Abstract:
Horizontal splitting of tasks within a homogenous pool of virtual machines. A primary virtual machine is provided to service requests from a request source during a session. The primary virtual machine services requests having a first priority received during the session. A secondary virtual machine is provided to service requests from the request source during the session. The secondary virtual machine services requests having a second priority received during the session. The first virtual machine and the second virtual machine run on a single physical computing platform.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application 61/421,989, entitled “Methods and Systems for making effective use of system resources in an on-demand environment,” by Vijayanth Devadhar, et al., filed Dec. 10, 2010, the entire contents of which are incorporated herein by reference. 
     
    
     COPYRIGHT NOTICE 
       [0002]    A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
       TECHNICAL FIELD 
       [0003]    Embodiments relate to techniques for task distribution in an environment having virtual machines. More particularly, embodiments relate to a horizontal distribution strategy that may be used in an environment having multiple virtual machines. 
       BACKGROUND 
       [0004]    Systems that service incoming requests have various mechanisms to respond to requests in an efficient manner. For example, load-balancing strategies have been developed to distribute requests. Other strategies have also been developed. However, these strategies typically focus on request quantities or request sources, which provide an improved, but not optimal result. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    Embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like reference numerals refer to similar elements. 
           [0006]      FIG. 1  is a block diagram of one embodiment of a networked system of request sources and application servers that my service requests. 
           [0007]      FIG. 2  is a block diagram of one embodiment of an application server having one or more processors. 
           [0008]      FIG. 3  is a flow diagram of one embodiment of a technique for utilizing a primary virtual machine and a secondary virtual machine. 
           [0009]      FIG. 4  illustrates a block diagram of an environment where an on-demand database service might be used. 
           [0010]      FIG. 5  is a block diagram of an embodiment of a multitenant environment. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In the following description, numerous specific details are set forth. However, embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure the understanding of this description. 
         [0012]    Techniques described herein may be utilized to provide automatic lifecycle management (e.g., startup, teardown) of multiple virtual machines (e.g., JAVA Virtual Machines, or JVMs) on a single server machine (e.g., an application server, or app server), and the ease of configuration of services that determines which virtual machine the service runs on. 
         [0013]    In one embodiment, a secondary app server virtual machine is provided on the same physical computing device as a primary app server. The secondary app server may be used for background jobs. In one embodiment, the secondary app server is run by one or more specified processor cores to prioritize certain types of requests. This may be accomplished by using LINUX utilities, for example, and may allow prioritization of real time requests over background jobs. 
         [0014]      FIG. 1  is a block diagram of one embodiment of a networked system of request sources and application servers that may service requests. The example of  FIG. 1  provides an example with two request sources, one load balancer and two application servers; however, any number of request sources, load balancers and application servers can be supported using the techniques described herein. 
         [0015]    Network  100  may be any type of network that provides connections between request sources  110  and  120  and application servers  160  and  170 . Network  100  can be, for example, the Internet, a local area network (LAN), and/or any combination of networks and subnetworks. Request sources  110  and  120  operate to request services and/or resources from application servers  160  and  170 . Request sources  110  and  120  can be, for example, computer systems running browser applications that allow a user thereof to interact with application servers  160  and  170 . Load balancer  140  may operate to distribute requests from the request sources to the application servers in order to more efficiently utilize the resources provided by the application servers. 
         [0016]    Application servers  160  and  170  include one or more processor cores that support execution of virtual machines that may be utilized in servicing requests from request sources  110  and  120 . Application servers  160  and  170  function to provide primary and secondary virtual machines as described herein to service requests from request sources  110  and  120 . 
         [0017]      FIG. 2  is a block diagram of one embodiment of an application server having one or more processors. Alternative application servers may include more, fewer and/or different components. 
         [0018]    Application server  200  includes bus  205  or other communication device to communicate information, and processor  210  coupled to bus  205  that may process information. While application server  200  is illustrated with a single processor, application server  200  may include multiple processors and/or co-processors. Application server  200  further may include random access memory (RAM) or other dynamic storage device  220  (referred to as main memory), coupled to bus  205  and may store information and instructions that may be executed by processor  210 . Main memory  220  may also be used to store temporary variables or other intermediate information during execution of instructions by processor  210 . 
         [0019]    One or more processors  210  support operation of multiple virtual machines as discussed herein. One or more processors  210  may further each include one or more processing cores. In one embodiment, individual processor cores may be assigned to virtual machines. 
