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J Internet Serv Appl (2010) 1: 7–18 DOI 10.1007/s13174-010-0007-6
O R I G I NA L PA P E R S
Cloud computing: state-of-the-art and research challenges
Qi Zhang · Lu Cheng · Raouf Boutaba
Received: 8 January 2010 / Accepted: 25 February 2010 / Published online: 20 April 2010 © The Brazilian Computer Society 2010
Abstract Cloud computing has recently emerged as a new paradigm for hosting and delivering services over the Inter- net. Cloud computing is attractive to business owners as it eliminates the requirement for users to plan ahead for pro- visioning, and allows enterprises to start from the small and increase resources only when there is a rise in service de- mand. However, despite the fact that cloud computing offers huge opportunities to the IT industry, the development of cloud computing technology is currently at its infancy, with many issues still to be addressed. In this paper, we present a survey of cloud computing, highlighting its key concepts, architectural principles, state-of-the-art implementation as well as research challenges. The aim of this paper is to pro- vide a better understanding of the design challenges of cloud computing and identify important research directions in this increasingly important area.
Keywords Cloud computing · Data centers · Virtualization
With the rapid development of processing and storage tech- nologies and the success of the Internet, computing re- sources have become cheaper, more powerful and more ubiquitously available than ever before. This technological trend has enabled the realization of a new computing model
Q. Zhang · L. Cheng · R. Boutaba () University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1 e-mail: email@example.com
Q. Zhang e-mail: firstname.lastname@example.org
L. Cheng e-mail: email@example.com
called cloud computing, in which resources (e.g., CPU and storage) are provided as general utilities that can be leased and released by users through the Internet in an on-demand fashion. In a cloud computing environment, the traditional role of service provider is divided into two: the infrastruc- ture providers who manage cloud platforms and lease re- sources according to a usage-based pricing model, and ser- vice providers, who rent resources from one or many in- frastructure providers to serve the end users. The emer- gence of cloud computing has made a tremendous impact on the Information Technology (IT) industry over the past few years, where large companies such as Google, Ama- zon and Microsoft strive to provide more powerful, reliable and cost-efficient cloud platforms, and business enterprises seek to reshape their business models to gain benefit from this new paradigm. Indeed, cloud computing provides sev- eral compelling features that make it attractive to business owners, as shown below.
No up-front investment: Cloud computing uses a pay-as- you-go pricing model. A service provider does not need to invest in the infrastructure to start gaining benefit from cloud computing. It simply rents resources from the cloud accord- ing to its own needs and pay for the usage.
Lowering operating cost: Resources in a cloud environ- ment can be rapidly allocated and de-allocated on demand. Hence, a service provider no longer needs to provision ca- pacities according to the peak load. This provides huge sav- ings since resources can be released to save on operating costs when service demand is low.
Highly scalable: Infrastructure providers pool large amount of resources from data centers and make them easily accessible. A service provider can easily expand its service to large scales in order to handle rapid increase in service demands (e.g., flash-crowd effect). This model is sometimes called surge computing .
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Easy access: Services hosted in the cloud are generally web-based. Therefore, they are easily accessible through a variety of devices with Internet connections. These devices not only include desktop and laptop computers, but also cell phones and PDAs.
Reducing business risks and maintenance expenses: By outsourcing the service infrastructure to the clouds, a service provider shifts its business risks (such as hardware failures) to infrastructure providers, who often have better expertise and are better equipped for managing these risks. In addi- tion, a service provider can cut down the hardware mainte- nance and the staff training costs.
However, although cloud computing has shown consid- erable opportunities to the IT industry, it also brings many unique challenges that need to be carefully addressed. In this paper, we present a survey of cloud computing, highlighting its key concepts, architectural principles, state-of-the-art im- plementations as well as research challenges. Our aim is to provide a better understanding of the design challenges of cloud computing and identify important research directions in this fascinating topic.
The remainder of this paper is organized as follows. In Sect. 2 we provide an overview of cloud computing and compare it with other related technologies. In Sect. 3, we describe the architecture of cloud computing and present its design principles. The key features and characteristics of cloud computing are detailed in Sect. 4. Section 5 surveys the commercial products as well as the current technologies used for cloud computing. In Sect. 6, we summarize the cur- rent research topics in cloud computing. Finally, the paper concludes in Sect. 7.
2 Overview of cloud computing
This section presents a general overview of cloud comput- ing, including its definition and a comparison with related concepts.
