Abstract:
Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for an advertisement auction system. In one aspect, a method includes operating bidders on a data processing apparatus, each bidder executing in a distinct virtual machine, wherein a host operating system that executes on the data processing apparatus performs operations including: receiving an advertisement bid request from a requestor using a first communication protocol, the bid request identifying a bidder, the data processing apparatus and the ad exchange being on a same internal network; sending the advertisement bid request to the virtual machine of the identified bidder using a second communication protocol; receiving an advertisement bid response for the bid request from the identified bidder using the second communication protocol; and sending the advertisement bid response to the requestor using the first communication protocol.

Description:
BACKGROUND 
     This specification relates to virtual machine communications and, in particular, advertising bidding using virtual machines. 
     Cloud computing is network-accessible computing in which typically large collections of servers housed in data centers or “server farms” provides computational resources and data storage as needed to remote end users. Some cloud computing services allow end users to run user-provided server software applications (e.g., e-commerce server application, web server, or file server) in the cloud. Some other cloud computing services provide access to software applications such as word processors and other commonly used applications to end users who interface with the applications through web browsers or other client-side software. Users&#39; electronic data files are usually stored in the server farm rather than on the users&#39; computing devices. Maintaining software applications and user data on the server farm may simplify management of end user computing devices. Some cloud computing services allow end users to execute software applications in virtual machines. 
     SUMMARY 
     The system described can efficiently implement a real-time bidding solution. The system can achieve this by placing bidders and an advertisement exchange on the same internal network. Using custom communication protocols, the advertisement exchange also can quickly communicate to a network of virtual machines through a load balancer. The load balancer communicates with a virtual machine through a host operating system. In this way, the virtual machine can place bids with the advertisement exchange. 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in methods that include the actions of operating one or more bidders on the data processing apparatus, each bidder executing in a distinct virtual machine that is a hardware virtualization and wherein the virtual machines are hosted by a host operating system that executes on the data processing apparatus, wherein the host operating system performs operations comprising: receiving an advertisement bid request from a requestor using a first communication protocol, the bid request identifying a bidder of the one or more bidders, wherein the data processing apparatus and the ad exchange are on a same internal network; sending the advertisement bid request to the virtual machine of the identified bidder using a second communication protocol; receiving an advertisement bid response for the bid request from the identified bidder using the second communication protocol; and sending the advertisement bid response to the requestor using the first communication protocol. Other embodiments of this aspect include corresponding systems, apparatus, and computer programs, configured to perform the actions of the methods, encoded on computer storage devices. 
     These and other embodiments can each optionally include one or more of the following features. Sending the advertisement bid request to the virtual machine further comprises creating a new advertisement bid request in a different format than the advertisement bid request and sending the new advertisement bid request to the virtual machine. Sending the advertisement bid response to the requestor further comprises creating a new advertisement bid response in a different format than the advertisement bid response and sending the new advertisement response to the requestor. The first protocol can be a Remote Procedure Call protocol. The second protocol can use shared memory to communicate data between the host operating system and the identified bidder. The advertisement bid request can include bid information and the advertisement bid response includes one or more of a bid amount and an advertisement that will be shown. The bid information comprises one or more of a cookie, offer price, and a uniform resource locator for a web page on which an advertisement will be placed. The requestor can select the bidder using a load balancing algorithm. The load balancing algorithm can select a bidder based on random selection, round-robin selection, work load of the bidder, or latency of the bidder. The requestor can be an ad exchange. The first and second protocols can be different. 
     Particular embodiments of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. The system can efficiently communicate with virtual machines to support real-time bidding by reducing latency between an advertisement exchange and bidders. The system can support an advertisement exchange and bidders on the same internal network. A bidder can be represented as one or more virtual machines. Therefore, a bidder does not need to implement the bidder&#39;s own load balancing solution or establish the bidder&#39;s own server to respond to the advertisement exchange. Another advantage is that the advertisement exchange can communicate with external servers and process external requests. The advertisement exchange can manage simultaneous bids between virtual machines using a load balancer. The load balancer can use statistics (e.g., latency measurements) to manage load. Another advantage is that the system can dynamically add more virtual machines based on load statistics. Yet another advantage is that the load balancer can consider whether a virtual machine, host machine, or network is overloaded in determining how to manage load. Host operating systems can quickly communicate with virtual machines using Transmission Control Protocol (TCP) or paravirtualized socket support to reduce latency. 
     The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic illustration of an example virtual machine system. 