         [0020]    Application server  200  may also include read only memory (ROM) and/or other static storage device  230  coupled to bus  205  that may store static information and instructions for processor  210 . Data storage device  240  may be coupled to bus  205  to store information and instructions. Data storage device  240  such as a magnetic disk or optical disc and corresponding drive may be coupled to application server  200 . 
         [0021]    Application server  200  may also be coupled via bus  205  to display device  250 , such as a cathode ray tube (CRT) or liquid crystal display (LCD), to display information to a user. Alphanumeric input device  260 , including alphanumeric and other keys, may be coupled to bus  205  to communicate information and command selections to processor  210 . Another type of user input device is cursor control  270 , such as a mouse, a trackball, or cursor direction keys to communicate direction information and command selections to processor  210  and to control cursor movement on display  250 . 
         [0022]    Application server  200  further may include network interface(s)  280  to provide access to a network, such as a local area network. Network interface(s)  280  may include, for example, a wireless network interface having antenna  285 , which may represent one or more antenna(e). Network interface(s)  280  may also include, for example, a wired network interface to communicate with remote devices via network cable  287 , which may be, for example, an Ethernet cable, a coaxial cable, a fiber optic cable, a serial cable, or a parallel cable. 
         [0023]    Instructions are provided to memory from a storage device, such as magnetic disk, a read-only memory (ROM) integrated circuit, CD-ROM, DVD, via a remote connection (e.g., over a network via network interface  280 ) that is either wired or wireless, etc. In alternative embodiments, hard-wired circuitry can be used in place of or in combination with software instructions. Thus, execution of sequences of instructions is not limited to any specific combination of hardware circuitry and software instructions. 
         [0024]    A computer-readable medium includes any mechanism that provides (i.e., stores) content (e.g., computer executable instructions) in a form readable by an electronic device (e.g., a computer, a personal digital assistant, a cellular telephone). For example, a computer-readable medium includes read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; etc. 
         [0025]      FIG. 3  is a flow diagram of one embodiment of a technique for utilizing a primary virtual machine and a secondary virtual machine. In one embodiment, the virtual machines are provided in a multitenant database environment. Example embodiments of multitenant database environments are set forth below. 
         [0026]    A primary virtual machine is launched,  310 . In one embodiment, the primary virtual machine provides a primary application server to service requests received from remote request sources. The remote request sources can be, for example, computing platforms that a user may utilize a browser or other application to access resources provided by the application server(s). The browser may generate requests that are received, and serviced, by the application server(s). 
         [0027]    The primary virtual machine launches the secondary virtual machine,  320 . The secondary virtual machine may provide a secondary application server. In one embodiment, the secondary application server runs alongside true application servers only (e.g., no capps, indexers, batch servers, etc). In one embodiment, the secondary application server is launched via an appserver startup task on the primary application server. In one embodiment, the primary application server is responsible for killing secondary application server when appropriate (e.g. mbean+JVM shutdown hooks). 
         [0028]    In one embodiment, the primary application server starts the secondary application server using current virtual machine arguments and class paths for the primary virtual machine. Certain properties (e.g., settings path, startup log) may be overridden. In one embodiment, the secondary application server is allowed only as many connections as needed. 
         [0029]    In one embodiment, the primary application server may monitor the secondary application server and revive the secondary application server if necessary. For robustness in development scenarios, the secondary application server may also monitor the primary application server, and may kill itself when the good primary application dies. For example, if two-way monitoring is implemented with a blocking read on a local socket connection, when it reaches an EOF, the other application has died. 
         [0030]    In one embodiment, the secondary application server may be configured with a unique appname for agent proxying. This appname may be configured in a CMS as a miniAppname, for example. In one embodiment, a CMS may model the secondary application server by storing a value for the application port on the application instances that run secondary application server. In one embodiment, an agent may start a core and a core application (e.g., the primary application server) and may start the secondary application server and manage the secondary application server&#39;s life cycle. However, in one embodiment, the secondary application server cannot start the core. 
         [0031]    In one embodiment, virtual machines are assigned to particular processing cores on the same computing platform,  330 . The core assignments may be accomplished by utilizing operating system utilities and commands. In one embodiment, the primary application server is utilized to process real time requests that are received from the request sources and the secondary application server is utilized to process background (or non-real time) requests. Thus, conceptually, this division of processing may be considered horizontal. 
