Patent Publication Number: US-11042503-B1

Title: Continuous data protection and restoration

Description:
This application claims benefit of priority to U.S. Provisional Application Ser. No. 62/590,227, entitled “Restore and Point-in-time Restore,” filed Nov. 22, 2017, and which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Several leading technology organizations are investing in building technologies that sell “software-as-a-service”. Such services provide access to shared storage (e.g., database systems) and/or computing resources to clients, or subscribers. Within multi-tier e-commerce systems, different resources may be allocated to subscribers and/or their applications from whole machines, to CPU, to memory, to network bandwidth, and to I/O capacity. 
     Database systems managing large amounts of data on behalf of users may distribute and/or replicate that data across two or more machines, often in different locations, for any of a number of reasons, including security issues, disaster prevention and recovery issues, data locality and availability issues, etc. These machines may be configured in a number of ways, including as a shared resource pool. For example, a database table may be split into two or more partitions, each of which may be replicated, and each replica may be stored on a different machine. Interaction between client applications and database servers typically includes read operations (read-only queries), write operations (to store data), and update operations that can be conceptualized using a read-modify-write workflow. 
     Clients with large-scale databases may create proprietary backup solutions that scan data from a database into scalable, durable storage, but these solutions suffer from a number of problems such as taking too much time to perform the backup, interrupting a customer&#39;s regular workflow, tying up resources (e.g., scan-based solutions can compete with production workloads for read throughput), and providing only limited options for restoring from static backups (e.g., periods of time between backups may be too long). Additionally, getting a consistent backup across a large number of storage nodes can be hard to do without the customer stopping all writes. Scan-based solutions can be expensive (e.g., a scan backup of a 100 TB table, etc.) both in compute costs and storage costs and may not provide a managed experience (e.g., a managed experience where customers can view characteristics of the backups, manage the backups, etc.). 
     Some systems perform timed, periodic backups by scanning the data store to create a copy of the data store. But, the period of time in-between backups of a periodic system may be of a length that prohibits restoring the data store to a particular point-in-time of interest (e.g., if the point-in-time of interest falls in-between to backups). 
     Some database systems require backup of the entire table (e.g., all partitions are backed-up at once, as a coherent whole). Generally, database systems require obtaining a copy of the entire authoritative database in order to generate the next full backup snapshot. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating logical relationships between components of a system for continuous data protection, according to some embodiments. 
         FIG. 2  is a data flow diagram illustrating logical relationships between a snapshot manager, a change log manager, and associated data within the context of a continuous data protection manager, according to some embodiments. 
         FIG. 3  is a flow chart illustrating a technique for continuous data protection, according to some embodiments. 
         FIG. 4  is a flow chart illustrating a technique for continuous data protection according to some embodiments. 
         FIG. 5  is a flow chart illustrating a technique for continuous data protection during a partition split, according to some embodiments. 
         FIG. 6  is a data flow diagram illustrating a continuous data protection system with a continuous data protection manager and a streaming data service, according to some embodiments. 
         FIG. 7  is a block diagram illustrating a service provider network that implements a continuous data protection manager service, according to some embodiments. 
         FIG. 8  is a block diagram illustrating logical relationships between components of a continuous data protection manager, a database service, and a storage service, according to some embodiments. 
         FIG. 9  is a process diagram illustrating a logical flow of an on-demand backup, according to some embodiments. 
         FIG. 10  is a process diagram illustrating a logical flow of a point-in-time restoration, according to some embodiments. 
         FIG. 11  is a chart illustrating creation of an aligned set of distinct snapshots of a table, according to some embodiments. 
         FIG. 12  is a block diagram illustrating a computer system configured to implement at least a portion of a system that includes continuous data protection, according to various embodiments. 
     
    
    
     While embodiments are described herein by way of example for several embodiments and illustrative drawings, those skilled in the art will recognize that the embodiments are not limited to the embodiments or drawings described. It should be understood, that the drawings and detailed description thereto are not intended to limit embodiments to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” be used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include”, “including”, and “includes” mean including, but not limited to. 
     DETAILED DESCRIPTION 
     Disclosed is a continuous data protection system that captures all of the changes happening on the data store (e.g., a database) and periodically builds system snapshots (sometimes referred to as copies, herein) by applying logs on the closest system snapshot, in embodiments. The system may be able to apply transaction logs to a previous logical backup to create a new point-in-time logical backup, without losing any customer data, in some instances. For example, system snapshots may be built at a partition level (e.g., for systems that partition data) by applying the change logs to prior snapshots. In some such embodiments, the continuous data protection system generates backups without any additional queries or scanning of the client&#39;s production data source by relying on prior snapshots and change log data to create new snapshots, instead. Accumulation of change log data for a table, as well as generation of updates to snapshots for the table may be performed independently for each partition (e.g., at a different time) of a same table, based on characteristics particular to each partition for example, in some embodiments. 
     In embodiments, a log apply service of the continuous data protection system is responsible for at least two core functionalities to support backup and restore. During a conversion process, the log apply service may convert partition snapshots (sometimes referred to as backups) from a physical format (e.g., mysql) to a logical format snapshot. The log apply service may also create subsequent point-in-time logical partition snapshots by applying transaction logs to a previous logical partition snapshot, and create a complete user backup, for example. 
     In some embodiments, continuous capture of individual changes to a table provide for a more fine-grained availability of those individual changes at a later time. For example, the accumulation of individual changes to the data-data that constitutes a state of the database table at a time, in embodiments, may be used to more accurately take the table—or partition—back to a particular state at any point-in-time along a continuum. Such features contrast with prior systems that could only take the system back to a select few points-in-time when snapshots of the system were taken. In some such prior systems, the snapshots added additional burden to the client&#39;s production system because the snapshots were created from scans of the production database, interrupting or delaying production services. Additionally, scan-based snapshots take relatively more time to create, and fail to provide as accurate a view of the database as the techniques disclosed herein, at least because, by the time the scan completes (at least for large data sets), data that has already been scanned may have been changed. 
     Additionally, in at least some embodiments, the techniques described herein are applied on a partition-by-partition basis. For example, snapshots and change log data for a particular partition may be kept in an uncoordinated manner, with respect to the other partitions of the table (e.g., according to different schedules). A relatively inactive partition of a table may have a snapshot generated based on a maximum duration of time threshold, while another relatively active partition of that same table may have snapshots generated more often, based on an amount of accumulated changes, as just one example of many. The times at which the snapshots are created for either partition may not have any relationship, and may be based upon the particular characteristics of that partition, in embodiments. 
     The above-noted process may be triggered when a customer enables backups for a given table. In embodiments, the continuous data protection manager  112  may initiate the first complete backup of the table, during the initial backup process. For example, for all partitions of the table, the continuous data protection manager or service may store the snapshots by exporting data from storage nodes to storage-level physical format into a durable storage. 
     In embodiments, a log apply process is used whenever the continuous data protection manager or service decides a new logical partition snapshot is required to bound the time taken for creating point-in-time backups. Log apply may also be used during restore to apply logs to a backup. 
     In some systems, log applying backups is an expensive operation (e.g., when there is a relatively greater amount of time and greater number of changes between backups). By relying on pre-existing incremental partition images to define a backup, the system may significantly reduce the load on the log apply service, saving compute costs. Additionally, by relying upon incremental partition images to define a backup, the system may allow users to create many backups partially sharing the same set of logs and base partition images, which may translate into storage costs savings. 
     In some embodiments, a periodicity at which system snapshots of the partitions are built is decided based on an amount of logs accumulated. For example, the periodicity may be based on a threshold amount of logs accumulated. In another example, the periodicity may be based upon a rate of change of the logs accumulated. For instance, if the system is becoming more active more changes are being generated . . . the increase in the rate of change of the number of logs may be used as a trigger to increase the periodicity. In some embodiments, the log apply service applies logs for a single partition on a single host. In some circumstances (e.g., large tables) each partition may be log applied in parallel to reduce the time to apply the logs for the table, e.g., by respective parallel processes. 
     In embodiments, both the newly-created snapshots as well as the change logs are stored to durable storage. In some such embodiments, the snapshots and the durably-stored change logs may be used to restore the partition. In some embodiments, the continuous data protection backup service provides an interface and functionality supporting unified management of the data, while optimizing customer costs and restore times via periodic log application and trimming. 
     Another benefit of some embodiments is a predictable time to recovery, by accurately identifying continuous backups, which, if restored, would cause the system to break SLAs and take appropriate action to bring the system back into compliance. 
     The systems and methods described herein may be employed in various combinations and in various embodiments to implement a network-based service that provides data storage services to storage service clients (e.g., user, subscribers, or client applications that access the data storage service on behalf of users or subscribers). The service may, in some embodiments, support the continuous data protection of tables that are maintained on behalf of clients in a data store, e.g., a non-relational database or other type of database. The service may provide a high level of durability and availability through replication, in some embodiments. For example, in some embodiments, the data storage service may store data in multiple partitions (e.g., partitions that each contain a subset of the data in a table being maintained on behalf of a client), and may store multiple replicas of those partitions on respective storage devices or virtual storage volumes of different storage nodes. 