         FIG. 1B  is a schematic diagram of an example bidding system. 
         FIG. 2  is a schematic illustration of an example system that provides efficient communication between virtual machines to support real-time bidding. 
         FIG. 3  is a flow chart of an example method of processing an advertisement exchange&#39;s bid request with an internal bidder. 
         FIG. 4  is a flow chart of an example method of communicating between a host operating system and a virtual machine. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1A  is a schematic illustration of an example virtual machine system  100 . The system  100  includes one or more host machines such as, for example, host machine  102  and host machine  104 . Generally speaking, a host machine is one or more data processing apparatus such as rack mounted servers or other computing devices. The data processing apparatus can be in different physical locations and can have different capabilities and computer architectures. Host machines can communicate with each other through an internal data communications network  116 . The internal network can include one or more wired (e.g., Ethernet) or wireless (e.g., WI-FI) networks, for example. In some implementations the internal network  116  is an intranet. Host machines can also communicate with devices on external networks, such as the Internet  122 , through one or more gateways  120  which are data processing apparatus responsible for routing data communication traffic between the internal network  116  and the external network  122 . Other types of external networks are possible. 
     Each host machine executes a host operating system or other software that virtualizes the underlying host machine hardware and manages concurrent execution of one or more virtual machines (e.g., a virtual machine monitor or hypervisor). For example, the host operating system  106  is managing virtual machine (VM)  110  and VM  112 , while host OS  108  is managing a single VM  114 . Each VM includes a simulated version of the underlying host machine hardware, or a different computer architecture. The simulated version of the hardware is referred to as virtual hardware (e.g., virtual hardware  110   a ,  112   a  and  114   a ). Software that is executed by the virtual hardware is referred to as guest software. In some implementations, guest software cannot determine if it is being executed by virtual hardware or by a physical host machine. If guest software executing in a VM, or the VM itself, malfunctions or aborts, other VMs executing on the host machine will not be affected. A host machine&#39;s microprocessor(s) can include processor-level mechanisms to enable virtual hardware to execute software applications efficiently by allowing guest software instructions to be executed directly on the host machine&#39;s microprocessor without requiring code-rewriting, recompilation, or instruction emulation. 
     Each VM (e.g., VMs  110 ,  112  and  114 ) is allocated a set of virtual memory pages from the virtual memory of the underlying host operating system and is allocated virtual disk blocks from one or more virtual disk drives for use by the guest software executing on the VM. For example, host operating system  106  allocates memory pages and disk blocks to VM  110  and VM  112 , and host operating system  108  does the same for VM  114 . In some implementations, a given VM cannot access the virtual memory pages assigned to other VMs. For example, VM  110  cannot access memory pages that have been assigned to VM  112 . A virtual disk drive can be persisted across VM restarts. Virtual disk blocks are allocated on physical disk drives coupled to host machines or available over the internal network  116 , for example. In addition to virtual memory and disk resources, VMs can be allocated network addresses through which their respective guest software can communicate with other processes reachable through the internal network  116  or the Internet  122 . For example, guest software executing on VM  110  can communicate with guest software executing on VM  112  or VM  114 . In some implementations, each VM is allocated one or more unique Internet Protocol (IP) version 4 or version 6 addresses and one or more User Datagram Protocol (UDP) port numbers. Other address schemes are possible. 
     A VM&#39;s guest software can include a guest operating system (e.g., guest operating systems  110   b ,  112   b  and  114   b ) which is software that controls the execution of respective guest software applications (e.g., guest applications  110   c ,  112   c  and  114   c ), within the VM and provides services to those applications. For example, a guest operating system could be a variation of the UNIX operating system. Other operating systems are possible. Each VM can execute the same guest operating system or different guest operating systems. In further implementations, a VM does not require a guest operating system in order to execute guest software applications. A guest operating system&#39;s access to resources such as networks and virtual disk storage is controlled by the underlying host operating system. 
     By way of illustration, and with reference to virtual machine  110 , when the guest application  110   c  or guest operating system  110   b  attempts to perform an input/output operation on a virtual disk, initiate network communication, or perform a privileged operation, for example, the virtual hardware  110   a  is interrupted so that the host operating system  106  can perform the action on behalf of the virtual machine  110 . The host operating system  106  can perform these actions with a process that executes in kernel process space  106   b , user process space  106   a , or both. 