         [0032]    As requests are received, they may be evaluated as background or real time and assigned to the appropriate virtual machine and application server,  340 . In one embodiment, the primary application server is exposed to realtime requests via the load balancer. As realtime requests come in, they are proxied to the application server port that the primary virtual machine is monitoring. 
         [0033]    In one embodiment, services that consume background (e.g., asynchronous) jobs do not run on the primary virtual machine. They are configured to launch on the secondary virtual machine. These services consume scheduled/queued jobs. The secondary virtual machine is not exposed to the load balancer so it will not receive/serve any realtime requests. Once the application server receives the requests, the requests may be serviced,  350 . 
         [0034]      FIG. 4  illustrates a block diagram of an environment  410  wherein an on-demand database service might be used. Environment  410  may include user systems  412 , network  414 , system  416 , processor system  417 , application platform  418 , network interface  420 , tenant data storage  422 , system data storage  424 , program code  426 , and process space  428 . In other embodiments, environment  410  may not have all of the components listed and/or may have other elements instead of, or in addition to, those listed above. 
         [0035]    Environment  410  is an environment in which an on-demand database service exists. User system  412  may be any machine or system that is used by a user to access a database user system. For example, any of user systems  412  can be a handheld computing device, a mobile phone, a laptop computer, a work station, and/or a network of computing devices. As illustrated in  FIG. 4  (and in more detail in  FIG. 5 ) user systems  412  might interact via a network  414  with an on-demand database service, which is system  416 . 
         [0036]    An on-demand database service, such as system  416 , is a database system that is made available to outside users that do not need to necessarily be concerned with building and/or maintaining the database system, but instead may be available for their use when the users need the database system (e.g., on the demand of the users). Some on-demand database services may store information from one or more tenants stored into tables of a common database image to form a multi-tenant database system (MTS). Accordingly, “on-demand database service  416 ” and “system  416 ” will be used interchangeably herein. 
         [0037]    A database image may include one or more database objects. A relational database management system (RDMS) or the equivalent may execute storage and retrieval of information against the database object(s). Application platform  418  may be a framework that allows the applications of system  416  to run, such as the hardware and/or software, e.g., the operating system. In an embodiment, on-demand database service  416  may include an application platform  418  that enables creation, managing and executing one or more applications developed by the provider of the on-demand database service, users accessing the on-demand database service via user systems  412 , or third party application developers accessing the on-demand database service via user systems  412 . 
         [0038]    The users of user systems  412  may differ in their respective capacities, and the capacity of a particular user system  412  might be entirely determined by permissions (permission levels) for the current user. For example, where a salesperson is using a particular user system  412  to interact with system  416 , that user system has the capacities allotted to that salesperson. However, while an administrator is using that user system to interact with system  416 , that user system has the capacities allotted to that administrator. 
         [0039]    In systems with a hierarchical role model, users at one permission level may have access to applications, data, and database information accessible by a lower permission level user, but may not have access to certain applications, database information, and data accessible by a user at a higher permission level. Thus, different users will have different capabilities with regard to accessing and modifying application and database information, depending on a user&#39;s security or permission level. 
         [0040]    Network  414  is any network or combination of networks of devices that communicate with one another. For example, network  414  can be any one or any combination of a LAN (local area network), WAN (wide area network), telephone network, wireless network, point-to-point network, star network, token ring network, hub network, or other appropriate configuration. As the most common type of computer network in current use is a TCP/IP (Transfer Control Protocol and Internet Protocol) network, such as the global internetwork of networks often referred to as the “Internet” with a capital “I,” that network will be used in many of the examples herein. However, it should be understood that the networks that the present invention might use are not so limited, although TCP/IP is a frequently implemented protocol. 
         [0041]    User systems  412  might communicate with system  416  using TCP/IP and, at a higher network level, use other common Internet protocols to communicate, such as HTTP, FTP, AFS, WAP, etc. In an example where HTTP is used, user system  412  might include an HTTP client commonly referred to as a “browser” for sending and receiving HTTP messages to and from an HTTP server at system  416 . Such an HTTP server might be implemented as the sole network interface between system  416  and network  414 , but other techniques might be used as well or instead. In some implementations, the interface between system  416  and network  414  includes load sharing functionality, such as round-robin HTTP request distributors to balance loads and distribute incoming HTTP requests evenly over a plurality of servers. At least as for the users that are accessing that server, each of the plurality of servers has access to the MTS&#39; data; however, other alternative configurations may be used instead. 
