Patent Publication Number: US-10762102-B2

Title: System and method for incremental replication

Description:
RELATED APPLICATIONS 
     This application claims the benefit as a Continuation of application Ser. No. 14/537,367, entitled “System And Method For Incremental Replication” filed Nov. 10, 2014 with claims the benefit of Continuation of application Ser. No. 13/922,437, entitled “System And Method For Incrementally Replicating Investigative Analysis Data,” issued as U.S. Pat. No. 8,886,601 on Nov. 11, 2014, the entire contents of which is hereby incorporated by reference as if fully set forth herein, under 35 U.S.C. § 120. The applicant(s) hereby rescind any disclaimer of claim scope in the parent application(s) or the prosecution history thereof and advise the USPTO that the claims in this application may be broader than any claim in the parent application(s). 
    
    
     TECHNICAL FIELD 
     A system and method are disclosed which generally relate to replication of database data, and more particularly to a system and method of incrementally replicating database data. 
     BACKGROUND 
     Making investigative decisions, especially those that have the potentially to impact lives and communities, requires access to up-to-date and accurate investigative information. Unfortunately, investigative information is often spread across multiple databases, computers, geographies, and clearance levels. For investigative organizations such as intelligence, defense, and law enforcement organizations to be successful, they need ways to share and find information quickly so that critical decisions can be made in time for them to have impact. 
     One complication to sharing investigative data between investigative teams is that some of teams may be located in geographic locations where network connectivity is unreliable or impractical. For example, a forward deployed military unit may have only periodic access to a satellite-based network. Thus, solutions for sharing data that presume highly-available network connectivity may be inadequate or inefficient. 
     Currently, there exist commercial software products for replicating database data between distributed database instances. These software products, for example, allow an administrator to export database data from a first database instance, copy the exported database data to a second database instance, and once copied, import the exported database data into the second database instance. This process of replicating database data can be tedious, time-consuming, or unreliable, especially when the data network connecting the first and second instances is unreliable and the amount of exported database data is large. 
     SUMMARY 
     The following is a summary of various aspects realizable according to various embodiments of the system and method of incrementally replicating investigative analysis data according to the present disclosure. It is provided as an introduction to assist those skilled in the art to more rapidly assimilate the details of the disclosure and does not and is not intended in any way to limit the scope of the claims that are appended hereto. 
     In one aspect, a method of incrementally replicating investigative analysis data is disclosed along with a system for performing the same. The method and system provide the ability to break a data replication job into multiple “replication chunks” which can be exported and imported separately. By doing so, the method and system can efficiently replicate large numbers of data object changes over an unreliable data network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example, reference will now be made to the accompanying drawings. 
         FIG. 1  is a block diagram illustrating a distributed computer system for incrementally replicating investigative analysis data. 
         FIG. 2  is a block diagram illustrating an investigative analysis computer system. 
         FIG. 3  illustrates an object-centric data model. 
         FIG. 4  illustrates a table of a revisioning database. 
         FIG. 5  is a flowchart illustrating a computer-implemented process for incrementally replicating investigative analysis data. 
         FIG. 6  is a block diagram illustrating an incremental replication plan. 
         FIG. 7  is a flowchart illustrating a computer-implemented process for executing an incremental replication job. 
         FIG. 8  is a flowchart illustrating a computer-implemented process for importing a replication chunk of an incremental replication job. 
         FIG. 9  is a block diagram of a computer system on which embodiments of the present invention may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention 
     Overview 
     A system and method of incrementally replicating investigative analysis data is disclosed herein. In one embodiment, the term “investigative analysis data” generally refers to any database data meaningful to an investigative organization. Investigative analysis data includes, but is not limited to, database data that represents people, places, things, events, documents, media, notes, properties, taken alone and in any combination thereof. 
     In one embodiment, a method and system are disclosed for incrementally replicating investigative analysis data from an exporting investigative analysis system (“exporting system”) to an importing investigative analysis system (“importing system”). The exporting system and the importing system may be operatively coupled by an unreliable data network such as a data network with high latency, low bandwidth, and/or intermittent or periodic availability. 
