Patent Publication Number: US-11640378-B2

Title: Archiving objects in a database environment

Description:
CROSS REFERENCE(S) 
     The present application is a continuation of and claims priority under 35 U.S.C. 120 to U.S. application Ser. No. 16/241,810, filed on Jan. 7, 2019, now U.S. Pat. No. 11,068,448 which is hereby expressly incorporated by reference herein in its entirety. 
    
    
     COPYRIGHT NOTICE 
     A portion of the disclosure of this patent document contains material which 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 
     The present disclosure relates generally to a computing device, and more specifically to systems and methods for archiving records in a database environment. 
     BACKGROUND 
     The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions. 
     Computer and software development is evolving away from the client-server model toward network-based processing systems that provide access to data and services via the Internet or other networks. In contrast to traditional systems that host networked applications on dedicated server hardware, a “cloud” computing model allows applications to be provided over the network “as a service” supplied by an infrastructure provider. The infrastructure provider typically abstracts the underlying hardware and other resources used to deliver a user-developed application so that a user (e.g., consumer of cloud-based services) no longer needs to operate and support dedicated server hardware. The cloud computing model can often provide substantial cost savings to the user over the life of the application because the user no longer needs to provide dedicated network infrastructure, electrical and temperature controls, physical security and other logistics in support of dedicated server hardware. 
     A cloud platform (i.e., a computing platform for cloud computing) may be employed by many users to store, manage, and process data using a shared network of remote servers. Users may develop applications on the cloud platform to handle the storage, management, and processing of data. In some cases, the cloud platform may utilize a multi-tenant database system. Users may access the cloud platform using various user devices (e.g., desktop computers, laptops, smartphones, tablets, or other computing systems, etc.). In one example, the cloud platform may support customer relationship management (CRM) solutions. This may include support for sales, service, marketing, community, analytics, applications, and the Internet of Things (IoT). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a block diagram of an example environment according to some embodiments. 
         FIG.  2    illustrates a block diagram of another example environment according to some embodiments. 
         FIG.  3    illustrates a diagram for archiving one or more records stored in a first storage device into a second storage device according to some embodiments. 
         FIG.  4    illustrates a diagram for archiving records into the second storage device according to some embodiments. 
         FIG.  5    illustrates a timing diagram of a current schema and a historical schema of an object over time based on field modifications according to some embodiments. 
         FIG.  6    illustrates a diagram for archiving records into the second storage device according to some embodiments. 
         FIG.  7    illustrates a diagram for storing a full record field in an archive record according to some embodiments. 
         FIG.  8    illustrates a diagram for archiving one or more records stored in a tenant data storage according to some embodiments. 
         FIG.  9    is a flowchart of a method for processing a query according to some embodiments. 
     
    
    
     In the figures, elements having the same designations have the same or similar functions.
         I. Example Environment   II. Archiving Records into a Second Storage Device
           A. Records Based on Customized Objects   B. Schema Drift Over Time   C. Migration of Original Records into the Second Storage Device   D. Example Timing Diagram   E. Maintenance of Metadata and a Historical Schema   
           III. Query Processing
           A. Query the First Data Storage Storing Values for Current Fields   B. Query the Second Data Storage Storing Archive Records
               1. The Subset of Fields Specified in the Query Matches the Current Fields   2. The Subset of Fields Specified in the Query Includes a Backdoor Field   3. The Subset of Fields Specified in the Query Includes a Non-Current Field   
               C. Standard Full Record Field   
           IV. Versions of an Object in a Multi-Tenancy   V. Operational Flow       

     DETAILED DESCRIPTION 
     This description and the accompanying drawings that illustrate aspects, embodiments, implementations, or applications should not be taken as limiting—the claims define the protected invention. Various mechanical, compositional, structural, electrical, and operational changes may be made without departing from the spirit and scope of this description and the claims. In some instances, well-known circuits, structures, or techniques have not been shown or described in detail as these are known to one skilled in the art. Like numbers in two or more figures represent the same or similar elements. 
     In this description, specific details are set forth describing some embodiments consistent with the present disclosure. Numerous specific details are set forth in order to provide a thorough understanding of the embodiments. It will be apparent, however, to one skilled in the art that some embodiments may be practiced without some or all of these specific details. The specific embodiments disclosed herein are meant to be illustrative but not limiting. One skilled in the art may realize other elements that, although not specifically described here, are within the scope and the spirit of this disclosure. In addition, to avoid unnecessary repetition, one or more features shown and described in association with one embodiment may be incorporated into other embodiments unless specifically described otherwise or if the one or more features would make an embodiment non-functional. 
     I. Example Environment 
     The system and methods of the present disclosure can include, incorporate, or operate in conjunction with or in the environment of a database, which in some embodiments can be implemented as a multi-tenant, cloud-based architecture. Multi-tenant cloud-based architectures have been developed to improve collaboration, integration, and community-based cooperation between customer tenants without sacrificing data security. Generally speaking, multi-tenancy refers to a system where a single hardware and software platform simultaneously support multiple user groups (also referred to as “organizations” or “tenants”) from a common data storage element (also referred to as a “multi-tenant database”). The multi-tenant design provides a number of advantages over conventional server virtualization systems. First, the multi-tenant platform operator can often make improvements to the platform based upon collective information from the entire tenant community. Additionally, because all users in the multi-tenant environment execute applications within a common processing space, it is relatively easy to grant or deny access to specific sets of data for any user within the multi-tenant platform, thereby improving collaboration and integration between applications and the data managed by the various applications. The multi-tenant architecture therefore allows convenient and cost-effective sharing of similar application features between multiple sets of users. 
       FIG.  1    illustrates a block diagram of an example environment  110  according to some embodiments. Environment  110  may include user systems  112 , network  114 , system  116 , processor system  117 , application platform  118 , network interface  120 , tenant data storage  122 , system data storage  124 , program code  126 , and process space  128  for executing database system processes and tenant-specific processes, such as running applications as part of an application hosting service. In other embodiments, environment  110  may not have all of the components listed and/or may have other elements instead of, or in addition to, those listed above. 
     In some embodiments, the environment  110  is an environment in which an on-demand database service exists. A user system  112  may be any machine or system that is used by a user to access a database user system. For example, any of user systems  112  can be a handheld computing device, a mobile phone, a laptop computer, a notepad computer, a work station, and/or a network of computing devices. As illustrated in  FIG.  1    (and in more detail in  FIG.  2   ) user systems  112  might interact via a network  114  with an on-demand database service, which is system  116 . 
     An on-demand database service, such as that which can be implemented using the system  116 , is a service that is made available to users outside of the enterprise(s) that own, maintain or provide access to the system  116 . As described above, such users do not need to necessarily be concerned with building and/or maintaining the system  116 . Instead, resources provided by the system  116  may be available for such users&#39; use when the users need services provided by the system  116 —e.g., on the demand of the users. Some on-demand database services may store information from one or more tenants into tables of a common database image to form a multi-tenant database system (MTS). Accordingly, the “on-demand database service  116 ” and the “system  116 ” will be used interchangeably herein. The term “multi-tenant database system” can refer to those systems in which various elements of hardware and software of a database system may be shared by one or more customers or tenants. For example, a given application server may simultaneously process requests for a great number of customers, and a given database table may store rows of data such as feed items for a potentially much greater number of customers. A database image may include one or more database objects. A relational database management system (RDBMS) or the equivalent may execute storage and retrieval of information against the database object(s). 
     The application platform  118  may be a framework that allows the applications of system  116  to run, such as the hardware and/or software infrastructure, e.g., the operating system. In an embodiment, on-demand database service  116  may include an application platform  118  that enables creating, 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  112 , or third-party application developers accessing the on-demand database service via user systems  112 . 