         [0042]    In one embodiment, system  416 , shown in  FIG. 4 , implements a web-based customer relationship management (CRM) system. For example, in one embodiment, system  416  includes application servers configured to implement and execute CRM software applications as well as provide related data, code, forms, webpages and other information to and from user systems  412  and to store to, and retrieve from, a database system related data, objects, and Webpage content. With a multi-tenant system, data for multiple tenants may be stored in the same physical database object, however, tenant data typically is arranged so that data of one tenant is kept logically separate from that of other tenants so that one tenant does not have access to another tenant&#39;s data, unless such data is expressly shared. 
         [0043]    In certain embodiments, system  416  implements applications other than, or in addition to, a CRM application. For example, system  416  may provide tenant access to multiple hosted (standard and custom) applications, including a CRM application. User (or third party developer) applications, which may or may not include CRM, may be supported by the application platform  418 , which manages creation, storage of the applications into one or more database objects and executing of the applications in a virtual machine in the process space of the system  416 . 
         [0044]    One arrangement for elements of system  416  is shown in  FIG. 4 , including a network interface  420 , application platform  418 , tenant data storage  422  for tenant data  423 , system data storage  424  for system data  425  accessible to system  416  and possibly multiple tenants, program code  426  for implementing various functions of system  416 , and a process space  428  for executing MTS system processes and tenant-specific processes, such as running applications as part of an application hosting service. Additional processes that may execute on system  416  include database indexing processes. 
         [0045]    Several elements in the system shown in  FIG. 4  include conventional, well-known elements that are explained only briefly here. For example, each user system  412  could include a desktop personal computer, workstation, laptop, PDA, cell phone, or any wireless access protocol (WAP) enabled device or any other computing device capable of interfacing directly or indirectly to the Internet or other network connection. User system  412  typically runs an HTTP client, e.g., a browsing program, such as Microsoft&#39;s Internet Explorer browser, Netscape&#39;s Navigator browser, Opera&#39;s browser, or a WAP-enabled browser in the case of a cell phone, PDA or other wireless device, or the like, allowing a user (e.g., subscriber of the multi-tenant database system) of user system  412  to access, process and view information, pages and applications available to it from system  416  over network  414 . 
         [0046]    Each user system  412  also typically includes one or more user interface devices, such as a keyboard, a mouse, trackball, touch pad, touch screen, pen or the like, for interacting with a graphical user interface (GUI) provided by the browser on a display (e.g., a monitor screen, LCD display, etc.) in conjunction with pages, forms, applications and other information provided by system  416  or other systems or servers. For example, the user interface device can be used to access data and applications hosted by system  416 , and to perform searches on stored data, and otherwise allow a user to interact with various GUI pages that may be presented to a user. As discussed above, embodiments are suitable for use with the Internet, which refers to a specific global internetwork of networks. However, it should be understood that other networks can be used instead of the Internet, such as an intranet, an extranet, a virtual private network (VPN), a non-TCP/IP based network, any LAN or WAN or the like. 
         [0047]    According to one embodiment, each user system  412  and all of its components are operator configurable using applications, such as a browser, including computer code run using a central processing unit such as an Intel Pentium® processor or the like. Similarly, system  416  (and additional instances of an MTS, where more than one is present) and all of their components might be operator configurable using application(s) including computer code to run using a central processing unit such as processor system  417 , which may include an Intel Pentium® processor or the like, and/or multiple processor units. 
         [0048]    According to one embodiment, each system  416  is configured to provide webpages, forms, applications, data and media content to user (client) systems  412  to support the access by user systems  412  as tenants of system  416 . As such, system  416  provides security mechanisms to keep each tenant&#39;s data separate unless the data is shared. If more than one MTS is used, they may be located in close proximity to one another (e.g., in a server farm located in a single building or campus), or they may be distributed at locations remote from one another (e.g., one or more servers located in city A and one or more servers located in city B). 
         [0049]    As used herein, each MTS could include one or more logically and/or physically connected servers distributed locally or across one or more geographic locations. Additionally, the term “server” is meant to include a computer system, including processing hardware and process space(s), and an associated storage system and database application (e.g., OODBMS or RDBMS) as is well known in the art. It should also be understood that “server system” and “server” are often used interchangeably herein. Similarly, the database object described herein can be implemented as single databases, a distributed database, a collection of distributed databases, a database with redundant online or offline backups or other redundancies, etc., and might include a distributed database or storage network and associated processing intelligence. 
