Abstract:
The present invention concerns a method for replicating one or more data items of a primary database system ( 10 ), the method comprising the following steps performed by the primary database system ( 10 );
       a. providing replication data ( 102, 104 ) to at least one replication database system ( 20 ), the replication data ( 102, 104 ) being usable for replicating the one or more data items of the primary database system ( 10 ) by the at least one replication database system ( 20 ); and   b. adding lock data ( 103 ) to the replication data ( 102, 104 ) sent in step a., wherein the lock data ( 103 ) relates to at least one lock on the one or more data items.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a primary database system, a replication database system and a method for replicating data items of a primary database system. 
       THE PRIOR ART 
       [0002]    Database systems are commonly used for storing, modifying and querying large amounts of data used within modern computing environments. An unexpected crash or inconsistency in such database systems, e.g. due to hardware or network failures, can result in enormous system down-time of the computing environment, since the data on which the computing environment operates may be lost or corrupted. The reliability and availability of database systems is thus of eminent importance. To this end, in addition to the processing of requests for storing, modifying and querying data (transaction processing), a database system may replicate its data at one or more replication database systems (replication processing). 
         [0003]    In case of failure of the (primary) database system, the lost data may then be restored from the replication database system(s), i.e. the latter serve as stand-by backup storages. In this failure-recovery use case, transaction processing is typically only performed by the primary database system and the replication database systems only participate in the replication processing. 
         [0004]    However, replication database systems may also be employed in other scenarios, e.g. for the mobile use of data. Here, a replication database system may be located on a mobile device such as a laptop or a smartphone and synchronize in regular intervals with a primary database system on a central server. In this use case, transaction processing may be performed directly on the replication database system in order to increase performance. 
         [0005]    Yet another important use case is using replication database systems for load balancing, i.e. for performing transaction processing by the primary and replication database systems simultaneously to also increase the performance of the transaction processing. 
         [0006]    However, if data of a replication database system is queried during transaction processing while a replication processing is in progress, the query may obtain inconsistent results. This is since the query processing may retrieve intermediate data that may change during the course of the replication processing (a phenomenon known as ‘dirty reads’). In order to overcome this problem, it is known to allow query processing and replication processing only in different time slots, is thereby guaranteeing that only consistent data can be queried. However, this approach comes at the cost of reduced performance, since query processing has to be postponed until any replication processing is finished, which is especially disadvantageous in the context of load balancing. Alternatively, simultaneous query processing and replication processing may be allowed for better performance, however, at the cost that consistency cannot be guaranteed. 
         [0007]    Furthermore, it may be desirable in certain scenarios to not only simultaneously query the data of the primary and replication database systems, but to also allow simultaneous modifications on the different data copies stored in the primary and replication database systems. Here, an additional problem is that the different data copies may transition to an unsynchronized, inconsistent state. To this end, it is known to employ complex mechanisms for detecting and resolving inconsistent data within the primary and replication databases. Alternatively, synchronous replication techniques may be used for preventing inconsistent data modifications in the first place (e.g. using the known two-phase commit protocol). However, these approaches also come at the cost of reduced performance due to the overhead of conflict resolution or synchronous replication. 
         [0008]    It is therefore the technical problem underlying the present invention to provide a database replication technique that allows an efficient and consistent transaction processing, thereby at least partly overcoming the above explained disadvantages of the prior art. 
       SUMMARY OF THE INVENTION 
       [0009]    This problem is according to one aspect of the invention solved by a method for replicating one or more data items of a primary database system in accordance with claim  1 , wherein the method comprises the following steps performed by the primary database system:
       a. providing replication data to at least one replication database system, the replication data being usable for replicating the one or more data items of the primary database system by the at least one replication database system; and   b. adding lock data to the replication data sent in step a., wherein the lock data relates to at least one lock on the one or more data items.       
 
         [0012]    Accordingly, replication data is provided in step a. by a primary database system to be used by at least one replication database system to replicate one or more data items of the primary database system. As a result, the at least one replication database system is enabled to obtain a local copy of the one or more data items of the primary database system. To this end, the primary database system may store the replication data and the at least one replication database system may request the stored replication data. Furthermore, the primary database system adds lock data to the replication data sent in step a. to the at least one replication database system, wherein the lock data relates to at least one lock on the one or more data items. 