     In some embodiments, the exporting system receives a user command to establish an incremental data replication relationship with the importing system and responds by creating an incremental data replication plan. The plan represents an incremental data replication job that is to be performed by the exporting system to replicate, to the importing system, changes to investigative data objects that the exporting system knows about that the exporting system determines the importing system does not yet know about. The number of changes to replicate may be large (e.g., on the order of hundreds of millions). For example, it may have been some time since a last replication exchange between exporting system and the importing system during which a large number of data objects changes were made by the exporting system. Among other information, the plan specifies the number of replication chunks that the incremental replication job is broken down into. After the plan is created, the exporting system separately exports each replication chunk to the importing system according to a user-configurable exportation schedule. For example, the exportation schedule can be configured to accommodate periodic data network availability or to avoid exporting replication chunks during peak usage times of the exporting or importing systems. Depending on the exportation schedule and the number of replication chunks, performance of the incremental replication job can span minutes, hours, days, or longer. During performance of the incremental replication job, the exporting system can continue to make changes to data objects. This is facilitated by an always increasing logical clock maintained at the exporting system that provides a total ordering for all data object changes made by the exporting system. When creating the incremental replication plan, the exporting system records a current logic clock value in the plan. The recorded value represents the most recent data object change that will be included in a replication chunk of the replication job (“maximum export logical clock value”). When exporting replication chunks of the replication job, the exporting system includes only data object changes associated with logic clock values that are less than or equal to the maximum export logical clock value. By doing so, the exporting system provides a consistent “snapshot” view of data object changes to the importing system in the replication chunks. At the same time, the exporting system can make additional data object changes without affecting this consistent view. Such additional data object changes can be replicated to the importing system in a subsequent replication job. 
     Investigative Analysis Computer System 
       FIG. 1  illustrates a system for incrementally replicating investigative analysis data. A distributed investigative analysis computer system  10  includes an exporting investigative analysis computer system (“exporting system”)  12  and an importing investigative analysis computer system (“importing system”)  14 . In replication examples described below, investigative analysis data is exported by investigative analysis computer system  12  and imported by investigative analysis computer system  14 . Hence, investigative analysis computer system  12  is labeled the “exporting system” and investigative analysis computer system  14  is labeled the “importing system”. However, the examples could just have easily involved investigative analysis data exported by investigative analysis computer system  14  and imported by investigative analysis computer system  12 . Further, investigative analysis computer system  12  can also function as an importing system and investigative analysis computer system  14  can also function as an exporting system. Thus, investigative analysis computer system  12  can import investigative analysis data as well as export investigative analysis data. Similarly, investigative analysis computer system  14  can export investigative analysis data as well import investigative analysis data. 
     In distributed investigative analysis system  10 , exporting system  12  and importing system  14  may be operatively coupled to each other by unreliable data network  16 . Data network  16  may be unreliable in the sense that it is only periodically or intermittently available (i.e., not highly-available), has high network communication latency, and/or has low network communication bandwidth. For example, data network  16  may be unreliable in that a user would find it frustrating or impractical to use for purposes of surfing the Internet. 
       FIG. 2  illustrates an investigative analysis computer system  100  which may be used as exporting system  12  and importing system  14  in distributed investigative analysis computer system  10 . That is, an instance of investigative analysis computer system  100  may be used as exporting system  12  and another separate instance of investigative analysis computer system  100  may be used as importing system  14 . Both instances may be operatively coupled to each other by unreliable data network  16 . 
     Investigative analysis computer system  100  includes one or more analyst clients  102 , one or more analysis servers  104 , and a revisioning database  106 . Clients  102  connect to analysis servers  104  to conduct various investigative analysis and management operations on investigative analysis data stored in revisioning database  106 . Investigative analysis operations include commanding analysis servers  104  to create, read, update, and delete investigative analysis data stored in revisioning database  106 . Management operations include configuring analysis servers  104  for incremental data replication as described in hereinafter. 
     In some embodiments, investigative analysis and management operations are conducted by users of clients  102  through a graphical user interface (GUI) or web browser-based user interface presented at clients  102 . Such presentation may be driven by analysis servers  104 , for example, through delivery of user interface and investigative analysis data according to standardized networking protocols and presentation formats such as the HyperText Transfer Protocol (HTTP), the Secure HyperText Transfer Protocol (HTTPS), the HyperText Markup Language (HTML), Cascading Style Sheets (CSS), JavaScript, etc. In other embodiments, operations are conducted by users through a command line interface (CLI) available at clients  102  or on servers  104 . 