         [0050]      FIG. 5  also illustrates environment  410 . However, in  FIG. 5  elements of system  416  and various interconnections in an embodiment are further illustrated.  FIG. 5  shows that user system  412  may include processor system  412 A, memory system  412 B, input system  412 C, and output system  412 D.  FIG. 5  shows network  414  and system  416 .  FIG. 5  also shows that system  416  may include tenant data storage  422 , tenant data  423 , system data storage  424 , system data  425 , User Interface (UI)  530 , Application Program Interface (API)  532 , PL/SOQL  534 , save routines  536 , application setup mechanism  538 , applications servers  500   1 - 500   N , system process space  502 , tenant process spaces  504 , tenant management process space  510 , tenant storage area  512 , user storage  514 , and application metadata  516 . In other embodiments, environment  410  may not have the same elements as those listed above and/or may have other elements instead of, or in addition to, those listed above. 
         [0051]    User system  412 , network  414 , system  416 , tenant data storage  422 , and system data storage  424  were discussed above in  FIG. 4 . Regarding user system  412 , processor system  412 A may be any combination of one or more processors. Memory system  412 B may be any combination of one or more memory devices, short term, and/or long term memory. Input system  412 C may be any combination of input devices, such as one or more keyboards, mice, trackballs, scanners, cameras, and/or interfaces to networks. Output system  412 D may be any combination of output devices, such as one or more monitors, printers, and/or interfaces to networks. As shown by  FIG. 5 , system  416  may include a network interface  420  (of  FIG. 4 ) implemented as a set of HTTP application servers  500 , an application platform  418 , tenant data storage  422 , and system data storage  424 . 
         [0052]    Also shown is system process space  502 , including individual tenant process spaces  504  and a tenant management process space  510 . Each application server  500  may be configured to tenant data storage  422  and the tenant data  423  therein, and system data storage  424  and the system data  425  therein to serve requests of user systems  412 . The tenant data  423  might be divided into individual tenant storage areas  512 , which can be either a physical arrangement and/or a logical arrangement of data. Within each tenant storage area  512 , user storage  514  and application metadata  516  might be similarly allocated for each user. For example, a copy of a user&#39;s most recently used (MRU) items might be stored to user storage  514 . Similarly, a copy of MRU items for an entire organization that is a tenant might be stored to tenant storage area  512 . A UI  530  provides a user interface and an API  532  provides an application programmer interface to system  416  resident processes to users and/or developers at user systems  412 . The tenant data and the system data may be stored in various databases, such as one or more Oracle™ databases. 
         [0053]    Application platform  418  includes an application setup mechanism  538  that supports application developers&#39; creation and management of applications, which may be saved as metadata into tenant data storage  422  by save routines  536  for execution by subscribers as one or more tenant process spaces  504  managed by tenant management process  510  for example. Invocations to such applications may be coded using PL/SOQL  534  that provides a programming language style interface extension to API  532 . A detailed description of some PL/SOQL language embodiments is discussed in commonly owned co-pending U.S. Provisional Patent Application 40/828,192 entitled, PROGRAMMING LANGUAGE METHOD AND SYSTEM FOR EXTENDING APIS TO EXECUTE IN CONJUNCTION WITH DATABASE APIS, by Craig Weissman, filed Oct. 4, 2006, which is incorporated in its entirety herein for all purposes. Invocations to applications may be detected by one or more system processes, which manages retrieving application metadata  516  for the subscriber making the invocation and executing the metadata as an application in a virtual machine. 
         [0054]    Each application server  500  may be communicably coupled to database systems, e.g., having access to system data  425  and tenant data  423 , via a different network connection. For example, one application server  500   1  might be coupled via the network  414  (e.g., the Internet), another application server  500   N-1  might be coupled via a direct network link, and another application server  500   N  might be coupled by yet a different network connection. Transfer Control Protocol and Internet Protocol (TCP/IP) are typical protocols for communicating between application servers  500  and the database system. However, it will be apparent to one skilled in the art that other transport protocols may be used to optimize the system depending on the network interconnect used. 