     The users of user systems  112  may differ in their respective capacities, and the capacity of a particular user system  112  might be entirely determined by permissions (permission levels) for the current user. For example, where a salesperson is using a particular user system  112  to interact with system  116 , that user system has the capacities allotted to that salesperson. However, while an administrator is using that user system  112  to interact with system  116 , that user system  112  has the capacities allotted to that administrator. 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. 
     The network  114  is any network or combination of networks of devices that communicate with one another. For example, the network  114  can be any one or any combination of a local area network (LAN), wide area network (WAN), 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 transfer control protocol and Internet protocol (TCP/IP) network, such as the global inter network 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 embodiments might use are not so limited, although TCP/IP is a frequently implemented protocol. 
     The user systems  112  might communicate with system  116  using TCP/IP and, at a higher network level, use other common Internet protocols to communicate, such as hypertext transfer protocol (HTTP), file transfer protocol (FTP), Andrew file system (AFS), wireless application protocol (WAP), etc. In an example where HTTP is used, user system  112  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  116 . Such an HTTP server might be implemented as the sole network interface between system  116  and network  114 , but other techniques might be used as well or instead. In some implementations, the interface between system  116  and network  114  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 for the users that are accessing that server, each of the plurality of servers has access to the MTS data; however, other alternative configurations may be used instead. 
     In some embodiments, the system  116 , shown in  FIG.  1   , implements a web-based customer relationship management (CRM) system. For example, in one embodiment, system  116  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  112  and to store to, and retrieve from, a database system related data, objects, and web page content. With a MTS, 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. In certain embodiments, the system  116  implements applications other than, or in addition to, a CRM application. For example, system  116  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  118 , which manages creation, storage of the applications into one or more database objects, and execution of the applications in a virtual machine in the process space of the system  116 . 
     One arrangement for elements of the system  116  is shown in  FIG.  1   , including the network interface  120 , the application platform  118 , the tenant data storage  122  for tenant data  123 , the system data storage  124  for system data  125  accessible to system  116  and possibly multiple tenants, the program code  126  for implementing various functions of the system  116 , and the process space  128  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  116  include database indexing processes. 
     Several elements in the system shown in  FIG.  1    include conventional, well-known elements that are explained only briefly here. For example, each of the user systems  112  could include a desktop personal computer, workstation, laptop, notepad computer, personal digital assistant (PDA), cellphone, 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. Each of the user systems  112  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, notepad computer, PDA or other wireless device, or the like, allowing a user (e.g., subscriber of the MTS) of the user systems  112  to access, process, and view information, pages, and applications available to it from the system  116  over the network  114 . Each of the user systems  112  also typically includes one or more user interface devices, such as a keyboard, 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, liquid crystal display (LCD) monitor, light emitting diode (LED) monitor, organic light emitting diode (OLED) monitor, etc.) in conjunction with pages, forms, applications, and other information provided by the system  116  or other systems or servers. For example, the user interface device can be used to access data and applications hosted by system  116 , 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. 
     According to one embodiment, each of the user systems  112  and all of its components are operator configurable using applications, such as a browser, including computer code run using a central processing unit (CPU) such as an Intel Pentium® processor or the like. Similarly, system  116  (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 CPU such as the processor system  117 , which may include an Intel Pentium® processor or the like, and/or multiple processor units. A computer program product embodiment includes a machine-readable storage medium (media) having instructions stored thereon/in which can be used to program a computer to perform any of the processes of the embodiments described herein. Computer code for operating and configuring the system  116  to intercommunicate and to process webpages, applications and other data and media content as described herein are preferably downloaded and stored on a hard disk, but the entire program code, or portions thereof, may also be stored in any other volatile or non-volatile memory medium or device as is well known, such as a read-only memory (ROM) or random-access memory (RAM), or provided on any media capable of storing program code, such as any type of rotating media including floppy disks, optical discs, digital versatile disk (DVD), compact disk (CD), microdrive, and magneto-optical disks, and magnetic or optical cards, nanosystems (including molecular memory integrated circuits (ICs)), or any type of media or device suitable for storing instructions and/or data. Additionally, the entire program code, or portions thereof, may be transmitted and downloaded from a software source over a transmission medium, e.g., over the Internet, or from another server, as is well known, or transmitted over any other conventional network connection as is well known (e.g., extranet, virtual private network (VPN), LAN, etc.) using any communication medium and protocols (e.g., TCP/IP, HTTP, HTTPS, Ethernet, etc.) as are well known. It will also be appreciated that computer code for implementing embodiments of the present disclosure can be implemented in any programming language that can be executed on a client system and/or server or server system such as, for example, C, C++, HTML, any other markup language, Java™, JavaScript, ActiveX, any other scripting language, such as VBScript, and many other programming languages as are well known may be used. (Java™ is a trademark of Sun MicroSystems, Inc.). 
     According to one embodiment, the system  116  is configured to provide webpages, forms, applications, data and media content to the user (client) systems  112  to support the access by the user systems  112  as tenants of the system  116 . As such, the system  116  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). 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., object-oriented database management system (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. 
       FIG.  2    also illustrates the environment  110 , which may be used to implement embodiments described herein.  FIG.  2    further illustrates elements of system  116  and various interconnections, according to some embodiments.  FIG.  2    shows that each of the user systems  112  may include a processor system  112 A, a memory system  112 B, an input system  112 C, and an output system  112 D.  FIG.  2    shows the network  114  and the system  116 .  FIG.  2    also shows that the system  116  may include the tenant data storage  122 , the tenant data  123 , the system data storage  124 , the system data  125 , a user interface (UI)  230 , an application program interface (API)  232 , a Salesforce.com object query language (SOQL)  234 , save routines  236 , an application setup mechanism  238 , applications servers  200   1 - 200   N , a system process space  202 , tenant process spaces  204 , a tenant management process space  210 , a tenant storage area  212 , a user storage  214 , and application metadata  216 . In other embodiments, environment  110  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. 
     The user systems  112 , the network  114 , the system  116 , the tenant data storage  122 , and the system data storage  124  were discussed above in  FIG.  1   . Regarding the user systems  112 , the processor system  112 A may be any combination of one or more processors. The memory system  112 B may be any combination of one or more memory devices, short term, and/or long term memory. The input system  112 C may be any combination of input devices, such as one or more keyboards, mice, trackballs, scanners, cameras, and/or interfaces to networks. The output system  112 D may be any combination of output devices, such as one or more monitors, printers, and/or interfaces to networks. As shown in  FIGS.  1  and  2   , the system  116  may include the network interface  120  (of  FIG.  1   ) implemented as a set of HTTP application servers  200 , the application platform  118 , the tenant data storage  122 , and the system data storage  124 . Also shown is system process space  202 , including individual tenant process spaces  204  and the tenant management process space  210 . Each application server  200  may be configured to access the tenant data storage  122  and the tenant data  123  therein, and the system data storage  124  and the system data  125  therein to serve requests of the user systems  112 . The tenant data  123  might be divided into individual tenant storage areas  212 , which can be either a physical arrangement and/or a logical arrangement of data. Within each tenant storage area  212 , the user storage  214  and the application metadata  216  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 the user storage  214 . Similarly, a copy of MRU items for an entire organization that is a tenant might be stored to the tenant storage area  212 . The UI  230  provides a user interface and the API  232  provides an application programmer interface to the system  116  resident processes and to users and/or developers at the user systems  112 . The tenant data and the system data may be stored in various databases, such as one or more Oracle™ databases. 
     The application platform  118  includes an application setup mechanism  238  that supports application developers&#39; creation and management of applications, which may be saved as metadata into tenant data storage  122  by the save routines  236  for execution by subscribers as one or more tenant process spaces  204  managed by the tenant management process  210 , for example. Invocations to such applications may be coded using SOQL  234  that provides a programming language style interface extension to the API  232 . Some embodiments of SOQL language are discussed in further detail in U.S. Pat. No. 7,730,478, filed September 2007, entitled, “Method and System For Allowing Access to Developed Applications Via a Multi-Tenant On-Demand Database Service,” which is incorporated herein by reference. Invocations to applications may be detected by one or more system processes, which manage retrieving the application metadata  216  for the subscriber, making the invocation and executing the metadata as an application in a virtual machine. 
