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CDDS Target Types
CDDS target type refers to the function a database performs within a CDDS. The target type determines whether changes to databases can be replicated and whether collisions are detected. Target types are defined separately for each CDDS; therefore, a database can have more than one target type if it is contained in more than one CDDS.
Ingres Replicator currently supports the following types of CDDS target types to carry out replication:
Full peer
Protected read-only
Unprotected read-only
A database provides services of a particular target type based on the CDDSs it contains. To plan database replication, you need to decide what role you want each database to play within each CDDS and assign one of the three target types. Your decision is based on the type of replication behavior you want the database to perform and the computing resources available in your system. By carefully analyzing how various work groups use data, you can determine which targets require full peer or read-only capabilities.
Full Peer Target
A full peer designation is required for production CDDSs where interactive users or other processes first manipulate data. Full peer CDDS targets make full use of the Ingres Replicator processing capabilities and maintain shadow tables, archive tables, as well as the input and distribution queues.
In a full peer CDDS target, each replicated table has a shadow table. Each change to a row designated for replication is tracked through a shadow record entry in the shadow table. Before-images of updates and deletes are maintained in the archive table. The shadow table itself contains the information necessary to allow the propagation of row changes to other targets.
If a row change is an update or a delete, the old image of the row is stored in an archive table. The archive table makes it possible to re-run the transactions that occurred in a database, in the proper order with the proper data, even if the replicated row has been changed many times. For each change in a replicated table of a full peer target, an entry is also made in the input queue table.
If a full peer source sends information to a full peer CDDS target (in a peer-to-peer configuration), database procedures are used to transmit the data, while the DBMS captures local changes to the database. As a result, all replicated tables in this CDDS must be registered with Ingres Replicator, not only to transmit information to other replicated CDDS targets, but to receive information as well.
As the workhorse of the Ingres Replicator system, a full peer target consumes more disk space than any other target type; it makes use of all support tables and many of the CPU cycles available in the Ingres Replicator system to process its replication.
Protected Read-Only Target
A protected read-only target is used to receive information from a full peer source. The term protected means that this target contains sufficient information to detect collisions against the source copy of the data. Users must not make changes to the data in a protected read-only target, although Ingres Replicator does not prevent this. The ability to detect and resolve collisions provides a protection against any change that is made to a replicated table from another source. Note that collisions resulting from updating data directly at the protected read-only target are generally not detectable.
A protected mode is important because more than one source can be transmitting to a single protected read-only target. If the target is not protected, transactions can arrive out of order. A protected read-only target maintains a shadow table for each replicated table but does not maintain archive tables or input and distribution queues. This allows the target to only accept replicated rows in order, thus ensuring that consistent data is available to users. Other sources replicate data to this target by database procedures; so although this target does not send data, all replicated tables in the protected read-only target must be registered with Ingres Replicator, and most Ingres Replicator support objects must be present.
Unprotected Read-Only Target
In unprotected mode, there is no safeguard against any change that is made to a replicated table from another source (or locally); hence, use unprotected read-only target only if one full peer source sends information to the read-only target. Because the unprotected read-only target does not maintain any support objects within the database, no table registration is required for the target’s tables. Collisions cannot be detected because of the absence of the shadow table.
All Enterprise Access and EDBC databases can only be designated as unprotected read-only.
Last modified date: 06/10/2024