         [0055]    In certain embodiments, each application server  500  is configured to handle requests for any user associated with any organization that is a tenant. Because it is desirable to be able to add and remove application servers from the server pool at any time for any reason, there is preferably no server affinity for a user and/or organization to a specific application server  500 . In one embodiment, therefore, an interface system implementing a load balancing function (e.g., an F5 Big-IP load balancer) is communicably coupled between the application servers  500  and the user systems  412  to distribute requests to the application servers  500 . 
         [0056]    In one embodiment, the load balancer uses a least connections algorithm to route user requests to the application servers  500 . Other examples of load balancing algorithms, such as round robin and observed response time, also can be used. For example, in certain embodiments, three consecutive requests from the same user could hit three different application servers  500 , and three requests from different users could hit the same application server  500 . In this manner, system  416  is multi-tenant, wherein system  416  handles storage of, and access to, different objects, data and applications across disparate users and organizations. 
         [0057]    As an example of storage, one tenant might be a company that employs a sales force where each salesperson uses system  416  to manage their sales process. Thus, a user might maintain contact data, leads data, customer follow-up data, performance data, goals and progress data, etc., all applicable to that user&#39;s personal sales process (e.g., in tenant data storage  422 ). In an example of a MTS arrangement, since all of the data and the applications to access, view, modify, report, transmit, calculate, etc., can be maintained and accessed by a user system having nothing more than network access, the user can manage his or her sales efforts and cycles from any of many different user systems. For example, if a salesperson is visiting a customer and the customer has Internet access in their lobby, the salesperson can obtain critical updates as to that customer while waiting for the customer to arrive in the lobby. 
         [0058]    While each user&#39;s data might be separate from other users&#39; data regardless of the employers of each user, some data might be organization-wide data shared or accessible by a plurality of users or all of the users for a given organization that is a tenant. Thus, there might be some data structures managed by system  416  that are allocated at the tenant level while other data structures might be managed at the user level. Because an MTS might support multiple tenants including possible competitors, the MTS should have security protocols that keep data, applications, and application use separate. Also, because many tenants may opt for access to an MTS rather than maintain their own system, redundancy, up-time, and backup are additional functions that may be implemented in the MTS. In addition to user-specific data and tenant specific data, system  416  might also maintain system level data usable by multiple tenants or other data. Such system level data might include industry reports, news, postings, and the like that are sharable among tenants. 
         [0059]    In certain embodiments, user systems  412  (which may be client systems) communicate with application servers  500  to request and update system-level and tenant-level data from system  416  that may require sending one or more queries to tenant data storage  422  and/or system data storage  424 . System  416  (e.g., an application server  500  in system  416 ) automatically generates one or more SQL statements (e.g., one or more SQL queries) that are designed to access the desired information. System data storage  424  may generate query plans to access the requested data from the database. 
         [0060]    Each database can generally be viewed as a collection of objects, such as a set of logical tables, containing data fitted into predefined categories. A “table” is one representation of a data object, and may be used herein to simplify the conceptual description of objects and custom objects according to the present invention. It should be understood that “table” and “object” may be used interchangeably herein. Each table generally contains one or more data categories logically arranged as columns or fields in a viewable schema. 
         [0061]    Each row or record of a table contains an instance of data for each category defined by the fields. For example, a CRM database may include a table that describes a customer with fields for basic contact information such as name, address, phone number, fax number, etc. Another table might describe a purchase order, including fields for information such as customer, product, sale price, date, etc. In some multi-tenant database systems, standard entity tables might be provided for use by all tenants. For CRM database applications, such standard entities might include tables for Account, Contact, Lead, and Opportunity data, each containing pre-defined fields. It should be understood that the word “entity” may also be used interchangeably herein with “object” and “table”. 
         [0062]    In some multi-tenant database systems, tenants may be allowed to create and store custom objects, or they may be allowed to customize standard entities or objects, for example by creating custom fields for standard objects, including custom index fields. U.S. patent application Ser. No. 10/817,161, filed Apr. 2, 2004, entitled “Custom Entities and Fields in a Multi-Tenant Database System”, and which is hereby incorporated herein by reference, teaches systems and methods for creating custom objects as well as customizing standard objects in a multi-tenant database system. In certain embodiments, for example, all custom entity data rows are stored in a single multi-tenant physical table, which may contain multiple logical tables per organization. It is transparent to customers that their multiple “tables” are in fact stored in one large table or that their data may be stored in the same table as the data of other customers. 
         [0063]    Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment. 
         [0064]    While the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.