Questões de Concurso Comentadas para uerj

                        Virtual network appliances: Benefits and drawbacks

There's lots of talk about network virtualization benefits, but are virtual network appliances all they're cracked up to be? Only in some scenarios.
Network virtualization benefits can be plentiful, but only in certain scenarios. Learn where virtual network appliances can work -- and where they can't.
If virtualization enables servers to be spun up and down on demand for cost efficiency and agility, wouldn't it make sense to implement virtual network components too? After all, virtual servers need to communicate inbound and outbound and still be firewall-protected and load balanced. That would seem to be best addressed by virtual network appliances that can be spun-up on demand, right? Only in some scenarios.
Many networking vendors have already begun to minimize development cost by using Intel-based platforms and commodity hardware. Examples of this range from the Cisco ASA firewall to F5 load balancers and Vyatta routers. The obvious next step for some of these vendors has been to offer their products in virtual appliance packaging. F5 took a small step forward with the Local Traffic Manager - Virtual Edition (LTM VE), while Vyatta claims to offer a full range of virtual appliance solutions. VMware was somewhat late to the game, but it also offers virtualized firewalls (vShield Zones and vShield App) and routers/load balancers (vShield Edge).


                        Virtual network appliances: What's the catch?

The problem is that unlike servers, networking appliances commonly perform I/O-intensive tasks, moving large amounts of data between network interfaces with minimal additional processing, relying heavily on dedicated hardware. All high-speed routing and packet forwarding, as well as encryption (both IPsec and SSL) and load balancing, rely on dedicated silicon. When a networking appliance is repackaged into a virtual machine format, the dedicated hardware is gone, and all these tasks must now be performed by the general- purpose CPU, sometimes resulting in extreme reduction in performance.

Implementing routers, switches or firewalls in a virtual appliance would just burn the CPU cycles that could be better used elsewhere -- unless, of course, you’ve over-provisioned your servers and have plenty of idle CPU cycles, in which case something has gone seriously wrong with your planning.

To make matters worse, the hypervisor software used in server virtualization solutions also virtualizes the network interfaces. That means that every I/O access path to virtualized hardware from the networking appliance results in a context switch to higher privilege software (the hypervisor), which uses numerous CPU cycles to decode what needs to be done and emulate the desired action. Also, data passed between virtual machines must be copied between their address spaces, adding further latency to the process.

There is some help in that the VMware hypervisor has the DVFilter API, which allows a loadable kernel module to inspect and modify network traffic either within the hypervisor (vNetwork Data Path Agent) or in combination with a virtual machine (vNetwork Control Path Agent). The loadable kernel module significantly reduces the VM context switching overhead.


                        Where virtual network appliances can work?

There are some use cases in which virtual network appliances make perfect sense. For instance, you could virtualize an appliance that performs lots of CPU-intensive processing with no reliance on dedicated hardware. Web application firewalls (WAFs) and complex load balancers are perfect examples (no wonder they’re commonly implemented as loadable modules in Apache Web servers or as Squid reverse proxy servers).
Also, if you’re planning to roll out multi-tenant cloud, the flexibility gained by treating networking appliances as click-to-deploy Lego bricks might more than justify the subpar performance. This is especially so if you charge your users by their actual VM/CPU usage, in which case you don’t really care how much CPU they’re using.
Virtualized networking also makes sense when firewall and routing functions are implemented as part of the virtual switch in each hypervisor. This could result in optimal traffic flow between virtual machines (regardless of whether they belong to the same IP subnet or not) and solve the problem of traffic trombones. Unfortunately, it seems that Cisco is still the only vendor that extends the VMware hypervisor switch using the Virtual Ethernet Module (VEM) functionality. While numerous security solutions already deploy the VMsafe APIs, the networking appliances I’ve seen so far (including the vShield Edge from VMware) rely on virtual machines to forward traffic between virtual (or physical) LANs.
Obviously the networking vendors have a very long way to go before reaching the true potential of virtualized networking.

                          Disponível em: http://searchnetworking.techtarget.com/tip/Virtual-network-appliances-Benefits-and- drawbacks
                                                                              Search Networking - Tech Target - Texto de Ivan Pepelnjak (Março de 2011)

                        Here’s the first line of code ever written by a US president

Barack Obama just became the first US president to write a line of computer code (assuming George W. Bush never secretly indulged in PHP). At the White House yesterday, Obama sat down with students who were learning the fundamentals of JavaScript, the popular programming language used to create most web pages.

The line he wrote was:

                  moveForward(100);

“So I make the F in higher case?” Obama asked, correctly observing that JavaScript is case sensitive. “Semicolon?” (That semicolon is optional, but Obama apparently has a knack for recognizing JavaScript best practices.)
Obama was playing with a Code.org tutorial based on the popular Disney movie Frozen. In his line of code, the President called a function-moveForward-pre-defined by Code.org for the exercise.
Calling a function in JavaScript is simple: write its name exactly as it has been defined, followed by parentheses that contain its “arguments.” In this case, a single argument tells the program how many pixels to move a Frozen character forward. Because it’s measured in pixels, the argument has to be a number. If Obama had written moveForward(“three steps”), the program would have failed, offering only a cryptic error message and exposing the president to the near-perpetual state of frustration most software developers live in.

“This is Elsa?” Obama asked, referring to the movie’s main character.

Obama was promoting Computer Science Education Week and Code.org’s Hour of Code campaign, which encourages kids to try programming for at least one hour. “It turns out the concepts are not that complicated,” Obama told the students at the White House, though his attempt to explain it suggested otherwise:

                  “The basic concept behind coding is that you take zeros and ones, you take two numbers, yes or no, and those can be translated into electrical messages that then run through the computer…. So all it’s doing is it’s saying yes or no over and over again, and the computer’s powerful enough that it can read a really long set of instructions really quickly.”

Something like that.

                                                            Disponível em: http://qz.com/308904/heres-the-first-line-of-code-ever-written-by-a-us-president/
                                                                                                            Quartz (9 de Dezembro de 2014) - Texto de Zachary M. Seward