     In one embodiment, the system  100  is embodied in a single computing device such as a laptop computer. In another embodiment, the system  100  is embodied in multiple computing devices such as one or more personal or workstation computing devices for the analysts&#39; clients  102 , one or more server computing devices for the analysis servers  104 , and one or more server computing devices for the revisioning database  106 . In some embodiments, one of the exporting system  12  or the importing system  14  is embodied in a single computing device such as a laptop computer and the other is embodied in multiple computing devices. This embodiment may represent a situation in which, for example, investigative analysis data is being shared between a team of analysts at a hub location such as a central office within the organization and an analyst, or team of analysts, in the field such as at a forward operating location. 
     Object-Centric Data Model 
     Investigative analysis data stored in revisioning database  106  may be conceptually stored and organized according to an object-centric data model.  FIG. 3  illustrates an object-centric conceptual data model  108 . Model  108  is centered on the notion of a data object  110 . At the highest level of abstraction, a data object  110  is a container for information representing things in the world. For example, a data object  110  can represent an entity such as a person, a place, an organization, or other noun. A data object  110  can represent an event that happens at a point in time or for a period of time. A data object  110  can represent a document or other unstructured data source such as an e-mail message, a news report, or a written paper or article. These are just some example of what a data object  110  can represent. A data object  110  may be associated with a unique identifier that uniquely identifies the data object to the investigative analysis computer system  100  among other data objects  110 . A data object  110  may also have a type (e.g., Person, Event, or Document) and a display name which may be the value of a particular property of the data object  110 . 
     A data object  110  may have one or more properties  112 . A property  112  is an attribute of a data object  110  that represents an individual data item. A property  112  may have a type and a value. Different types of data objects  110  may have different types of properties  112 . For example, a Person data object  110  might have an Eye Color property and an Event data object  110  might have a Date property. In one embodiment, the set of data object types and the set of property types for each type of data object supported by the investigative analysis system  100  are defined according to a pre-defined, user-defined, or dynamically-defined ontology or other hierarchical structuring of knowledge through sub-categorization of object types and property types according to their relevant and/or cognitive qualities. In addition, data model  108  may support property multiplicity. In particular, a data object  110  may be allowed to have more than one property  112  of the same type. For example, a Person data object might have multiple Address properties or multiple Name properties. 
     A link  114  represents a connection between two data objects  110 . In one embodiment, the connection is either through a relationship, an event, or through matching properties. A relationship connection may be asymmetrical or symmetrical. For example, Person data object A may be connected to Person data object B by a Child Of relationship (where Person data object B has an asymmetric Parent Of relationship to Person data object A), a Kin Of symmetric relationship to Person data object C, and an asymmetric Member Of relationship to Organization data object X. The type of relationship between two data objects may vary depending on the types of the data objects. For example, Person data object A may have an Appear In relationship with Document data object Y or have a Participate In relationship with Event data object E. As an example of an event connection, two Person data objects may be connected by an Airline Flight data object representing a particular airline flight if they traveled together on that flight, or by a Meeting data object representing a particular meeting if they both attended that meeting. In one embodiment, when two data objects are connected by an event, they are also connected by relationships, in which each object has a specific relationship to the event, such as, for example, an Appears In relationship. As an example of a matching properties connection, two Person data objects representing a brother and a sister, may both have an Address property that indicates where they live. If the brother and the sister live in the same home, then their Address properties likely contain similar, if not identical information. In one embodiment, a link  114  between two data objects may be established based on similar or matching properties of the data objects. The above are just some examples of the types of connections that may be represented by a link  114  and other types of connections may be represented. Thus, it should be understood that embodiments of the invention are not limited to any particular types of connections between data objects  110 . For example, a document might contain two different tagged entities. A link  114  between two data objects  110  may represent a connection between these two entities through their co-occurrence within the same document. 
     A data object  110  can have multiple links  114  with another data object  110  to form a link set  116 . For example, two Person data objects representing a husband and a wife could be linked through a Spouse Of relationship, a matching property (Address), and an event (Wedding). 
     Revisioning Database 
     Investigative analysis computer system  100  employs a revisioning database system for tracking changes made to investigative analysis data stored in revisioning database  106 . In some embodiments, the revisioning database system is implemented by analysis servers  104  as an application on top of a conventional database management system (not shown). For example, the database management system may be a relational database management system such as those commercially available from the Oracle Corporation of Redwood Shores, Calif. and the Microsoft Corporation of Redmond, Wash. 