     The kernel process space  106   b  is virtual memory reserved for the host operating system  106 &#39;s kernel  106   d  which can include kernel extensions and device drivers, for instance. The kernel process space has elevated privileges (sometimes referred to as “supervisor mode”); that is, the kernel  106   d  can perform certain privileged operations that are off limits to processes running in the user process space  106   a . Examples of privileged operations include access to different address spaces, access to special functional processor units in the host machine such as memory management units, and so on. The user process space  106   a  is a separate portion of virtual memory reserved for user mode processes. User mode processes cannot perform privileged operations directly. 
     In various implementations, a portion of VM network communication functionality is implemented in a communication process (e.g., communication process  106   c ). In some implementations, the communication process executes in the user process space (e.g., user process space  106   a ) of a host operating system (e.g., host operating system  106 ). In other implementations, the communication process can execute in the kernel process space (e.g., kernel process space  106   d ) of the host operating system or be implemented in hardware. There can be a single communication process for all VMs executing on a host machine or multiple communication processes, one for each VM executing on a host machine. In yet further implementations, some portion of the communication process executes in the user process space and another portion executes in the kernel process space. The communication process communicates with a directory service (e.g., VM registry service  118 ) in order to establish a virtual network pair (VNP) between two VMs. A virtual network pair (VNP) is a logical computer network that is implemented on top of one or more physical (wired or wireless) computer networks. A VNP can be implemented with a tunneling protocol, for example. A VNP routes traffic between two endpoints using one or more virtual connections or links. By way of illustration, a VNP between virtual machine  110  and virtual machine  114  would route packets sent between VNP endpoints managed respectively by communication processes  106   c  and  108   c  over internal network  116 . The VM registry service  118  is one or more data processing apparatus that execute software for keeping track of assignments of network addresses (e.g., IP addresses) to VMs, and for keeping track of network addresses (e.g., IP addresses) of host machines that the VMs are executing on. The data processing apparatus can be in different locations and can have different capabilities and computer architectures. 
       FIG. 1B  is a schematic diagram of an example bidding system  124 . The system  124  can support real-time bidding for advertisement placements or placement of other assets. In some implementations, a user having a user device  126  (e.g., a data processing apparatus such as a smart phone or a portable computer) views a resource provided by a publisher  128  server. Resources can be web pages, mobile applications, videos, electronic games, electronic books, documents, or other types of content. The resource can include executable instructions such as JavaScript or other information, for example, that causes an advertisement to be fetched and provided with the content of the resource. 
     In some implementations, when the user device  126  processes the resource, the user device can send a request to a server specified in the instructions of the resource for the server to provide an advertisement to the user device  126 . In response, the publisher  128  can fetch the advertisement and subsequently send the advertisement to the user device  126 . Alternatively, the publisher  128  can obtain the advertisement before sending the resource to the user device  126  and provide the advertisement with the resource. In some implementations, the advertisement is chosen by an advertisement exchange  130  (or “ad exchange”). The ad exchange  130  is a system that facilitates the buying and selling of advertisements through an automated auction. The ad exchange  130  communicates with one or more bidders  132  (e.g., data processing apparatus) to auction an advertisement spot for placement with a resource by sending bid requests to the bidders  132 . Bidders  132  respond with bid responses. Bidders  132  can be part of advertisement networks that serve ads for multiple clients, or bidders  132  can be individual entities running advertisement campaigns. If a bidder wins an auction, the ad exchange  130  provides the bidder&#39;s advertisement to the publisher  128 , which in turn sends the advertisement to the user device  126 . The user can then view the advertisement using the user device  126 . Because the user device  126 , publisher  128 , ad exchange  130 , and bidders  132  can be different machines in different geographic locations, network latency can delay the delivery of the advertisements to the user device  126 . 
       FIG. 2  is a schematic illustration of an example system  200  that provides efficient communication between virtual machines to support real-time bidding. To reduce latency, the system  200  includes the internal network  116  that facilitates communication between the ad exchange  130  and internal bidders  202  without requiring their communications to flow over the public Internet  122 . By decreasing latency between the ad exchange  130  and the internal bidders  202 , the overall advertisement serving latency experienced by the user device  126  can be reduced. This also allows the internal bidders  202  to run more complex algorithms and better determine how to respond to a bid request from the ad exchange  130 . In various implementations, the ad exchange  130  is implemented as one or more VMs (e.g., VM  110 , VM  114 ). 