     Each application server  200  may be communicably coupled to database systems, e.g., having access to the system data  125  and the tenant data  123 , via a different network connection. For example, one application server  200   1  might be coupled via the network  114  (e.g., the Internet), another application server  200   N-1  might be coupled via a direct network link, and another application server  200   N  might be coupled by yet a different network connection. TCP/IP are typical protocols for communicating between application servers  200  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 connection used. 
     In certain embodiments, each application server  200  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  200 . 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  200  and the user systems  112  to distribute requests to the application servers  200 . In one embodiment, the load balancer uses a least connections algorithm to route user requests to the application servers  200 . 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  200 , and three requests from different users could hit the same application server  200 . In this manner, the system  116  is multi-tenant, wherein the system  116  handles storage of, and access to, different objects, data and applications across disparate users and organizations. 
     As an example of storage, one tenant might be a company that employs a sales force where each salesperson uses the system  116  to manage his or her 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 the tenant data storage  122 ). In an example of a MTS arrangement, since all 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. 
     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 the users for a given organization that is a tenant. Thus, there might be some data structures managed by the system  116  that are allocated at the tenant level while other data structures might be managed at the user level. Because a 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 a 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, the system  116  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. 
     In certain embodiments, the user systems  112  (which may be client systems) communicate with the application servers  200  to request and update system-level and tenant-level data from the system  116  that may require sending one or more queries to the tenant data storage  122  and/or the system data storage  124 . The system  116  (e.g., an application server  200  in the system  116 ) automatically generates one or more structured query language (SQL) statements (e.g., one or more SQL queries) that are designed to access the desired information. The system data storage  124  may generate query plans to access the requested data from the database. 
     In a database system, such as system  116  shown and described with respect to  FIGS.  1  and  2   , data or information may be organized or arranged in categories or groupings. 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. 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. Each row or record of a table contains an instance of data for each category defined by the fields. 
     In a CRM system, for example, these categories or groupings can include various standard entities, such as account, contact, lead, opportunity, group, case, knowledge article, etc., each containing pre-defined 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 MTS, standard entity tables might be provided for use by all tenants. 
     In some MTSs, 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. Systems and methods for creating custom objects as well as customizing standard objects in a MTS are described in further detail in U.S. Pat. No. 7,779,039, filed Apr. 2, 2004, entitled “Custom Entities and Fields in a Multi-Tenant Database System,” which is incorporated herein by reference. 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. 
     II. Archiving Records into a Second Storage Device 
     Various users interacting with a system, such as a MTS, may manage and control data stored in the MTS. The term “user” may refer to an “entity,” a “tenant,” or an “organization.” Users may store data via an object, which may come with a set of standard fields. Each tenant may customize its own version of the object based on, for example, its business practices and the information it desires to save and maintain. For example, an accounting firm may desire to add customized fields for storing client information (e.g., client names, client addresses, billing arrangement, end of the client&#39;s fiscal year, etc.). In another example, a temp agency may desire to add customized fields for employees who have been provided with temporary work (e.g., employee names, employee addresses, hourly rates, place of temporary employment, etc.). Accordingly, multiple versions of an object may exist, with each version being specific to a tenant. As will be explained further below, it may be helpful to archive records at a tenant-specific level from the MTS to the second storage device, considering each tenant&#39;s set of custom fields. 
     A. Records Based on Customized Objects 
       FIG.  3    illustrates a diagram  300  for archiving one or more records stored in a first storage device into a second storage device according to some embodiments. For example, components of diagram  300  may be, in some examples, implemented as part of the example environment  110 . In  FIG.  3   , a computing device  302  includes an archive manager  304  and a query engine  306 . The archive manager  304  includes a schema engine  308  and an archive engine  310 . The archive engine  310  archives data stored in the tenant data storage  122  into a second storage device  312 . 
     A plurality of users may store and maintain data in the tenant data storage  122 , which may be a MTS having a relational schema. Some users may have a huge amount of data stored in the MTS. For example, a user may store thousands of records in the tenant data storage  122  and desire to archive a subset of these records into a second storage device that is more cost-effective than the tenant data storage  122 . Aspects of the disclosure may provide benefits such as easing the workload and thus reducing the burden on the tenant data storage  122 . Additionally, although the second storage device  312  may be slower than the tenant data storage  122 , the user may benefit from lower storage costs by archiving records into and retrieving records from the second storage device  312  rather than maintaining them in the tenant data storage  122 . The archive manager  304  and the query engine  306  may allow the user continued access to the archive records, using existing platform semantics as was previously used by the user for the tenant data storage  122 . 
     The schema engine  308  may maintain and track a current schema  316  and a historical schema  320  of an object  314  in the system data storage  124 . The current schema  316  and the historical schema  320  are dynamic and may change over time. The current schema  316  is a subset of the historical schema  320 . Additionally, the historical schema  320  maintains fields in a particular order and uses slots as placeholders for each field that was included in the object  314 . 
     In the example illustrated in  FIG.  3   , the user may insert records into an Object table  305  stored in the tenant data storage  122 . Records  318  and  319  may be based on the object  314  having a schema that defines a set of standard fields and a set of custom fields. The current schema  316  defines an ordered set of current fields of the object  314 . A standard field is included in each representation of an object. A custom field may be added, removed, or modified by a user. The current schema  316  defines two standard fields and two custom fields, &lt;S1, S2, C1, C2&gt;. The standard fields S1 and S2 may be a person&#39;s first name and last name, respectively. The custom fields C1 and C2 may be the person&#39;s age and sex, respectively. As shown in the record  318 , Kyle Anand is a 26 year-old male. As shown in the record  319 , Anne Chan is a 32 year-old female. The user may modify the current schema  316  of the object  314  by adding fields to, removing fields from, and/or modifying the custom fields (e.g., changing a data type). Accordingly, the current schema  316  of the object  314  may change over time. 
     Although  FIGS.  3 - 7    discuss a single tenant modifying its version of the object  314 , it should be understood that multiple tenants may modify their versions of the same object  314 , which may each include the two standard fields S1 and S2 and zero or more custom fields specific to the tenant. 
     B. Schema Drift Over Time 
     Schema drift occurs when a field definition of an object  314 &#39;s schema changes over time. For example, if the user removes the field C1 from the current schema  316 , the user may no longer be able to access the field C1 in the future. To overcome this problem, the schema engine  308  preserves historical fields of the object  314  by maintaining a historical schema  320  of the object  314 . If the user modifies the current schema  316 , the schema engine  308  may detect the modification and update the current schema  316  and the historical schema  320  accordingly. The historical schema  320  defines an ordered set of fields based on previous and current schemas of the object  314 . In other words, the historical schema  320  keeps track of the current and non-current fields of the object  314  based on an order that defines the serialization format for an archive record. A non-current field is a field that is not present in the object  314 &#39;s current schema and may also be referred to a historical field. The schema engine  308  may determine the historical schema  320  by snapshotting the current schema  316  and adding fields to an end of the historical schema  320 , which defines a superset of fields that have been defined in the object  314 . 
     Schema drift is inherent in the archiving process as users change the current schema  316  of the object  314 . For each field modification of the current schema  316 , the schema engine  308  may insert the new field at an end of the historical schema  320  to update it. The schema engine  308  maintains the order of the fields defined in the historical schema  320 , which provides a timeline of field modification in the object  314 . For example, if a first field is stored in a slot preceding a second field in the historical schema  320 , the first field may have been added to the current schema  316  before the second field. If the first field is stored in a slot succeeding the second field in the historical schema  320 , the second field may have been added to the current schema  316  before the first field. When archiving records into the second storage device  312 , the historical schema  320  may be used to insert values into the correct location in the second storage device  312 . The historical schema  320  may include current and non-current fields, where non-current fields are those fields that no longer exist in the current schema  316  of the object  314 . As discussed in further detail below, when querying from the second storage device  312 , the query engine  306  may be used to retrieve those values corresponding to the current fields. Additionally, the query engine  306  may determine, based on the historical schema  320 , the physical structure of records stored in the second storage device  312  and retrieve values corresponding to the applicable fields. 