The main idea behind cloud computing is not a new one. John McCarthy in the 1960s already envisioned that com- puting facilities will be provided to the general public like a utility . The term “cloud” has also been used in vari- ous contexts such as describing large ATM networks in the 1990s. However, it was after Google’s CEO Eric Schmidt used the word to describe the business model of provid- ing services across the Internet in 2006, that the term re- ally started to gain popularity. Since then, the term cloud computing has been used mainly as a marketing term in a variety of contexts to represent many different ideas. Cer- tainly, the lack of a standard definition of cloud computing
has generated not only market hypes, but also a fair amount of skepticism and confusion. For this reason, recently there has been work on standardizing the definition of cloud com- puting. As an example, the work in  compared over 20 different definitions from a variety of sources to confirm a standard definition. In this paper, we adopt the definition of cloud computing provided by The National Institute of Standards and Technology (NIST) , as it covers, in our opinion, all the essential aspects of cloud computing:
NIST definition of cloud computing Cloud computing is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal man- agement effort or service provider interaction.
The main reason for the existence of different percep- tions of cloud computing is that cloud computing, unlike other technical terms, is not a new technology, but rather a new operations model that brings together a set of ex- isting technologies to run business in a different way. In- deed, most of the technologies used by cloud computing, such as virtualization and utility-based pricing, are not new. Instead, cloud computing leverages these existing technolo- gies to meet the technological and economic requirements of today’s demand for information technology.
2.2 Related technologies
Cloud computing is often compared to the following tech- nologies, each of which shares certain aspects with cloud computing:
Grid Computing: Grid computing is a distributed com- puting paradigm that coordinates networked resources to achieve a common computational objective. The develop- ment of Grid computing was originally driven by scien- tific applications which are usually computation-intensive. Cloud computing is similar to Grid computing in that it also employs distributed resources to achieve application-level objectives. However, cloud computing takes one step further by leveraging virtualization technologies at multiple levels (hardware and application platform) to realize resource shar- ing and dynamic resource provisioning.
Utility Computing: Utility computing represents the model of providing resources on-demand and charging cus- tomers based on usage rather than a flat rate. Cloud comput- ing can be perceived as a realization of utility computing. It adopts a utility-based pricing scheme entirely for economic reasons. With on-demand resource provisioning and utility- based pricing, service providers can truly maximize resource utilization and minimize their operating costs.
Virtualization: Virtualization is a technology that ab- stracts away the details of physical hardware and provides
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virtualized resources for high-level applications. A virtual- ized server is commonly called a virtual machine (VM). Vir- tualization forms the foundation of cloud computing, as it provides the capability of pooling computing resources from clusters of servers and dynamically assigning or reassigning virtual resources to applications on-demand.
Autonomic Computing: Originally coined by IBM in 2001, autonomic computing aims at building computing sys- tems capable of self-management, i.e. reacting to internal and external observations without human intervention. The goal of autonomic computing is to overcome the manage- ment complexity of today’s computer systems. Although cloud computing exhibits certain autonomic features such as automatic resource provisioning, its objective is to lower the resource cost rather than to reduce system complexity.
In summary, cloud computing leverages virtualization technology to achieve the goal of providing computing re- sources as a utility. It shares certain aspects with grid com- puting and autonomic computing but differs from them in other aspects. Therefore, it offers unique benefits and im- poses distinctive challenges to meet its requirements.
3 Cloud computing architecture
This section describes the architectural, business and various operation models of cloud computing.
3.1 A layered model of cloud computing
Generally speaking, the architecture of a cloud comput- ing environment can be divided into 4 layers: the hard- ware/datacenter layer, the infrastructure layer, the platform layer and the application layer, as shown in Fig. 1. We de- scribe each of them in detail:
The hardware layer: This layer is responsible for man- aging the physical resources of the cloud, including phys- ical servers, routers, switches, power and cooling systems. In practice, the hardware layer is typically implemented in data centers. A data center usually contains thousands of servers that are organized in racks and interconnected through switches, routers or other fabrics. Typical issues at hardware layer include hardware configuration, fault- tolerance, traffic management, power and cooling resource management.
The infrastructure layer: Also known as the virtualiza- tion layer, the infrastructure layer creates a pool of storage and computing resources by partitioning the physical re- sources using virtualization technologies such as Xen , KVM  and VMware . The infrastructure layer is an essential component of cloud computing, since many key features, such as dynamic resource assignment, are only made available through virtualization technologies.
The platform layer: Built on top of the infrastructure layer, the platform layer consists of operating systems and application frameworks. The purpose of the platform layer is to minimize the burden of deploying applications directly into VM containers. For example, Google App Engine oper- ates at the platform layer to provide API support for imple- menting storage, database and business logic of typical web applications.
The application layer: At the highest level of the hierar- chy, the application layer consists of the actual cloud appli- cations. Different from traditional applications, cloud appli- cations can leverage the automatic-scaling feature to achieve better performance, availability and lower operating cost.
Compared to traditional service hosting environments such as dedicated server farms, the architecture of cloud computing is more modular. Each layer is loosely coupled with the layers above and below, allowing each layer to evolve separately. This is similar to the design of the OSI
Fig. 1 Cloud computing architecture
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model for network protocols. The architectural modularity allows cloud computing to support a wide range of applica- tion requirements while reducing management and mainte- nance overhead.