     In one aspect, the revisioning database system differs from other types of database systems in that the revisioning database system is capable of answering a query about the state of investigative analysis data stored in revisioning database  106  at a point in time in the past as opposed to only being able to answer a query about the current state of the investigative analysis data. With the revisioning database system, investigative analysts can determine when a particular piece of data was added or edited in revisioning database  106 . Thus, the revisioning database system, as a result of its capability to track changes to investigative analysis data stored in the revisioning database  106 , enables investigative analysts to determine what was known when. 
     In one embodiment, revisioning database system is capable of tracking all changes made to investigative analysis data over a period of time. To do so, the revisioning database system creates a new database change record in revisioning database  106  for every creation, edit, or deletion of a data object  110 , property  112 , or link  114 , thereby creating a historical record of all changes. To track the ordering of the changes, the revisioning database system employs an always increasing logical clock that models all of the changes as a linear sequence of database events. The logical clock provides a total ordering for all changes. In addition, the logical clock provides atomicity for changes as multiple changes can occur at the same point in the linear sequence of database events represented by the logical clock (and hence be associated with the same logical clock value). 
       FIG. 4  illustrates tracking changes made to revisioning database  106  by an example and according to an embodiment of the invention. As shown in  FIG. 4 , each record  118 ,  120 ,  122 ,  124 , and  126  in table  128  represents a creation, edit, or deletion of a data object  110  or a creation, edit, or deletion of a property  122  of a data object  110 . The fields of each change record include a ‘obj_comp_id’ field identifying the data object  110  or property  112  that was created, edited, or deleted by the change, an ‘obj_id’ field identifying the data object  110  that was created, edited, or deleted by the change, a ‘logical_clk’ field that identifies the order of the change in a total ordering of all changes made to revisioning database  106 , a ‘deleted’ field indicating whether the change was a deletion of a data object  110  or a property  112 , and a ‘&lt;values’&gt; field indicating, for changes that create or edit a value, the value that resulted from the change or, for changes that delete a value, the value that was deleted. 
     For example, referring to  FIG. 4 , at logical clock event  1 , a data object  110  of type “Person” was created. Also at logical clock event  1 , a “Name” property of the data object was created and given the value “John Smith”. Later, at logical clock event  2 , a “Phone #” property of the object was created and given the value “415-222-1234”. At logical clock event  3 , the “Name” property of the object that was created at logical clock event  2  was edited with the value “Jonathan Smith”. At logical clock event  4 , the “Phone #” property that was created at logical clock event  3  was deleted. As a result of the changes at logical clock events  1 ,  2 , and  3 , the state of the object at logical clock event  4  is an object  110  of type “Person” with the property “Name” having a value “Jonathan Smith”. 
     By preserving all changes made to an object  110  in the form of change records, the revisioning database system is able to provide the state of an object  110  at a point in time in the past. For example, referring again to  FIG. 4 , it can be seen from change records  118 ,  120 , and  122  that the state of the object with obj_id=10 at logical clock event  2  was an object  110  of type “Person” with a property “Name” having a value “John Smith” and a property “Phone #” having a value “415-222-1234”. 
     Note that while table  128  contains change records for only one data object with an identifier of 10, table  128  could contain change records for multiple data objects. 
       FIG. 4  illustrates but one example scheme that the revisioning database system could employ to track changes to revisioning database  106 . However, the invention should not be construed as being limited to only the one example scheme or be construed as requiring all details of the one example scheme. For example, instead of storing change records for all data objects in a single table as depicted in  FIG. 4 , the change records might be stored across multiple tables. Further, the change records may contain other fields that are not depicted in  FIG. 4 . For example, each change record may have an additional version field that serves as a single primary key for the change record as opposed to using a combination of the ‘obj_comp_id’ and the ‘logical_clk’ fields as the primary key. 
     Process for Incremental Data Replication 
       FIG. 5  is a process for incrementally replication investigative data from an exporting system to an importing system. The process may be performed by one or more computing devices. For example, the process may be performed by one or more analysis servers  104  of exporting system  20 . 
     At step  502 , the exporting system receives a command from a user to execute an incremental replication job. The command may be provided by the user through a graphical user interface such as a graphical user interface presented at an analyst client  102 , for example. Alternatively, the command may be provided by the user through a command line interface at an analyst client  102  or at an analysis server  104 , as some examples. 
     In some embodiments, the command includes a specification of an identifier of the importing system that the exporting system is to export investigative analysis data to. The specification can be any identifier that the exporting system can use to identity the importing system. For example, the identifier can be a network address, domain name, or assigned identifier of the importing system. 
