Abstract:
Methods and apparatus for detecting a change in demand for server resources across a load-bearing system having one or more content servers hosting identical content, the load-bearing system being connected to a network of content servers; and automatically modifying the number of content servers on the load-bearing system in response to the change in demand.

Description:
BACKGROUND 
     This invention relates to automated server replication. 
     The popularity of the World Wide Web as a communications medium lies in the richness of its information content and ease of use. Information in this medium exists as objects in a widely distributed collection of internetworked servers, each object uniquely addressable by its own Uniform Resource Locator (URL). The proliferation of commercial applications on the World Wide Web brings with it an increasing number of users making ever-increasing numbers of requests for web content. The problems of latency and bandwidth considerations manifest themselves in delay and lost information. 
     Network architects respond using an array of solutions, one of which is the server farm. This involves the use of multiple web servers with identical content, or the segmentation based upon functionality. For example, two servers for web functions, two for File Transfer Protocol (FTP), two as a database, and so forth. The use of multiple servers solves one problem at the expense of creating another. If there are multiple servers, how does the end user locate a particular web site? Presently, names and Universal Resource Locator (URLs) are resolved into unique single addresses by a Domain Name Service (DNS) residing in a DNS server. DNS servers maintain a list of domain names cross referenced to individual Internet Protocol (IP) addresses. However, if multiple web servers or server farms are used, a modified version of DNS service is used. A common approach to this problem is to modify the DNS system to be aware of a one-to-many mapping of names-to-IP-addresses. Thus, the DNS will return an IP address that comes from a list of possible IP addresses that correspond to a particular web object. Thus, from one moment to the next, a DNS query will resolve to different IP addresses. In this example, the modified DNS decides which IP address to return based on how busy each of the servers is. 
     In current network management systems, there are various methods of detecting and monitoring the load across a server or a server farm. One system uses a load capacity detection agent to monitor the load across a server or a server farm. In this system, when the load detection agent detects that a server farm, for example, is experiencing excess load, the agent notifies a system administrator of the system. The system administrator may decide to manually take action to either reduce the load across the server farm, or alternatively, increase the available load capacity by adding a server to the server farm. Generally, the system administrator adds a server by manually identifying an additional available server, and then modifying the entries in the load management system to include the IP address of the recently-added content server. 
     SUMMARY 
     In general, in one aspect, the invention provides a method and apparatus, including a computer program apparatus, implementing techniques for detecting a change in demand for server resources across a load-bearing system having one or more content servers hosting identical content, the load-bearing system being connected to a network of content servers; and automatically modifying the number of content servers on the load-bearing system in response to the change in demand. 
     Each content server on the network may be categorized as active or idle. The hosted content may be assigned a priority level, and a number defining a maximum number of content servers on the load-bearing system. A content server may be added to the load-bearing system if the change in demand is greater than a pre-configured threshold and the number of content servers on the load-bearing system is less than a maximum number assigned to the hosted content. The number of content servers may be modified by selecting an available content server on the network; loading hosted content onto the selected content server; and adding the selected content server to the load-bearing system. All of the content servers on the load-bearing system including the selected content server host identical content. The available content server having the lowest priority level, the highest available load capacity, or the least number of active sessions may be selected. 
     The hosted content may be assigned a number defining a minimum number of content servers on the load-bearing system. A content server may be removed from the load-bearing system if the change in demand is greater than a pre-configured threshold and the number of content servers on the load-bearing system is greater than a minimum number assigned to the hosted content. The content server having the highest available load capacity or the least number of active sessions may be removed. 
     Embodiments may have one or more of the following advantages. The invention uses scripting, or other software techniques, to automate the addition or removal of a content server from a load-bearing system. Automating the addition and removal of content servers gives, for example, a web hosting operation a way to manipulate server resources between high-activity servers and low-activity servers without requiring any user intervention. In particular, the invention serves the needs of “seasonal” or “spiky” web applications, such as tax preparation services or Superbowl-related web sites, which experience a tremendous increase in the number of hits at specific times of the year. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIGS. 1   a - 1   d  are block diagrams illustrating an internetwork topology including an network of content servers and a management server. 
         FIG. 2  is a flowchart of a process for automatically modifying the number of content servers on a load-bearing system. 
       Like reference symbols in the various drawings indicate like elements. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1   a - 1   d , an exemplary internetwork  100  includes a network of servers  102  connected to a network of clients  104  through the Internet  106 . The network of servers  102  includes a management server  108  and a group of content servers  110 . 
     The content servers  110  are nodes on the network  102  that perform the actual serving of content, for example, Web pages or FTP files. Although only nine content servers  110  are shown in  FIGS. 1   a  through  1   d , it will be understood that any number of content servers may be connected to the network  102 . Each content server  110  is capable of receiving queries from clients, doing substantially all the processing necessary to formulate responses to the queries, and providing these responses to the clients. Web servers, for example, respond to requests from clients (e.g., through browser software) for access to files and data. Other types of servers allow clients to share access to network devices, such as shared printers or disk storage. 