3.2 Business model
Cloud computing employs a service-driven business model. In other words, hardware and platform-level resources are provided as services on an on-demand basis. Conceptually, every layer of the architecture described in the previous sec- tion can be implemented as a service to the layer above. Conversely, every layer can be perceived as a customer of the layer below. However, in practice, clouds offer services that can be grouped into three categories: software as a ser- vice (SaaS), platform as a service (PaaS), and infrastructure as a service (IaaS).
1. Infrastructure as a Service: IaaS refers to on-demand provisioning of infrastructural resources, usually in terms of VMs. The cloud owner who offers IaaS is called an IaaS provider. Examples of IaaS providers include Ama- zon EC2 , GoGrid  and Flexiscale .
2. Platform as a Service: PaaS refers to providing platform layer resources, including operating system support and software development frameworks. Examples of PaaS providers include Google App Engine , Microsoft Windows Azure  and Force.com .
3. Software as a Service: SaaS refers to providing on- demand applications over the Internet. Examples of SaaS providers include Salesforce.com , Rackspace  and SAP Business ByDesign .
The business model of cloud computing is depicted by Fig. 2. According to the layered architecture of cloud com- puting, it is entirely possible that a PaaS provider runs its cloud on top of an IaaS provider’s cloud. However, in the current practice, IaaS and PaaS providers are often parts of the same organization (e.g., Google and Salesforce). This is why PaaS and IaaS providers are often called the infrastruc- ture providers or cloud providers .
Fig. 2 Business model of cloud computing
3.3 Types of clouds
There are many issues to consider when moving an enter- prise application to the cloud environment. For example, some service providers are mostly interested in lowering op- eration cost, while others may prefer high reliability and se- curity. Accordingly, there are different types of clouds, each with its own benefits and drawbacks:
Public clouds: A cloud in which service providers of- fer their resources as services to the general public. Pub- lic clouds offer several key benefits to service providers, in- cluding no initial capital investment on infrastructure and shifting of risks to infrastructure providers. However, pub- lic clouds lack fine-grained control over data, network and security settings, which hampers their effectiveness in many business scenarios.
Private clouds: Also known as internal clouds, private clouds are designed for exclusive use by a single organiza- tion. A private cloud may be built and managed by the orga- nization or by external providers. A private cloud offers the highest degree of control over performance, reliability and security. However, they are often criticized for being simi- lar to traditional proprietary server farms and do not provide benefits such as no up-front capital costs.
Hybrid clouds: A hybrid cloud is a combination of public and private cloud models that tries to address the limitations of each approach. In a hybrid cloud, part of the service in- frastructure runs in private clouds while the remaining part runs in public clouds. Hybrid clouds offer more flexibility than both public and private clouds. Specifically, they pro- vide tighter control and security over application data com- pared to public clouds, while still facilitating on-demand service expansion and contraction. On the down side, de- signing a hybrid cloud requires carefully determining the best split between public and private cloud components.
Virtual Private Cloud: An alternative solution to address- ing the limitations of both public and private clouds is called Virtual Private Cloud (VPC). A VPC is essentially a plat- form running on top of public clouds. The main difference is that a VPC leverages virtual private network (VPN) technol- ogy that allows service providers to design their own topol- ogy and security settings such as firewall rules. VPC is es- sentially a more holistic design since it not only virtualizes servers and applications, but also the underlying commu- nication network as well. Additionally, for most companies, VPC provides seamless transition from a proprietary service infrastructure to a cloud-based infrastructure, owing to the virtualized network layer.
For most service providers, selecting the right cloud model is dependent on the business scenario. For exam- ple, computation-intensive scientific applications are best deployed on public clouds for cost-effectiveness. Arguably, certain types of clouds will be more popular than others.
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In particular, it was predicted that hybrid clouds will be the dominant type for most organizations . However, vir- tual private clouds have started to gain more popularity since their inception in 2009.
4 Cloud computing characteristics
Cloud computing provides several salient features that are different from traditional service computing, which we sum- marize below:
Multi-tenancy: In a cloud environment, services owned by multiple providers are co-located in a single data center. The performance and management issues of these services are shared among service providers and the infrastructure provider. The layered architecture of cloud computing pro- vides a natural division of responsibilities: the owner of each layer only needs to focus on the specific objectives associ- ated with this layer. However, multi-tenancy also introduces difficulties in understanding and managing the interactions among various stakeholders.
Shared resource pooling: The infrastructure provider of- fers a pool of computing resources that can be dynamically assigned to multiple resource consumers. Such dynamic re- source assignment capability provides much flexibility to in- frastructure providers for managing their own resource us- age and operating costs. For instance, an IaaS provider can leverage VM migration technology to attain a high degree of server consolidation, hence maximizing resource utiliza- tion while minimizing cost such as power consumption and cooling.