     If the user adds a field to the current schema  316 , the schema engine  308  inserts the added field at an end of the historical schema  320  to update it. If the user modifies a field&#39;s data type, the field with the new data type is considered a new field, and schema engine  308  inserts the new field at an end of the historical schema  320  to update it. In this example, if the user inserts a new record, the user writes into the new field (the field with the new data type) rather than the field with the old data type. If the user renames a field in the current schema  316 , the field with the new name is considered a new field, and the schema engine  308  inserts the new field at an end of the historical schema  320  to update it. In this example, if the user inserts a new record, the user writes into the field with the new name rather than the field with the old name. 
     In another example, if the user removes a field from the current schema  316 , the schema engine  308  updates the historical schema  320  by leaving it as-is. In other words, if a field is removed from the current schema  316 , the schema engine  308  does not modify the historical schema  320 . The historical schema  320  preserves all historical field values, and the fields defined in the current schema  316  and the historical schema  320  are dynamic. 
     Additionally, a field that was historical can be made current again. If a field is deleted, the query engine  306  would not write into that field anymore when new records are added to the tenant data storage  122 . If a user re-adds that field in the same way, the query engine  306  may start writing into that field again. If the historical field is pulled back into the current set of fields, then the values that were previously archived for that field may be made available again, even though for a period of time in the past they were not available due to being in the historical set. If the field has a different datatype, however, the query engine  306  may create a new field and write to this new field, even if the field has the same name as the one that had its datatype modified. 
     Using the historical schema  320 , a user may have access to all field(s) that have ever been included in the object  314 , even in cases where the metadata no longer exists in the original object for those fields. A user may project and filter on all fields with values in the archive records stored in the second storage device  312 , including fields that have been deleted or undergone non-backwards compatible data type changes on the schema of the object  314 . For example, if a user removes the field C1 from the current schema  316 , users may still be able to project and filter on the field C1 in the archive records because the historical schema  320  maintains its knowledge of the field. Additionally, the users may retrieve values for records that were archived when the field C1 was current on the object  314 . 
     In response to an indication to archive an original record stored in the tenant data storage  122 , the archive manager  304  generates, at the second storage device  312 , an archive record corresponding to and having the same schema as the original record. The archive manager  304  may copy data stored in each field of the original record to the corresponding archive record stored in the second storage device. After copying the data, the archive manager  304  removes the original record from the tenant data storage  122 . 
     C. Migration of Original Records into the Second Storage Device 
     Each record includes a record identifier (ID) (e.g., Record_ID column) that identifies a record. In some examples, the archive engine  310  archives original records stored in the tenant data storage  122  by querying record IDs, inserting original records identified by the record IDs into the second storage device  312 , and removing the original records from the tenant data storage  122 . The administrator for the tenant may provide the record IDS to the archive manager  304  for archiving into the second storage device  312 . Other mechanism for identifying and archiving records are within the scope of the disclosure. 
       FIG.  4    illustrates a diagram  400  for archiving records into the second storage device  312  according to some embodiments. A user may opt into usage of the archive manager  304  by marking one or more records stored in the tenant data storage  122  for archiving into the second storage device  312 . In an example, the archive records may be immutable such that once they are archived, they are not modifiable in the second storage device  312 . The archive engine  310  maintains a representation of the archive records such that users can interact with the data using familiar platform features and semantics as used by the tenant data storage  122 . The archive engine  310  may provide a unified view of the archive and non-archive data via entity interfaces. In an example, the archive records are Parquet files. In an example, the archive records are stored as S3 blobs. In an example, the archive records are stored in HBase®, and Phoenix® is an open source SQL layer on top of HBase®. 
     In the example illustrated in  FIG.  4   , the archive engine  310  includes a memory scanner  402  and one or more message handlers  403 . The memory scanner  402  scans the memory in the tenant data storage  122  for records that have been marked by one or more users for archiving into the second storage device  312 . In an example, the memory scanner  402  is implemented via a cron job, which is a scheduled task. Cron is a LINUX® utility that schedules a command or script to run automatically at a specified time and date. Trademarks are the property of their respective owners. An administrator for the tenant data storage  122  may schedule the memory scanner  402  to execute, for example, once per day. For each user and each marked record, the memory scanner  402  enqueues a message to a message queue  304 . Each message  406  may specify the record ID of the record for archiving. The memory scanner  402  enqueues to the message queue  404  a message  406  including the Record_ID R001, which identifies the record  318 , and a message  407  including the Record_ID R002, which identifies the record  319 . 
     Additionally, each record itself may be related to one or more other records stored in the tenant data storage  122 . The memory scanner  402  may create an archive record graph including records related to the marked records. The memory scanner  402  may identify related records by identifying a cascade-relationship for any marked records and mark these related records for archiving into the second storage device  312 . In an example, the record  318  is related to a comment record storing comments entered by one or more users. The comment record may be separate from the record  318  (e.g., stored in an object table different from the record  318 ) and have a cascade-relationship with the record  318  such that if the record  318  is deleted, the comment record should be deleted as well. The memory scanner  402  enqueues to the message queue  404 , messages including the record IDs of records related to the records marked for archiving into the second storage device  312 . 
     The message handler  403  identifies the records for archiving by dequeuing messages from the message queue  404  and processing the messages. One message handler  403  may execute per-user, per-record for archiving. For each message in the message queue  404 , the message handler  403  identifies the particular record specified by the message and creates an archive record having the same schema and storing the same data as the particular record at the time of archive. In an example, the archive record  418  mirrors the schema of and data stored in the original record  318 , and the archive record  419  mirrors the schema of and data stored in the original record  319 . 
     The schema engine  308  maintains metadata  422  including information about the schemas for archive records. The information may include the schema ID, the schema, and a specific point in time at which the record corresponding to the schema was archived (e.g., a timestamp). The schema ID included in an archive record identifies the schema of the respective archive record. The historical schema  320  and/or the metadata  422  enables the archive manager  304  and the query engine  306  to represent data without destroying the visibility of specific information that was stored in the object  314 &#39;s modified field. 
     The schema engine  308  may maintain the current schema  316 , the historical schema  320 , and the metadata  422  for each separate object, per tenant. The schema engine  308  identifies the schema of an archive record, assigns the schema a Schema ID, and inserts the schema ID along with the schema into the metadata  422 . For example, the schema engine  308  inserts into the metadata  422 , a record  424  including the Schema ID S001, which identifies the schema &lt;S1, S2, C1, C2&gt;, and a timestamp T1 of the archive of the records  318  and  319 . In this way, the schema engine  308  may attach the Schema ID S001 with the archive records AR001 and AR002, allowing the data to be synchronized with the appropriate schema. If the data is queried in the future (e.g., read or write access), the query engine  306  may apply the correct schema to the data stored in the second storage device  312  and provide the user with a view of the data at the correct point in time in the history of the object  314 . 
     The archive engine  310  may maintain an Object_Archive table  405  that stores records archived from the tenant data storage  122  (e.g., Object table  305  and other tables). The Object table  305  may map to the Object_Archive table  405 . In another example, the archive engine  310  creates a table in the second storage device  312  having the same name as the table from which the records were archived (e.g., “Object” table). Each of the archive records may be assigned an archive record ID and include the schema ID and data included in the original record. For example, the archive record  418  is assigned the archive record ID AR001 and includes the schema ID S001 and the data included in the original record  318 , and the archive record  419  is assigned the archive record ID AR002 and includes the schema ID S001 and the data included in the original record  319 . 