     In one exemplary commercial setting, each content server  110  is a managed node on the network  102  maintained by a network operator, such as Genuity Inc. of Woburn, Mass. By the term “managed node”, it is meant that each content server  110  runs a management process that allows another computer to query the managed node for information. For example, Simple Network Management Protocol (SNMP) describes how a computer formats a message to send to the managed node. Software on the managed node, called an SNMP agent (not shown), examines SNMP messages it receives, and responds accordingly. Each SNMP agent maintains a local database of variables that describe the state of the content server  110  and may, optionally, affect its operation. Each SNMP agent&#39;s local database includes, but need not be limited to the following: the number of packets and bytes sent and received from the Internet  106 , the number of broadcasts, the current output queue size, the current transaction rate, the current processor utilization, and the current disk utilization. 
     Content servers  110  are the systems which store information that may be accessed using web browser software such as Netscape Communicator® and Microsoft&#39;s Internet Explorer®. Content servers transmit their information in response to receiving a message of a format specified by Hyper Text Transfer Protocol (HTTP). The format of the server&#39;s response is also specified by HTTP, and is understood by web browser software. 
     Network management on the network  102  can be done from management servers, an example of which is shown as management server  108 , which are general-purpose computers running management software. The management server  108  interacts with the SNMP agents in the content servers  110  using the SNMP protocol. This protocol allows the management server  108  to query the state of an agent&#39;s local variables, and change them if necessary. For example, the management server  108  can issue commands and get responses that allow the management server  108  to monitor the resources available (i.e., available load capacity) on each content server  110 , and express them in a way that enables the content servers  110  to be categorized as active or idle. In one example, the management server  108  only considers the CPU resource, and neglects all others. In other implementations, the management server  108  uses a more complex combination of considerations, with load metrics ranging from the instantaneous CPU utilization and IO queue length, through to a linear combination of CPU, memory and IO queue lengths. The management server  108  can also modify network routing tables, and change the status of network links and devices. The collection of all possible variables available via SNMP is given in a data structure called the Management Information Base (MIB), which is formally defined in the Internet Engineering Task Force (IETF) Request For Comment (RFC)  1213 . The IETF is a large open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and smooth operation of the Internet—it is open to any individual. 
     The management server  108  also houses several other software components, which include in one implementation, an image manager  116 , and a content storage system  118 . In the illustrated examples on  FIGS. 1   a - 1   d , a single server implements the software components of the management server  108 . However, it should be noted that two or more servers connected to the network  102  may implement the software components. 
     The router  114  uses any one of a number of dynamic routing algorithms (e.g., distance vector routing and link state routing) to decide where to send packets addressed to a particular IP address; for example, how to get to a particular content server which are all distinguished based on their assigned IP address. Dynamic routing algorithms change their routing decisions in response to changes in the topology of the network  102 . A set of files is stored in the content storage system  118 . Each file contains content data, applications and all the necessary software required to implement a web site presence on the Internet  106 . Each file can be loaded onto a content server  110  by the image manager  116 . In one implementation, the image manager  116  maintains a table (shown below) that includes, for each file, a priority level, a minimum number and a maximum number of content servers  110  that can be used to implement the web site. The priority level indicates to the management server  108  the relative importance—to the network operator, Genuity Inc., for example—of implementating a particular web site in situations in which several web sites are vying for limited server capacity. In one implementation, a five-point scale is used to designate a priority level: “highest”, “high”, “medium”, “low” and “lowest”. Generally, the higher the priority level assigned to a file, the more server resources (up to the maximum number of content servers  110  identified in the table) are allocated to implement the web site. By default, content servers  110  that are idle are assigned a “lowest” priority level. When a file having a “highest” priority level is loaded onto a content server  110  that is idle, for example, the priority level of the content server  110  changes from “lowest” to “highest”. 
     Assume, for example, that three files—file “A”, file “B”, and file “C”—are stored in the content storage system  118 . The second, third and fourth table entries are populated with the following data: 
                                                                           File   Priority Level   Min. Number   Max. Number                                        A   highest   1   7           B   medium   1   3           C   low   1   3                        
Initially, each file is loaded and run on one or more content servers  110 , indicated in dashed lines in  FIG. 1   a  as load-bearing system A  120 , load-bearing system B  122 , and load-bearing system C  124 . If a load-bearing system, such as load-bearing system A  120 , has multiple content servers  110 , the load on the system is distributed using one of the following schemes: (1) a load-sharing scheme; (2) a load-balancing scheme; or (3) a load-leveling scheme. Generally, content servers  110  in a load-sharing system that utilizes the load-sharing scheme are viewed in binary. That is, the server is either idle or busy, and load may only be placed on idle servers. Load-balancing schemes attempt to ensure that the load on each content server  110  in the system is within a small degree of the load present on every other content server in the system. Load-leveling schemes can be viewed as the middle ground between the load-sharing and load-balancing schemes. Rather than trying to obtain an even distribution across the system, or utilizing only the content servers  110  that are idle, the load-leveling scheme distributes load in a manner that minimizes congestion.