     In some embodiments, the command includes a specification of a replication chunk size. The specification can be a number that represents the maximum number of replication chunks to divide the incremental data replication job into. Alternatively, the specification can be a number that represents the maximum number of data objects to include change data for in a replication chunk of the incremental data replication job. As yet another possible alternative, the specification can be a maximum number of replication chunks to divide the incremental replication job into. As used herein, the term “change data” refers broadly to data representing a change to a data object. Change data can include the data of the change itself (e.g., the values that were created, edited, or deleted) and any associated metadata. Such metadata may include information representing the version of the change and may include, for example, logical clock values and vector clock information for determining causality of the change with respect to other changes made to the data object at the importing system. 
     The command may also include a specification of an exportation schedule. The specification may include a start time when the exporting system is to begin execution of the incremental data replication job. For example, the user may specify a start time that is in the middle of the night or other time when the exporting system or the importing system is not being heavily used. As another example, the start time may correspond to when network connectivity between the exporting system and the importing system is expected to be available. For example, if the network connectivity is satellite-based, then the start time may correspond to when the satellite is in range of the exporting system or the importing system. 
     At step  504 , the exporting system creates an incremental data replication plan for the incremental data replication job. The plan may be stored persistently such as in revisioning database  106  or other non-volatile data storage medium so that it is not lost in the event of power failure or other failure of the exporting system. By persistently storing the plan, the exporting system can resume the incremental data replication job from the stored plan after a failure. For example, if some but not all of the chunks were successfully exported or all chunks were successfully exported but not all chunks were successfully received by the importing system, the missing or failed chunks can be exported individually. Accordingly, in some embodiments, a received command to execute an incremental replication job specifies one or more particular chunks to export. The exporting system then exports the specified chunks based on the previously stored plan. 
     Incremental Data Replication Plan 
       FIG. 6  is a block diagram illustrating an incremental data replication plan according to an embodiment of the present invention. As mentioned, data representing an incremental replication plan may be persistently stored in non-volatile data storage media, for example, as one or more rows in one or more database tables. During job execution, data representing an incremental replication plan may also be stored in volatile computer memory, for example, as one or more data structures or software objects. 
     As shown, plan  130 , representing an incremental data replication job, includes a unique plan identifier  132 , a snapshot time  134 , the number  136  of replication chunks the job is divided into, an identifier  136  of the importing system, and one or more specifications  138 A-N of the replication chunk, one for each of the number  136  of replication chunks. 
     Plan identifier  132  may be any identifier that the exporting system and importing system can use to refer to or identify the corresponding replication job represented by the plan  130 . 
     Snapshot time  134  is a current logical clock value from exporting system&#39;s logical clock used by the exporting system to provide a total ordering of changes to data objects made by the exporting system. Snapshot time  134  may be obtained from the logical clock in response to receiving the command to execute the incremental data replication job for which plan  130  is created. By recording snapshot time  134  in plan  130 , ongoing changes can be made to investigative analysis data by the exporting system without affecting which changes will be included in the incremental data replication job. 
     As mentioned, peer system identifier  138  is an identifier of the importing system that exporting system will be exporting changes to in the incremental replication job represented by plan  130 . 
     As mentioned, the incremental data replication job is divided into the number  136  of replication chunks based on the replication chunk size information specified in the command to execute the job. Plan  130  also includes a replication chunk specification  140  for each of the number  136  of replication chunks. Each replication chunk specification  140  includes a chunk identifier  142 , a minimum data object identifier  144 , a maximum data object identifier  146 , and a complete flag  148 . 
     In some embodiments, the replication chunks of the job represented by plan  130  are ordered. The chunk identifier  142  indicates the order of the corresponding replication chunk. For example, the chunk identifier  142  can be an ordinal number such as 1, 2, 3, etc. 
     Minimum data object identifier  144  specifies the lowest valued identifier of all data objects for which change data will be included in the corresponding replication chunk. Maximum data object identifier  146  specifies the highest value identifier of all data objects for which change data will be included in the corresponding replication chunk. 
     Complete flag  148  is used by the exporting system to track if the corresponding replication chunk has been exported. Complete flag  148  is initially set to zero, false, or other similar value. After the corresponding replication chunk has been successfully exported, which may or may not be after the importing system has imported or even received the replication chunk, the exporting system sets the complete flag  148  to one, true, or other similar value. 