     In some embodiments, the data storage systems described herein may provide mechanisms for backing up a database table as a synchronous operation while the database continues to receive, accept, and service read and/or write operations that are directed to the table. In some embodiments, in response to a request to back up a table, the system may create a backup of each individual partition independently and (in some cases) in parallel (i.e., substantially concurrently). In embodiments, when a request to back up a table is received, the system may guarantee that all write operations that were directed to the table up to that point are included in the backup. In some embodiments, such a guarantee may not be made. 
     In some embodiments, backup operations may be initiated by data storage service users (e.g., customers, service subscriber, and/or client applications) using a “CreateBackup” application programming interface (API). In some embodiments, the systems described herein may support the scheduling of backups (e.g., every day at a particular time, or according to a published, but not necessarily periodic, schedule). In response to receiving a request to back up a table, these systems may back up each partition of the table as an individual item in a remote storage system (e.g., a key-value durable storage system), and may store metadata about the backup that is subsequently usable when restoring the backup to a new database (e.g., a new database table). In some embodiments, the system may be configured to initiate separate backup operations for each of the partitions of a table automatically (e.g., programmatically and without user intervention) in response to a request to back up the table, and to manage those backup operations on a per-partition basis (again, without user involvement). 
     In various embodiments, the data storage service described herein may provide an application programming interface (API) that includes support for some or all of the following operations on the data in a table maintained by the service on behalf of a storage service client: put (or store) an item, get (or retrieve) one or more items having a specified primary key, delete an item, update the attributes in a single item, query for items using an index, and scan (e.g., list items) over the whole table, optionally filtering the items returned. The amount of work required to satisfy service requests that specify these operations may vary depending on the particular operation specified and/or the amount of data that is accessed and/or transferred between the storage system and the client in order to satisfy the request. 
     In embodiments, the system disclosed herein may implement an application program interface (API) that provides access to configuration setting associated with the partition, the configuration settings including, but not limited to: a maximum backup time that indicates a maximum period of time between snapshots of the partition; a minimum backup time that indicates a minimum period of time between snapshots of the partition; and a duration of time to retain snapshots of the partition. 
     Another API may allow consumers to update the current settings for a table within the database service, for example, to enable or disable the continuous backups and modify the duration of time to retain backups. 
     Yet another API may provide the option to enable continuous backups for a table. The triggered action may initiate the creation of a continuous backup through the workflow described herein, such as by initiation of the archival copy of logs for a table and creation of an initial backup of a table in a logical format. 
     In various embodiments, the systems described herein may store data in replicated partitions on multiple storage nodes (which may be located in multiple data centers) and may implement a single master failover protocol. For example, each partition may be replicated on two or more storage nodes (or storage devices thereof) in a distributed database system, where those replicas make up a replica group. In some embodiments, membership in various replica groups may be adjusted through replicated changes, and membership and other updates in the system may be synchronized by synchronizing over a quorum of replicas in one or more data centers at failover time. 
     As described herein, when a database table is created or restored from backup, various resources may be provisioned for the implementation of that table, including storage resources (e.g., disk capacity), and throughput capacity (which may, e.g., be specified in terms of input/output requests per second, or IOPS, for read operations and/or write operations). If the table is divided into two or more partitions (e.g., if various data items are stored on different ones of the partitions according to their primary key values), the provisioned resources may also be divided among the partitions. For example, if a database table is divided into two partitions, each partition may have access to half of the total amount of storage and/or throughput resources that are provisioned and/or committed for the implementation of the table. 
     In some embodiments of the distributed database systems described herein, each storage node may include multiple storage devices or logical volumes, each of which stores various partition replicas. For example, in one embodiment, each storage node of the distributed database system may include five storage devices or logical storage volumes. In some embodiments, one or more mechanisms may be implemented on each of the storage nodes for determining, on a local level (e.g., on a storage node basis) whether and/or how to split a partition or move a partition (or a given replica of a partition), based on the current utilization of provisioned resources and/or other information. For example, one of the storage nodes may be configured to determine that a partition for which a replica is stored on one of its storage devices (e.g., disks) or logical storage volumes should be split into two new partitions, and may divide the data in the partition by hash ranges, by key space ranges, or using other criteria to divide the data between the two new partitions. In another example, a storage node may be configured to determine that one or more partitions (or replicas thereof) should be moved from a given storage device or logical storage volume to another storage device or logical storage volume, e.g., in order to reduce the amount of provisioned storage capacity or throughput capacity on the given storage device or logical storage volume. 
     As noted above, from a user&#39;s perspective, a backup operation generally operates to create a backup of a whole table, but internally, the system may back up each partition of the table independently, such that consistency is guaranteed only up to a particular transaction or write operation on a per partition basis (rather than across the whole table). In some embodiments, the system may be configured to maintain metadata about the table (e.g., to keep track of the table schema, and the state of the world from the perspective of the table and of each partition). In some embodiments, this metadata may be stored in the data storage system itself, and a copy of the metadata may also be stored in the remote storage system into which tables are backed up. 
       FIG. 1  is a block diagram illustrating a system for continuous data protection, according to some embodiments. Provider network  100  may be a private or closed system, in one embodiment, or may be set up by an entity such as a company or a public-sector organization to provide one or more services (such as various types of cloud-based storage) accessible via the Internet and/or other networks to clients  160 , in another embodiment. In one embodiment, provider network  100  may be implemented in a single location or may include numerous data centers hosting various resource pools, such as collections of physical and/or virtualized computer servers, storage devices, networking equipment and the like (e.g., computing system  1200  described below with regard to  FIG. 12 ), needed to implement and distribute the infrastructure and storage services offered by the provider network  100 . In one embodiment, provider network  100  may implement various computing resources or services, such as a database service  110  or other data processing (e.g., relational or non-relational (NoSQL) database query engines, data warehouse, data flow processing, and/or other large scale data processing techniques), data storage services (e.g., an object storage service, block-based storage service, or data storage service that may store different types of data for centralized access), virtual compute services, and/or any other type of network based services (which may include various other types of storage, processing, analysis, communication, event handling, visualization, and security services not illustrated). 
     In some embodiments, the provider network  100  may include a continuous protection data manager  112  configured to handle or manage backups of databases that are stored with or maintained by the database service  110 . The backups may be maintained by one or more data storage services. The continuous data protection manager  112  may manage snapshots from the database service (e.g., in a native format illustrated as  144 ) as well as change log data (e.g., in a native format illustrated as  154 ) from the database service  110 , in some embodiments. In at least some embodiments, a storage node may convert the change log data, from a format native to the database service to a non-native format, prior to providing the change log data to the continuous data protection manager  112 . 
     In some embodiments, the data storage services may include a snapshot data store  142  and a change log archive data store  152 . The snapshot data store  142  may be configured to store complete backups of partitions (e.g., partitions  124 A-N) of the database at a particular point-in-time. For example, a particular snapshot stored at the snapshot data store  142  may be generated at a particular point-in-time such that the data in the particular snapshot is at the state in which the database existed at that particular point-in-time. In other embodiments, the database service  110  may be configured to store current or active data for the database such that data of the partition of the database is the most recent version of the data. The change log archive data store  152  may be configured to store logs  152  indicating changes, mutations or events that occur with respect to the database or any data corresponding to the database managed by the database service  110 . 
     In at least some embodiments, archives may be immutable. In some examples, immutable archived data may not be changed or edited, but only read or deleted. In some examples, archived snapshots or change log data may be not be changed or edited, but only read or deleted, in durable storage (e.g., storage service  290 ), for example. 
     In various embodiments, the components illustrated in  FIG. 1  may be implemented directly within computer hardware, as instructions directly or indirectly executable by computer hardware (e.g., a microprocessor or computer system), or using a combination of these techniques. For example, the components of  FIG. 1  may be implemented by a system that includes a number of computing nodes (or simply, nodes), in one embodiment, each of which may be similar to the computer system embodiment illustrated in  FIG. 12  and described below. In one embodiment, the functionality of a given system or service component (e.g., a component of database service  110 ) may be implemented by a particular node or may be distributed across several nodes. In some embodiments, a given node may implement the functionality of more than one service system component (e.g., more than one data store component, such as snapshot data store  142  or change log archive  152 ). 
     Database service  110  may include various types of database services, in embodiments (e.g., relational and non-relational) for storing, querying, and updating data. Such services may be enterprise-class database systems that are highly scalable and extensible. In one embodiment, queries may be directed to a database in database service  110  that is distributed across multiple physical resources (e.g., computing nodes or database nodes), and the database system may be scaled up or down on an as needed basis. The database system may work effectively with database schemas of various types and/or organizations, in different embodiments. In one embodiment, clients/subscribers may submit queries in a number of ways, e.g., interactively via a SQL interface to the database system. In other embodiments, external applications and programs may submit queries using Open Database Connectivity (ODBC) and/or Java Database Connectivity (JDBC) driver interfaces to the database system. In one embodiment, database service  110  may also be any of various types of data processing services to perform different functions (e.g., query or other processing engines to perform functions such as anomaly detection, machine learning, data lookup, or any other type of data processing operation). Various other distributed processing architectures and techniques may be implemented by database service  110  (e.g., grid computing, sharding, distributed hashing, etc.) in another embodiment. 
