Abstract:
A system, method, apparatus, means and computer program code for processing a request to create a snapshot version of a database are provided which include identifying a current state of a converter table, the converter table including information identifying a plurality of page identifiers and a physical location of each of the page identifiers in a data area, saving the current state of the converter table, and marking each of the plurality of page identifiers to prevent overwriting.

Description:
FIELD 
     Embodiments of the present invention relate to database systems. More specifically, some embodiments relate to systems and methods for managing data in a database system. 
     BACKGROUND 
     Many database systems allow administrators or other authorized users to restore the database in the event of a database crash or other error. For example, in the event of a crash, the last known good version of the database may be retrieved and brought up to date to the time of the crash using data from a data log. 
     Database systems may also allow administrators or other authorized users to reset the database to an older state. Generally, such resets require that data be retrieved from an external backup of the database. For example, if the data volumes storing the database are backed up daily, an administrator may be able to reset the database to the previous day&#39;s state by retrieving the backup data from the previous day. Such backup techniques are desirable for many purposes, and ensure that data is recoverable in the event of catastrophic system errors. 
     However, in some situations, an administrator or other user may wish to more frequently restart a database in an earlier known state. In such situations, the use of existing techniques would decrease the availability of the database system and require valuable time, hardware and other resources. It would be desirable to provide improved systems and methods for restarting databases in an known earlier state. 
     SUMMARY 
     Pursuant to some embodiments, a system, method, apparatus, means and computer program code for processing a request to create a snapshot version of a database are provided which include identifying a current state of a converter table, the converter table including information identifying a plurality of page identifiers and a physical location of each of the page identifiers in a data area, saving the current state of the converter table, and marking each of the plurality of page identifiers to prevent overwriting. Pursuant to some embodiments, saving the current state of the converter table includes copying a current restart record to a snapshot restart record. 
     Pursuant to some embodiments, the snapshot version is restored upon receiving a request to restore the database to the snapshot version, and copying the snapshot restart record onto a current restart record of the database. 
     Pursuant to some embodiments, a system, method, apparatus, means and computer program code for setting up a training database are provided which include selecting a database as the training database, the training database having an initial state, creating a snapshot version of the initial state of the training database by saving a copy of a current restart record as a snapshot restart record, making the training database available for use by participants in a training class, and restoring the training database to the initial state by copying the snapshot restart record to a current restart record. 
     With these and other advantages and features that will become hereafter apparent, a more complete understanding of the nature of the invention can be obtained by referring to the following detailed description and to the drawings appended hereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a system according to some embodiments. 
         FIG. 2  is a flow diagram of a method according to some embodiments. 
         FIG. 3  is an illustration of a data cache, converter, and data volumes at a first point in time according to some embodiments. 
         FIG. 4  is an illustration of the data cache, converter and data volumes at a second point in time according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     To alleviate problems inherent in the art, embodiments of the present invention introduce systems, methods, computer program code and means for restarting a database to an earlier state. Pursuant to some embodiments, a database may be easily and efficiently restarted at an earlier state without requiring reference to backup volumes or the performance of an initialization process. 
     To illustrate features of some embodiments, an example environment will be described. The example is for illustration only, and is not intended to limit the scope of the present invention (e.g., those skilled in the art will appreciate that embodiments may be used in other environments as well). In the example, a database system is used in a hands-on training class to train new database administrators how to use, maintain and interact with the database system. Each training class starts with the data in the database system in a known state. As each training class progresses, trainees are allowed to enter, manipulate, and otherwise modify data in the database to follow a training curriculum. 
     At the end of the class, the data in the database may be significantly different than at the start of the class. In the illustrative example, once a training class is over, a subsequent class may need use of the same database system. The subsequent class needs the database system to be reset to the initial or known state so that the curriculum can be followed from a known set of data. Embodiments allow such a database system to be easily and efficiently reset or restarted to the known state. As will be described further below, pursuant to the present invention, a “snapshot” of the database is created at the start of the training course. At the end of each class, an administrator may reset the database to the snapshot version with a simple database command. Further details and features of some embodiments will be provided below. 
     For the purposes of illustration, but not limitation, a particular type of database system will be used to describe certain features of embodiments. More particularly, a relational database management system will be described, although those skilled in the art will recognize that the techniques and embodiments described herein may be implemented with desirable results in other types of database systems. 