     When the ad exchange  130  receives a request to fetch an advertisement from a publisher, the ad exchange  130  can send bid requests to advertisers or aggregators that have requested to receive bids. Each advertiser or aggregator can be associated with a group of bidders. The ad exchange  130  can direct a bid request to one or more bidders within the group of bidders. In some implementations, the one or more bidders are selected based partially on the content of the advertisement. Bidders can reside in an internal network  116  (e.g., internal bidders  202 ) or outside of the internal network  116  (e.g., external bidders  132 ). In various implementations, internal bidders  202  are implemented using one or more virtual machines (e.g., VM  110 , VM  114 ). External bidders  132  can be implemented using data processing apparatus (e.g., servers). Communication between the ad exchange  130  and the external bidders  132  travels over the Internet  122  and through zero or more gateways  120 , which can introduce latency in the communications. 
     The ad exchange  130  can send bid requests to numerous bidders and simultaneously process incoming bid responses from bidders using a load balancer  214 . The load balancer  214  can subscribe to a directory of available bidders. The directory of available bidders can include a directory of internal bidders  202  and, optionally, external bidders  132 . In the case of internal bidders  202 , the directory can include the identification of the virtual machine (s) used by a given internal bidder. The directory of bidders can be managed by the VM Registry Service  118 , for example. 
     After the ad exchange  130  selects an advertiser or aggregator, the load balancer  214  can select one or more of an internal bidder (e.g., via a VM&#39;s address) and an external bidder (e.g., via an Internet Protocol “IP” address) associated with the advertiser or aggregator. In some implementations, the load balancer  214  selects a bidder using a selection algorithm. The selection algorithm can utilize random selection of bidders or selection through a round robin of bidders, for example. Other selection techniques are possible. In some implementations, internal bidders  202  publish the amount of work (or “load”) they are performing to the load balancer  214 , and the load balancer  214  selects one or more internal bidders having the lightest load. In some implementations, the load balancer  214  periodically checks the load of each internal bidder and selects one or more of the internal bidders  202  having the lightest load. In some implementations, the load balancer  214  records the latency of an internal bidder&#39;s responses to bid requests over time and increases or decreases an estimation of the internal bidder&#39;s load based on the latency. In some implementations, the algorithm is a hybrid of one or more of the above techniques. 
     In some implementations, the load balancer  214  communicates with each internal bidder through the internal bidder&#39;s host operating system (e.g., host operating system  106 ). Each internal bidder (e.g., a VM) is associated with a host operating system. For example, VM  110  is monitored by host operating system  106 . Likewise, VM  114  is monitored by host operating system  108 . A host operating system can be the primary interface between the internal bidder and the load balancer. The host operating system of an internal bidder can ensure the internal bidder is running. In some implementations, once an internal bidder is selected, the load balancer  214  communicates to the host operating system of the selected internal bidder using a remote procedure call (RPC), for example. RPC is an inter-process communication that can be used to allow a computer program executing on one computer to cause a method, function, subroutine or procedure to execute on another computer on a shared network without the programmer explicitly having to code the details of how information is communicated to the other computer. In further implementations, the load balancer  214  communicates with the host operating system of the selected internal bidder using a switched fabric networking technology like infiniband, or other hardware accelerated networking solutions. 
       FIG. 3  is a flow chart of an example method  300  of processing an ad exchange&#39;s bid request with an internal bidder. As mentioned above, the ad exchange (e.g., ad exchange  130 ) first receives a request to serve an advertisement  302 . The ad exchange can select advertisers or aggregators interested in bidding for the request. After selecting an advertiser or aggregator, the ad exchange then sends a bid request to the load balancer  303  (e.g., load balancer  214 ) using RPC. In some implementations, the ad exchange sends a bid request for each advertiser or aggregator. The load balancer selects one or more internal bidders (e.g., internal bidders  202 ) on the internal network associated with the advertiser or aggregator, as described above  304 . The load balancer communicates to the selected internal bidder(s) through each internal bidder&#39;s host operating system. The host operating system converts the RPC request to a network protocol request that the internal bidder can process. For example, the host operating system can form an HTTP request that the internal bidder can process  306 . The host operating system can communicate the bid request to the internal bidder in multiple ways as will be described further below. In some implementations, a bid request includes bid information about the request to fetch an advertisement (e.g., a cookie) and a page URL that will be displaying the advertisement. Once the internal bidder processes the bid request to generate a bid response  308 , the internal bidder sends the bid response to the ad exchange  310 . For example, after receiving a bid request that asks for $0.25 to display an advertisement, the internal bidder can process the amount and generate a ‘yes’ or a ‘no’ to send back to the ad exchange. The bid response can also include the bid amount and the actual advertisement that will be shown. The ad exchange can then process the bid response by sending more bid requests to multiple internal or external bidders, or the ad exchange can select a winner of the auction  312 . In some implementations, the bid response uses the same protocols that the bid request uses. 