     In one embodiment, clients  160  may encompass any type of client configurable to submit network-based requests to provider network  100  via network  170 , including requests for database service  110  (e.g., to query a database managed by the database service  110 ) or data storage service(s) (e.g., a request to create, read, write, obtain, or modify data in data storage service(s), etc.). For example, in one embodiment a given client  160  may include a suitable version of a web browser, or may include a plug-in module or other type of code module configured to execute as an extension to or within an execution environment provided by a web browser. Alternatively, in a different embodiment, a client  160  may encompass an application such as a database application (or user interface thereof), a media application, an office application or any other application that may make use of storage resources in data storage service(s) to store and/or access the data to implement various applications. In one embodiment, such an application may include sufficient protocol support (e.g., for a suitable version of Hypertext Transfer Protocol (HTTP)) for generating and processing network-based services requests without necessarily implementing full browser support for all types of network-based data. That is, client  160  may be an application configured to interact directly with provider network  100 , in one embodiment. In one embodiment, client  160  may be configured to generate network-based services requests according to a Representational State Transfer (REST)-style network-based services architecture, a document- or message-based network-based services architecture, or another suitable network-based services architecture. 
     In one embodiment, a client  160  may be configured to provide access to provider network  100  to other applications in a manner that is transparent to those applications. For example, client  160  may be configured to integrate with a database on database service  110 . In such an embodiment, applications may not need to be modified to make use of the storage system service model. Instead, the details of interfacing to the database service  110  may be coordinated by client  160 . 
     The clients  160  may communicate with the database service  110  from within the provider network  100 , in some embodiments. For example, the clients  160  may be implemented on computing nodes of a computing service offered by the provider network  100 . The clients  160  may convey network-based services requests to and receive responses from provider network  100  via network  170 , in one embodiment. In one embodiment, network  170  may encompass any suitable combination of networking hardware and protocols necessary to establish network-based-based communications between clients  160  and provider network  100 . For example, network  170  may encompass the various telecommunications networks and service providers that collectively implement the Internet. In one embodiment, network  170  may also include private networks such as local area networks (LANs) or wide area networks (WANs) as well as public or private wireless networks. For example, both a given client  160  and a provider network  100  may be respectively provisioned within enterprises having their own internal networks. In such an embodiment, network  170  may include the hardware (e.g., modems, routers, switches, load balancers, proxy servers, etc.) and software (e.g., protocol stacks, accounting software, firewall/security software, etc.) necessary to establish a networking link between given client  160  and the Internet as well as between the Internet and provider network  100 . It is noted that in one embodiment, clients  160  may communicate with provider network  100  using a private network rather than the public Internet. 
     The clients  160  may send a request to perform an operation to respective databases managed by the database service  110 . For example, a given client  160  may send a PUT (or SET) command and corresponding data to request that the data be stored in the database at the database service  110 . In another example, a given client  160  may send a CHANGE (or UPDATE) command and corresponding data to request that some data existing in the database at the database service  110  be changed based on the sent data. In yet another example, a given client  160  may send a DELETE (or REMOVE) command and identify data to request that the data existing in the database at the database service  110  be deleted or removed from the database and the database service  110 . In other embodiments, a given client  160  may provide another command to perform some operation to the data stored in the database at the database service  110 . 
     The database service  110  may be configured to maintain a backup system for partitions of databases managed by the database service  110 . In some embodiments, the backup system may perform backups for single partitions of the databases or single-partition databases. In other embodiments, the backup system may perform backups for multiple partitions of the databases. The backup system may include a continuous data protection manager  112  configured to manage change logs and backups or snapshots of partitions of the databases stored in storage services of the database service  110 . The continuous data protection manager  112  may generate a complete backup of a partition of the database (e.g., a snapshot) by applying logs to a closest existing snapshot. A periodicity at which the continuous data protection manager  112  generated backups may be based on the amount of logs accumulated for the partition or table. Periodicity may be based on an amount of time, in some embodiments. In some embodiments the periodicity may be based on an amount of changes or a period of time between backups, whichever happens sooner. 
     A change log (e.g., change log data  154 ) may indicate one or more changes to the database over a period of time or sequence of events. For example, the change log (e.g., change log  154 ) may indicate that data was added, changed or deleted from the database in a period of time. The change log may be stored at a log store (e.g., change log data store  604  in  FIG. 6 , described below, or change log archive  152 ). The log store may be accessible to the database service  110  and the continuous data protection manager  112 . In some embodiments, the database service  110  may generate or update a log segment in response to an event at the database. For example, the database service  110  may indicate in the log segment that the event occurred, and some data in the database has changed. The log segment may include metadata indicating a sequence start identifier, a sequence end identifier, a time start identifier, a time end identifier, one or more checksums, a previous cumulative size of the change log, a lineage of the log segment, or any combination thereof. The sequence start identifier may indicate a sequence number for a first event in a sequence of events that is logged in the log segment. The sequence end identifier may indicate a sequence number for a last event in the sequence of events that is logged in the log segment. The time start identifier may indicate a timestamp for the first event in the sequence of events that is logged in the log segment. The time end identifier may indicate a timestamp for the last event in the sequence of events that is logged in the log segment. The one or more checksums may correspond to the data in the partition of the database, the log segment, etc. The one or more checksums may be used by the continuous data protection manager  112  or the database service  110  to determine whether application of at least a portion of the log segment or the change log to the database has resulted in the correct data being restored or retrieved. The previous cumulative size of the change log may indicate a size of the change log prior to the respective log segment. The lineage of the log segment may indicate parent or child partitions associated with the log segment. 
     The log segment may be added to the change log in response to satisfying one or more criteria. In some embodiments, the log segment may be added to the change log based on a determination that the log segment satisfies a size threshold. For example, the database service  110  may add events to the log segment continuously until the log segment reaches a specified size or a size limit. After the log segment is the specified size, the log segment may be committed to the change log. In other embodiments, the log segment may be added to the change log based on a determination that a number of operations tracked or identified in the log segment satisfies a threshold number of operations. For example, the database service  110  may perform multiple CHANGE operations to the database. The multiple CHANGE operations on a data entry may require a higher amount of storage space in the log segment despite the storage space used by the data entry has not significantly increased. The database service  110  may track a number of operations and corresponding types and save the number of operations and corresponding types to the log segment. 
     The database service  110  may receive an indication that an event has occurred with a partition of a given database managed by the database service  110 . In some embodiments, the event may be based on a request to perform an operation sent from a given client  160  to the database service  110 . For example, the event may be based on a PUT command, a CHANGE command, a DELETE command or any other type of request from the given client  160 . The event may also indicate a type of the operation, such as PUT, CHANGE, DELETE, etc. The event may also indicate data that used to perform the operation. For example, the indicated data may include new data to be stored with the PUT command, changed data to be updated with the CHANGE command, data to be deleted with the DELETE command. 
     The clients  160  may send a request to the database service  110  to generate a backup of a partition of a database. The database service  110  may also implement timed, periodic or continuous backups automatically or after scheduling by the clients  160 . In some embodiments, a backup of the partition the database (e.g., a snapshot) may be generated at least in part by the continuous data protection manager  112 . The continuous data protection manager  112  may have access to the database stored at the database service  110 , the snapshot data store  142  and the change log archive  152 . 
     In an example, the continuous data protection manager  112  may determine that the snapshot type is a log-based snapshot. The continuous data protection manager  112  may generate data that indicates a point in the change log that, when used in conjunction with a stored snapshot, may be used to represent a snapshot of the partition of the database. The continuous data protection manager  112  may store the generated data as metadata in the snapshot data store  142 . 
     In some embodiments, the log-based snapshot may be used to generate a full snapshot based on applying at least a portion the change log to a stored snapshot. The log-based snapshot may indicate the portion of the change log that may include logged events that have occurred since generation of the stored snapshot through the current time. The continuous data protection manager  112  may retrieve the stored snapshot from the snapshot data store  142 , such as by sending a request to the snapshot data store  142  for the previous snapshot. The continuous data protection manager  112  may retrieve the change log from the change log archive  152 , such as by sending a request to the change log archive  152  for the change log. The continuous data protection manager  112  may be configured to apply the change log to the stored snapshot. In some embodiments, applying the change log to the stored snapshot includes starting with the stored snapshot and stepping through the change log and reenacting events included in the change log in chronological order to the stored snapshot. In other embodiments, applying the change log to the stored snapshot includes starting with the stored snapshot and stepping through the change log and undoing events included in the change log in reverse chronological order. 
     In at least the embodiment illustrated in  FIG. 1 , the system may instruct components of the system (e.g., forward/reverse converter  206  in  FIG. 2 , described below) to convert partition data in a native format to a preferred format (e.g., a binary and text interchangeable format, a typed JSON-superset, etc.) and then persist the preferred format to backup storage (e.g., snapshot data store  142 ) distinct from the source storage system (e.g., database service  110 ). In embodiments, the conversion from the service or native format is not done directly into the preferred format. In some embodiments the native format is converted to an intermediate model (e.g., a Java data type) first and then serialized into the preferred format. 