     Return to the process of  FIG. 5 , at step  506 , the exporting system executes the incremental replication job according to the plan created at step  504 . The exporting system may start execution at the start time specified in the command received at step  502 . If no start time is specified in the command, then the exporting system may start execution of the job immediately or in response to a future triggering of the job. For example, after the plan is created at step  504 , a user may interact with a graphical user interface or a command line interface to command the exporting system to start execution of the job. As an alternative to a user interacting with a user interface, a script or cron job may be used to trigger execution of the job at a point in the future after the plan is created at step  504 . 
     Incremental Replication Job Execution 
       FIG. 7  is a flow diagram illustrating a process performed by the exporting system for executing an incremental replication job from an incremental replication plan according to an embodiment of the invention. The process illustrated in  FIG. 6  may be performed as part of step  506  of the process illustrated in  FIG. 5  for performing an incremental data replication. The process illustrated in  FIG. 6  may be performed by one or more computing devices. For example, the process illustrated in  FIG. 6  may be performed by one or more analysis servers  104  of the exporting system. 
     As shown steps  702 ,  704 , and  706  are performed for each replication chunk specification in the incremental data replication plan. For example, step  702 ,  704 , and  706  may be performed by the exporting system for each replication chunk specification  140  in a plan  130 . Further, steps  702 ,  704 , and  706  are performed for each replication chunk specification in the order of their assigned replication chunk identifiers. For example, steps  702 ,  704 , and  706  may be performed by the exporting system for each replication chunk specification  140  in a plan  130  in order of the respective chunk identifiers  142 . 
     At step  702 , change data for the data objects included in the current replication chunk are collected from the revisioning database at the exporting system. Generally, this involves the exporting system reading change records from the revisioning database corresponding to changes to data objects included in the replication chunk that are associated with logical clock values that are less than or equal to the snapshot time recorded in the incremental data replication plan. For example, for the current replication chunk specification  140 , exporting system may read all records from table  128  where obj_id is greater than or equal to minimum data object identifier  144  and obj_id is less than or equal to maximum data object identifier  146  and where logical_clk is less than or equal to snapshot time  134 . This filter may be further refined to exclude change records that exporting system “knows” the importing system has already received. Such knowledge by exporting system can be based on previous replication exchanges between the exporting system and the importing system. For example, the importing system may provide version vector information and/or acknowledgement vector information to the exporting system in such replication exchanges that indicate the version of investigative analysis data the importing system has in its revisioning database at the time of the exchanges. Note the knowledge the exporting system has about the importing system may be out-of-date at the time the exporting system executes the incremental replication job. This may be because the importing system has received and imported change data from other investigative analysis systems since the last replication exchange between the exporting system and the importing system. In this case, the exporting system may send change data for data object changes in the incremental data replication that the importing system has previously received and considered. In some embodiments, the importing system simply discards the duplicate change data. 
     In some embodiments, change data collected for inclusion in a replication chunk includes change data for revisioning database entities that depend on multiple data objects depend on that multiple data objects depend on. For example, a link  114  that is changed may depend on two data objects  110 . As another example, multiple data objects  110  may depend on a data source. In this context, a data source represents a file, data base, a document, or other source of information that backs one or more data objects  110  and provides a lineage for the source of information that forms one or more data objects  110 . In some embodiments, where change data such as for a link depends on change data for one or more data objects to be exported as part of the replication job, the dependent change data is included in same chunk as the chunk that includes the last of the one or more data objects that are depended on. In other embodiments, the dependent change data is included in the last chunk of the replication job. In some embodiments, where change data such as for a data source is depended on by one or more data objects to be exported as part of the replication job, the depended upon change data is included in the same chunk as the chunk that includes the first of the one or more dependent data objects. In other embodiments, the depended upon change data is included in the first chunk of the replication job. 
     At step  704 , the change data collected at step  702  is written to a local file. For example, the file may be one stored on analysis servers  104 . As well as the change data, the file may include other incremental replication data such as replication metadata. The replication metadata may include version vector information and access control information. The contents of the file may also be encrypted for security. 
     At step  706 , the exporting system sends the file written to in step  704  to the importing system. The exporting system may send the file to the importing system using any suitable reliable network transport protocol such as the Transmission Control Protocol/Internet Protocol (TCP/IP). 