         [0013]    Such locks may be created by the primary database system when creating, modifying, deleting and/or querying one or more data items in response to a transaction received by the primary database system and generally serve for restricting the access of other transactions to the one or more data items, when being processed by the primary database system. By supplementing the replication data sent to the at least one replication database system with locking data relating to these locks, the at least one replication database system is enabled to also process one or more transactions in accordance with the received lock data and preferably simultaneously to any replication processing and/or transaction processing performed by the primary database system. Consequently, it is ensured that transactions are processed by the at least one replication database system in an efficient yet transaction-consistent manner, as will be explained further below. 
         [0014]    In synchronous replication techniques known from the prior art, the primary database system has to wait until the sent replication data is confirmed by the at least one replication database system. In other words, the primary database system cannot process any further transactions until the replication data has been processed by the at least one replication database system. The present invention departs from this concept in that an asynchronous replication technique is provided, i.e. the primary database system can perform its transaction processing in parallel and independently from the replication processing. This is because the consistency is ensured by the at least one replication database system based on the received lock data related to the one or more data items to be replicated. 
         [0015]    Since lock data restricts the access of a transaction to one or more data items, the efficiency of the at least one replication database system can be further improved by minimizing the amount of locking data transferred from the primary database system to the at least one replication database system. In other words, the amount of transferred locking data should be reduced to the necessary minimum. 
         [0016]    Accordingly, in one aspect of the invention, the lock data may be added to the replication data provided in step a., only if the at least one lock is requested by an update transaction received by the primary database system. An update transaction is a transaction that creates, modifies or deletes one or more data items, as opposed to a read-only transaction that only queries one or more data items (without changing them). While both types of transactions may acquire locks on one or more data items, locks relating to read-only transactions are preferably not added to the replication data. Still, the processing of read-only transactions by the at least one replication database system in a transaction-consistent manner is ensured, since the at least one replication database system has the locks relating to update transactions available. 
         [0017]    In another aspect, the lock data may be added to the replication data provided in step a., only if the related at least one lock is an exclusive lock. Accordingly, the present invention supports different kinds of locks, e.g. exclusive locks, shared locks, etc. known from the prior art. However, locking data relating to shared locks is preferably not added to the replication data, since such shared locks cannot cause conflicts with the processing of the at least one replication database. Consequently, the amount of lock data transferred is further minimized, thereby increasing on the one hand the efficiency of the at least one replication database system and on the other hand the efficiency of the primary database system, since it has to process only the necessary minimum of replication data. 
         [0018]    According to yet another aspect of the invention, the lock data may be added to the replication data provided in step a., only if the one or more data items related to the at least one lock have been modified during a processing of an update transaction by the primary database system. Since locks relating to one or more data items that are not modified during the processing of a transaction by the primary database system cannot cause conflicts at the at least one replication database system, such locks are omitted from the transferred replication data in order to further decrease the amount of transferred lock data. 
         [0019]    In a further aspect, step b. may be only performed, if at least one replication database system adapted for simultaneous queries is registered at the primary database system, as will be further explained in the detailed description below. 
         [0020]    The present invention also relates to a method for replicating one or more data items of a primary database system by a replication database system in accordance with claim  5 , wherein the method comprises the following steps performed by the replication database system:
       a. receiving replication data from the primary database system and replicating the one or more data items of the primary database system in accordance with the replication data;   b. wherein the replication data received in step a. comprises lock data relating to at least one lock on the one or more data items.       
 
         [0023]    Accordingly, as already explained above, the replication database may use the received lock data for processing one or more transactions, preferably read-only transactions, so that the one or more transactions are processed in a transaction-consistent manner, preferably simultaneously to any replication processing and/or independently from any transaction processing of the primary database system. 