Geo-distribution and ubiquitous network access: Clouds are generally accessible through the Internet and use the Internet as a service delivery network. Hence any device with Internet connectivity, be it a mobile phone, a PDA or a laptop, is able to access cloud services. Additionally, to achieve high network performance and localization, many of today’s clouds consist of data centers located at many locations around the globe. A service provider can easily leverage geo-diversity to achieve maximum service utility.
Service oriented: As mentioned previously, cloud com- puting adopts a service-driven operating model. Hence it places a strong emphasis on service management. In a cloud, each IaaS, PaaS and SaaS provider offers its service accord- ing to the Service Level Agreement (SLA) negotiated with its customers. SLA assurance is therefore a critical objective of every provider.
Dynamic resource provisioning: One of the key features of cloud computing is that computing resources can be ob- tained and released on the fly. Compared to the traditional model that provisions resources according to peak demand, dynamic resource provisioning allows service providers to acquire resources based on the current demand, which can considerably lower the operating cost.
Self-organizing: Since resources can be allocated or de- allocated on-demand, service providers are empowered to manage their resource consumption according to their own needs. Furthermore, the automated resource management feature yields high agility that enables service providers to respond quickly to rapid changes in service demand such as the flash crowd effect.
Utility-based pricing: Cloud computing employs a pay- per-use pricing model. The exact pricing scheme may vary from service to service. For example, a SaaS provider may rent a virtual machine from an IaaS provider on a per-hour basis. On the other hand, a SaaS provider that provides on-demand customer relationship management (CRM) may charge its customers based on the number of clients it serves (e.g., Salesforce). Utility-based pricing lowers service oper- ating cost as it charges customers on a per-use basis. How- ever, it also introduces complexities in controlling the oper- ating cost. In this perspective, companies like VKernel  provide software to help cloud customers understand, ana- lyze and cut down the unnecessary cost on resource con- sumption.
In this section, we present the state-of-the-art implementa- tions of cloud computing. We first describe the key technolo- gies currently used for cloud computing. Then, we survey the popular cloud computing products.
5.1 Cloud computing technologies
This section provides a review of technologies used in cloud computing environments.
5.1.1 Architectural design of data centers
A data center, which is home to the computation power and storage, is central to cloud computing and contains thou- sands of devices like servers, switches and routers. Proper planning of this network architecture is critical, as it will heavily influence applications performance and throughput in such a distributed computing environment. Further, scala- bility and resiliency features need to be carefully considered.
Currently, a layered approach is the basic foundation of the network architecture design, which has been tested in some of the largest deployed data centers. The basic layers of a data center consist of the core, aggregation, and access layers, as shown in Fig. 3. The access layer is where the servers in racks physically connect to the network. There are typically 20 to 40 servers per rack, each connected to an access switch with a 1 Gbps link. Access switches usually connect to two aggregation switches for redundancy with
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Fig. 3 Basic layered design of data center network infrastructure
10 Gbps links. The aggregation layer usually provides im- portant functions, such as domain service, location service, server load balancing, and more. The core layer provides connectivity to multiple aggregation switches and provides a resilient routed fabric with no single point of failure. The core routers manage traffic into and out of the data center.
A popular practice is to leverage commodity Ethernet switches and routers to build the network infrastructure. In different business solutions, the layered network infrastruc- ture can be elaborated to meet specific business challenges. Basically, the design of a data center network architecture should meet the following objectives [1, 21–23, 35]:
Uniform high capacity: The maximum rate of a server- to-server traffic flow should be limited only by the available capacity on the network-interface cards of the sending and receiving servers, and assigning servers to a service should be independent of the network topology. It should be possi- ble for an arbitrary host in the data center to communicate with any other host in the network at the full bandwidth of its local network interface.
Free VM migration: Virtualization allows the entire VM state to be transmitted across the network to migrate a VM from one physical machine to another. A cloud comput- ing hosting service may migrate VMs for statistical multi- plexing or dynamically changing communication patterns to achieve high bandwidth for tightly coupled hosts or to achieve variable heat distribution and power availability in the data center. The communication topology should be de- signed so as to support rapid virtual machine migration.
Resiliency: Failures will be common at scale. The net- work infrastructure must be fault-tolerant against various types of server failures, link outages, or server-rack failures. Existing unicast and multicast communications should not be affected to the extent allowed by the underlying physical connectivity.
Scalability: The network infrastructure must be able to scale to a large number of servers and allow for incremental expansion.
Backward compatibility: The network infrastructure should be backward compatible with switches and routers running Ethernet and IP. Because existing data centers have commonly leveraged commodity Ethernet and IP based de- vices, they should also be used in the new architecture with- out major modifications.
Another area of rapid innovation in the industry is the de- sign and deployment of shipping-container based, modular data center (MDC). In an MDC, normally up to a few thou- sands of servers, are interconnected via switches to form the network infrastructure. Highly interactive applications, which are sensitive to response time, are suitable for geo- diverse MDC placed close to major population areas. The MDC also helps with redundancy because not all areas are likely to lose power, experience an earthquake, or suffer ri- ots at the same time. Rather than the three-layered approach discussed above, Guo et al. [22, 23] proposed server-centric, recursively defined network structures of MDC.