     As an alternative to writing the collected change data to a local file (step  704 ) and then sending the file (step  706 ) to the importing system, the exporting system can stream the change data and associated replication metadata to the importing system over the network as it is collected without first writing the change data and associated replication metadata to a local file. 
     In some embodiments, the exporting system includes a “local ack” vector clock as part of the replication metadata included in the last (highest ordered) replication chunk exported as part of the job. In the parlance of causality in distributed systems, the local ack vector clock “happens after” all data object changes in all replication chunks of the job. The lock ack vector clock indicates to the importing system that once the importing system has imported all replication chunks in their specified order the importing system has seen all data object changes that the exporting system knows about up to a given vector clock indicating by the lock ack vector clock. 
     Process for Importing a Replication Chunk 
       FIG. 8  is a flow diagram illustrating a process performed by the importing system for importing a replication chunk according to an embodiment of the invention. The process illustrated in  FIG. 8  may be performed by one or more computing devices. For example, the process illustrated in  FIG. 8  may be performed by one or more analysis servers  104  of the importing system. 
     At step  802 , the importing system receives a replication chunk of an incremental replication job from the exporting system. For example, the importing system may receive the chunk from the exporting system over a data network or from portable physical media (e.g., a USB drive or a flash drive) physically transported from the exporting system to the importing system. Among other information including change data, the chunk contains an identifier of the incremental replication job that the chunk is a part of. In addition, the chunk contains a chunk identifier. For example, the received chunk may contain plan identifier  132  from the corresponding replication plan  130  created by the exporting system and chunk identifier  142  from the corresponding replication chunk specification  140 . The identifier of the job may be used by the importing system to determine the replication job to which the received chunk belongs to. The identifier of the chunk may be used by the importing system to determine the order of the received chunk in the sequence of chunks the exporting system has broken the job into. 
     In some embodiments, the exporting system assigns the first chunk in the sequence of chunks of a replication job an initial ordinal such as the number 1. Each chunk thereafter is assigned the next ordinal relative to the ordinal assigned to previous chunk. For example, the second chunk in the sequence can be assigned the number 2, the third chunk in the sequence assigned the number 3, and so on. An alternative ordinal sequence could be ‘A’, ‘B’, ‘C’, ‘D’ . . . , for example. 
     In some embodiments, the exporting system includes “final chunk” information in the last chunk in the sequence of chunks to indicate to the importing system that the chunk is the last chunk of the job. For example, the final chunk information can be a flag or other data that indicates to the importing system that the chunk having the final chunk information is the last chunk of the job. 
     At step  804 , the importing system imports the replication chunk received at step  802  into the revisioning database at the importing system. This importing includes incorporating the change data contained in the replication chunk into the revisioning database. Importing the change data may include performing causality detection between the change data in the chunk and change data already stored in the revisioning database. In particular, change data in the chunk and existing change data in the revisioning database at the importing system may be versioned with version vectors (also known as “vector clocks”) suitable for detecting causality relationships between the change data in the replication chunk and corresponding change data in the revisioning database. In particular, the version vectors may be used to determine whether change data in the chunk “happened after”, “happened before”, or neither “happened after” nor “happened before” corresponding change data in the revisioning database. Change data in the chunk is incorporated into the revisioning database at the importing system if the change data “happened after” the corresponding change data in the revisioning database. Change data in the chunk is not incorporated into the revisioning database at the importing system if the change data “happened before” the corresponding change data in the revisioning database. If change data in the chunk neither “happened after” nor “happened before” the corresponding change data in the revisioning database, then a conflict exists between the change data in the chunk and the corresponding change in the revisioning database. In some scenarios, the conflict is automatically resolved by the importing system. In other scenarios, a user must manually resolve the conflict. In both cases, the resolution of the conflict is incorporated into the revisioning database after the conflict is resolved. More information on the “happened before” and the “happened after” relations between events in a distributed computing system can be found in a paper by Leslie Lamport entitled “Time, Clocks and the Ordering of Events in a Distributed System”, Communications of the ACM, 21(7), pp. 558-565 (1978), the entire contents of which is hereby incorporated by reference as if fully set forth herein. 