         [0024]    In another aspect, the lock data may relate to at least one exclusive lock on the one or more data items, at least one read-only transaction may request a second lock on the one or more data items and the method may comprise the step of postponing a processing of the at least one read-only transaction until the at least one exclusive lock is released by the replication database system. Accordingly, in case a read-only transaction is to be processed by the replication database system that serves for reading data items already exclusively locked (e.g. while a replication transaction is in progress), the read-only transaction is forced to wait until the exclusive lock is released (e.g. at the end of the replication transaction). In other words, replication processing takes precedence over transaction processing by the replication database system for the sake of best overall performance. 
         [0025]    According to yet another aspect of the invention, the replication database system may request a third lock for replication purposes on the one or more data items, wherein the third lock may be already held by the at least one read-only transaction and wherein the method may comprise the further step of forcing the at least one read-only transaction to release the third lock, e.g. by a rollback of the at least one read-only transaction. Accordingly, situations may occur where the replication database system intends to acquire a lock on certain data items for replicating them, while the data items are already locked by a read-only transaction currently in progress. In particular if the read-only transaction itself intends to acquire a lock that is already held by the replication database system, each of the two entities would have to wait for the other to release the corresponding lock (a phenomenon referred to as ‘cyclic waiting’ or ‘deadlock’). In order to resolve this situation, the replication database system may force the read-only transaction to release the lock by a rollback, so that the read-only transaction may be restarted, possibly at a later stage. 
         [0026]    In summary, the replication database system can be effectively used for load-balancing, i.e. for serving read-only transactions simultaneously to the primary database system. 
         [0027]    The present invention further concerns a primary database system, comprising a primary database server adapted for sending replication data to at least one replication database system to be used for replicating one or more data items of the primary database system by the at least one replication database system, wherein the replication data comprises lock data relating to at least one lock on the one or more data items. 
         [0028]    Furthermore, a replication database system is provided, comprising a replication database server adapted for receiving replication data from a primary database system and for replicating one or more data items of the primary database system in accordance with the replication data, wherein the replication data comprises lock data relating to at least one lock on the one or more data items. 
         [0029]    Further advantageous modifications of embodiments of the primary database system and the replication database system of the invention are defined in further dependent claims. 
         [0030]    The present invention is further directed to a system comprising a primary database system and at least one replication database system as described above. Lastly, the present invention provides a computer program comprising instructions adapted for implementing any of the above methods. 
     
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
         [0031]    In the following detailed description, presently preferred embodiments of the invention are further described with reference to the following figures: 
           [0032]      FIG. 1 : A schematic overview of a primary database system and a replication database system according to an embodiment of the invention; 
           [0033]      FIG. 2 : An example of redo log data created from lock table entries according to an embodiment of the invention; 
           [0034]      FIG. 3 : A flowchart of adding lock data when update data is to be created on the redo log according to an embodiment of the invention; 
           [0035]      FIG. 4 : A flowchart of the creation of redo log data when locks are deleted according to an embodiment of the invention; 
           [0036]      FIG. 5 : An exemplary view of data stored in a redo log according to an embodiment of the invention; 
           [0037]      FIG. 6 : A schematic overview of the use of lock data by a replication database system according to an embodiment of the invention; and 
           [0038]      FIG. 7 : A schematic overview of the creation of lock data for a replication database system according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    Modern database systems generally serve for processing transactions initiated by users and/or applications that operate on data items stored in at least one database of the database system. A transaction may be a read-only transaction that serves for querying certain data items from the database, or an update transaction that serves for creating, modifying and/or deleting data items of the database. A transaction may comprise one or more individual steps/operations. 
         [0040]    Modern database systems are typically designed to ensure the known database properties ACID (‘atomicity’, ‘consistency’, ‘isolation’ and ‘durability’; cf. http://en.wikipedia.org/wiki/ACID). Atomicity refers to the ability of a database system to guarantee that either all of the steps/operations of a transaction are performed or none of them are. The consistency property ensures that the database remains in a consistent state before the start of a transaction and after the transaction is over (whether successful or not). Isolation refers to the requirement that other steps/operations cannot access or see the data in an intermediate state during a given transaction. Lastly, durability refers to the guarantee that once a user who initiated a transaction has been notified of success of the transaction, the transaction will persist, i.e. it will not be undone. 