5.1.2 Distributed file system over clouds
Google File System (GFS)  is a proprietary distributed file system developed by Google and specially designed to provide efficient, reliable access to data using large clusters of commodity servers. Files are divided into chunks of 64 megabytes, and are usually appended to or read and only extremely rarely overwritten or shrunk. Compared with tra- ditional file systems, GFS is designed and optimized to run on data centers to provide extremely high data throughputs, low latency and survive individual server failures.
Inspired by GFS, the open source Hadoop Distributed File System (HDFS)  stores large files across multi- ple machines. It achieves reliability by replicating the data across multiple servers. Similarly to GFS, data is stored on multiple geo-diverse nodes. The file system is built from a cluster of data nodes, each of which serves blocks of data over the network using a block protocol specific to HDFS. Data is also provided over HTTP, allowing access to all con- tent from a web browser or other types of clients. Data nodes can talk to each other to rebalance data distribution, to move copies around, and to keep the replication of data high.
5.1.3 Distributed application framework over clouds
HTTP-based applications usually conform to some web ap- plication framework such as Java EE. In modern data center environments, clusters of servers are also used for computa- tion and data-intensive jobs such as financial trend analysis, or film animation.
MapReduce  is a software framework introduced by Google to support distributed computing on large data sets
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on clusters of computers. MapReduce consists of one Mas- ter, to which client applications submit MapReduce jobs. The Master pushes work out to available task nodes in the data center, striving to keep the tasks as close to the data as possible. The Master knows which node contains the data, and which other hosts are nearby. If the task cannot be hosted on the node where the data is stored, priority is given to nodes in the same rack. In this way, network traffic on the main backbone is reduced, which also helps to im- prove throughput, as the backbone is usually the bottleneck. If a task fails or times out, it is rescheduled. If the Master fails, all ongoing tasks are lost. The Master records what it is up to in the filesystem. When it starts up, it looks for any such data, so that it can restart work from where it left off.
The open source Hadoop MapReduce project  is in- spired by Google’s work. Currently, many organizations are using Hadoop MapReduce to run large data-intensive com- putations.
5.2 Commercial products
In this section, we provide a survey of some of the dominant cloud computing products.
5.2.1 Amazon EC2
Amazon Web Services (AWS)  is a set of cloud services, providing cloud-based computation, storage and other func- tionality that enable organizations and individuals to deploy applications and services on an on-demand basis and at com- modity prices. Amazon Web Services’ offerings are acces- sible over HTTP, using REST and SOAP protocols.
Amazon Elastic Compute Cloud (Amazon EC2) enables cloud users to launch and manage server instances in data centers using APIs or available tools and utilities. EC2 in- stances are virtual machines running on top of the Xen virtu- alization engine . After creating and starting an instance, users can upload software and make changes to it. When changes are finished, they can be bundled as a new machine image. An identical copy can then be launched at any time. Users have nearly full control of the entire software stack on the EC2 instances that look like hardware to them. On the other hand, this feature makes it inherently difficult for Amazon to offer automatic scaling of resources.
EC2 provides the ability to place instances in multiple lo- cations. EC2 locations are composed of Regions and Avail- ability Zones. Regions consist of one or more Availability Zones, are geographically dispersed. Availability Zones are distinct locations that are engineered to be insulated from failures in other Availability Zones and provide inexpensive, low latency network connectivity to other Availability Zones in the same Region.
EC2 machine images are stored in and retrieved from Amazon Simple Storage Service (Amazon S3). S3 stores
data as “objects” that are grouped in “buckets.” Each object contains from 1 byte to 5 gigabytes of data. Object names are essentially URI  pathnames. Buckets must be explic- itly created before they can be used. A bucket can be stored in one of several Regions. Users can choose a Region to opti- mize latency, minimize costs, or address regulatory require- ments.
Amazon Virtual Private Cloud (VPC) is a secure and seamless bridge between a company’s existing IT infrastruc- ture and the AWS cloud. Amazon VPC enables enterprises to connect their existing infrastructure to a set of isolated AWS compute resources via a Virtual Private Network (VPN) connection, and to extend their existing management capabilities such as security services, firewalls, and intrusion detection systems to include their AWS resources.
For cloud users, Amazon CloudWatch is a useful man- agement tool which collects raw data from partnered AWS services such as Amazon EC2 and then processes the in- formation into readable, near real-time metrics. The metrics about EC2 include, for example, CPU utilization, network in/out bytes, disk read/write operations, etc.
5.2.2 Microsoft Windows Azure platform
Microsoft’s Windows Azure platform  consists of three components and each of them provides a specific set of ser- vices to cloud users. Windows Azure provides a Windows- based environment for running applications and storing data on servers in data centers; SQL Azure provides data services in the cloud based on SQL Server; and .NET Services offer distributed infrastructure services to cloud-based and local applications. Windows Azure platform can be used both by applications running in the cloud and by applications run- ning on local systems.
Windows Azure also supports applications built on the .NET Framework and other ordinary languages supported in Windows systems, like C#, Visual Basic, C++, and others. Windows Azure supports general-purpose programs, rather than a single class of computing. Developers can create web applications using technologies such as ASP.NET and Win- dows Communication Foundation (WCF), applications that run as independent background processes, or applications that combine the two. Windows Azure allows storing data in blobs, tables, and queues, all accessed in a RESTful style via HTTP or HTTPS.
SQL Azure components are SQL Azure Database and “Huron” Data Sync. SQL Azure Database is built on Mi- crosoft SQL Server, providing a database management sys- tem (DBMS) in the cloud. The data can be accessed using ADO.NET and other Windows data access interfaces. Users can also use on-premises software to work with this cloud- based information. “Huron” Data Sync synchronizes rela- tional data across various on-premises DBMSs.
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Table 1 A comparison of representative commercial products
Cloud Provider Amazon EC2 Windows Azure Google App Engine
Classes of Utility Computing Infrastructure service Platform service Platform service
Target Applications General-purpose applications General-purpose Windows applications
Traditional web applications with supported framework
Computation OS Level on a Xen Virtual Machine
Microsoft Common Language Runtime (CLR) VM; Predefined roles of app. instances
Predefined web application frameworks
Storage Elastic Block Store; Amazon Simple Storage Service (S3); Amazon SimpleDB
Azure storage service and SQL Data Services
BigTable and MegaStore
Auto Scaling Automatically changing the number of instances based on parameters that users specify
Automatic scaling based on application roles and a configuration file specified by users
Automatic Scaling which is transparent to users
The .NET Services facilitate the creation of distributed applications. The Access Control component provides a cloud-based implementation of single identity verification across applications and companies. The Service Bus helps an application expose web services endpoints that can be accessed by other applications, whether on-premises or in the cloud. Each exposed endpoint is assigned a URI, which clients can use to locate and access a service.
All of the physical resources, VMs and applications in the data center are monitored by software called the fabric controller. With each application, the users upload a config- uration file that provides an XML-based description of what the application needs. Based on this file, the fabric controller decides where new applications should run, choosing phys- ical servers to optimize hardware utilization.
5.2.3 Google App Engine
Google App Engine  is a platform for traditional web applications in Google-managed data centers. Currently, the supported programming languages are Python and Java. Web frameworks that run on the Google App Engine include Django, CherryPy, Pylons, and web2py, as well as a custom Google-written web application framework similar to JSP or ASP.NET. Google handles deploying code to a cluster, monitoring, failover, and launching application instances as necessary. Current APIs support features such as storing and retrieving data from a BigTable  non-relational database, making HTTP requests and caching. Developers have read- only access to the filesystem on App Engine.
Table 1 summarizes the three examples of popular cloud offerings in terms of the classes of utility computing, tar- get types of application, and more importantly their models of computation, storage and auto-scaling. Apparently, these cloud offerings are based on different levels of abstraction
and management of the resources. Users can choose one type or combinations of several types of cloud offerings to satisfy specific business requirements.
6 Research challenges
Although cloud computing has been widely adopted by the industry, the research on cloud computing is still at an early stage. Many existing issues have not been fully addressed, while new challenges keep emerging from industry applica- tions. In this section, we summarize some of the challenging research issues in cloud computing.
6.1 Automated service provisioning
One of the key features of cloud computing is the capabil- ity of acquiring and releasing resources on-demand. The ob- jective of a service provider in this case is to allocate and de-allocate resources from the cloud to satisfy its service level objectives (SLOs), while minimizing its operational cost. However, it is not obvious how a service provider can achieve this objective. In particular, it is not easy to de- termine how to map SLOs such as QoS requirements to low-level resource requirement such as CPU and memory requirements. Furthermore, to achieve high agility and re- spond to rapid demand fluctuations such as in flash crowd effect, the resource provisioning decisions must be made on- line.
Automated service provisioning is not a new problem. Dynamic resource provisioning for Internet applications has been studied extensively in the past [47, 57]. These ap- proaches typically involve: (1) Constructing an application performance model that predicts the number of application instances required to handle demand at each particular level,
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in order to satisfy QoS requirements; (2) Periodically pre- dicting future demand and determining resource require- ments using the performance model; and (3) Automatically allocating resources using the predicted resource require- ments. Application performance model can be constructed using various techniques, including Queuing theory , Control theory  and Statistical Machine Learning .