       FIG. 2  is a data flow diagram illustrating logical relationships between a snapshot manager, a change log manager, and associated data within the context of a continuous data protection manager  112 , according to some embodiments. The snapshot manager  212 , and a change log manager  214  depicted in  FIG. 2  illustrate components of the continuous data protection manager  112  also depicted in  FIG. 1 . These managers, in combination with the other depicted components may perform one or more of the steps of the processes illustrated in  FIGS. 3-5 , in embodiments. Different components (e.g., illustrated or non-illustrated components) may perform various of the steps, in various embodiments, without departing from the scope of this disclosure.  FIG. 3  is a flow chart illustrating a technique used for continuous data protection, according to some embodiments. 
     An initial copy of a database partition is obtained and saved as a snapshot in a snapshot data store (block  302 ). For example, in at least the illustrated embodiment of FIG.  2 , continuous data protection manager  112  receives partition data  144  (e.g., in a format native to the database), for example, obtaining a copy (e.g., a full partition snapshot). The copy may be from a scan or an export of a replica of the database partition, for example. The snapshot manager  212  may store the initial copy of the partition data (e.g., from a replica) in a format native to the database to a data store (e.g., snapshot data store  142  in storage service  290 ). 
     In some embodiments, the continuous data protection manager may direct a process external to the continuous data protection manager (e.g., a database service process) to obtain a copy of the database partition and to store the copy to durable storage (e.g., in either of a native or non-native format). 
     In some embodiments, the snapshot may be converted from a native format to a non-native format. In an example, forward/reverse converter  206  may convert the initial copy of the partition data stored in snapshot data store  142  into a snapshot in a non-native format  145 , for example, forward converting each item of the full partition snapshot to a non-native format, and then store the converted items to durable storage (e.g., to snapshot data store  142 , or another data store distinct from snapshot data store  142 ) as partition snapshot data in a non-native format. In some such embodiments, the durable storage may be used at part of a transport mechanism to transport snapshots obtained in a native format to the continuous data protection manger  112  in a non-native format, with the native format snapshots being deleted from durable storage subsequent to conversion. In at least some embodiments, forward/reverse conversion may be performed by a forward/reverse converter process of the database service that is not part of the continuous data protection manager  112 . 
     In embodiments, the conversion process may include forward transforming the replica from the native format into an intermediate object (e.g., an intermediate JAVA based record) to represent the data in-memory (an in-memory representation) before continuing to transform the native-format record to the non-native format. 
     In embodiments, a non-native format may be a format that provides dual-format interoperability (e.g., applications can seamlessly consume the data in either its text or binary forms without loss of data fidelity) which enables users to take advantage of the ease of use of a text format while capitalizing on the efficiency of a binary format. For example, some forms of dual-format interoperability make the text form easy to prototype, test, and debug, while the binary format saves space and parsing effort. 
     A non-native format may include a rich type system, e.g., extending JSON&#39;s type system, adding support for types suitable for a wider variety of uses, including precision-sensitive applications and portability across languages and runtimes, in some embodiments. In addition to strings, Booleans, arrays (e.g., lists), objects (e.g., structs), and nulls, the non-native format may add support for arbitrary-precision timestamps, embedded binary values, and symbolic expressions. Some embodiments of a non-native format may expand JSON&#39;s number specifications by defining distinct types for arbitrary-size integers, IEEE-754 binary floating-point numbers, and infinite-precision decimals. 
     A non-native format may include Ion, a richly-typed, self-describing, hierarchical data serialization format that provides interchangeable binary and text representations, in embodiments. 
     In some embodiments, a non-native format is a self-describing format, giving its readers and writers the flexibility to exchange data without needing to agree on a schema in advance. For example, in some embodiments, a self-describing format may not require external metadata (i.e. a schema) in order to interpret the structural characteristics of data denoted by the format. For example, the payloads of the self-describing format may be free from build-time binding that inhibits independent innovation and evolution across service boundaries (data may be sparsely encoded and the implicit schema may be changed without explicit renegotiation of the schema among all consumers). 
     Continuing with  FIGS. 2 and 3 , change log data corresponding to changes to the database partition may be accumulated (block  304 ), to change log storage, for instance. For example, change log storage may include any one of the database service  110 , or the change log manager  214 , or a change log data store  604  ( FIG. 6 , described below) may accumulate the change log data, or the change log data may be accumulated to durable storage (e.g., change log archive  152  of storage service  290 ). The change log data may be accumulated in a format native to the database from which the change log data is obtained, or may be converted into a non-native format prior to accumulation, in various embodiments. In some embodiments, the change log data may be accumulated in a process that continues during one or more of the other steps of the process. For example, the accumulation of the change log data (block  304 ) may continue during other steps such as any or all of steps  306 - 312 . 
     If a backup is not triggered (block  306 , no) the system may continue to accumulate change log data. When a backup is triggered (block  306 , yes) a previous snapshot of the database partition may be accessed (block  308 ) from the snapshot data store  142 , by the snapshot manager  212 , for example. In some embodiments, instead of using a previous snapshot of the database partition (e.g., from durable storage), the system may obtain a copy of the database partition (e.g., from a replica of the partition). Changes from the accumulated change log since the previous snapshot was created may be applied (block  310 ) to the previous snapshot to generate a new snapshot of the database partition. For instance, the snapshot manager  212  may call the log apply service  213  to apply the accumulated change log. Log-apply may sometimes be referred to herein as materialization. Materialization may include starting with one or more snapshots, applying the corresponding logs, and creating a coherent set of artifacts that represent the requested backup, for example. The results of materialization may be put directly into a database service table or a materialized set of artifacts may be stored to durable storage, in embodiments. 
     The snapshot manager  212  may store (block  312 ) the new snapshot to the snapshot data store  142 . The system may return to accumulating change log data (block  304 ) in at least the illustrated embodiment. In some embodiments, accumulation may not be interrupted, and continues as a parallel process. 
     In some embodiments, a first copy or a snapshot of a partition is stored to durable storage (e.g., snapshot datastore  142 ) and change log data (e.g., from the database service) may be continuously archived (e.g., to change log archive  152 ) to durable storage. At some point, (e.g., when the change log archive gets to a certain size threshold, or some time threshold) the change log data is downloaded from the archive and used (e.g., via a log apply function) to generate a new snapshot, based on the first snapshot. This process may iterate, at least as long as changes are being made to the partition or database, for example. 
     In at least the illustrated embodiments, either of snapshot manager  212  or change log manager  214  may call an error handler  217  to trigger any of various error routines such as retrying the conversion/reverse-conversion process, retrying the process using a different partition replica, retrying the process on different host machine, reporting failure, flagging the error and continuing, etc. 
     In at least the illustrated embodiments, a checksum generator (not illustrated) may generate a checksum on the snapshot in the non-native format prior to upload of the snapshot to the snapshot data store  142 . In embodiments, the uploaded data may also be processed with a streaming checksum. In some embodiments, the checksum is performed after the upload is complete. The checksum may help protect integrity of the overall process while uploading the output file to storage, in embodiments. Some such features are useful when the entire data set cannot fit into memory (e.g., partitions). 
       FIG. 4  is a flow chart illustrating a technique for continuous data protection according to some embodiments.  FIG. 6  is a data flow diagram illustrating a continuous data protection system with a continuous data protection manager and a change log data store (e.g., a streaming data service data store), according to some embodiments. 
     The steps illustrated in  FIG. 4  may be performed by various components of the systems described herein (e.g., components in continuous data protection system or service  112 ), or by components not illustrated herein, in some embodiments without departing from the scope of this disclosure. At least some of the illustrated steps may be performed by one or more of the components depicted in  FIGS. 1-2 &amp; 6 . The following description will describe various components of  FIG. 6  that implement the functionality described and illustrated in  FIG. 4 , in embodiments. 
     Partition data (e.g.,  144 ) is obtained from a storage node in a storage format native to the database (block  402 ). The partition data may be read into memory (e.g., by forward/reverse converter  206 ) transforming the data to an in-memory format. A forward conversion process may be applied to convert each item of the in-memory partition data to a non-native format for the snapshot (block  404 ), by the forward/reverse converter  206 , for example. A checksum may be generated for the snapshot, in embodiments. The partition snapshot may be stored (block  406 ) in a non-native format to a durable snapshot data store  142 , by the snapshot manager  212  of the continuous data protection manager  112 , for example. 
     Native-database-format change log data corresponding to a database partition is accumulated (block  408 ) via a change log data store  604 . In some embodiments, change log data store  604  may be implemented in durable storage (e.g., of storage service  290 ). In some embodiments, change log data store  604  may be implemented externally to the storage service  290 , by a data stream service that streams the changes, for example. In some embodiments, the database service  110  may provide the streams of change data. If a change log amount threshold is not reached (block  410 ) the continuous protection manager may determine whether a change log time period threshold has been reached (block  412 ). If the change log time period threshold is not reached, the process may return to the accumulation step at  408 . But, if either of the change log amount threshold is reached (block  410 , yes) or the change log time period threshold is reached (block  412 , yes), the process may continue by obtaining (block  415 ) a previous snapshot from the snapshot data store  142  and converting the obtained snapshot to the native format (e.g., by forward/reverse converter  206  of the continuous data protection manager  112 ). 
     In some embodiments, instead of applying native format change log data to a native version of a previous snapshot to generate an updated snapshot, stored native format change log data may be converted to non-native format change log data and then may be log-applied to an existing snapshot in non-native format to generate the updated snapshot. 