     Implementation Mechanism 
     According to one embodiment, the techniques described herein are implemented by one or more special-purpose computing devices. The special-purpose computing devices may be hard-wired to perform the techniques, or may include digital electronic devices such as one or more application-specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs) that are persistently programmed to perform the techniques, or may include one or more general purpose hardware processors programmed to perform the techniques pursuant to program instructions in firmware, memory, other storage, or a combination. Such special-purpose computing devices may also combine custom hard-wired logic, ASICs, or FPGAs with custom programming to accomplish the techniques. The special-purpose computing devices may be desktop computer systems, portable computer systems, handheld devices, networking devices or any other device that incorporates hard-wired and/or program logic to implement the techniques. 
     For example,  FIG. 9  illustrates a computer system upon which one or more embodiments may be implemented. Computer system  900  includes a bus  902  or other communication mechanism for communicating information, and a hardware processor  904  coupled with bus  902  for processing information. Hardware processor  904  may be, for example, a general purpose microprocessor. 
     Computer system  900  also includes a main memory  906 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  902  for storing information and instructions to be executed by processor  904 . Main memory  906  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  904 . Such instructions, when stored in non-transitory storage media accessible to processor  904 , render computer system  900  into a special-purpose machine that is customized to perform the operations specified in the instructions. 
     Computer system  900  further includes a read only memory (ROM)  908  or other static storage device coupled to bus  902  for storing static information and instructions for processor  904 . A storage device  910 , such as a magnetic disk, optical disk, or solid-state drive is provided and coupled to bus  902  for storing information and instructions. 
     Computer system  900  may be coupled via bus  902  to a display  912 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device  914 , including alphanumeric and other keys, is coupled to bus  902  for communicating information and command selections to processor  904 . Another type of user input device is cursor control  916 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  904  and for controlling cursor movement on display  912 . This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. 
     Computer system  900  may implement the techniques described herein using customized hard-wired logic, one or more ASICs or FPGAs, firmware and/or program logic which in combination with the computer system causes or programs computer system  900  to be a special-purpose machine. According to one embodiment, the techniques herein are performed by computer system  900  in response to processor  904  executing one or more sequences of one or more instructions contained in main memory  906 . Such instructions may be read into main memory  906  from another storage medium, such as storage device  910 . Execution of the sequences of instructions contained in main memory  906  causes processor  904  to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. 
     The term “storage media” as used herein refers to any non-transitory media that store data and/or instructions that cause a machine to operate in a specific fashion. Such storage media may comprise non-volatile media and/or volatile media. Non-volatile media includes, for example, optical disks, magnetic disks, or solid-state drives, such as storage device  910 . Volatile media includes dynamic memory, such as main memory  906 . Common forms of storage media include, for example, a floppy disk, a flexible disk, hard disk, solid-state drive, magnetic tape, or any other magnetic data storage medium, a CD-ROM, any other optical data storage medium, any physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, NVRAM, any other memory chip or cartridge. 
     Storage media is distinct from but may be used in conjunction with transmission media. Transmission media participates in transferring information between storage media. For example, transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  902 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications. 
     Various forms of media may be involved in carrying one or more sequences of one or more instructions to processor  904  for execution. For example, the instructions may initially be carried on a magnetic disk or solid-state drive of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system  900  can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus  902 . Bus  902  carries the data to main memory  906 , from which processor  904  retrieves and executes the instructions. The instructions received by main memory  906  may optionally be stored on storage device  910  either before or after execution by processor  904 . 
     Computer system  900  also includes a communication interface  918  coupled to bus  902 . Communication interface  918  provides a two-way data communication coupling to a network link  920  that is connected to a local network  922 . For example, communication interface  918  may be an integrated services digital network (ISDN) card, cable modem, satellite modem, or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  918  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface  918  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  920  typically provides data communication through one or more networks to other data devices. For example, network link  920  may provide a connection through local network  922  to a host computer  924  or to data equipment operated by an Internet Service Provider (ISP)  926 . ISP  926  in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet”  928 . Local network  922  and Internet  928  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  920  and through communication interface  918 , which carry the digital data to and from computer system  900 , are example forms of transmission media. 
     Computer system  900  can send messages and receive data, including program code, through the network(s), network link  920  and communication interface  918 . In the Internet example, a server  930  might transmit a requested code for an application program through Internet  928 , ISP  926 , local network  922  and communication interface  918 . 
     The received code may be executed by processor  904  as it is received, and/or stored in storage device  910 , or other non-volatile storage for later execution. 
     EXTENSIONS AND ALTERNATIVES 
     In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. The sole and exclusive indicator of the scope of the invention, and what is intended by the applicants to be the scope of the invention, is the literal and equivalent scope of the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction.