Abstract:
A method for electing a master blade in a virtual application distribution chassis (VADC), includes: sending by each blade a VADC message to each of the other blades; determining by each blade that the VADC message was not received from the master blade within a predetermined period of time; in response, sending a master claim message including a blade priority by each blade to the other blades; determining by each blade whether any of the blade priorities obtained from the received master claim messages is higher than the blade priority of the receiving blade; in response to determining that none of the blade priorities obtained is higher, setting a status of a given receiving blade to a new master blade; and sending by the given receiving blade a second VADC message to the other blades indicating the status of the new master blade of the given receiving blade.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of co-pending U.S. patent application Ser. No. 13/363,055, filed on Jan. 31, 2012, which in turn is a continuation of U.S. patent application Ser. No. 13/004,861, filed on Jan. 11, 2011. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field 
     This invention relates generally to data communications, and more specifically, to a virtual application delivery chassis system. 
     2. Background 
     Web services and cloud computing are deployed in an unprecedented pace. New servers are unloaded and installed at datacenters every day. Demands of web services and corporate computing come from all directions. Consumer oriented services include iPhone™ apps, mobile applications such as location based services, turn-by-turn navigation services, e-book services such as Kindle™, video applications such as YouTube™ or Hulu™, music applications such as Pandora™ or iTunes™, Internet television services such as Netflix™, and many other fast growing consumer Web services. On the corporate front, cloud computing based services such as Google™ docs, Microsoft™ Office Live and Sharepoint™ software, Salesforce.com™&#39;s on-line software services, tele-presence and web conferencing services, and many other corporate cloud computing services. 
     As a result more and more servers are deployed to accommodate the increasing computing needs. These servers are typically managed by server load balancers (SLB) or application delivery controllers (ADC). ADC is typically a network appliance, such as A10 Network&#39;s AX-Series traffic manager. ADC manages the load balancing and delivery of service sessions from client host computers to servers based on incoming service requests. As more servers are deployed, more ADC&#39;s are deployed accordingly. Similarly as more servers are pooled together within a data center or spread across multiple data centers to provide a scalable solution for services, ADC&#39;s become a bottleneck. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment of the present invention, a method for electing a master blade in a virtual application distribution chassis, the virtual application distribution chassis comprising a plurality of blades, comprises: (a) sending by each blade of the plurality of blades a virtual application distribution chassis message to each of the other blades in the virtual application distribution chassis; (b) determining by each blade whether the virtual application distribution chassis message was received from the master blade within a predetermined period of time; (c) in response to determining that the virtual application distribution message was not received from the master blade within the predetermined period of time, sending a master claim message by each blade to each of the other blades in the virtual application distribution chassis, each master claim message comprising a blade priority for the sending blade; (d) determining by each blade receiving the master claim messages whether any of the blade priorities obtained from the received master claim messages is higher than the blade priority of the receiving blade; (e) in response to determining that by a given receiving blade that none of the blade priorities obtained from the received master claim messages is higher than the blade priority of the given receiving blade, setting a status of the given receiving blade to a new master blade; and (f) sending by the given receiving blade a second virtual application distribution chassis message to the other blades in the virtual application distribution chassis, the second virtual application distribution chassis message indicating the status of the new master blade of the given receiving blade. 
     In one aspect of the present invention, the plurality of blades comprises a master slave and one or more slave blades, wherein if the at least one slave blade is operating properly, the at least one slave blade sends the virtual application distribution chassis message to each of the other blades in the virtual application distribution chassis, and wherein if the master blade is operating properly, the master blade sends the virtual application distribution chassis message to each of the other blades in the virtual application distribution chassis. 
     In one aspect of the present invention, the plurality of blades comprises a master blade and one or more slave blades, where the determining (b) further comprises: (b1) determining by each blade whether the virtual application distribution chassis message was received from each slave blade of the virtual application distribution chassis within the predetermined period of time; (b2) in response to determining that the virtual application distribution chassis message was not received from a given slave blade within the predetermined period of time, marking the given slave blade as “failed” by each blade receiving the virtual application distribution messages; and (b3) in response to determining that the virtual application distribution chassis message was received from the given slave blade within the predetermined period of time, marking the given slave blade as properly operating by each blade receiving the virtual application distribution chassis messages. 