     After the records  418  and  419  have been archived into the second storage device  312 , the archive engine  310  removes the original records  318  and  319  from the tenant data storage  122 , as shown by the dashed lines. If the archive engine  310  is unable to remove a record, the archive engine  310  may log the removal failure into a log file for a re-try later. The message handler  403  continues to archive records by dequeuing remaining messages from the message queue  404 . 
     D. Example Timing Diagram 
       FIG.  5    illustrates a timing diagram  500  of the current schema  316  and the historical schema  320  of the object  314  over time based on field modifications according to some embodiments. At time T1, the current schema  316  is &lt;S1, S2, C1, C2&gt; and the historical schema  320  is &lt;S1, S2, C1, C2&gt;, as shown in  FIG.  1   . At time T2, in response to a user adding a custom field C3 to the current schema  316 , the schema engine  308  updates the current schema  316  to define two standard fields and three custom fields &lt;S1, S2, C1, C2, C3&gt;. Additionally, the schema engine  308  updates the historical schema  320  by adding the new field C3 to an end of the historical schema  320  such that the new field occupies the last slot of the historical schema. The updated historical schema  320  becomes &lt;S1, S2, C1, C2, C3&gt;. 
     At time T3, in response to a user removing the custom field C3 from the current schema  316 , the schema engine  308  updates the current schema  316  to define two standard fields and two custom fields &lt;S1, S2, C1, C2&gt;. The historical schema  320  includes a superset of the fields defined in the object  314  and accordingly remains the same. At a later point in time, the user may make the field C3 current again. If the historical field C3 is pulled back into the current set of fields, then the values that were previously archived for that field will be made available again, even though for a period of time in the past they were not available due to being in the historical set. 
     At time T4, in response to a user adding a custom field C4 to the current schema  316 , the schema engine  308  updates the current schema  316  to define two standard fields and three custom fields &lt;S1, S2, C1, C2, C4&gt;. Additionally, the schema engine  308  updates the historical schema  320  by adding the new field C4 to an end of the historical schema  320  such that the new field occupies the last slot of the historical schema. The updated historical schema  320  becomes &lt;S1, S2, C1, C2, C3, C4&gt;. The historical schema  320  provides a timeline of the field modifications in the current schema  316  relative to each other. For example, the C4 field was added to the object  314  after the C3 field was added. Accordingly, the slot filled by the C4 field succeeds the slot filled by the C3 field in the historical schema  320 . At time T5, in response to a user removing the custom field C4 from the current schema  316 , the schema engine  308  updates the current schema  316  to define two standard fields and two custom fields &lt;S1, S2, C1, C2&gt;. The historical schema  320  remains the same. 
     From time T1 to T5, users may insert records into, remove records from, or modify records stored in the tenant data storage  122 . The records inserted into the tenant data storage  122  are defined by the current schema  316  at the time of insertion. For example, between times T2 and T3, the records added are defined by two standard fields and three custom fields &lt;S1, S2, C1, C2, C3&gt;. Between times T3 and T4, the records added are defined by two standard fields and two custom fields &lt;S1, S2, C1, C2&gt;. Between times T4 and T5, the records added are defined by two standard fields and three custom fields &lt;S1, S2, C1, C2, C4&gt;. From time T5 until the next modification to the current schema of the object  314 , the records added are defined by two standard fields and two custom fields &lt;S1, S2, C1, C2&gt;. The archive manager may archive at least some of these records into the second storage device  312 . 
     E. Maintenance of Metadata and a Historical Schema 
       FIG.  6    illustrates a diagram  600  for archiving records into the second storage device  312  according to some embodiments. To aid in understanding aspects of  FIG.  6   ,  FIG.  5    will be discussed in relation to  FIG.  6   . As shown in  FIG.  6   , each of archive records  418 ,  419 ,  602 ,  606 , and  608  stored in the second storage device  312  may have the same (or common) or different schemas relative to each other. Each record&#39;s schema reflects the shape of the original entity at the time of the archive. 
     The archive record  602  is defined by a schema &lt;S1, S2, C1, C2, C3&gt; corresponding to time T2 in  FIG.  5   . The schema engine  308  assigns a Schema ID S002 to the schema &lt;S1, S2, C1, C2, C3&gt; and inserts into the metadata  422 , a record  604  including the schema ID S002, the schema &lt;S1, S2, C1, C2, C3&gt;, and a timestamp T2 of the archive of the corresponding original record. The archive engine  310  archives the corresponding original record stored in the tenant data storage  122 , assigns it an archive record ID AR003, includes the Schema ID S002 in the record  602 , and removes the applicable record from the tenant data storage  122 . 
     Additionally, the archive record  606  is defined by a schema &lt;S1, S2, C1, C2&gt; corresponding to time T3 in  FIG.  5   . The schema engine  308  assigns a Schema ID S003 to the schema &lt;S1, S2, C1, C2&gt; and inserts into the metadata  422 , a record  610  including the schema ID S003, the schema &lt;S1, S2, C1, C2&gt;, and a timestamp T3 of the archive of the corresponding original record. The archive engine  310  archives the corresponding original record stored in the tenant data storage  122 , assigns it an archive record ID AR004, includes the Schema ID S003 in the record  606 , and removes the applicable record from the tenant data storage  122 . 
     Additionally, the archive record  608  is defined by a schema &lt;S1, S2, C1, C2, C4&gt; corresponding to time T4 in  FIG.  5   . The schema engine  308  assigns a Schema ID S004 to the schema &lt;S1, S2, C1, C2, C4&gt; and inserts into the metadata  422 , a record  611  including the schema ID S004, the schema &lt;S1, S2, C1, C2, C4&gt;, and a timestamp T4 of the archive of the corresponding original record. The archive engine  310  archives the corresponding original record stored in the tenant data storage  122 , assigns it an archive record ID AR005, includes the Schema ID S004 in the record  608 , and removes the applicable record from the tenant data storage  122 . 
     Between time T1 and T2, between time T3 and T4, or after time T5, the user may insert records  612  and  614  into the tenant data storage  122 . The records  612  and  614  are based on the current schema  316  of the object  314  corresponding to time T1, time T3, or time T5 in  FIG.  5    and are assigned record IDs R009 and R014, respectively. Additionally, based on the removal of the C4 field in  FIG.  5   , the schema engine  308  updates the current schema  316  to &lt;S1, S2, C1, C2&gt; and the historical schema  320  to &lt;S1, S2, C1, C2, C3, C4&gt;. 
     III. Query Processing 
     It may be desirable to allow a user to access records stored in both the tenant data storage  122  and the second storage device  312 . The query engine  306  may use the current schema  316  and the historical schema  320  of the object  314  to access records in both storage devices. Additionally, the query engine  306  allows the user to access data in a similar way in which the user originally accessed data from the tenant data storage  122 . 
     During design time, users may modify or redefine fields of the current schema  316 . The query engine  306  may determine, based on the historical schema  320 , the physical structure of records stored in the second storage device  312 . During runtime, the query engine  306  may account for changes to the current schema  316  that occurred during design time and execute queries submitted by users against the tenant data storage  122  and/or the second storage device  312 . The query engine  306  receives a query from a user and decomposes the query to determine whether to submit the query to and retrieve a result set from the tenant data storage  122  and/or the second storage device  312 . The query engine  308  may process the query and perform calculations for presenting a view of the data to a user. 
     The second storage device  312  may support dynamic columns, which may be added at runtime. For example, if during design time, a user modifies a field of the current schema  316  by, for example, adding a field, changing the data type, etc., the archive engine  310  may add a new column to the corresponding record in the Object_Archive table  405 . Accordingly, the rows in the Object_Archive table  405  may have different schemas relative to each other. 