       FIG. 4  is a flow chart of an example method  400  of communicating between a host operating system  108  and a VM  114  on a host machine (e.g., host machine  108 ). In some implementations, the host operating system  108  communicates with the VM  114  using Transmission Control Protocol (TCP)  404 . As mentioned above, the host operating system can form an HTTP request that the VM  114  can process. The host operating system  108  can use standard TCP protocols to break down the HTTP request into one or more packets and communicate those packets to the VM  114 . In other words, because VMs can simulate isolated environments, communicating using TCP requires both the VM  114  and the host operating system  108  to abide by standard protocols. On the other hand, communicating using TCP will not require modification to the host operating system  108 . Communicating using TCP can also be optimized because TCP can benefit systems that are not physically close to each other by addressing issues inherent in a network environment. For example, TCP includes a reliable transmission functionality that ensures a packet is delivered over a network. However, here, because the VM  114  is on the same host machine as the host operating system  108 , TCP&#39;s reliable transmission, error checking, and other functionality optimized for a network environment can be simplified for a single system environment to reduce latency. 
     In some implementations, the host operating system  108  communicates to the VM  114  using a paravirtualized socket  406 . Using a paravirtualized socket is a virtualization technique that presents a software interface to virtual machines that is optimized for a virtualized environment. A paravirtualized socket can remove latency issues introduced by using a protocol like TCP. For example, a paravirtualized socket can require much less hardware emulation and can reduce the number of slow transitions out of the VM&#39;s context, thus improving the efficiency of the communication. A paravirtualized socket can also allow the host operating system  108  and the VM  114  to communicate without requiring network error checking and network error recovery. In some implementations, a paravirtualized socket can utilize a shared memory ring buffer between the host operating system  108  and the VM  114 . The host operating system  108  can send and receive the data from the VM  114  using the shared memory ring buffer. In some implementations, a shared memory buffer is a special section of memory that is mapped into both the VM and the host operating system. A ring buffer allows the VM and host operating system to both be reading and writing at the same time with a minimum of copying. In some implementations, to ensure compatibility, the kernel of the VM  114  creates packets encapsulating the data from the shared memory ring buffer to mimic a protocol such as TCP. 
     Embodiments of the subject matter and the operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions can be encoded on an artificially-generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. A computer storage medium can be, or be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium can be a source or destination of computer program instructions encoded in an artificially-generated propagated signal. The computer storage medium can also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices). 
     The operations described in this specification can be implemented as operations performed by a data processing apparatus on data stored on one or more computer-readable storage devices or received from other sources. 
     The term “data processing apparatus” encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, a system on a chip, or multiple ones, or combinations, of the foregoing The apparatus can include special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). The apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a cross-platform runtime environment, a virtual machine, or a combination of one or more of them. The apparatus and execution environment can realize various different computing model infrastructures, such as web services, distributed computing and grid computing infrastructures. 
     A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, object, or other unit suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language resource), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub-programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
     The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform actions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). 
     Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for performing actions in accordance with instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) receiver, or a portable storage device (e.g., a universal serial bus (USB) flash drive), to name just a few. Devices suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
     To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending resources to and receiving resources from a device that is used by the user; for example, by sending web pages to a web browser on a user&#39;s client device in response to requests received from the web browser. 
     Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back-end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front-end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation of the subject matter described in this specification, or any combination of one or more such back-end, middleware, or front-end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), an inter-network (e.g., the Internet), and peer-to-peer networks (e.g., advertisement hoc peer-to-peer networks). 
     The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. In some embodiments, a server transmits data (e.g., an HTML page) to a client device (e.g., for purposes of displaying data to and receiving user input from a user interacting with the client device). Data generated at the client device (e.g., a result of the user interaction) can be received from the client device at the server. 
     A system of one or more computers can be configured to perform particular operations or actions by virtue of having software, firmware, hardware, or a combination of them installed on the system that in operation causes or cause the system to perform the actions. One or more computer programs can be configured to perform particular operations or actions by virtue of including instructions that, when executed by data processing apparatus, cause the apparatus to perform the actions. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Thus, particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. In addition, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.