     In one aspect of the present invention, the determining (d) and the setting (e) comprises: (d1) comparing, by the given receiving blade, the blade priority obtained from a given master claim message with the blade priority of the given receiving blade; (e1) in response to determining that the blade priority of the given receiving blade is higher than the blade priority obtained from the given master claim message, repeating the comparing (d1) with another received master claim message; and (e2) in response to determining that the blade priority of the given receiving blade is higher than the blade priorities obtained from each of the other received master claim messages, setting the status of the given receiving blade to the new master blade. 
     In one aspect of the present invention, the determining (d) and the setting (e) comprise: (d1) comparing, by the given receiving blade, the blade priority obtained from a given master claim message with the blade priority of the given receiving blade; and (e1) in response to determining that the blade priority obtained from the given master claim message is higher than the blade priority of the given receiving blade, maintaining a slave status of the given receiving blade. 
     In one aspect of the present invention, each master claim message further comprises a blade identity of the sending blade, wherein the comparing (d1), the repeating (e1), and the setting (e2) comprises: (d1i) determining by the given receiving blade that the blade priority obtained from the given master claim message is equal to the blade priority of the given receiving blade; (d1ii) in response to determining that the blade priority obtained from the given master claim message is equal to the blade priority of the given receiving blade, determining whether the blade priority obtained from the given master claim message is numerically smaller than the blade priority of the given receiving blade; (e1i) in response to determining that the blade priority obtained from the given master claim message is numerically smaller than the blade priority of the given receiving blade, repeating the comparing (d1) with another received claim message; and (e2i) in response to determining that the blade priority of the given receiving blade is either higher than, or equal to and numerically smaller than, the blade priorities obtained from each of the other received master claim messages, setting the status of the given receiving blade to the new master blade. 
     In one aspect of the present invention, the sending (f) comprises: (f1) determining by a given slave blade of the plurality of blades whether the second virtual application distribution chassis message was received within a second predetermined period of time; and (f2) in response to determining by the given slave blade that the second virtual application distribution chassis message was not received within the second predetermined period of time, setting a status of the given slave blade to the new master blade and sending the second virtual application distribution message by the given slave blade to the other blades in the virtual application distribution chassis. 
     In one aspect of the present invention, the method further comprises: (g) adding a new blade to the virtual application distribution chassis; (h) sending a second master claim message by the new blade to each of the other blades in the virtual application distribution chassis; (i) in response to receiving the second master claim message from the new blade, determining by each of the other blades whether the master blade is marked as properly operating; and (j) in response to determining by each of the other blades that the master blade is marked as properly operating, ignoring the second master claim message, wherein otherwise, performing the sending (c), the determining (d), the setting (e), and the sending (f). 
     In one aspect of the present invention, the plurality of blades are operatively coupled as the virtual application distribution chassis within a data network, the data network comprising a network switch wherein the method further comprises: (g) assigning a first blade of the plurality of blades as an active blade to a virtual address, and assigning a second blade of the plurality of blades as a backup blade to the virtual address, wherein service requests destined for the virtual address are sent to the first blade by the network switch. 
     In one aspect of the present invention, the method further comprises: (h) determining by each blade that the virtual application distribution chassis message was not received from the first blade within the predetermined period of time; (i) in response to determining that the virtual application distribution chassis message was not received from the first blade within the predetermined period of time, changing a status of the second blade to the active blade for the virtual address; and (j) sending a virtual address change message to the network switch to inform the network switch to send future service requests destined for the virtual address to the second blade. 
     System and computer program products corresponding to the above-summarized methods are also described and claimed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE FIGURES 
         FIG. 1  illustrates a virtual application delivery chassis distributing a service session from a host to a server according to an embodiment of the present invention. 
         FIG. 2  illustrates components of an application delivery blade, a host and a server according to an embodiment of the present invention. 
         FIG. 3  illustrates connectivity of application delivery blades in the virtual application distribution chassis according to an embodiment of the present invention. 
         FIG. 4  illustrates the roles of blades in the virtual application distribution chassis according to an embodiment of the present invention. 
         FIG. 5  illustrates a process to elect a master blade in the virtual application distribution chassis according to an embodiment of the present invention. 
         FIG. 6  illustrates a blade joining the virtual application distribution chassis according to an embodiment of the present invention. 