     A. Query the First Data Storage Storing Values For Current Fields 
     The query includes a command specifying a subset of fields or columns included in a set of records based on the object  314 . The query engine  306  determines whether the current schema  316  of the object  314  includes the subset of fields. If so, the query engine  306  submits the query against the tenant data storage  122  and retrieves a result set of the query from the tenant data storage  122 . If not, the query engine  306  determines that the second storage device  312  does not store data applicable to the query. 
     In an example, the query is a Structured Query Language (SQL) query (Q1): “SELECT S1, S2, C1, C2 FROM Object WHERE C1&gt;30.” The query engine  306  may receive the query Q1 from a user and retrieve a result set including values from the &lt;S1, S2, C1, C2&gt; fields in records  612  and  614 . Records  612  and  614  include the specified fields and satisfy the condition C1&gt;30. The result set of the query Q1 from the tenant data storage  122  may be {&lt;S1=‘Tyson’, S2=‘Henry’, C1=42, C2=‘M’, &lt;S1=‘Molly’, S2=‘Smith’, C1=35, C2=‘F’&gt;}. The query engine  306  may return the result set from the tenant data storage  122  to the user. 
     B. Query the Second Data Storage Storing Archive Records 
     The query engine  306  may reconstruct, based on the historical schema  320  and the metadata  422 , the data stored in the archive records. The archive manager  304  and the query engine  306  may serialize, based on the historical schema  320  and the metadata  422 , data in the second storage device  312 . Additionally, the archive manager  304  and the query engine  306  may deserialize, based on the historical schema  320  and the metadata  422 , the serialized records stored in the second storage device  312 . 
     In some examples, the query engine  306  generates, based on the historical schema  320  and the metadata  422 , a view of the archive data. In an example, the second storage device  312  is HBase, and the query engine  306  implements the open source SQL layer, providing a low-level operation that translates SQL statements into HBase raw scans (e.g., GETs and PUTs). If a field has a particular name and particular data type, the query engine  306  may create a specific column bound to that field in the record that has that particular name and data type. The query engine  306  translates the historical schema  320  and the metadata  422  into columns bounded to particular fields included in the correct schema. Each field in the historical schema  320  may be represented as Phoenix columns, which may be associated with a timestamp. 
     The query engine  306  may generate a view including all applicable columns defined in a record. If a field in the object  314  is modified (e.g., removed, data type modified, etc.), the query engine  306  may continue to generate a view including the column representing the modified field. Accordingly, the query engine  306  may reconstruct, based on the historical schema  320  and the metadata  422 , views including columns corresponding to current and non-current fields. Any of the columns included in a view may be queried for data. Accordingly, fields that no longer exist on an object  314  may be queried and the applicable data returned to a user. The view defines the serialization format for the row, and the view may provide a superset of all current and historical fields. 
     The user may submit a query including a command specifying a subset of fields based on the object. Additionally, the query may indicate one or more non-current fields, where non-current fields are absent from the current schema. In an example, the query includes a backdoor field that maps to a non-current field. The current schema and the historical schema are devoid of the backdoor field. In another example, the query includes the non-current field. The query engine  306  may obtain the query and process the query. 
     1. The Subset of Fields Specified in the Query Matches the Current Fields 
     In an example, the query engine  306  submits the query Q1 to the second storage device  312 . In this example, the query engine  306  may receive an error because the second storage device  312  does not store an Object table. In another example, the archive manager  304  may maintain table mappings, where a table mapping includes an entry including an original table name from which an original record is archived and an archive table name storing the applicable archive record. For example, the archive manager  304  may map the Object table  305  to the Archive_Object table  405 . The query engine  306  may rewrite the query Q1 by determining that the Object table stored in the tenant data storage  122  maps to the Object_Archive table  405  stored in the second storage device  312 . The query engine  306  may rewrite the query Q1 by replacing the Object table with the Object_Archive table, resulting in the rewritten query (RQ1): “SELECT S1, S2, C1, C2 FROM Object_Archive WHERE C1&gt;30.” 
     In some examples, the user submits the query Q1 to the tenant data storage  122 . The query engine  306  may obtain the query Q1 and submit the query Q1 to the tenant data storage  122 , rewrite the Q1 to RQ1, and submit the RQ1 to the second storage device  312 . The query engine  306  may retrieve a first result set from the tenant data storage  122  and a second result set from the second storage device  312 . The query engine  306  may return a final result set to the user, the final result set being based on the first and second result sets. 
     In some examples, the user includes the name of the table stored in the second storage device  312 . In an example, the user may submit a query (Q2): “SELECT S1, S2, C1, C2 FROM Object_Archive WHERE C1&gt;30.” The fields included in the historical schema  320  are in an order that is followed by the records to ensure that when data is pulled, the query engine  306  knows which fields specified in the query map to which fields/slots in the record. For the query Q2, the query engine  306  determines whether the subset of fields specified in the query is included in the historical schema  320 . If so, the query engine  306  may search the metadata  422  for schemas that match the subset of fields specified in the query. A schema matches the subset of fields specified in the query Q2 if the schema includes the subset of fields &lt;S1, S2, C1, C2&gt;. The query engine  306  may determine the Schema IDs assigned to the matching schemas and retrieve a result set including archive records that include the Schema IDs and satisfy the conditions in the query. 
     In this example, the query engine  306  identifies Schema IDs S001, S002, S003, and S004 as being assigned to a schema including the fields &lt;S1, S2, C1, C2&gt; specified in the query Q2. The query engine  306  retrieves a result set including values from the fields &lt;S1, S2, C1, C2&gt; in records  419 ,  602 , and  606 , which include Schema ID S001, S002, S003, or S004 and also satisfy the condition C1&gt;30. The result set of the query Q2 from the Object_Archive table  405  stored in the second storage device  312  may be {&lt;S1=‘Anne’, S2=‘Chan’, C1=32, C2=‘F’&gt;, &lt;S1=‘John’, S2=null, C1=37, C2=‘M’&gt;, &lt;S1=‘Sean’, S2=‘Khan’, C1=36, C2=‘M’&gt;}. The query engine  306  may return the result set from the second storage device  312  to the user. 
     2. The Subset of Fields Specified in the Query Includes a Backdoor Field 
     As fields in the object  314  are modified, the query engine  306  may successfully query archive records storing data that may not be exposed on the current schema  316  of the object  314 . The query engine  306  may read the historical schema  320  and the metadata  422  in relation to a command (e.g., a read or a write command) and apply the correct schema on the data in accordance with the command. The query engine  306  may translate the historical schema  320  and/or the metadata  422  into a storage layer that allows a query “back in time.” A user may believe that any removed fields are no longer relevant because they were removed, but these removed fields are still relevant in the second storage device  312  to decode serialized records storing that information. 
     In some examples, the query engine  306  exposes a backdoor field that the user may include in a query to retrieve values from fields that are not in the current schema  316  of the object  314 . The backdoor field may be optional. For example, if the user does not specify the backdoor field in the query, then the backdoor field is not computed and no value is returned for any non-current fields. If the user specifies the backdoor field in the query, the user may access fields that no longer exist in the current schema  316  of the object  314 . In an example, the backdoor field is of a JSON field type. As long as the user performing the query has knowledge of the backdoor field, the user may access non-current fields of the object  314  to retrieve their stored values. If the user specifies the backdoor field in a query, the query engine  306  may retrieve the values to which the backdoor field is mapped (e.g., values in the non-current fields). 