Additionally, there is a distinction between proactive and reactive resource control. The proactive approach uses predicted demand to periodically allocate resources before they are needed. The reactive approach reacts to immedi- ate demand fluctuations before periodic demand prediction is available. Both approaches are important and necessary for effective resource control in dynamic operating environ- ments.
6.2 Virtual machine migration
Virtualization can provide significant benefits in cloud com- puting by enabling virtual machine migration to balance load across the data center. In addition, virtual machine mi- gration enables robust and highly responsive provisioning in data centers.
Virtual machine migration has evolved from process migration techniques . More recently, Xen  and VMWare  have implemented “live” migration of VMs that involves extremely short downtimes ranging from tens of milliseconds to a second. Clark et al.  pointed out that migrating an entire OS and all of its applications as one unit allows to avoid many of the difficulties faced by process- level migration approaches, and analyzed the benefits of live migration of VMs.
The major benefits of VM migration is to avoid hotspots; however, this is not straightforward. Currently, detecting workload hotspots and initiating a migration lacks the agility to respond to sudden workload changes. Moreover, the in- memory state should be transferred consistently and effi- ciently, with integrated consideration of resources for appli- cations and physical servers.
6.3 Server consolidation
Server consolidation is an effective approach to maximize resource utilization while minimizing energy consumption in a cloud computing environment. Live VM migration tech- nology is often used to consolidate VMs residing on multi- ple under-utilized servers onto a single server, so that the remaining servers can be set to an energy-saving state. The problem of optimally consolidating servers in a data center is often formulated as a variant of the vector bin-packing problem , which is an NP-hard optimization problem. Various heuristics have been proposed for this problem [33, 46]. Additionally, dependencies among VMs, such as
communication requirements, have also been considered re- cently .
However, server consolidation activities should not hurt application performance. It is known that the resource usage (also known as the footprint ) of individual VMs may vary over time . For server resources that are shared among VMs, such as bandwidth, memory cache and disk I/O, maximally consolidating a server may result in re- source congestion when a VM changes its footprint on the server . Hence, it is sometimes important to observe the fluctuations of VM footprints and use this information for effective server consolidation. Finally, the system must quickly react to resource congestions when they occur .
6.4 Energy management
Improving energy efficiency is another major issue in cloud computing. It has been estimated that the cost of powering and cooling accounts for 53% of the total operational expen- diture of data centers . In 2006, data centers in the US consumed more than 1.5% of the total energy generated in that year, and the percentage is projected to grow 18% an- nually . Hence infrastructure providers are under enor- mous pressure to reduce energy consumption. The goal is not only to cut down energy cost in data centers, but also to meet government regulations and environmental standards.
Designing energy-efficient data centers has recently re- ceived considerable attention. This problem can be ap- proached from several directions. For example, energy- efficient hardware architecture that enables slowing down CPU speeds and turning off partial hardware components  has become commonplace. Energy-aware job schedul- ing  and server consolidation  are two other ways to reduce power consumption by turning off unused machines. Recent research has also begun to study energy-efficient net- work protocols and infrastructures . A key challenge in all the above methods is to achieve a good trade-off between energy savings and application performance. In this respect, few researchers have recently started to investigate coordi- nated solutions for performance and power management in a dynamic cloud environment .
6.5 Traffic management and analysis
Analysis of data traffic is important for today’s data cen- ters. For example, many web applications rely on analysis of traffic data to optimize customer experiences. Network operators also need to know how traffic flows through the network in order to make many of the management and plan- ning decisions.
However, there are several challenges for existing traf- fic measurement and analysis methods in Internet Service Providers (ISPs) networks and enterprise to extend to data
16 J Internet Serv Appl (2010) 1: 7–18
centers. Firstly, the density of links is much higher than that in ISPs or enterprise networks, which makes the worst- case scenario for existing methods. Secondly, most existing methods can compute traffic matrices between a few hun- dreds end hosts, but even a modular data center can have several thousand servers. Finally, existing methods usually assume some flow patterns that are reasonable in Internet and enterprises networks, but the applications deployed on data centers, such as MapReduce jobs, significantly change the traffic pattern. Further, there is tighter coupling in appli- cation’s use of network, computing, and storage resources, than what is seen in other settings.
Currently, there is not much work on measurement and analysis of data center traffic. Greenberg et al.  report data center traffic characteristics on flow sizes and concur- rent flows, and use these to guide network infrastructure de- sign. Benson et al.  perform a complementary study of traffic at the edges of a data center by examining SNMP traces from routers.