         FIG. 7  illustrates service connectivity between virtual application delivery chassis and the host according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     The invention can take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment containing both hardware and software elements. In a preferred embodiment, the invention is implemented in software, which includes but is not limited to firmware, resident software, microcode, etc. 
     Furthermore, the invention can take the form of a computer program product accessible from a computer-usable or computer-readable medium providing program code for use by or in connection with a computer or any instruction execution system. For the purposes of this description, a computer-usable or computer readable medium can be any apparatus that can contain, store, communicate, propagate, or transport eh program for use by or in connection with the instruction execution system, apparatus, or device. 
     The medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device) or a propagation medium. Examples of a computer-readable medium include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Current examples of optical disks include compact disk—read only memory (CD-ROM), compact disk—read/write (CD-R/W) and DVD. 
     A data processing system suitable for storing and/or executing program code will include at least one processor coupled directly or indirectly to memory elements through a system bus. The memory elements can include local memory employed during actual execution of the program code, bulk storage, and cache memories which provide temporary storage of at least some program code in order to reduce the number of times code must be retrieved from bulk storage during execution. 
     Input/output or I/O devices (including but not limited to keyboards, displays, point devices, etc.) can be coupled to the system either directly or through intervening I/O controllers. 
     Network adapters may also be coupled to the system to enable the data processing system to become coupled to other data processing systems or remote printers or storage devices through intervening private or public networks. Modems, cable modem and Ethernet cards are just a few of the currently available types of network adapters. 
     The block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified local function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams, and combinations of blocks in the block diagrams, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
       FIG. 1  illustrates a virtual application delivery chassis  300  delivering an application service session  401  from a server  200  to host  100  according to an embodiment of the present invention. The virtual application delivery chassis  300  receives from host  100  a service request  403  of application service session  401  between host  100  and server  200 . Service request  403  is delivered over a data network  153 . In one embodiment, service session  401  is a Web service session and service request  403  is a Web service request such as a HTTP (Hypertext Transport Protocol) request, a secure HTTP request, a FTP (File Transfer Protocol) request, a file transfer request, an SIP (Session Initiation Protocol) session request, a request based on Web technology, a video or audio streaming request, a Web conferencing session request, or any request over the Internet or corporate network. 
     In one embodiment, service session  401  is a session for a non-Web service such as a remote access service, a file and print service, a name or directory service, an enterprise application service, a database application service or a messaging service. Service request  403  is a corresponding request of service session  401 . 
       FIG. 2  illustrates components of an application delivery blade, a host, and a server according to an embodiment of the present invention. Host  100  is a computing device with network access capabilities. As shown in  FIG. 2 , the host  100  is operationally coupled to a processor  103 , a computer readable medium  104  and a network interface  105 . The computer readable medium  104  stores computer readable program code, which when executed by the processor  103 , implements the various embodiments of the present invention as described herein. The network interface  105  connects to data network  153 . Examples of network interface  105  include Ethernet, WiFi, mobile network interface, Bluetooth, WiMAX, digital subscriber line (DSL), cable interface, broadband network interfaces such as T1 or T3, optical network interfaces, wireless network interfaces or other data network interfaces. In one embodiment, host  100  is a workstation, a desktop personal computer or a laptop personal computer. In one embodiment, host  100  is a Personal Data Assistant (PDA), a smartphone, a tablet PC, or a cellular phone. In one embodiment, host  100  is a set-top box, an Internet media viewer, an Internet media player, a smart sensor, a smart medical device, a net-top box, a networked television set, a networked DVR, a networked Blu-ray player, or a media center. 
     In  FIG. 1 , the virtual application delivery chassis  300  includes a plurality of blades, such as blade  310 , blade  312 , blade  314  and blade  316 . Each blade, for example blade  310 , as illustrated in  FIG. 2 , is operationally coupled to a processor  303 , a computer readable medium  304  and a network interface  305 . The computer readable medium  304  stores computer readable program code, which when executed by the processor  303 , implements the various embodiments of the present invention as described herein. In some embodiments, virtual application delivery chassis  300  is implemented to function as a server load balancer, an application delivery controller, a service delivery platform, a traffic manager, a security gateway, a component of a firewall system, a component of a virtual private network (VPN), a load balancer for video servers, or a service gateway to distribute load to a plurality of servers. The network interface  305  connects to data network  155 . In  FIG. 1 , blades  310 ,  312 ,  314 ,  316  also connect to data network  151 . The blades  310 - 312 ,  314 ,  316  form virtual application delivery chassis  300  through their connections to the data network  151 . In one embodiment, the network interface  305  includes various embodiments of the network interface  105 . In one embodiment, the network interface  305  further includes an optical network interface. 