     Referring to  FIGS.  5  and  6   , at time T3, the user may submit a query (Q3): “SELECT S1, S2, C1, C2, backdoor FROM Object_Archive WHERE S1=‘John’.” The query engine  306  may submit the query Q3 to the second storage device  312 . In an example, the backdoor field may be mapped to the non-current fields of the object  314 . The query engine  306  may retrieve, based on one or more candidate schemas, a result set for the query Q3. The query engine  306  may determine one or more candidate schemas by searching the metadata  422  for a schema including the specified fields &lt;S1, S2, C1, C2&gt; and one or more additional fields. The candidate schemas may be schema &lt;S1, S2, C1, C2, C3&gt; and schema &lt;S1, S2, C1, C2, C4&gt;, which are identified by Schema IDs S002 and S004, respectively. The query engine  306  searches the Object_Archive  405  for one or more records including the Schema IDs S002 and S004 and satisfying the condition S1=‘John’. In this example, a result set of the query Q3 from the second storage device  312  may correspond to the record  602  and may be {&lt;S1‘John’, S2=null, C1=37, C2=‘M’, backdoor=‘{C3:“94542”}’&gt;}. Accordingly, the result set includes the values of the fields that were specified in the query along with the value for the non-current C3 field. If the record  602  included more non-current fields aside from the non-current field C3, the query engine  306  may also provide values for these one or more non-current fields in the result set. 
     In another example, the backdoor field may be mapped to the current and the non-current fields of the object  314 . In this example, a result set of the query Q3 from the second storage device  312  may be {&lt;S1=‘John’, S2=null, C1=37, C2=‘M’, backdoor=‘{S1:“John”, S2: null, C1:“37”, C2:“M”, C3:“94542”}’&gt;}. The current schema and the historical schema may be devoid of the backdoor field. Accordingly, the object  314  does not include the backdoor field in its schema. For example, the backdoor field is not an actual field that is included in the current schema of the object  314 , but is used as a mechanism for gaining access to historical fields. 
     3. The Subset of Fields Specified in the Query Includes a Non-Current Field 
     In some examples, the user specifies a non-current field in the query. In this example, as long as the user has knowledge of the name of a non-current field for specification in the query, the user may retrieve values stored in the non-current fields from the archive records. The user may submit a query (Q4): “SELECT S2, C3 FROM Object_Archive. The query engine  306  may determine one or more candidate schemas by searching the metadata  422  for a schema including the specified field S2 and the specific non-current field C3. The candidate schema may be schema &lt;S1, S2, C1, C2, C3&gt;, which is the only schema including the non-current field C3 and is identified by the Schema ID S002. The query engine  306  searches the Object_Archive  405  for one or more records including the Schema ID S002 and satisfying conditions specified in the query. In this example, a result set of the query Q4 from the second storage device  312  may be {&lt;S2=null, C3=94542&gt;}. 
     C. Standard Full Record Field 
     In some examples, each archive record includes a standard full record field containing data stored in fields corresponding to the original record.  FIG.  7    illustrates a diagram  700  for storing a full record field  702  in an archive record according to some embodiments. In the example illustrated in  FIG.  7   , an Object_Archive table  705  is stored in a second storage device  712 . The Object_Archive table  705  includes one or more archive records, each having a set of standard and custom fields. In the Object_Archive table  705 , the full record field  702  may be of a JSON field type and contain a fully serialized immutable archive record that provides a JSON representation of the data. The full record field  702  may contain the values corresponding to a schema snapshot and the schema snapshot. The schema snapshot is a snapshot of the schema at the point in time view of the record. The data stored in fields of the object  314  at the time of archive may be duplicated in the corresponding full record field  702 . By utilizing the full record field  702 , the archive manager  304  may maintain a snapshot of the record exactly as it was at the point of archive and allows access to field values where the metadata no longer exists on the original entity for those fields (e.g., on custom field deletion). 
     The Object_Archive table  705  may store the data for fields S1, S2, C1, C2, C3, and/or C4 as shown in  FIG.  7   . Each of the full records  702  may be immutable and store the data for fields S1, S2, C1, C2, C3, and/or C4 in a serialized format. For example, a record  704  may store the values &lt;S1=‘Kyle’, S2=‘Anand’, C1=26, C2=‘M’&gt; for the applicable fields in a serialized format, a record  706  may store the values &lt;S1=‘Anne’, S2=‘Chan’, C1=32, C2=‘F’&gt; for the applicable fields in a serialized format, a record  708  may store the values &lt;S1=‘John’, S2=null, C1=37, C2=‘M’, C3=‘94542’&gt; for the applicable fields in a serialized format, a record  710  may store the values &lt;S1=‘Sean’, S2=‘Khan’, C1=36, C2=‘M’&gt; for the applicable fields in a serialized format, and a record  711  may store the values &lt;S1=‘Diane’, S2=‘Le’, C1=28, C2=‘F’, C4=‘Yes’&gt; for the applicable fields in a serialized format. If the user submits a query specifying the full record field  702 , the query engine  306  may return the values stored in the applicable full record (e.g., record  704 ,  706 ,  708 ,  710 , and/or  711 ). 
     In an example, for the query (Q4): “SELECT full_record FROM Object_Archive WHERE C2=‘F’, the query engine  306  may submit the query Q4 against the second storage device  712 . A result set of the query may include the full records  706  and  711 . 
     IV. Versions of an Object in a Multi-Tenancy 
     An object may be specific to tenants and their business practices. Each tenant may control and customize its own version of the object  314  and may add fields to, remove fields from, and/or modify custom fields (e.g., change a data type) within the version controlled by the tenant. The schema engine  308  and the archive engine  310  may maintain an object  314  for multiple tenants and separately track changes to the object  314  per-tenant. In relation to  FIGS.  3 - 7   , although not explicitly shown, the schema engine  308  and the archive engine  310  may maintain different tenant-specific versions of the object  314  and archive records at a tenant-specific level. 
       FIG.  8    illustrates a diagram  800  for archiving one or more records stored in the tenant data storage  122  according to some embodiments. The tenant data storage  122  may be a MTS, and the schema engine  308  may maintain the current schema  316  and the historical schema  320  for the object  314  per-tenant. Each tenant may have its own view of an object, and as tenants customize their objects, the schema engine  308  tracks their corresponding historical schemas and metadata. Metadata may be maintained to reflect the changes made by each tenant to its object version. Tenants may provide an indication to archive their version of the object. In this example, an archive manager may snapshot a schema of each of the object versions, where each schema includes the set of standard fields and a set of custom fields specific to a particular tenant of the MTS. By taking a snapshot of each schema corresponding to a particular tenant, the archive manager is able to maintain the tenant&#39;s set of custom fields. 
     The current schemas and historical schemas for each of the object versions may be stored in the system data storage  214 . For example, for a tenant  801 , the schema engine  308  may maintain the current schema  316 , the historical schema  320 , and the metadata  422  for the tenant  801 &#39;s version of the object  314   a  (as shown in  FIGS.  3 - 7   ). For a tenant  802 , the schema engine  308  may maintain a current schema  816 , a historical schema  820 , and the metadata for the tenant  802 &#39;s version of the object  314   b . The tenant  801 &#39;s current schema and historical schema of the object  314   a  may be different from the tenant  802 &#39;s. Each tenant&#39;s schema may include a common set of standard fields (e.g., S1 and S2 fields), but the object  314 &#39;s customized fields may differ based on what kind of data the tenant desires to store. The tenant  802  may customize its version of the object  314   b  such that its schema defines two standard fields and three custom fields, &lt;S1, S2, C1, C5, C7&gt;. Accordingly, for the tenant  802 &#39;s version of the object  314   b , the current schema  816  is &lt;S1, S2, C1, C5, C7&gt; and the historical schema  820  is &lt;S1, S2, C1, C5, C6, C7&gt;. 
     Additionally, the archive engine  310  may maintain tenant data  214   a  for the tenant  801  and maintain tenant data  214   b  for the tenant  802 . Referring to  FIG.  4   , the memory scanner  402  maintains a list of tenants (e.g., tenant IDs) and records for archiving, and the message queues  404  may be specific to a tenant. The archive engine  310  may archive records based on the object  314 , per-tenant. 