     In some embodiments, the storage node may directly provide the change log data in the non-native format (e.g., instead of a conversion being performed by the continuous data protection manager  112 ). 
     At block  416 , accumulated native-format change log data is applied to the native version of the previous snapshot to generate an updated snapshot (e.g., by the log apply service  213  of the continuous data protection manager  112 ). At  417 , the updated snapshot is converted (block  417 ) to a non-native format and the updated snapshot is stored to the snapshot data store  142 . At block  419 , each applied change item is converted to a non-native format and archived to the change log archive  152  (e.g., by the change log manager  214  and the forward/reverse converter  206 ). The process may return to the accumulation step at block  408 , in some embodiments. 
       FIG. 4  illustrates logical steps of a process that may be implemented in various different ways. For example, while the accumulation of change log data in block  408  is illustrated as a step that is performed prior to the applying of the accumulated change log data in block  416  and prior to the conversion of the updated snapshot in  417 , and prior to the conversion of change items in block  419  in at least some embodiments, some of the functionality described in the steps  402 - 419  may be implemented by one or more processes (e.g., one process for each partition, or the like) and may be performed in parallel with the other steps of the process. For example, an instantiated process that is executing the accumulation of the change log data in block  408  may continue to accumulate the change log data while another process is converting change items to a non-native format as in block  419 . 
       FIG. 5  is a flow chart illustrating a technique for continuous data protection during a partition split, according to some embodiments. The steps illustrated in  FIG. 5  may be performed by various of the components depicted in  FIGS. 1-2 and 6, 7 and 8 , in embodiments. At block  502 , a data base split procedure is triggered that splits a partition into a first and a second partition at a point-in-time (e.g., by the database service  110 ). Responsive to the partition split, a corresponding change log is split into a first and a second change log at the point-in-time (block  504 ). For example, the first change log includes changes since the split for the first partition, and the second change log include changes since the split for the second partition. At block  506 , a continuous data protection backup is triggered (e.g., by continuous data protection manager  112 ). A previous snapshot corresponding to the database partition prior to the split is split into a first and a second snapshot (block  508 ). 
     At block  510 , some of the changes from the corresponding change log that are prior to the point-in-time, as well as changes from the first change log, are applied to the first snapshot to create an updated first snapshot. Some of the changes from the corresponding change log that are prior to the point-in-time, as well as changes from the second change log, are applied to the second snapshot to create an updated second snapshot (block  512 ). In some embodiments, the updated first and second snapshots are stored (block  514 ) to durable storage (e.g., at the snapshot data store  142 ). 
     In some embodiments, logs and snapshots may be stored to durable storage in the native format and converted to the non-native format (e.g., on the fly) when needed. For example, native format change logs  603  may be stored to change log data store  604  in storage service  290  and then converted by continuous data protection manager  112  to the non-native format  155 . Similarly, native format snapshots or copies of partitions may be stored to durable storage (e.g., snapshot data store  142 , via continuous data protection manager  112  or via the database service, directly) and then converted to non-native snapshots. The non-native format changes may be log-applied to a prior snapshot in order to generate another snapshot (e.g., for an on-demand backup or to create a periodic snapshot). The change logs in native-format, may remain in durable storage, in embodiments. 
     In some embodiments, the first and second change logs are stored to a change log archive  152 . 
     In at least some embodiments, a checksum and the non-native format (e.g., logical format) snapshot of the partition may be sent to the snapshot data store (e.g., data store  142 ). In some embodiments, the system may send the snapshot without the checksum. 
     The recipient of the snapshot (e.g., snapshot data store  142 ) may use the checksum to verify whether the snapshot has been received without error and send a message back to the snapshot manager  212  indicating success or not. For example, the snapshot manager  212  receives back the checksum conformation, and invokes an error handler  217  or reports a successful snapshot. The error handler  217  may perform any of various handling processes, such as retrying the transmission of the snapshot, encoding the failure, reporting the failure and continuing, etc. The system may respond to a successful checksum by marking the process successful, in embodiments. 
       FIG. 7  is a block diagram illustrating a service provider network that implements a continuous data protection manager service, according to some embodiments. Depicted is a continuous data protection manager service  112  similar to those illustrated in  FIGS. 1, 2, and 6  that may perform some of the steps illustrated in at least  FIGS. 3-5 . The continuous data protection manager service  112  is illustrated as part of service provider network  100  that includes database service  110 , storage service  290 , computer service  640 , clients  160  as well as other storage services  620  and other services  610 . In at least the illustrated embodiment, the service provider network  100  is communicatively coupled to client networks  760  and clients  160  via network  170 . Service provider network  100  may provide one or more services to a plurality of distinct customers, each distinct customer comprising a distinct customer network. One or more of the components in  FIG. 7  may be implemented by one or more of the computing nodes  1200  illustrated in  FIG. 12 . 
       FIG. 8  is a block diagram illustrating logical relationships between components of a continuous data protection manager, a database service, and a storage service, according to some embodiments. In at least the illustrated embodiment, the system has already obtained initial copies of the partitions and accumulated changes log data, as described above. 
     In some embodiments, the illustrated components may perform one or more of the steps depicted in  FIGS. 9 and 10 . Other components may perform the steps. For example, some of the steps depicted in  FIGS. 9 and 10  may be performed by components of the continuous data protection manager  112  depicted in  FIG. 2 . 
     Illustrated is, a database service  110 , change log archive  152 , and snapshot data store  142 , in storage service  290 . Various components of the continuous data protection manager  112  may direct the data transfers, in some embodiments. Various components of continuous data protection manager  112  may obtain and/or write the change data  855  (in native or non-native format), and/or the snapshot data  845  (in native or non-native format) from/to the storage service  290 . Backup data  865  may be stored to snapshot data store  142 , in embodiments. 
     On-Demand Backup 
       FIG. 9  is a process diagram illustrating a logical flow of an on-demand backup request, according to some embodiments. In some embodiments, the depicted process illustrates that independent partition copies are coalesced into a coherent backup copy of a table for a particular point-in-time, at the time of the on-demand backup, for example. In some embodiments, the target time for the on-demand backup may be a time in the future (or past), with respect to when the request backup was requested. On-demand backups of the full database table may be made for a variety of reasons, not limited to, but including compliance, or to support subsequent restoration, for example. 
     In at least the illustrated embodiment, an on-demand request to backup a database table to durable storage is received (block  902 ), by restoration and backup interface  826 , from a requesting entity, such as a client device, for example. A target on-demand time associated with the request is stored and the requested backup is scheduled (block  904 ) as an asynchronous process, by the continuous data protection manager  112 , for example. The process may be asynchronous in that an acknowledgement that the requested backup has been successful may be sent to the requesting entity prior to actually generating the requested backup (block  906 ). This relatively quick response may be an unexpected result, especially for clients requesting backup of a large database table at least because a scanning-type backup of a large database table—a prior way of performing a backup of an entire table—takes a relatively longer amount of time to complete. The system may be able to respond with the acknowledgement of success prior to performing the backup because the system has all the data it needs to create the backup, in embodiments. For example, a combination of existing snapshots and change log data may be used to generate the requested on-demand backup after the confirmation of success has been sent. Some such workflows may be beneficial for a variety of scenarios, such as, but not limited to, creation of compliance backups that are required on some schedule, but are not necessarily used right away, for example. 
     In some embodiments, a backup may be deleted, overwritten or otherwise removed from the system without ever being materialized. For instance, a backup may be requested and deleted, without being exported or used in a restore, and without the backup ever being materialized, in at least some embodiments. 
     A most-recent snapshot for a partition of the table may be obtained for each partition (block  908 ), from the snapshot data store  142 , by the snapshot manager  212  (see  FIG. 2 ) of the continuous data protection manager  112 , for example. Separate sets of changes to the partition are identified from change logs for each partition between the most recent snapshot and the target time of the backup (block  910 ), by the change log manager  214  (see  FIG. 2 ) of the continuous data protection manager  112 , for example. 
     The identified change logs corresponding to the partition may be log-applied to the most recent snapshot for each partition, to generate an updated snapshot of the partition in a state at the target time (block  912 ), by log apply service  213  (see  FIG. 2 ) of the continuous data protection manager  112 , for example. The generated snapshot for each partition may be stored to durable storage with a time stamp indicating the target time of the on-demand backup (block  914 ), by the snapshot manager  212  of the continuous data protection manager  112 , for example. Block  916  illustrates the system may mark the on-demand backup for the target time complete. 
     In embodiments, the resulting table may have a different partitioning than the prior table. Partitions may be merged or split for example. 
     Some embodiments, herein, describe techniques that operate on distinct partitions of a table at distinct times. For example, a first partition of a table (e.g., a relatively active table) may accumulate changes to the first partition in a change log more quickly than a second partition accumulates changes to the second partition, and the system may dynamically respond to each of those partitions differently, by performing updates to snapshots or producing backups of the more active partition more often and at different times from updates or backups of the less-active second partition of the same table. 