     To illustrate features of some embodiments of the present invention, reference is first made to  FIG. 1  where an illustrative database system  100  is shown. For simplicity, only several components of database system  100  are shown in  FIG. 1 ; those skilled in the art will appreciate that a database system typically may include a number of other components including, for example, I/O devices, drivers, APIs, and the like. Database system  100  may be, for example, a structured query language (“SQL”) database system. For the purpose of illustration, but not limitation, database server  104  may be the MaxDB® system offered by mySQL AB and SAP AG of Walldorf Germany. Database system  100  includes one or more client devices  102  interacting with a database server  104 . For example, each client device  102  includes software configured to allow one or more users or applications to interact with database server  104  to submit queries and receive database responses. As a simple example, client  102  may be a computing device (such as a personal computer) operating Internet browser software (such as Microsoft&#39;s Internet Explorer®) to interact with database server  104  over a network connection by submitting SQL queries to database server  104  and by receiving responses from the database server  104 . As an example (continuing the training environment example introduced above), one or more client devices  102  may be operated by trainees during a training class. 
     While the system  100  of  FIG. 1  only shows two client devices  102  and a single database server  104 , those skilled in the art will recognize that database system  100  may include any number of clients interacting with one or more database servers. Further, for the purpose of illustration but not limitation, some or all of the client devices  102  and database server  104  may be implemented on computing devices such as, for example, computing devices implemented with Intel® x86 processors running a version of Microsoft Windows® or LINUX. 
     Database server  104  may consist of a number of elements (some of which are shown) interacting to create databases, store data, and respond to queries, including, for example, a command analyzer  106 , a SQL component  108 , a log. manager  110 , a data cache  112 , a converter  114 , a data volume  116  and a data log  118 . In general, database server  104  receives a database request from client  102  and responds to the request. Command analyzer  106  operates to analyze requests to identify the specific database interactions to be taken and passes SQL commands to SQL system  108  for application to data stored in data volume  116 . Data volume  116  may consist of one or more devices or components (e.g., such as an array of mass storage devices). 
     Frequently, SQL commands received by database server  104  will require modification of data stored in (or to be added to) a database. When information from the database is to be modified, the data is retrieved from data volume  116  and manipulated in data cache  112 . To increase the speed of manipulation of data, data cache  112  may be implemented using volatile memory such as random access memory (RAM) of the server. Once the data manipulation is complete (or after a series of manipulations has completed), the modified data is written from (or “committed”) data cache  112  to data volume  116  to update the database. Further, a log entry may be written in data log  118  (e.g., to allow the database to be restored to a consistent state if an error occurs). 
     For convenience, clarity and ease of exposition, a number of terms are used herein. For example, the term “server” is used to refer to software configured to perform tasks in response to requests from a “client”. One or more servers may be implemented on a given computing device. In the context of the illustrative example where the database system is a relational database system, several terms will be used to describe features of some embodiments. A relational database management system organizes data in two-dimensional “tables”, each having one or more “rows” and “columns”. Each row is assigned a “primary key” (or some other internally assigned system key, both of which will be generally referred to herein as a “primary key” or simply, a “key”). 
     A number of rows, ordered in key sequence, are stored in fixed size “pages” (for example, in the MaxDB system, pages are each 8 kbytes in size) in data volume  116 . A number of pages from the same table are referred to as a “file”. In the example system, each of the pages of a file are organized in a b*tree, with the data pages in leaf level “0” and ordered by key sequence (those skilled in the art will appreciate that other organization techniques may be used). 
     When a new page is created, the page is assigned a unique identifier or “page identifier” or “page ID”. All page accesses are addressed using this page ID. Pages are written to “blocks” of the same size on the attached data volumes. The “data area” is equal to the sum or entirety of all area designated for the database system in the attached data volumes  116 . The data volumes  116  may be, for example, any of a number of different types of mass storage devices, including for example, magnetic media or the like. 
     Pursuant to some embodiments, when data (on a page) is modified, the page(s) to be modified are read into the data cache  112  and the data is updated. A log entry may be written to the log data  118  (to allow recovery in the event of an error). When a number of modified pages in data cache  112  exceeds a predefined threshold (or after a predetermined time period passes), all of the modified pages are written back to the data volumes  116 . 
     Pursuant to some embodiments, the modified data pages are not rewritten to their original locations; instead, modified data pages are written to new free blocks on in the data volumes  116 . This results in two instances of the data page on the volume: the “last known good” page and the updated page. In case of a recovery of a crashed database, the updated pages are ignored, and the “last known good” pages are used instead. Database content is then reconstructed by redoing all of the log entries. As mentioned above, periodically (e.g., every few minutes), a synchronization of log data  118  and data volumes  116  are performed. All modified data pages are flushed to the data volumes  116  and marked as the “last known good” version. This point in time is marked as a “savepoint”, and is written to log data  118 . Each savepoint has an unique identifier called savepoint version or converter version or still version. This version is stored in the restart record which will be flushed at the end of each savepoint. The blocks on the data volumes  116  which contained the previous (and now superceded) “last known good” pages are marked as free and ready for a reassignment. 