6.6 Data security
Data security is another important research topic in cloud computing. Since service providers typically do not have ac- cess to the physical security system of data centers, they must rely on the infrastructure provider to achieve full data security. Even for a virtual private cloud, the service provider can only specify the security setting remotely, with- out knowing whether it is fully implemented. The infrastruc- ture provider, in this context, must achieve the following objectives: (1) confidentiality, for secure data access and transfer, and (2) auditability, for attesting whether secu- rity setting of applications has been tampered or not. Con- fidentiality is usually achieved using cryptographic proto- cols, whereas auditability can be achieved using remote at- testation techniques. Remote attestation typically requires a trusted platform module (TPM) to generate non-forgeable system summary (i.e. system state encrypted using TPM’s private key) as the proof of system security. However, in a virtualized environment like the clouds, VMs can dynami- cally migrate from one location to another, hence directly using remote attestation is not sufficient. In this case, it is critical to build trust mechanisms at every architectural layer of the cloud. Firstly, the hardware layer must be trusted using hardware TPM. Secondly, the virtualization platform must be trusted using secure virtual machine monitors . VM migration should only be allowed if both source and destination servers are trusted. Recent work has been de- voted to designing efficient protocols for trust establishment and management [31, 43].
6.7 Software frameworks
Cloud computing provides a compelling platform for host- ing large-scale data-intensive applications. Typically, these
applications leverage MapReduce frameworks such as Hadoop for scalable and fault-tolerant data processing. Re- cent work has shown that the performance and resource con- sumption of a MapReduce job is highly dependent on the type of the application [29, 42, 56]. For instance, Hadoop tasks such as sort is I/O intensive, whereas grep requires significant CPU resources. Furthermore, the VM allocated to each Hadoop node may have heterogeneous character- istics. For example, the bandwidth available to a VM is dependent on other VMs collocated on the same server. Hence, it is possible to optimize the performance and cost of a MapReduce application by carefully selecting its con- figuration parameter values  and designing more effi- cient scheduling algorithms [42, 56]. By mitigating the bot- tleneck resources, execution time of applications can be significantly improved. The key challenges include perfor- mance modeling of Hadoop jobs (either online or offline), and adaptive scheduling in dynamic conditions.
Another related approach argues for making MapReduce frameworks energy-aware . The essential idea of this ap- proach is to turn Hadoop node into sleep mode when it has finished its job while waiting for new assignments. To do so, both Hadoop and HDFS must be made energy-aware. Fur- thermore, there is often a trade-off between performance and energy-awareness. Depending on the objective, finding a de- sirable trade-off point is still an unexplored research topic.
6.8 Storage technologies and data management
Software frameworks such as MapReduce and its various implementations such as Hadoop and Dryad are designed for distributed processing of data-intensive tasks. As men- tioned previously, these frameworks typically operate on Internet-scale file systems such as GFS and HDFS. These file systems are different from traditional distributed file sys- tems in their storage structure, access pattern and application programming interface. In particular, they do not implement the standard POSIX interface, and therefore introduce com- patibility issues with legacy file systems and applications. Several research efforts have studied this problem [4, 40]. For instance, the work in  proposed a method for sup- porting the MapReduce framework using cluster file sys- tems such as IBM’s GPFS. Patil et al.  proposed new API primitives for scalable and concurrent data access.
6.9 Novel cloud architectures
Currently, most of the commercial clouds are implemented in large data centers and operated in a centralized fashion. Although this design achieves economy-of-scale and high manageability, it also comes with its limitations such high energy expense and high initial investment for construct- ing data centers. Recent work [12, 48] suggests that small- size data centers can be more advantageous than big data
J Internet Serv Appl (2010) 1: 7–18 17
centers in many cases: a small data center does not con- sume so much power, hence it does not require a power- ful and yet expensive cooling system; small data centers are cheaper to build and better geographically distributed than large data centers. Geo-diversity is often desirable for re- sponse time-critical services such as content delivery and interactive gaming. For example, Valancius et al.  stud- ied the feasibility of hosting video-streaming services using application gateways (a.k.a. nano-data centers).
Another related research trend is on using voluntary re- sources (i.e. resources donated by end-users) for hosting cloud applications . Clouds built using voluntary re- sources, or a mixture of voluntary and dedicated resources are much cheaper to operate and more suitable for non-profit applications such as scientific computing. However, this ar- chitecture also imposes challenges such managing heteroge- neous resources and frequent churn events. Also, devising incentive schemes for such architectures is an open research problem.
Cloud computing has recently emerged as a compelling par- adigm for managing and delivering services over the Inter- net. The rise of cloud computing is rapidly changing the landscape of information technology, and ultimately turning the long-held promise of utility computing into a reality.
However, despite the significant benefits offered by cloud computing, the current technologies are not matured enough to realize its full potential. Many key challenges in this domain, including automatic resource provisioning, power management and security management, are only starting to receive attention from the research community. There- fore, we believe there is still tremendous opportunity for re- searchers to make groundbreaking contributions in this field, and bring significant impact to their development in the in- dustry.
In this paper, we have surveyed the state-of-the-art of cloud computing, covering its essential concepts, architec- tural designs, prominent characteristics, key technologies as well as research directions. As the development of cloud computing technology is still at an early stage, we hope our work will provide a better understanding of the design chal- lenges of cloud computing, and pave the way for further re- search in this area.
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