     In at least the illustrated embodiment, a technique for producing an on-demand backup for an entire table that includes a number of distinct partitions that each have updated snapshots or backups at distinct times, is disclosed. In some embodiments where distinct partitions of a table have durably-stored snapshots that were generated at distinct times from one another, the system may use a log-apply service to apply logs for the respective partitions to the corresponding snapshots to bring the state of the data for each partition to a common point-in-time—an aligned-in-time set of snapshots for the partitions of the table. A request for an on-demand backup may inherently indicate that a target time for the requested backup of the table is when the request is made or received. In some embodiments, an on-demand backup may be requested for another time, the on-demand backup request may indicate a time in the future for performing the backup, for example. 
       FIG. 11  illustrates, among other things, timing characteristics associated with change log streams of a system that implement the techniques described herein, in embodiments. For example, the illustrated example of change log streams for table A depict five change log streams (SA 1 -SA 5 ) with snapshots  1105 - 1114  taken at various times t. Also illustrated is a target time of a requested point-in-time restoration of table A, and change log data  1108 CLD- 1111 CLD for the periods of time between the last snapshot for the respective change stream and the target time.  FIG. 11  illustrates that the system may use the change log data particular to each change log stream to bring the snapshot for that partition to a state at a time common for each of the partitions—a time aligned set of snapshots that together constitute a full backup of a table at the requested time (in some embodiments) and that can be used to restore the table to a requested point-in-time (in some embodiments). 
     As described herein, the aligned set of partitions (the partitions of a table) may be coherent (e.g., the aligned set of backups may represent a state of the database table at a target time (or at least relatively close points-in-time)), but causal consistency is not necessarily guaranteed. 
     In embodiments, the system may perform an on-demand backup in a similar fashion for a single partition of a table with multiple partitions, or as depicted in  FIG. 11  for Table A, for the table as a whole, using a partition-by-partition process to accomplish the task. 
     Point-in-Time Restore 
       FIG. 10  is a process diagram illustrating a logical flow of a point-in-time restoration, according to some embodiments. A point-in-time restoration may be performed for a portion of a database table (e.g., a partition) or for the entire table, in some embodiments. As illustrated in  FIG. 10 , a request to restore a database table to a state of the database table at a target point-in-time (e.g., a point-in-time specified in the request, or otherwise) is received (block  1002 ). For example, a requesting entity, such as a client, a client device, a process executing on a service provider or client machine, or the like may request the point-in-time restore. 
     Requesting clients may wish to restore a table or partition in various ways. For example, a client may wish to restore over the existing table, or restore to a newly-created table while retaining the existing table. In some way, a table in a database service is identified for the restoration (block  1004 ), as directed by backup manager  824 , for example. 
     In some embodiments, restoration and backup interface  826  may provide a customer-facing interface for a customer to experience a fully managed restoration or backup service. For instance, restoration and backup interface  826  may provide an interface (e.g., a graphical or program interface) that client devices or processes of a customer can access to manage restorations and backups, such as viewing the number of available backups as well as characteristics thereof, requesting on-demand backups or restorations, and the like. 
     In at least some embodiments, the restoration and backup interface  826  may provide a customer-facing interface that gives the customer the ability to configure table properties at the time of a restoration. For example, when a restoration is performed using the client-facing interface to a database service (e.g.,  110 ) the restoration interface (e.g.,  826 ) may provide access to the features provided via a client-facing interface of the database service. Managed-database service features, such as configuration of table properties, partition management, secondary indexing services and the like may be passed though from the database service to the client via the restoration and backup interface  826 . 
     The disclosed technique may be performed by one or more processes. In one example, one process may be called to create the new table and another process may be called to perform the rest of the restoration. In some instances, a distinct process or node may perform steps  1006 - 1012  for a respective partition, the node hosting the partition, for example, may perform the steps  1006 - 1012  for that partition. 
     At block  1006 , a separate archived partition is identified for each partition, based on the target point-in-time. For example, restoration manager  822  may identify an archived snapshot of a partition of the database table that is closest in time (either prior to the time, or subsequent to the time) to the target point-in-time.  FIG. 11  illustrates a particular example for a point-in-time restoration. Change log stream SA 3 , in particular, illustrates that snapshot  1114  has been selected instead of snapshot  1109  because snapshot  1112  is closer in time to the requested time, a concept also applicable to on-demand backups that are scheduled, instead of performed at the requested time, in some embodiments. It is contemplated that archived partitions may be selected, in some embodiments, based on the amount of changes between the requested time and the archived partition. It may be preferable to select partitions associated with fewer changes between the partition and the requested time, even when the amount of time between the selected partition and the requested time is greater than another available partition, in embodiments. 
     For each partition, a segment of changes in a change log corresponding to the partition is identified for a period of time between the identified snapshot and the target point in time (block  1008 ), by the change log manager  214  in  FIG. 2 , for example. Block  1010  illustrates that for each partition, the identified segment is log-applied to the corresponding partition to generate an updated snapshot of the partition at the target point-in-time. Some combination of the change log manager  214  snapshot manager  212 , log apply service  213 , and the restore manager may perform such functions, in some embodiments, for example. 
     Going back to the illustrated example in  FIG. 11 , the change log data  1109 CLD (represented by the shaded area) may be applied to the snapshot  1109  to walk the state of the partition backward in time to the state of the partition at the requested point-in-time. Items (e.g., restoration data  875 ) from the updated snapshot of the identified portion of the database (e.g., for a partition) are sent to the database service to create entries in the newly-identified database table (block  1012 ) in the illustrated embodiment, by restoration manager  822 , for example. 
     The items may be sent as puts via a client-facing interface of the database service to a newly-created table, or to overwrite an existing table, in a few non-limiting examples. Successful completion of the point-in-time restore of the table may be acknowledged to the requesting entity (block  1014 ), via restoration and backup interface  826 , for example. 
       FIG. 10  is a logical flow of a process that may be implemented various ways. For example, various different processes or nodes may each perform steps  1006 - 1012  for a respective partition, in parallel or sequentially without departing from the scope of this disclosure. 
     Restoration of on-demand backups may be performed, even if the on-demand backup has not been created or materialized—if the coherent artifacts have not been generated. For example, a process similar to that described above for point-in-time restore may be performed to create the table requested for on-demand restoration. If an on-demand backup has already been materialized and stored to durable storage, the log-apply process may be skipped and the items may be sent directly to the database service (e.g., via the client-facing interface or otherwise). For example, the items from separate partitions in durable storage that represent the backup may be streamed to the database service. In some embodiments, such a transfer may include a conversion (e.g., via forward/reverse converter  206 ) from non-native format to a native format of the database, as described above. 
     Client-Facing Interface of Database Service 
     Clients may generally access production database services of a service provider via a client-facing interface (e.g., a “front-door” of the database service, such as request routers of the database service). The service provider&#39;s internal processes may also access the database service via the client-facing interface, or may use a proprietary or “backend” interface to manage, restore, obtain partition copies, or otherwise access the database service, in embodiments. By, using the client-facing interface of the database service to enter the restore data into the table, the system may take advantage of the existing features of the database service that optimize the table performance. 
     The client-facing interface of the database service, or the restoration and backup interface  826 , for example, may make the data that is being restored available to clients before the entire restoration has completed, in embodiments. In a particular example, clients may be given access to the restored data while the restoration data is simultaneously being put into the database. Such a technique may have the advantage of giving the client access to at least some of the data in (on average) half the time of the total restoration time, for example. 
     In some embodiments, the items from the updated snapshots (e.g.,  875 ) may be sent by the restore manager  822  to create entries in the newly-identified database in database service  110  via either of the client-facing interface, or the backend interface of the database service. In some embodiments, by using the client-facing interface, the restoration process may leverage the existing functionality of the database service (e.g., the benefits of a fully-managed database service, managed table configurations—placement optimization at the time of restore etc., managed partitioning—fixing IOPS dilution via repartitioning etc., local and global secondary indexing services, or the like). Use of the client-facing alternative may decouple the restore process from the proprietary or technical features particular to the internal storage nodes of the database service. 
     Using the client-facing interface to the database service provides clients with access to existing features of the database service, such as automatically creating secondary index(es) as the restored partitions are loaded, changing the partitioning, streaming all the data changes out as items are added in, etc. For example, some database services provide managed features, such as providing streams of data indicating changes to the database table to which a customer may register a listener. In one example of many, such streams may provide full-text characteristics of database that does not support full-text search, for example. Clients may use such data for a variety of reasons. Another benefit of using the client-facing interface of such a database service is that it leverages such already-existing features of the database service without having to replicate them, as would be required in a back-end interface restoration scenario. In at least some embodiments, the restoration and backup interface  826  may provide for configuration of such features, such as turning the above-noted streaming feature off or on. 
     Use of the client-facing interface also has the benefit of leveraging the existing global secondary indexing (GSI) management of the existing database service. 
     Benefits of using a backend interface include faster data transfers to the storage nodes and greater control over the data. For example, restoring via a backend interface could provide clients with greater flexibility in controlling whether a new GSI is created or a LSI is deleted, instead of relying on the defaults, or non-existence of such features in the client-facing interface for such features. 
     The methods described herein may in various embodiments be implemented by any combination of hardware and software. For example, in one embodiment, the methods may be implemented by a computer system that includes a processor executing program instructions stored on a non-transitory, computer-readable storage medium coupled to the processor. The program instructions may be configured to implement the functionality described herein (e.g., the functionality of various servers and other components that implement the scalable, distributed data storage systems and/or remote key-value durable backup storage systems described herein (or another type of remote storage system suitable for backing up tables and/or partitions thereof). 