         [0041]    With respect to the above-presented isolation property, it is known from the prior art to process transactions at certain isolation levels cf. http://en.wikipedia.org/wiki/Isolation_(database_systems)). For example, the isolation level ‘serializable’ specifies that all transactions occur in a completely isolated fashion, i.e. as if all transactions in the database system had executed serially, one after the other. The isolation level ‘read committed’ specifies that data retrieved by a query is not prevented from modification by other transactions (probably modifying the retrieved data). The isolation level ‘read uncommitted’ specifies that one transaction may see uncommitted changes made by some other transaction. 
         [0042]    In order to support different isolation levels such as the above examples, database systems may offer the ability for transactions (and their individual steps/operations, respectively) and/or their initiating users/applications, respectively, to acquire locks on one or more data items. A lock generally restricts the access of other transactions to the corresponding data items, e.g. in the form of an exclusive lock (i.e. no other transaction is allowed to access the data items) or a shared lock (i.e. other transactions may only read, but not modify, the data items). 
         [0043]    In the context of database replication, it is oftentimes desired to process transactions not only by the primary database system that stores the data items, but also by at least one replication database system that stores replicated data items. Processing transactions, preferably queries (i.e. read-only transactions), simultaneous to replication processing and at any isolation level is the prerequisite for using replication databases for load balancing purposes, as already presented further above. However, when data is replicated that relates to transactions that are still in progress, the data may very well change later due to updates or due to a rollback of the respective transaction. This is the case, whenever the first modification of the transaction has already been processed, but the transaction itself has not been finished (committed). Even if the transaction comprises only a single modification step, the transaction comprises two steps; the first step is the modification and the second step is the transaction end (commit). Hence, queries on this replicated data might not get a transaction-consistent view to the data, which results in incorrect results obtained by the queries (thereby violating the above-presented consistency property). 
         [0044]    In order to ensure transaction-consistency in the context of database replication, the known database replication methods of the prior art presented further above either do not support simultaneous queries on the primary and replication database systems at all, or replication methods (e.g. based on redo log data in a traditional way containing modification data; see further below) only cannot offer other isolation levels for simultaneous queries than ‘read uncommitted’. In particular, the usage of enhanced locking techniques on replication databases, such as hierarchical locking, is not possible without locking data from the originating database. 
         [0045]    It is therefore an objective of the present invention to provide an improved way for enabling the processing of transactions, preferably queries (i.e. read-only transactions), simultaneous to replication processing and at any isolation level. 
         [0046]    This is in one aspect achieved by storing additional information in a database system, such as the primary database system  10  shown in  FIG. 1 . As can be seen, the primary database system  10  comprises at least one primary database  101  and at least one redo log  102 . 
         [0047]    To allow the processing of transactions (such as the update transactions  106  and/or the read-only transactions  107  directed to the primary database system  10  in  FIG. 1 ) by the primary database system  10  at any isolation level, locks for data items may be maintained within a lock table  103  of the primary database system  10 . The lock table  103  may comprise lock entries for update transactions  106  intended to modify one or more data items of the primary database  101 , as well as lock entries for read-only transactions  107  intended to query one or more data items of the primary database  101 . Both types of transactions  106 ,  107  may be initiated by users and/or applications and the respective lock table entries may comprise a reference to the respective user/application and/or information on the respective lock. 
         [0048]    However, if one of the steps/operations of an update transaction  106  fails for whatever reason, it may be necessary to roll-back or undo the respective update transaction, i.e. all steps/operations that have already been processed. As already explained further above, the redo log  102  of the primary database system  10  may to this end store information on the transactions  106  processed by the primary database system  10  which may then be used for a roll-back by the replication database server. Furthermore, a rollback on the primary database system may use another log, such as a journal log. If a read-only transaction  107  fails, it is necessary to release all locks obtained during this transaction. 
         [0049]    The basic principle to enable transaction-consistent queries on a replication database  201  provided by the present invention is to add the entries of the lock table  103  needed for this purpose to the redo log  104 ,  102  generated by the primary database server  100 . 