     Converter  114  is used to store the current mapping between the page IDs and the block addresses of the individual data volumes  116 . Pursuant to some embodiments, to ensure ready recovery in the event of an error, converter  114  maps the addresses of both the “last known good” page data and the updated page. In some embodiments, converter  114  will be flushed to the data volumes at the end of a savepoint particularly after all modified data pages are written and before the restart record is flushed. The restart record contains a reference to the starting point of the current converter  114 . Converter  114  may also store information identifying the blocks that are now free and ready for a reassignment. 
     In some embodiments, a subcomponent of converter  114  (referred to in  FIG. 1  as the “FreeBlockMgmt” subcomponent), is used to identify free and used blocks as well as to assemble adjacent blocks into clusters as described further below. For simplicity, the FreeBlockMgmt subcomponent will be referred to simply as the “converter” below. Those skilled in the art will appreciate that other subcomponents or modules may be used to implement certain functions as described herein. 
     Reference is now made to  FIG. 2 , where a process  200  is shown for writing data to data volumes  116 . The process depicted in  FIG. 2  (and other processes described herein) does not imply a fixed order to the process steps, and embodiments of the present invention may be performed in any order that is practicable. Process  200  may be performed using code implemented on database server  104 . 
     Process  200  begins at  202  with the receipt of a request to create a database snapshot. For example, the request may be received from an administrator or other authorized user operating user device  102 . The request may be presented as a SQL command while the authorized user has placed the database system in an administrative operational state. 
     After database server  104  receives the request at  202 , processing continues at  204  where server  104  identifies, reads and stores the current version of converter  114 . More particularly, in some embodiments, server  104  creates a copy of a restart record (also called a “snapshot restart record”) which contains the starting point of converter  114 . Snapshot restart record is written in a data volume  116  and the address is stored in the original restart record. In some embodiments, a timestamp may also be created, indicating the time when the snapshot was defined. 
     Processing continues at  206  where server  104  operates to identify and mark each of the physical data pages (in data volumes  116 ) that are addressed by the converter version identified at  204 . More particularly, in some embodiments, each of the data pages that represent the state of the database system at the time the snapshot request is received are identified and associated with the snapshot (e.g., by reading and rebuilding the table information for management of block records also known as “FreeBlockManagement” in the MaxDB system). Further, each of the data pages are marked, tagged, or otherwise identified as being part of a snapshot so that the pages will not be overwritten until a snapshot removal command is received. 
     Put another way, the database system writes changes that are made after the creation of the snapshot to other physical pages in the data volumes  116 . If a table associated with the snapshot version is deleted, the system does not physically delete the table. Instead, the table is only marked as deleted (e.g., the table can no longer be accessed by SQL statements). An example of this marking and identification will be provided below in conjunction with the description of  FIG. 3  and  FIG. 4 . 
     Upon completion of the processing associated with  204  and  206 , a snapshot is created. Processing continues at  208  where the snapshot is retained and preserved until a snapshot removal command of some sort is identified. That is, the data associated with the snapshot (including the converter information and the page information), are preserved and prevented from being overwritten, despite any database changes occurring after creation of the snapshot. 
     Pursuant to some embodiments, several different snapshot removal commands may be received, including a command to restore the database to the snapshot. For example, in some embodiments, an administrator or other authorized user may place the database instance in an administrative state and enter a SQL command to revert to the snapshot state. In this situation, the database system copies the “frozen” snapshot restart record onto the restart record and therefore the starting point of the latest converter is replaced by the converter of the snapshot. The log is cleared, and all pages that changed after the creation of the snapshot are released for overwriting. Effectively, the database system is placed back in the state it was in at the time of the snapshot. Pursuant to some embodiments, after a snapshot is restored, it remains valid until deleted, dropped or replaced. 
     Other snapshot removal commands may include, for example: (i) a request by an administrator or other authorized user to create a new database instance (in which case the snapshot will be rendered irrelevant and deleted), (ii) a request by an administrator or other authorized user to drop or delete the snapshot (e.g., where the user determines that it is no longer desirable or necessary to revert to the snapshot), or (iii) a request by an administrator or other authorized user to create a new snapshot (e.g., to supercede the first snapshot and replace it with a new snapshot). Those skilled in the art will appreciate that other situations may exist in which snapshots may be removed or deleted. Upon removal of a snapshot, the pages marked or preserved in conjunction with the snapshot are released in the data volumes  116  for overwriting. 
     Returning to the training course illustrative example introduced above, process  200  may be described as follows (from the perspective of an administrator operating user device  102 ). A database administrator may interact with database server  104  via user device  102  to set the database server  104  up for a training session by setting the database to a known configuration (e.g., by importing or opening a known training database). The database administrator may place the database system in an administrative state and submit a snapshot SQL command to the system. The database system may confirm that the snapshot has been created by returning a confirmation message to the administrator. 