       FIG. 12  is a block diagram illustrating a computing node configured to implement at least a portion of a continuous data protection system, according to various embodiments described herein. For example, computing node  1200  may represent a computing node that implements one or more of the techniques or components described herein for providing continuous data protection, according to various embodiments. In various embodiments, computing node  1200  may be configured to implement any or all of the components of a system that implements a scalable, distributed data storage system and a corresponding continuous data protection system, or multiple computing nodes similar to or different from computing node  1200  may collectively provide this functionality. For example, in various embodiments, one or more computing nodes  1200  may implement database service  110 , snapshot manager  212 , change log manager  214 , snapshot data store  142 , change log archive  152 , or clients  160 . Additionally, any number of storage node instances may each host one or more replicas of various data partitions and/or metadata associated therewith. For example, any given storage node instance may host a replica acting as master replica for its replica group and/or a replica acting as a slave replica in its replica group. 
     Similarly, one or more computing nodes  1200  may implement a key-value durable backup storage system (or an interface or other component thereof), in different embodiments. Computing node  1200  may be any of various types of devices, including, but not limited to, a personal computer system, desktop computer, laptop or notebook computer, mainframe computer system, handheld computer, workstation, network computer, a consumer device, application server, storage device, telephone, mobile telephone, or in general any type of computing device. 
     In some embodiments that include multiple computing nodes  1200 , all of the computing nodes  1200  may include the same or similar hardware components, software components, and functionality, while in other embodiments the computing nodes  1200  comprising a computing system configured to implement the functionality described herein may include a wide variety of hardware components, software components, and functionality. In some embodiments, multiple computing nodes  1200  that collectively implement a data storage service or a remote storage system may be components of a larger shared resource system or grid computing system. It is noted that different elements of the system described herein may be implemented by different computing nodes  1200 . For example, a computer system that supports the functionality described herein for performing continuous data protection may be implemented on the same computing nodes  1200  on which clients (through which a customer or subscriber may access the system) execute, or on one or more other computing nodes  1200 , in different embodiments. In another example, different subsystems (e.g., a Web service interface, an admission control subsystem, a service request subsystem; and/or one or more Web servers or other components) may be implemented on or across multiple ones of the computing nodes, and each of the computing nodes may be similar to computing node  1200 . 
     In the illustrated embodiment, computing node  1200  includes one or more processors  1210  (any of which may include multiple cores, which may be single or multi-threaded) coupled to a system memory  1220  via an input/output (I/O) interface  1230 . Computing node  1200  further includes a network interface  1240  coupled to I/O interface  1230 . In various embodiments, computing node  1200  may be a uniprocessor system including one processor  1210 , or a multiprocessor system including several processors  1210  (e.g., two, four, eight, or another suitable number). Processors  1210  may be any suitable processors capable of executing instructions. For example, in various embodiments, processors  1210  may be general-purpose or embedded processors implementing any of a variety of instruction set architectures (ISAs), such as the x86, PowerPC, SPARC, or MIPS ISAs, or any other suitable ISA. In multiprocessor systems, each of processors  1210  may commonly, but not necessarily, implement the same ISA. Similarly, in a distributed computing system such as one that collectively implements a scalable database service or a remote storage service in which tables are baked up, each of the computing nodes may implement the same ISA, or individual computing nodes and/or replica groups of nodes may implement different ISAs. 
     The computing node  1200  also includes one or more network communication devices (e.g., network interface  1240 ) for communicating with other systems and/or components over a communications network (e.g. Internet, LAN, etc.). For example, a client application executing on computing node  1200  may use network interface  1240  to communicate with a server application executing on a single server or on a cluster of servers that implement a distributed system. In another example, an instance of a server application executing on computing node  1200  may use network interface  1240  to communicate with other instances of the server application that may be implemented on other computer systems. 
     In the illustrated embodiment, computing node  1200  also includes one or more persistent storage devices  1260  and/or one or more I/O devices  1280 . In various embodiments, persistent storage devices  1260  may correspond to disk drives, tape drives, solid state memory, other mass storage devices, or any other persistent storage device. Computing node  1200  (or a distributed application or operating system operating thereon) may store instructions and/or data in persistent storage devices  1260 , as desired, and may retrieve the stored instruction and/or data as needed. 
     Computing node  1200  includes one or more system memories  1220  that are configured to store instructions and/or data (shown as program instructions  1225  and data store  1245 , respectively) that are accessible by processor(s)  1210 . In various embodiments, system memories  1220  may be implemented using any suitable memory technology, (e.g., one or more of cache, static random-access memory (SRAM), DRAM, RDRAM, EDO RAM, DDR 10 RAM, synchronous dynamic RAM (SDRAM), Rambus RAM, EEPROM, non-volatile/Flash-type memory, or any other type of memory). System memory  1220  may contain program instructions  1225  that are executable by processor(s)  1210  to implement the methods and techniques described herein. 
     In the illustrated embodiment, program instructions and data implementing desired functions, methods or techniques (such as functionality for backing up tables, and/or restoring tables from backup using any or all of the mechanisms described herein), are shown stored within system memory  1220  as program instructions  1225 . For example, program instruction  1225  may include program instructions that when executed on processor(s)  1210  implement any or all of a continuous data protection system  112 , storage service  290 , various data stores and archives, and/or any other components, modules, or sub-modules of a system that provides the data storage system and services described herein. Program instructions  1225  may also include program instructions configured to implement additional functionality of a system that implements a data storage service not described herein. In some embodiments, program instructions  1225  may include program instructions configured to implement functionality of a key-value durable backup storage system or another type of remote storage system suitable for backing up tables and/or partitions thereof. In some embodiments, program instructions  1225  may implement multiple separate clients, server nodes, and/or other components. 
     It is noted that in some embodiments, program instructions  1225  may include instructions and data implementing desired functions that are not directly executable by processor(s)  1210  but are represented or encoded in an abstract form that is translatable to instructions that are directly executable by processor(s)  1210 . For example, program instructions  1225  may include instructions specified in an ISA that may be emulated by processor  1210 , or by other program instructions  1225  executable on processor(s)  1210 . Alternatively, program instructions  1225  may include instructions, procedures or statements implemented in an abstract programming language that may be compiled or interpreted in the course of execution. As non-limiting examples, program instructions  1225  may be encoded in platform native binary, any interpreted language such as Java™ byte-code, or may include code specified in a procedural or object-oriented programming language such as C or C++, a scripting language such as perl, a markup language such as HTML or XML, or any other suitable language or in any combination of languages. 
     In some embodiments, program instructions  1225  may include instructions executable to implement an operating system (not shown), which may be any of various operating systems, such as UNIX, LINUX, Solaris™, MacOS™, Windows™, etc. Any or all of program instructions  1225  may be provided as a computer program product, or software, that may include a non-transitory computer-readable storage medium having stored thereon instructions, which may be used to program a computer system (or other electronic devices) to perform a process according to various embodiments. A non-transitory computer-readable storage medium may include any mechanism for storing information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). Generally speaking, a non-transitory computer-accessible medium may include computer-readable storage media or memory media such as magnetic or optical media, e.g., disk or DVD/CD-ROM coupled to computing node  1200  via I/O interface  1230 . A non-transitory computer-readable storage medium may also include any volatile or non-volatile media such as RAM (e.g. SDRAM, DDR SDRAM, RDRAM, SRAM, etc.), ROM, etc., that may be included in some embodiments of computing node  1200  as system memory  1220  or another type of memory. In other embodiments, program instructions may be communicated using optical, acoustical or other form of propagated signal (e.g., carrier waves, infrared signals, digital signals, etc.) conveyed via a communication medium such as a network and/or a wireless link, such as may be implemented via network interface  1240 . 
     In other embodiments, program instructions and/or data as described herein for implementing a data storage service that employs the techniques described above may be received, sent or stored upon different types of computer-readable media or on similar media separate from system memory  1220  or computing node  1200 . Program instructions and data stored on a computer-readable storage medium may be transmitted to a computing node  1200  for execution by a processor  1210  by transmission media or signals such as electrical, electromagnetic, or digital signals, which may be conveyed via a communication medium such as a network and/or a wireless link, such as may be implemented via network interface  1240 . In some embodiments, system memory  1220  may include data in data store  1245  and/or program instructions  1225  and/or special purpose instructions  1226 , which may be configured as described herein. In some embodiments, data store  1245  may store the snapshots, or the change stream items, for example. In some embodiments, special purpose program instructions  1226  may include instructions that implement the continuous data protection manager  112 , the storage service  290 , or the database service  110 , for example. 
     In some embodiments, system memory  1220  may include data store  1245 , which may be configured as described herein. For example, the information described herein as being stored by the scalable, distributed data storage system (e.g., table data, metadata for tables, partitions and backups, transaction information, configuration information for tables and/or partitions, or other information used in performing the methods described herein may be stored in data store  1245  or in another portion of system memory  1220  on one or more nodes, in persistent storage  1260 , and/or in remote storage  1270 , in various embodiments. In some embodiments, and at various times, system memory  1220  (e.g., data store  1245  within system memory  1220 ), persistent storage  1260 , and/or remote storage  1270  may store copies of table data (e.g., partition data) backup copies of table and/or partition data, metadata associated with tables, partitions, backups, transactions and/or their states, database configuration information, and/or any other information usable in implementing the methods and techniques described herein. In some embodiments, remote storage  1270  may be a key-value durable storage system in which tables (and/or partitions thereof) are backed up and from which they are restored, as described herein. 