     The archive manager  304  and the query engine  306  may maintain and retrieve data that is specific to a tenant. The query engine  308  may submit, based on the historical schema and the metadata of an object for a particular tenant, queries to the tenant data storage  122  and/or the second storage device as discussed in the present disclosure. Additionally, the query engine  306  may retrieve, based on the historical schema and the metadata of an object for a particular tenant, the applicable information as discussed in the present disclosure. 
     If the tenant  801  accesses its own data, other tenants storing data in the system data storage  124 , the tenant data storage  122 , and/or the second storage device  312  activity be unaffected in terms of performance or reliability. In some examples, the query engine  306  utilizes tenant-specific views and maintains a middle layer to create a mapping between the modifications in the tenant&#39;s customized object. The intermediate layer may maintain these mappings that include the versioning of the schemas and is applied appropriately for a given tenant. The query engine  306  utilizes the middle layer for operating on a particular object, with each tenant having its own list of changes. 
     The archive manager  304  may archive original records stored in the Object table  805  into the second storage device  312  (e.g., in the Object_Archive table  405 ). In some examples, the archive data (e.g., archive records stored in the Object_Archive table  405 ) may have the same view, but the accessed view will change per tenant. The query engine  306  applies the customizations implemented on the object  314  by a tenant for providing the tenant&#39;s view of the result set of a query. 
     V. Operational Flow 
     In the foregoing description, numerous details are set forth. It will be apparent, however, to one of ordinary skill in the art having the benefit of this disclosure, that the present disclosure may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present disclosure. 
       FIG.  9    is a flowchart of a method  900  for processing a query according to some embodiments. One or more of the processes  902 - 912  of the method  900  may be implemented, at least in part, in the form of executable code stored on non-transitory, tangible, machine-readable media that when run by one or more processors may cause the one or more processors to perform one or more of the processes  902 - 912 . In some embodiments, method  900  can be performed by one or more computing devices in systems or diagrams  100 ,  200 ,  300 ,  400 ,  600 ,  700 , and  800  of  FIGS.  1 ,  2 ,  3 ,  4 ,  6 ,  7 , and  8   , respectively, including the archive engine  310 , the schema engine  308 , and/or the query engine  306 . Aspects of the processes  902 - 912  of method  900  have been covered in the description for  FIGS.  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 , and  8   ; and additional aspects are provided below. 
     At a process  902 , a computing device (e.g., via schema engine  308 ) creates a historical schema defining an ordered set of fields based on a current schema of an object and a field modification of the current schema. The field modification of a current schema may refer to an addition of a field to an object, a removal of a field from the object, or a field definition modification (e.g., change in data type, change in the field&#39;s name, etc.). The current schema  316  and the historical schema  320  are dynamic and may change over time. The archive manger  304  may maintain the current schema  316  and the historical schema  320  for an object on a per-tenant basis. 
     At a process  904 , a computing device (e.g., via schema engine  308 ) inserts, based on the field modification, a new field at an end of the historical schema. The schema engine  308  may determine the historical schema by snapshotting the current schema and adding fields to an end of the historical schema, which defines a superset of fields that have been defined in the object. The schema engine  308  may perform this action, per object and per tenant. Additionally, the order of the fields in the historical schema remains the same. 
     At a process  906 , a computing device (e.g., via archive engine  310 ) archives a plurality of records stored in a first storage device into a second storage device, each record of the plurality of records being based on the object and corresponding to an archive record having a first schema common to the respective record at a time of archive, and the first schema following a field order in accordance with the historical schema. Accordingly, the query engine  306  may determine, based on the historical schema, the physical structure of records stored in a second storage device. 
     At a process  908 , a computing device (e.g., via query engine  306 ) receives a query including a command specifying a subset of fields based on the object, the query indicating a non-current field absent from the current schema. The non-current field may be inaccessible via the tenant data storage  122  because it is no longer exposed via the current schema of the object. In an example, the query includes a backdoor field that maps to the non-current field(s) of the object. The current schema and the historical schema may be devoid of the backdoor field. The backdoor field may map to the current schema of the object as well as the non-current fields of the object. In another example, the query includes the non-current field. In some examples, the subset of fields may include the non-current field. In some examples, the subset of fields is separate from the non-current field. 
     At a process  910 , a computing device (e.g., via query engine  306 ) searches in the second storage device for a set of archive records having a second schema including the subset of fields and the non-current field, the second schema following the field order in accordance with the historical schema. In an example, the query engine  306  searches metadata corresponding to the object for the second schema including the subset of fields and identifies a schema ID that identifies the second schema. The set of archive records may include the schema ID and satisfy the conditions specified in the query. The metadata may include one or more entries, each entry including a given schema ID assigned to a given schema, the given schema, and a timestamp corresponding to a given time at which at least one archive record defined by the given schema was archived. 
     In some examples, at least one archive record corresponding to an original record of the plurality of records includes a full record containing values from the original record corresponding to the first schema. The full record contains a serialized immutable archive record storing the values and may be of a JSON type. 
     At a process  912 , a computing device (e.g., via query engine  306 ) accesses the subset of fields and the non-current field in accordance with the command. In an example, the query engine  306  may return values for each field of the subset of fields and return a value for each non-current field indicated in the query in a result set to the user. 
     In some embodiments, one or more actions illustrated in processes  902 - 912  may be performed for any number of objects per-tenant. It is also understood that additional processes may be performed before, during, or after processes  902 - 912  discussed above. It is also understood that one or more of the processes of method  900  described herein may be omitted, combined, or performed in a different sequence as desired. 
     For example, each tenant may customize and maintain its own version of the object  314  in accordance with the data the tenant desires to store and maintain. As discussed, the archive manager  304  and the schema engine  308  may maintain a historical schema and a current schema of the object  314  for each tenant. In an example, the schema engine  308  creates the historical schema  820  defining a second ordered set of fields based on the current schema  816  of the object  314   b  and a second field modification of the current schema  816 . The historical schema  320  and the current schema  316  may be based on the first tenant  801 &#39;s version of the object  314   a , and the historical schema  820  and the current schema  816  may be based on the second tenant  802 &#39;s version of the object  314   b . Additionally, the current schema  316  and the current schema  816  may include a common set of standard fields &lt;S1, S2&gt;. 
     The schema engine  308  may insert, based on the second field modification, a second new field at an end of the second historical schema to update the historical schema  820 . Additionally, the archive engine  310  may archive a second plurality of records stored in the first storage device into the second storage device, where each record of the second plurality of records is based on the second tenant&#39;s version of the object  314   b  and corresponds to an archive record having a third schema common to the respective record at a second time of archive, and where the third schema follows a second field order in accordance with the historical schema  820 . 
     The query engine  306  may receive a second query including a second command specifying a second subset of fields based on the second tenant&#39;s version of the object  314   b . The second query may indicate a second non-current field absent from the second current schema  816 . In response to the second query, the query engine  306  may search in the second storage device for a second set of archive records having a fourth schema including the second subset of fields and the second non-current field, where the fourth schema follows the second field order in accordance with the historical schema  820 . Additionally, the query engine  306  may access the second subset of fields and the second non-current field in accordance with the second command. 
     Unless specifically stated otherwise as apparent from the following discussion, it is appreciated that throughout the description, discussions utilizing terms such as “determining,” “creating,” “inserting,” “archiving,” “receiving,” “comparing,” “identifying,” “searching,” “accessing,” “submitting,” “removing,” “snapshotting,” and the like, refer to the actions and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (e.g., electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     Certain examples of the present disclosure also relate to an apparatus for performing the operations herein. This apparatus may be constructed for the intended purposes, or it may include a general-purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, or any type of media suitable for storing electronic instructions. 
     Although illustrative embodiments have been shown and described, a wide range of modifications, changes and substitutions is contemplated in the foregoing disclosure and in some instances, some features of the embodiments may be employed without a corresponding use of other features. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. Thus, the scope of the disclosure should be limited only by the following claims, and it is appropriate that the claims be construed broadly and in a manner consistent with the scope of the embodiments disclosed herein.