         [0050]    The redo log  102  (with the added data from the lock table  103 ) may then be transferred to the replication database server  200  and applied there. To this end, a replication database server  200  of the replication database system  20  (cf.  FIG. 1 ) may use the lock table records of the redo log  102  to add the locks to a lock table  203  of the replication database system  20  similar as it was done by the originating update transaction  106  on the primary database system  10 . Transactions  207  to be processed by the replication database system  20  may then use locks dependent on the desired isolation level, so that there is no difference in the locks to be acquired on the primary database system  10  and the replication database system  20 . 
         [0051]    As can be seen in  FIG. 1 , the redo log data is transmitted via a link  105  to a replication database system  20  to be used as input data  204  for the replication database server  200  to update the replication database  201  and to maintain the lock table  203  of the replication database system  20 . Using the same lock table  203  for query processing  207  allows resolving data access conflicts between the read-only transactions  207  and the replicated update transactions  106  of the primary database system  10 . 
         [0052]    According to one aspect of the present invention, when creating a record for the redo log  102  to store the modified data of an update transaction step in the primary database  101  for replication, the entries of the lock table  103  that relate to the respective update transaction  106  (e.g. to its initiating user) may be scanned for exclusive locks. A new entry in the redo log  102  may be created to store data only about these locks held by the update transaction  106  prior to creating the redo log record with the modified data of the update transaction step. 
         [0053]    In another aspect, shared locks are not created on the redo log  102 , since they cannot cause lock conflicts with users on the replication database system  20  who are preferably not allowed to execute update transactions (note that  FIG. 1  shows only read-only transactions  207  processed by the replication database system  20 ). 
         [0054]    Moreover, since read-only transactions  107  processed by the primary database system are preferably not visible on the redo log  102 , redo log records with the lock data of such transactions need also not to be created. 
         [0055]    Exclusive locks for data items that are not modified within the respective update transaction  106  may be deleted at any time. Subsequently, the corresponding data items may be locked by another transaction. Consequently, a redo log record with this information is preferably immediately created in order to propagate that the respective lock is no longer held by the transaction that deleted the lock. Locks that are not explicitly deleted are preferably released implicitly at the end of the respective transaction, originated by a commit or rollback command. 
         [0056]    Furthermore, a read-only transaction  207  directed to the replication database system  20  that requests a lock for a data item on the replication database system  20  which is already locked exclusively by the replication database server  200  currently executing an update transaction (e.g. during a replication) preferably has to wait until the lock is released. 
         [0057]    For the replication database system  20 , a different policy for deadlock resolution may be used to avoid a large offset to the primary database system  10  caused by slowing down the replication processing. In case a read-only transaction  207  is waiting for a lock held by the replication database system  20  and the replication database system  20  tries to acquire a lock already held by the read-only transaction  207 , the operation of the replication database system  20  as requestor of the lock provoking the deadlock will not be aborted. On the contrary, the replication processing by the replication database system  20  preferably has a higher priority, so that the read-only transaction  207  is aborted and/or forced to be restarted when the conflicting lock is released. 
         [0058]    As already presented above, additional lock table records on the redo log  102  are only created when needed by the replication database system  20 . This is illustrated in  FIG. 2 , that shows exemplary entries of the lock table  103  maintained by the primary database system  10 . As can be seen, the entries  121  of the lock table  103  comprise lock table entries of an update transaction initiated by a user with user ID ‘U 2 ’, which are to be used to create a lock table record  122  in the redo log buffer  104  and/or the redo log  102  of the primary database system  10 . 
         [0059]    More precisely, the lock table record  122  shown in  FIG. 2  comprises lock data for resources (e.g. a data items) with the resource IDs ‘ 1003 ’ and ‘ 1004 ’. This lock table record  122  is preferably inserted into the redo log buffer  104  and/or the redo log  102  prior to creating an update data record. As can be further seen in  FIG. 2 , data for the shared lock of the resource with ID ‘ 1005 ’ is not inserted into the lock table entry of the redo log record  122 , since this information is not needed on a replication database system  20  where users are not allowed to perform update transactions and thus conflicts with shared locks are not possible. 