     Illustrated in  FIG. 2 , server  200  is operationally coupled to a processor  203 , a computer readable medium  204  and a network interface  205 . The computer readable medium  204  stores computer readable program code, which when executed by the processor  203 , implements the various embodiments of the present invention as described herein. In some embodiments, the computer readable program code implements server  200  to function as a Web server, a file server, a video server, a database server, a messaging server, a file server, a printer server, an application server, a voice system, a conferencing server, a media megateway, a SIP server, a remote access server, a VPN server, or a media center. The network interface  205  connects to data network  155 . 
     Back to  FIG. 1 , in one embodiment, data network  153  connects the host  100  and the virtual application delivery chassis  300 . In one embodiment, data network  153  is an Internet Protocol (IP) network. In one embodiment, data network  153  is a corporate data network or a regional corporate data network, including but not limited to an intranet, a private network cloud, a virtual private network, or a campus area network. In one embodiment, data network  153  is an Internet service provider network. In one embodiment, data network  153  is a residential data network, including but not limited to a broadband network such as a DSL network, a cable network, or a personal area network. In one embodiment, data network  153  includes a wired network such as Ethernet. In one embodiment, data network  153  includes a wireless network such as a WiFi network, or cellular network. In one embodiment, the network interface  205  includes various embodiments of the network interface  105  and the network interface  305 . 
     Data network  151  connects virtual application delivery chassis  300  to data network  153 . In one embodiment, data network  151  includes various embodiments of data network  153 . In one embodiment, data network  151  resides in a data center, spans across multiple data centers over a wide area network such as optical network, or spreads over multiple buildings in a campus area network. 
     Data network  155  connects the server  200  to data network  151 . In one embodiment, data network  155  is a sub-network of includes data network  151 . In one embodiment, data network  155  includes various embodiments of data network  151 . In one embodiment, data network  151  is a different network from data network  155 . 
     In an embodiment, the host  100  initiates an application service session  401  with the server  200 . The host  100  sends a service request  403  to virtual application delivery chassis  300 . After virtual application delivery chassis  300  receives service request  403 , virtual application delivery chassis  300  establishes a server session  405  and relays service request  403  to the server  200 . Virtual application delivery chassis  300  receives response through server session  405  from the server  200 . Virtual application delivery chassis  300  subsequently relays the response through server session  405  as a response to service request  403 . 
       FIG. 3  illustrates connectivity of the blades of a virtual application delivery chassis according to an embodiment of the present invention. In a preferred embodiment, blade  310  is a piece of hardware separated from other blades. Blade  310  connects to other blades, such as blade  312 , blade  314  and blade  316 , over data network  151 . In one embodiment, data network  151  includes a plurality of network switches such as switch  414 , switch  412  and switch  416 . Network switch  412  or network switch  414  is a networking device connecting a plurality of blades and other network switches. In one embodiment, switch  412  is an Ethernet switch, an IP router, an optical network switch, WAN network equipment, an ATM switch, a MPLS switch, a layer-2 network switch/hub/bridge, or a layer-3 network switch/router. In one embodiment as illustrated in  FIG. 3 , switch  412  connects blade  312 , blade  310  and switch  416 ; switch  414  connects blades  314 , blade  316  and switch  416 . In this embodiment, blade  312  communicates with blade  310  using switch  412 , and to blade  316  using switch  412 , switch  416  and switch  414 . 
     In one embodiment, switch  412 , switch  414  and switch  416  are located in the same geographic location, such as a data center. In one embodiment, switch  412 , switch  414  and switch  416  are located in different geographic locations, such as two or more data centers. In this embodiment, blades  310 ,  312 ,  314  and  316  are geographically located in different data centers. 
