s all!
Tips for writing great unit tests
Enough theoretical discussion – time for some practical advice. Here’s some guidance for writing unit tests that sit comfortably at Sweet Spot on the preceding scale, and are virtuous in other ways too.
·         Make each test orthogonal (i.e., independent) to all the others
Any given behavior should be specified in one and only one test. Otherwise if you later change that behavior, you’ll have to change multiple tests. The corollaries of this rule include
·         Test passes but not testing the actual feature
Beware of such test cases making place in your code repository which seems to doing lots of stuff in code, but in actual they were doing nothing. They were sending requests to server and no matter what server respond, they were passing. Horror!! They will become liability on you to carry them, build them and running them every time but without adding any value.
·         Testing irrelevant things
This one is another sign of bad test case. I have seen developers checking multiple irrelevant things so that code passes with doing SOMETHING, well not necessarily the correct thing. Best approach is to follow single responsibility principle, which says, one unit test case should test only one thing and that’s all.
·         Don’t make unnecessary assertions
Which specific behavior are you testing? It’s counterproductive to Assert () anything that’s also asserted by another test: it just increases the frequency of pointless failures without improving unit test coverage one bit. This also applies to unnecessary Verify () calls – if it isn’t the core behavior under test, then stop making observations about it! Sometimes, Testing folks express this by saying “have only one logical assertion per test”. Remember, unit tests are a design specification of how a certain behavior should work, not a list of observations of everything the code happens to do.
·         Don't ship code with tests that fail, even if it doesn't matter that they fail.
It's not uncommon, particularly in test-driven development, to change your mind during design about which tests are correct or relevant, or to make an initial implementation that only covers some of the test suite. But that means you end up with failed tests that you don't actually care about. Remove them, or at very least, document them: anyone running your tests should be able to assume that a failed test indicates broken code.
·         Consider using a Code coverage tool to check how much of your code is actually being tested. Coverage doesn't tell you everything: it only measures static execution paths, but it can give you some idea of things you might have missed altogether.
·         Whenever you find a bug in “finished code”, add a test for it.
Make sure the test fails in the buggy code and passes when it is fixed. Areas of code you've found bugs in are more likely to be fragile in general, and bugs that have already been found are relatively highly likely to reappear.
·         Test only one code unit at a time
Your architecture must support testing units (i.e., classes or very small groups of classes) independently, not all chained together. Otherwise, you have lots of overlap between tests, so changes to one unit can cascade outwards and cause failures everywhere.
If you can’t do this, then your architecture is limiting your work’s quality – consider using Inversion of Control.
·         Separate out all external services and state
Otherwise, behavior in those external services overlaps multiple tests, and state data means that different unit tests can influence each other’s outcome. You’ve definitely taken a wrong turn if you have to run your tests in a specific order, or if they only work when your database or network connection is active. (By the way, sometimes your architecture might mean your code touches static variables during unit tests. Avoid this if you can, but if you can’t, at least make sure each test resets the relevant statics to a known state before it runs.)
·         Avoid unnecessary preconditions
Avoid having common setup code that runs at the beginning of lots of unrelated tests. Otherwise, it’s unclear what assumptions each test relies on, and indicates that you’re not testing just a single unit. An exception: Sometimes I find it useful to have a common setup method shared by a very small number of unit tests (a handful at the most) but only if all those tests require all of those preconditions. This is related to the context-specification unit testing pattern, but still risks getting unmaintainable if you try to reuse the same setup code for a wide range of tests.
·         Don’t unit-test configuration settings
By definition, your configuration settings aren’t part of any unit of code (that’s why you extracted the setting out of your unit’s code). Even if you could write a unit test that inspects your configuration, it merely forces you to specify the same configuration in an additional redundant location.
Without doubt, unit testing can significantly increase the quality of your project. Many in our industry claim that any unit tests are better than none, but I disagree: a test suite can be a great asset, or it can be a great burden that contributes little. It depends on the quality of those tests, which seems to be determined by how well its developers have understood the goals and principles of unit testing.

Monday, September 1, 2014

MPEG2 PS/TS/Timecodes

MPEG-2 Program Stream Muxing

ffmpeg -genpts 1 -i ES_Video.m2v -i ES_Audio.mp2 -vcodec copy -acodec copy -f vob output.mpg

Note : In order to mux multiple audio tracks into the same file :
ffmpeg -genpts 1 -i ES_Video.m2v -i ES_Audio1.mp2 -i ES_Audio2.mp2 -vcodec copy -acodec copy -f vob output.mpg -newaudio

Note : In order to remux a PS file with multiple audio tracks :
ffmpeg -i input.mpg -vcodec copy -acodec copy -f vob output.mpg -acodec copy -newaudio

MPEG-2 Program Stream Demuxing

ffmpeg -i input.mpg -vcodec copy -f mpeg2video ES_Video.m2v -acodec copy -f mp2 ES_Audio.mp2

Note : This also works for files containing multiple audio tracks :
ffmpeg -i input.mpg -vcodec copy -f mpeg2video ES_Video.m2v -acodec copy -f mp2 ES_Audio1.mp2 -acodec copy -f mp2 ES_Audio2.mp2

MPEG-2 Start Timecode

ffmpeg -i <input_file> -timecode_frame_start <start_timecode> -vcodec mpeg2video -an output.m2v

Note : Start timecode is set as number of frames. For instance, if you want to start at 18:12:36:15, you will have to set -timecode_frame_start to 1638915 ( for 25 fps content ).