         [0060]    The grey fields in  FIG. 2  further show lock table entries which are not yet used for creating lock table records  122  on the redo log  104 / 102 , either because a user ‘U 1 ’ is holding only shared locks from a read-only transaction  107  or locks are from a user ‘U 3 ’ that has not yet modified any data. However, if the user ‘U 3 ’ (i.e. the respective transaction initiated by this user) performs an actual data modification, a lock table record  122  with lock data for the resource with ID ‘ 1006 ’ will be created. 
         [0061]    Consequently, the redo log  102 / 104  used as replication data preferably comprises only data from users with update transactions  106  that have actually modified the primary database  101 . Users with read-only transactions  107  may have been also using locks to conform to a certain isolation level (see further above), however, this locking data is not required for replication processing. Therefore, creating a lock table record on the redo log is preferably delayed until a point in time when a transaction step with data modification results in creating an update data record on the redo log. 
         [0062]    The flow chart depicted in  FIG. 3  shows in more detail the steps performed by a database system of the present invention before an update data record for a transaction of a given user is created in the redo log  104 : Firstly, the database system checks in step  131 , preferably during start-up, whether at least one replication database system  20  allowing simultaneous queries is registered and if so, a flag is set to indicate the database system as the primary database system  10  for replication with simultaneous queries. Consequently, any database system performing this step can take the role of a primary database system. If this flag is not set (step  131 ), no lock data at all is required to be written to the redo log, but only the up-date data record (step  134 ) is created. This aspect further minimizes the amount of lock data written by the primary database system and thereby increases its performance. If the flag is set (decision step  131 ), the lock table is scanned for locks held by the user that initiated the respective transaction (step  132 ) to create a lock table record with one or more lock data entries (step  133 ) and, preferably immediately after that, the update data record (step  134 ) is created. 
         [0063]    With hierarchical locking models, not only the data items modified are locked, but also additional resources may be locked that may be released before the transaction is finished. After a lock is released by a user (i.e. by its transaction), this lock may be acquired by another user (i.e. its transaction) and a lock table record is created on the redo log in case a modification is logged by an update data record. Before this lock table record is created, the lock data records on the redo log must ensure that the previous lock is removed from the lock table of the replicated database. The flow chart depicted in  FIG. 4  depicts when a lock table record needs to be created when releasing a lock. Firstly, the database system checks in step  140 , whether at least one replication database system  20  allowing simultaneous queries is registered and if so, a flag is set to indicate the database system as the primary database system  10  for replication with simultaneous queries. Consequently, any database system performing this step can take the role of a primary database system. If this flag is not set (step  140 ), no lock data at all is required to be written to the redo log, and the processing flow proceeds to the ‘end’ step. If a lock table record was provided for adding the lock (decision step  141 ), then a redo log record with the information about the released lock (step  142 ) is, preferably immediately, created to avoid a lock conflict on the replication database. Only if the user releasing the lock did not update any data, a lock table record with the data adding the lock is not created and a redo log record with the data that a lock was deleted is not needed. 
         [0064]    Creating lock table records only when needed by the replication database system  20  and creating a lock table record with (multiple) lock table entries at the latest time possible shortens the time window for locks held, as demonstrated in  FIG. 5 . Here, a sequence of transaction steps  150  is shown as well as the redo log entries created for this transaction. More precisely, the transaction steps  150  shown in  FIG. 5  comprise four lock requests  151  (‘L 1 ’, ‘L 2 ’, ‘L 3 ’ and ‘L 4 ’), one lock release  152  (‘R 4 ’), three modification requests  153  (‘M 1 ’, ‘M 2 ’ and ‘M 3 ’) and a transaction end step  154  (‘ET’). The grey parts of the diagram represent transaction steps of other users and redo log entries created by transactions of these other users. Within the example of  FIG. 5 , the data to add a lock  151  for the data items with the IDs ‘ 1 ’, ‘ 2 ’ and ‘ 3 ’ is put into a single lock table record  133  before creating the corresponding update data records  134  with the first modification  153  on a given data item.  FIG. 5  further shows that a further lock for the data item with ID ‘ 4 ’ was released (see the transaction step ‘R 4 ’  152 ) before creating the first redo log record for this transaction, thus no data for adding and releasing is to be put into a lock table record. The optimizations mentioned above require less space in the redo log and therefore less I/O processing time is needed for writing and reading the redo log data. Maintaining the locks of a user in a lock table and creating the lock table records by scanning the lock table for new locks acquired when creating the next update data log record results in the shortest time period possible for locks held by the replication database system  20 . 