     Blade  310  communicates with other blades in the virtual application distribution chassis  300  by sending and receiving a virtual application delivery chassis message (VCS)  500 . In one embodiment blade  310  sends virtual application delivery chassis message  500  using a broadcast network address such as an Ethernet broadcast address. In one embodiment, the virtual application delivery chassis message  500  is sent using a multicast address such as an Ethernet multicast address, an IP multicast address, or other multicast network address. In one embodiment, blade  310  establishes a communication session (not shown) with blade  312  and sends virtual application delivery chassis message  500  to blade 312  using the communication session. Blade  310  also establishes separate communication sessions with the other blades in the virtual application distribution chassis  300  and sends virtual application delivery chassis message  500  to the other blades over their respective communication sessions. Examples of the communication session can be an UDP session, a TCP session, an IP-based communication  session, a peer-to-peer communication session, a unicast communication session or other form of data communication session. In one embodiment, blade  310  receives virtual application delivery chassis message  500  using a broadcast, a multicast address or a communication session with blade  312  or other blades in the virtual application distribution chassis  300 . 
       FIG. 4  illustrates the roles of blades in the virtual application distribution chassis according to an embodiment of the present invention. Each blade is configured with either a master or slave role in virtual application delivery chassis  300 , with only one blade configured with the master role. For example, assume that blade  312  is a master blade, blades  310 ,  314  and  316  are slave blades. Each slave blade, including slave blade  310 , sends a slave report message  510  to the other blades in the virtual application distribution chassis  300 . The slave report message  510  is a form of virtual application delivery chassis message  500 . Receipt of the slave report message  510  itself from slave blade  310  indicates to the other blades that slave blade  310  is operating properly. Optionally slave report message  510  includes other information about blade  310 , useful for functions outside the scope of the present invention. In an embodiment, slave blade  310  includes a timer  320 . The timer  320  measure a pre-configured period of time. When the timer  320  expires, slave blade  310  sends slave report message  510  to the other blades in the virtual application distribution chassis  300 . In one embodiment, slave blade  310  sends message  510  periodically every 30 seconds, 5 seconds, or 3 minutes. In one embodiment, slave blade  310  sends message  510  when there is a change of information in slave blade  310 , such as changes to a software version due to an upgrade. In one embodiment, slave blade  310  sends message  510  when slave blade  310  receives a corresponding slave report message or master advertisement message  504  from another blade in the virtual application distribution chassis  300 . 
     In an example illustrated in  FIG. 4 , slave blade  314  and master blade  312  receives slave report message  510  from slave blade  310 . Contrary to including status indication within the slave report message  510 , the receipt of the slave report message  510  itself indicates to slave blade  314  and master blade  312  that slave blade  310  is operating properly. Slave blade  314  and master blade  312  each marks slave blade  310  as being in proper running status. In one embodiment, slave blade  314  includes a timer  324 . If slave blade  314  does not receive message  510  from slave blade  310  before the timer  324  expires, slave blade  314  considers slave blade  310  as “failed”, i.e., not available to process service requests. Similarly master blade  312  considers slave blade  310  as “failed” if master blade  312  does not receive message  510  from slave blade  310  before timer  322  expires. 
     Master blade  312  sends out master advertisement message  504 , another form of virtual application delivery chassis message  500 , to inform the slave blades in the virtual application delivery chassis  300  that master blade  312  is running properly. In one embodiment, master blade  312  includes a timer  322  configured to measure a predetermined period of time, and sends master advertisement message  504  when the timer  322  expires. In one embodiment, master blade  312  sends master advertisement message  504  periodically, every half a second, 5 seconds, 30 seconds or 1 minute. In an embodiment, master blade  312  sends advertisement message  504  whenever there is a change of information of master blade  312 , such as changes to a software version due to an upgrade. 
     In one embodiment, each slave blade, including slave blade  314 , receives master advertisement message  504  from master blade  312 . Contrary to including status indication within the master advertisement message  504 , the receipt of the master advertisement message  504  itself indicates to the slave blades that master blade  312  is in proper running status. In the example illustrated in  FIG. 4 , when slave blade  314  receives the master advertisement message  504  before the timer  324  expires, slave blade  314  marks master blade  312  as running in normal operation. In an embodiment, if slave blade  314  does not receive master advertisement message  504  from master blade  312  before the timer  324  expires, slave blade  314  marks master blade  312  as “failed”. In this case, slave blades  310  and  316  similarly does not receive the master advertisement message  504  from master blade  312  before their respective timers  320  and  326  expire, and mark master blade  312  as “failed”. 