     The administrator may then place the database in an online operational state. Students may then be given access to the database system to conduct the training session. The students may make any changes to the data and also be allowed to start and stop the database instance. At the end of the training session, and before the start of the next training session, the database administrator may place the system in an administrative state and submit a SQL request to revert to the snapshot. The database system may confirm that it has successfully reverted to the snapshot, and the administrator may then place the system back into an operational state for the next class. In this manner, each training class is able to start the session from a known configuration of data and is also free to modify, update and otherwise interact with the system. Previous systems and solutions required administrators to restart the database system using backup data—a solution that is resource intensive and time consuming. 
     Reference is now made to  FIG. 3 , where selected components of database system  100  are shown and will be used to describe the storage of data and the creation of a “snapshot” pursuant to some embodiments. More particularly,  FIG. 3  depicts data cache  112 , converter  114  and data volumes  116  at a particular point in time where data cache  112  has been flushed. For example, the point in time depicted is immediately after data pages modified and stored in the cache have been written to data volumes  116  and a “savepoint” has been created. As shown, the savepoint is savepoint “V.21”, and converter  114  stores a page of converter information referred to as converter page “C4”. 
     Converter page “C 4 ” is stored in the first volume of the three data volumes at block  20 . At the time of creation of savepoint “V.21”, converter  114  stored information identifying two pages: page  4711  (stored in volume  1  at block  300 ) and page  4712  (stored in volume  2  at block  2350 ). Pursuant to some embodiments, if a user (e.g., such as an administrator or other authorized user) elected to create a “snapshot” at this point in time, the database system would save a reference to the current starting point in converter  114  in the restart page of the database instance. 
     Further, each of the pages that are addressed by converter  114  would be marked or flagged to indicate that the pages are part of a snapshot and cannot be overwritten (even if the data in the pages becomes outdated). That is, if an administrator creates a snapshot at the point in time represented in  FIG. 3 , the following blocks would be saved: 1/20 (including the converter image “C 4 ”), 1/300 (including page number  4711 ), and 2/2350 (including page number  4712 ). The database system would ensure that none of these blocks are overwritten or used until an authorized removal command is received (e.g., such as a command to delete the snapshot, a command to revert to the snapshot, a command to create a new database instance, or a command to create a new snapshot). 
     Reference is now made to  FIG. 4 , where the same selected components of database system  100  are shown as were shown in  FIG. 3 , but at a slightly later point in time. More particularly,  FIG. 4  depicts data cache  112 , converter  114  and data volumes  116  after pages have been updated by the database system and data cache  112  is in the process of a further flush operation (to store the updated page information in data volumes). As shown, page  4711  has been updated and is shown as being written from cache  112  to data volume  2  (at block  177 ). Because this update affects converter image “C 4 ”, the converter image is also updated and written to data volumes  116  (in particular, the updated image is stored in data volume  3  at block  100 ). Pursuant to some embodiments, because a snapshot was created at the point in time shown in  FIG. 3 , the previous versions of pages 4711 and 4712 (as well as the previous converter image C 4 ) are all saved and prevented from being overwritten. That is, the information saved at 1/20 (“C 4 ”) and 1/300 (old page  4711 ) are saved and prevented from being overwritten. The other page associated with the snapshot (page number  4712 ) has not been modified in this example, and, accordingly, the information stored at 2/2350 is still relevant. 
     In this manner, a new version of the converter is gradually created. Over time (from the creation of the snapshot), the new converter differs increasingly from the version of the converter that was saved for the snapshot. Any pages that change after the snapshot are released for overwriting as normal. The pages and converter information stored for the snapshot are retained and prevented from overwriting until an authorized snapshot removal command is received (e.g., as discussed above, this may include either the creation of a new database instance, restoring to the snapshot, dropping the snapshot, or creating a new snapshot). 
     Pursuant to some embodiments, when pages are written to the data area, the version information of the page and the snapshot version stored in the converter can be used to determine whether a particular page is relevant to the snapshot (and, if so, whether a page should not be overwritten). Note that the version information of a page is part of the page header and will be updated with each write operation. For example, if the page version is less than or equal to the snapshot version, the page is relevant for the snapshot and must not be overwritten. Otherwise, the page could be overwritten without impacting the snapshot. In this manner, embodiments allow an authorized user to quickly and efficiently revert to a known state of the database, without needing to retrieve backup data or the like. Although the present invention has been described above with respect to some embodiments thereof, it should be noted that the above-described embodiments may be altered to create completely or partially different embodiments without departing from the spirit and scope of the appended claims.