     Data store  1245  may in various embodiments include collections of data maintained by a data storage service on behalf of its clients/users, and/or metadata used by a computing system that implements such a service, as described herein (including, but not limited to, tables managed and maintained on behalf of clients/users of the service, metadata tables, business rules, partition maps, routing tables, indexes, namespaces and/or partitions thereof, service level agreement parameter values, subscriber preferences and/or account information, performance data, resource capacity data, resource usage data, provisioned resource utilization data, reserved resource data, resource reservation IDs, resource reservation timeout period values, parameter values for various partition management policies, limits, or constraints, and/or information about candidate partition management operations). 
     In one embodiment, I/O interface  1230  may be configured to coordinate I/O traffic between processor(s)  1210 , system memory  1220  and any peripheral devices in the system, including through network interface  1240  or other peripheral interfaces. In some embodiments, I/O interface  1230  may perform any necessary protocol, timing or other data transformations to convert data signals from one component (e.g., system memory  1220 ) into a format suitable for use by another component (e.g., processor  1210 ). In some embodiments, I/O interface  1230  may include support for devices attached through various types of peripheral buses, such as a variant of the Peripheral Component Interconnect (PCI) bus standard or the Universal Serial Bus (USB) standard, for example. In some embodiments, the function of I/O interface  1230  may be split into two or more separate components, such as a north bridge and a south bridge, for example. Also, in some embodiments, some or all of the functionality of I/O interface  1230 , such as an interface to system memory  1220 , may be incorporated directly into processor  1210 . 
     Network interface  1240  may be configured to allow data to be exchanged between computing node  1200  and other devices attached to a network, such as other computer systems  1290  (which may implement one or more server nodes and/or clients of a scalable, distributed data storage system and/or a remote key-value durable storage system), for example. In addition, network interface  1240  may be configured to allow communication between computing node  1200  and various I/O devices  1250  and/or remote storage  1270 ), or between other nodes in a system providing shared computing services. In general, network interface  1240  may be configured to allow data to be exchanged between computing node  1200  and any of a variety of communication devices, external storage devices, input/output devices and/or other computing devices, in different embodiments. 
     Input/output devices  1250  may, in some embodiments, include one or more display terminals, keyboards, keypads, touchpads, scanning devices, voice or optical recognition devices, or any other devices suitable for entering or retrieving data by one or more computing nodes  1200 . Multiple input/output devices  1250  may be present in computing node  1200  or may be distributed on various nodes of a distributed system that includes computing node  1200 . In some embodiments, similar input/output devices may be separate from computing node  1200  and may interact with one or more nodes of a distributed system that includes computing node  1200  through a wired or wireless connection, such as over network interface  1240 . Network interface  1240  may commonly support one or more wireless networking protocols (e.g., Wi-Fi/IEEE 802.11, or another wireless networking standard). However, in various embodiments, network interface  1240  may support communication via any suitable wired or wireless general data networks, such as other types of Ethernet networks, for example. Additionally, network interface  1240  may support communication via telecommunications/telephony networks such as analog voice networks or digital fiber communications networks, via storage area networks such as Fibre Channel SANs, or via any other suitable type of network and/or protocol. In various embodiments, computing node  1200  may include more, fewer, or different components than those illustrated in  FIG. 12  (e.g., displays, video cards, audio cards, peripheral devices, other network interfaces such as an ATM interface, an Ethernet interface, a Frame Relay interface, etc.) 
     Storage service clients (e.g., users, subscribers and/or client applications) may interact with a data storage service such as that described herein in various ways in different embodiments, such as to submit requests for service (including, but not limited to, requests to create and/or partition tables, requests to store, retrieve and/or update items in tables, or requests to split, move, or otherwise repartition a table), and to receive results. For example, some subscribers to the service may have physical access to computing node  1200 , and if so, may interact with various input/output devices  1250  to provide and/or receive information. Alternatively, other clients/users may use client computing systems to access the system, such as remotely via network interface  1240  (e.g., via the Internet and/or the World Wide Web). In addition, some or all of the computing nodes of a system providing the service may provide various feedback or other general types of information to clients/users (e.g., in response to user requests) via one or more input/output devices  1250 . 
     It is noted that any of the distributed system embodiments described herein, or any of their components, may be implemented as one or more web services. For example, a front-end module or administrative console of a Web services platform may present data storage services and/or database services to clients as web services. In some embodiments, a web service may be implemented by a software and/or hardware system designed to support interoperable machine-to-machine interaction over a network. A web service may have an interface described in a machine-processable format, such as the Web Services Description Language (WSDL). Other systems may interact with the web service in a manner prescribed by the description of the web service&#39;s interface. For example, the web service may define various operations that other systems may invoke, and may define a particular application programming interface (API) to which other systems may be expected to conform when requesting the various operations. 
     In various embodiments, a web service may be requested or invoked through the use of a message that includes parameters and/or data associated with the web services request. Such a message may be formatted according to a particular markup language such as Extensible Markup Language (XML), and/or may be encapsulated using a protocol such as Simple Object Access Protocol (SOAP). To perform a web services request, a web services client may assemble a message including the request and convey the message to an addressable endpoint (e.g., a Uniform Resource Locator (URL)) corresponding to the web service, using an Internet-based application layer transfer protocol such as Hypertext Transfer Protocol (HTTP). 
     In some embodiments, web services may be implemented using Representational State Transfer (“RESTful”) techniques rather than message-based techniques. For example, a web service implemented according to a RESTful technique may be invoked through parameters included within an HTTP method such as PUT, GET, or DELETE, rather than encapsulated within a SOAP message. 
     Those skilled in the art will appreciate that computing node  1200  is merely illustrative and is not intended to limit the scope of embodiments. In particular, the computing system and devices may include any combination of hardware or software that can perform the indicated functions, including computers, network devices, internet appliances, PDAs, wireless phones, pagers, etc. Computing node  1200  may also be connected to other devices that are not illustrated, in some embodiments. In addition, the functionality provided by the illustrated components may in some embodiments be combined in fewer components or distributed in additional components. Similarly, in some embodiments the functionality of some of the illustrated components may not be provided and/or other additional functionality may be available. 
     Those skilled in the art will also appreciate that, while various items are illustrated as being stored in memory or on storage while being used, these items or portions of them may be transferred between memory and other storage devices for purposes of memory management and data integrity. Alternatively, in other embodiments some or all of the software components may execute in memory on another device and communicate with the illustrated computing system via inter-computer communication. Some or all of the system components or data structures may also be stored (e.g., as instructions or structured data) on a computer-readable storage medium or a portable article to be read by an appropriate drive, various examples of which are described above. In some embodiments, instructions stored on a computer-readable storage medium separate from computing node  1200  may be transmitted to computing node  1200  via transmission media or signals such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as a network and/or a wireless link. Various embodiments may further include receiving, sending or storing instructions and/or data implemented in accordance with the foregoing description upon a computer-readable storage medium. Accordingly, different embodiments may be practiced with other computer system configurations. 
     Note that while several examples described herein are directed to the application of various techniques in systems that include a non-relational database, in other embodiments these techniques may be applied in systems in which the distributed data store is implemented using a different storage paradigm. 
     The various methods as illustrated in the figures and described herein represent example embodiments of methods. The methods may be implemented manually, in software, in hardware, or in a combination thereof. The order of any method may be changed, and various elements may be added, reordered, combined, omitted, modified, etc. 
     Those skilled in the art will appreciate that in some embodiments the functionality provided by the methods discussed above may be provided in alternative ways, such as being split among more software modules or routines or consolidated into fewer modules or routines. Similarly, in some embodiments illustrated methods may provide more or less functionality than is described, such as when other illustrated methods instead lack or include such functionality respectively, or when the amount of functionality that is provided is altered. In addition, while various operations may be illustrated as being performed in a particular manner (e.g., in serial or in parallel) and/or in a particular order, those skilled in the art will appreciate that in other embodiments the operations may be performed in other orders and in other manners. Those skilled in the art will also appreciate that the data structures discussed above may be structured in different manners, such as by having a single data structure split into multiple data structures or by having multiple data structures consolidated into a single data structure. Similarly, in some embodiments illustrated data structures may store more or less information than is described, such as when other illustrated data structures instead lack or include such information respectively, or when the amount or types of information that is stored is altered. The various methods as depicted in the figures and described herein represent illustrative embodiments of methods. The methods may be implemented in software, in hardware, or in a combination thereof in various embodiments. Similarly, the order of any method may be changed, and various elements may be added, reordered, combined, omitted, modified, etc., in various embodiments. 
     From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the appended claims and the elements recited therein. In addition, while certain aspects are presented below in certain claim forms, the inventors contemplate the various aspects in any available claim form. For example, while only some aspects may currently be recited as being embodied in a computer readable storage medium, other aspects may likewise be so embodied.