     After the failure of master blade  312  is recognized by slave blades  310 ,  314  and  316 , the slave blades will elect a new master blade.  FIG. 5  illustrates a process to elect a master blade in the virtual application distribution chassis according to an embodiment of the present invention. In  FIG. 5 , slave blades  310 ,  314  and  316  each consider themselves eligible to become the new master blade. Each blade  310 ,  314 ,  316  sends a master claim message  520 ,  524 ,  526  to the other blades in the virtual application delivery chassis  300 , where the master claim message includes a blade priority for the sending blade. The blade priorities are preconfigured and set forth the sequence of blades to be assigned a master role based on factors configurable by a chassis administrator. The master claim message is a form of virtual application delivery chassis message  500 . 
     For example, in  FIG. 5 , slave blade  310  sends master claim message  520  containing its blade priority  530 ; slave blade  314  sends master claim message  524  containing its blade priority  534 ; slave blade  316  sends master claim message  526  containing its blade priority  536 . Slave blade  314  receives master claim message  520  from slave blade  310  and obtains blade priority  530  from the master claim message  520 . Slave blade  314  compares its own blade priority  534  to the blade priority  530  from the master claim message  520  and determines that blade priority  530  is higher than blade priority  534 . In response, slave blade  314  maintains the role of a slave. Similarly slave blade  316  receives master claim message  520  from slave blade  310  and obtains blade priority  530  from the master claim message  520 . Slave blade  316  compares its own blade priority  536  with the blade priority  530  from the master claim message  520  and determines that blade priority  530  is higher than its own blade priority  536 . In response, slave blade  316  maintains the role of a slave. 
     Slave blade  310  receives master claim message  524  from slave blade  314  and obtains blade priority  534  from the master claim message  524 . Slave blade  310  compares its own blade priority  530  with the blade priority  534  from the master claim message  524  and determines that blade priority  530  is higher than blade priority  534 . Slave blade  310  also receives master claim message  526  from slave blade  316  and obtains blade priority  536  from the master claim message  526 . Slave blade  310  compares its own blade priority  530  with the blade priority  536  and determines that blade priority  530  is higher than blade priority  536 . In one embodiment, in response, slave blade  310  changes its role to master blade. In one embodiment, slave blade  310  includes the timer  320  and does not receive any master claim messages containing a blade priority higher than blade priority  530  before the timer  320  expires. In response, slave blade  310  changes its role to master blade. The new master blade  310  then informs the other blades in the virtual application distribution chassis  300  of its new master role by sending master advertisement message  515  to each of the other blades. 
     In one embodiment, slave blade  310  determines blade priority  530  equals blade priority  534 . In response, slave blade  310  uses a pre-determined arbitration process to determine if slave blade  310  or slave blade  314  has a higher priority. For example, in one embodiment, slave blade  310  is assigned a blade identity  330 . Slave blade  310  includes blade identity  330  in master claim message  520 . Similarly slave blade  314  is assigned blade identity  334  and includes blade identity  334  in master claim message  524 . Upon receiving master claim message  524 , slave blade  310  compares the blade identity  334  from the master claim message  524  with its own blade identity  330  and determines that blade identity  334  is numerically smaller than blade identity  330 . In response, slave blade  310  determines itself to have a higher priority than blade  314 . Assuming slave blade  310  does not receive any other master claim messages with either a higher blade priority or an equal blade priority and a higher blade identity, slave blade  310  changes its role to master blade. In one embodiment, the pre-determined arbitration process prefers numerically smaller blade identities instead of higher blade identities. The pre-determined arbitration process may use information other than blade identity without departing from the spirit and scope of the present invention. 
     When the blade  310  changes its role to master blade, blade  310  sends a master advertisement message  515  to each of the other blades in the virtual application distribution chassis  300 . The receipt of a master advertisement message  515  itself from blade  310  indicates to each of the other blades that blade  310  is a master blade. For example, when slave blade  314  receives master advertisement message  515  from master blade  310 , slave blade  314  records that master blade  310  is the master blade of the virtual application delivery chassis  300 . 