         [0065]    Consequently, the data used for lock table records preferably comprises multiple lock data entries describing the resources locked in different lock modes and additional data about reapplying multiple lock entries. For further optimizations, this kind of data are preferably represented by two data structures: a basic lock table entry and a lock table entry extension. For database servers where replication databases are not registered (see above), only the basic lock table entry data structure will be used to further reduce memory consumption. 
         [0066]      FIG. 6  shows the usage of the redo log data to replicate contents of the primary database  101 . Here, the redo log data is received in an input data stream  261  by the replication processor  262  of the replication database system  20 . Lock table records on the redo log are used to maintain the lock table  203  of the replication database server  200 . Query users with read-only transactions  207  executed by the query processor  263  of the replication database system  20  may also use the lock table  203 . 
         [0067]    If a transaction is trying to acquire a lock that is already held by the replication processor  262 , the respective transaction (the ‘requestor’ of the lock) is put into a waiting state. As an example,  FIG. 7  shows a sequence of transaction steps  271  executed by the query processor  263  and a transaction step  272  from the redo log to be replicated resulting in the creation of entries in the lock table  203  of the replication database server  200 . With this example, a query user ‘Q 3 ’ tries to acquire a lock for a resource with the ID ‘ 1003 ’ which is already held by the replication processor  262 , thus the user ‘Q 3 ’ has to wait until the lock for the resource with ID ‘ 1003 ’ is released. 
         [0068]    Deadlock situations that might occur with query users and the replication processor are preferably resolved in favour of the replication processor to avoid replication processing to be slowed down, with the effect that other locks already held by the replication processor to replicate transactions running in parallel could create lock conflicts with other query users. To this end, the read-only transactions of query users in case of a deadlock are aborted. This approach is appropriate to achieve the best overall performance. 
         [0069]    As already presented further above, the lock data stored within redo log records may comprise different lock types, such as explicit locks, shared locks and/or intention lock types. Additional lock operations may be provided, such as upgrading a lock to stronger lock or downgrading to a weaker lock. Creating lock table records on the redo log for upgrading locks may be performed similar as adding locks (see above). Downgrading a lock may be performed similar to deleting a lock. 
         [0070]    In summary, the above embodiment of the present invention enables transaction consistent queries on a replication database in an especially efficient way by supplementing the redo log data  102 / 104  (i.e. the replication data) with locking data, preferably from the lock table  103 . The primary database server  100  reduces the amount of lock data to be added to the redo log to a minimum by filtering only those locks which are necessary to process the updates to be replicated. These locks are preferably exclusive locks of update transactions  106 . Shared locks and locks of read-only transactions  107  on the primary database  101  and exclusive locks within update transactions  106  without update processing are not included in the redo log data  104 ,  102 . In case no replication database system  20  is registered with a primary database system  10 , preferably no locking information at all is included in the redo log. By using the additional locking data, the replication database server  200  of a replication database  201  may effectively offer the same lock modes and isolation levels as the primary database server  100  of the primary database  10  for queries (read-only transactions  207 ) running in parallel to the replication processing. 
         [0071]    It will be appreciated that the example database systems described herein may include any suitable combination of programmed logic circuitry (e.g., hardware, software, firmware, and/or the like). For example, the example database systems described herein may include a processor that helps execute instructions and/or commands. In certain example embodiments, the data records, logs, and the like, may be stored in a suitable storage location (such as, for example, one or more databases, files, or other tangible and/or non-transitory memory locations). It will be appreciated that certain example embodiments may comprise a non-transitory computer readable storage medium storing tangible instructions that, when executed by a processor (e.g., of one or more of the example database systems described herein), perform the above-described method steps. It also will be appreciated that the example methods disclosed herein may be executed, in whole or in part, by one or more of the example database systems and/or a processor thereof.