 
     Referring to  FIG. 2 , a process  200  residing in the management server  108  periodically checks ( 202 ) the available load capacity on each load-bearing system. In one implementation, the process  200  polls each content server  110  in a load-bearing system to determine its available load capacity. 
     If the process  200  detects ( 204 ) that the load on the load-bearing system B  122 , for example, is greater than a preconfigured threshold level (i.e., there is insufficient aggregate available load capacity), the process then determines ( 206 ) whether the number of content servers  110  in the load-bearing system B  122  is less than the maximum number that may be allocated to that system. If so, the process  200  will add a content server  110  to the load-bearing system B  122  if one is available. By use of the term “available”, it is meant that the content server  110  is idle and may be added to a load-bearing system, or alternatively, the content server has a lower priority level and may be removed from a particular load-bearing system and allocated to a different load-bearing system. For example, as shown in  FIG. 1   b , if the load-bearing system C  124  is implemented by a single content server  124   a , the content server  124   a  is not available for re-allocation to either of the other two load-bearing systems  120  and  122 , because the network operator has designated in the table that at least one content server  110  in the network  102  must be allocated to the load-bearing system C  124 . 
     The process  200  first polls all of the content servers  110  on the network  102  to determine ( 210 ) if there is an available content server  110  on the network  102 . If none of the content servers  110  are available, the process  200  logs and signals the event ( 208 ) to the network operator maintaining the network  102 . Otherwise, the process  200  selects ( 212 ) one of the available content servers for addition to the load-bearing system B  122 . If there are multiple available content servers  110 , the process  200  will typically select one that is idle for addition to the load-bearing system B  122 . However, if all of the available content servers  110  on the network  102  are active, as shown in  FIG. 1   c , the process  200  makes the selection as follows:
         (1) If there is only one available content server  110 , the process  200  selects that content server  110 .   (2) If there are multiple available content servers  110 , the process  200  selects the content server  110  having the lowest priority level.   (3) If there are multiple available content servers  110  having the lowest priority level, the process  200  polls each of those content servers  110  to determine which content server  110  has the highest available load capacity and selects that content server  110 . Alternatively, the process  200  polls each of those content servers  110  to determine which content server  110  has the least number of running processes in progress (called “active sessions”) and selects that content server  110 .       

     Once the selection is made, say, for example, the process  200  selects the content server having an IP address of “128.11.234.59”  124   b  in the load-bearing system C  124 , as shown in  FIG. 1   c , the process  200  can be configured to immediately stop all future client queries directed to the load-bearing system C  124  from being sent to the selected content server  124   b  for processing. The process  200  can also be configured to wait until all of the active sessions on the selected content server  124   b  have been terminated before removing that content server  124   b  from the load-bearing system C  124 . Once removed, the process  200  retrieves the file “B” from the content storage system  118  and uses the image manager  116  to load ( 214 ) it onto the selected content server  124   b . Referring to  FIG. 1   d , the process  200  then starts the selected content server  124   b  having the recently-loaded file “B”, and modifies the DNS system to add ( 216 ) the content server having an IP address of “128.11.234.59”  124   b  to the load-bearing system B  122 . Thereafter, when a client requests a service by entering in a web browser the URL for the web site implemented by the load-bearing system B  122 , the request can be directed to the content server  124   b.    
     Referring to  FIG. 1   c ,  1   f  the process  200  detects ( 218 ) that the load on the load-bearing system B  122 , for example, is less than a preconfigured threshold level, the process  200  then determines ( 220 ) whether the number of content servers  110  in the load-bearing system B  122  is more than the minimum number that may be allocated to that system  122 . If so, the process  200  may select ( 222 ) a content server  110  from the load-bearing system B  122  for removal if the aggregate available load capacity of the load-bearing system B, the historical demand for services implemented by the load-bearing system B, as well as damping considerations, among others, warrants removal of a content server. The process  200  selects a content server for removal much in the same manner described above. That is, the selection may be made by polling each of the content servers  110  in the load-bearing system B  122  to determine which content server  110  has the highest available load capacity and selecting that content server  110 . Alternatively, the selection may be made by polling each of those content servers  110  to determine which content server  110  has the least number of active sessions and selecting that content server  110 . The process  200  can be configured to wait until all of the active sessions on the selected content server  110  have been terminated before removing ( 224 ) that content server  110  from the load-bearing system B  122 , and categorizing it as idle. 
     Other embodiments are within the scope of the following claims.