     In one embodiment, slave blade  314  includes a timer  324  and does not receive any master advertisement messages before the timer  324  expires. Slave blade  314  determines that a master blade has not been determined. In response, slave blade  314  elects itself to be the master blade and sends a master advertisement message to the other blades in the virtual application distribution chassis  300 . 
       FIG. 6  illustrates a process when a blade is added to a virtual application delivery chassis according to an embodiment of the present invention. Blade  316  is added to the virtual application delivery chassis  300 , which has blade  312  as the master blade. Upon being added, blade  316  sends a master claim message  526  containing its blade priority  536 . The other blades  310 ,  312 ,  314  in the virtual application delivery chassis  300  receive master claim message  526  from blade  316 , determines that they are each in report mode (i.e., the master blade  312  is marked as properly in running status) and ignores the message  526 . Master blade  312  sends out master advertisement message  504  upon the expiration of its timer  322  ( FIG. 4 ). In one embodiment, master advertisement message  504  includes the blade priority  532  of the master blade  312 . In response to receiving master advertisement message  504 , blade  316  sets its role to a slave blade. The receipt of the master advertisement message  504  itself indicates to blade  316  that the virtual application distribution chassis  300  already has a properly running master blade. Subsequently, upon the expiration of its timer  326  ( FIG. 4 ), slave blade  316  sends out slave report message  516  to the other blades in the virtual application distribution chassis  300 . The receipt of the slave report message  516  from the slave blade  316  by the other blades, including master blade  312 , indicates that blade  316  has newly joined the virtual application delivery chassis  300 , as described above with reference to  FIG. 4 . In this embodiment, the above described process may be used for adding a blade to an existing chassis or to a newly formed chassis. When forming a new virtual application distribution chassis, each blade that joins the chassis would send out a master claim message. Since there is not yet a master blade, i.e., none of the blades receives a master advertisement message before the expiration of its respective timers, the blades enter into the process of electing a master blade, as described above with reference to  FIG. 5 . 
     Revisiting the connectivity between virtual application delivery chassis  300  and the host  100 ,  FIG. 7  illustrates the connectivity between host  100  and virtual application delivery chassis  300  according to an embodiment of the present invention. In this embodiment, the host  100  communicates with virtual application delivery chassis  300  through a network switch  418 . Network switch  418  connects to blade  310  and blade  314 . Virtual application delivery chassis  300  includes a virtual address  423 . Virtual address  423  is a network address that the host  100  uses to communicate with virtual application delivery chassis  300 . In one embodiment, virtual address  423  is an IP address, or other network address. Blade  310  is designated as an active blade for virtual address  423 . Blade  314  is designated as a backup blade for virtual address  423 . Thus, an active and backup blade in the virtual application distribution chassis  300  are designated for each virtual address of the data network  151 . Network switch  418  delivers data packets destined for virtual address  423  to active blade  310 . The host  100  sends service request  403  to virtual application delivery chassis  300  using virtual address  423 . When network switch  418  receives service request  403  packet, network switch  418  sends the packet to active blade  310  destined for virtual address  423 . Active blade  310  receives service request  403  and processes service request  403 . 
     In one embodiment, active blade  310  fails and does not send slave report message  510 , or master advertisement message if active blade  310  is a master blade, upon expiration of timer  320 . Backup blade  314  does not receive slave report message  510  from active blade  310  before timer  324  expires. In response, backup blade  314  determines that active blade  310  has failed. Backup blade  314  changes its role to the active blade for virtual address  423 . Active blade  314  sends a virtual address change message  446  to inform network switch  418  that future packets for virtual address  423  should be sent to active blade  314 . In one embodiment, virtual address  423  is an IP address and network switch  418  is an Ethernet switch. Active blade  314  informs network switch  418  using an Address Resolution Protocol (ARP) message, which includes virtual address  423  and a MAC address of the network interface of active blade  314 . Upon receiving the virtual address change message  446 , network switch  418  changes state such that future packets destined for virtual address  423  will be sent to active blade  314 . 
     In one embodiment, service session between the host  100  and the server  200  based on service request  403  fails when active blade  310  fails. In an embodiment, the host  100  subsequently sends service request  407  for another service session  409  using virtual address  423 . Network switch  418  receives service request  407  and sends service request  407  with virtual address  423  to active blade  314 . Active blade  314  processes service request  407  and sends the request to the server  200 . 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.