The concept of synchronization applies mainly to table-to-table replication, where Post performs integrity checks to make certain that only one row in the target matches the row change that is being replicated. It does not apply to file, JMS, Kafka, and change-history targets, which contain a record of every operation replicated by Post, some of which may be identical over time. The Post process does not perform integrity checks on those targets.
The basic characteristics of synchronized source and target tables are as follows (unless the transformation feature is used).
Ensuring data integrity is the responsibility of the Post process. Post applies a WHERE clause to compare the key values and the before values of the SQL operations that it processes. Post uses the following logic to validate synchronization between source and target tables:
Post applies a replicated INSERT but a row with the same key already exists in the target. Post applies the following logic:
Post applies a replicated UPDATE but either cannot find a row in the target with the same key value as the one in the UPDATE or Post finds the correct row but the row values do not match the before values in the UPDATE. Post applies the following logic:
Note: You can configure Post so that it returns an out-of-sync message if the current values in the target row match the after values of the UPDATE. See the SP_OPO_SUPPRESSED_OOS parameter in the SharePlex Reference Guide.
Post only verifies the integrity of the rows that are being changed by its current SQL operation. It does not verify whether other rows in that table, or other tables, are out of synchronization in the target database. Hidden out-of-sync conditions may not show up until much later, when a change is eventually made by SharePlex to that data or a discrepancy is detected in the course of using that data.
Example of a detectable out-of-sync condition
Someone logs into the target and updates the COLOR column in the target table from “blue” to “red” in Row1. Then, an application user on the source system makes the same change to the source table, and SharePlex replicates it to the target. In the WHERE clause used by Post, the pre-image for the target table is “blue,” but the current value in the target row is “red.” Post generates an out-of-sync error alerting you to the out-of-sync condition.
Example of a hidden out-of-sync condition
Someone logs into the target and updates the COLOR column in the target table from “blue” to “red” in Row2, but the change is not made to the source table and is not replicated. The two tables are now out-of-sync, but there is no error message as in the previous example, because there is no Post involvement. No matter how many subsequent updates are made to other columns in the row (SIZE, WEIGHT), the hidden out-of-sync condition for the COLOR column persists (and users on the target have inaccurate information) until someone updates the COLOR column in the source table. When that change is replicated, Post compares the pre-images, and only then is there an error message.
The majority of time, the cause of out-of-sync data is not anything done wrong by replication, but rather DML applied on the target, an incomplete backup restore, or some other hidden out-of-sync condition that can go undetected a long time until an operation applied by Post finally affects that data and an error is returned. Solving out-of-sync conditions can be time-consuming and disruptive to user activity. Once replication is started, it is recommended that you:
Because SharePlex replicates over LAN and WAN connections, you can put a replica database to work as a reliable, continuously updated alternate database that can be used in many different ways. The following strategies enable you to get the right data to the people who need it, when they need it.
Targets maintained by SharePlex are ideal for offloading report and query processing because they are accessible while being kept up-to-date, and they can be optimized with keys and indexes designed for optimal query performance. You can run reports all day long, without complaints about performance from your OLTP users. Even during busy reporting times such as the end of the month or quarter, application response time will be unaffected by heavy reporting. And, your organization’s decision-makers will appreciate the accuracy of the data reflected in the reports.
When many remote users access or use data stored in a primary database, you can move their processing to one or more secondary databases that are kept current through SharePlex replication. That way, you can keep the primary database and system optimized for transactions. SharePlex also can cascade ndata through an intermediary system to remote systems, providing access for remote users who have no direct network connection to the primary database.
SharePlex can replicate from numerous source systems to one target system. It is ideal for consolidating data in a data warehouse or a data mart so that information is available enterprise-wide for queries and reports. A high degree of granularity in the data that you replicate and the option to transform replicated data to conform to a different target structure are unique SharePlex features that enable you to populate your data warehouse with the specific, timely information that users need to make good decisions.
SharePlex can be used to maintain duplicate databases over local or wide-area networks. Production can move to the alternate sites in an emergency or in a planned manner when routine maintenance is performed on the primary server. SharePlex replication enables the secondary database to be used for queries and reporting.
Figure 1: SharePlex replication strategies at a glance
Before you implement SharePlex on production systems, Quest recommends performing tests in a mirror of the production environment to ensure that you configured SharePlex properly to support your requirements. Testing can uncover issues such as configuration errors and unexpected environmental issues, for example network or resource issues that affect replication performance or availability.
Additionally, it is assumed that your organization has in place an infrastructure that supports the use of enterprise applications such as SharePlex. These include, but are not limited to, the following:
The SharePlex Professional Services team can help you prepare for, install, and deploy SharePlex in your environment.
This chapter contains instructions for running SharePlex on UNIX, Linux, and Windows
On Unix and Linux systems, you start SharePlex by running the sp_cop program. After you activate a configuration, sp_cop spawns the necessary child replication processes on the same system. Each instance of sp_cop that you start is a parent to its own set of child replication processes. The sp_cop process must be started on each system that is part of the replication configuration.
You can start sp_cop in one of two ways:
IMPORTANT: Run SharePlex from either the korn (ksh) or C shell (csh) shell.
When you start systems that are involved in replication, start the components in this order:
Verify that the SharePlex processes are started by issuing the lstatus command in sp_ctrl.
To start SharePlex, you must log onto the system as a SharePlex Administrator. Your user name must be assigned to the SharePlex admin group in the /etc/group file. For more information about user groups, see Assign SharePlex users to security groups.
Table 1: SharePlex startup syntax
|From root, with full path||$ /productdir/bin/sp_cop [-uidentifier] [-s] &|
|CD to the product directory||
$ cd /productdir/bin
$ ./sp_cop [-uidentifier] [-s] &
|From a startup script||
nohup sp_cop [-uidentifier] [-s] &
Table 2: Description of SharePlex startup syntax
|&||Causes SharePlex to run in the background.|
|nohup||Directs the startup of SharePlex to continue in the background after the current user logs out.|
Starts sp_cop with a unique identifier. Use this option when there are multiple instances of sp_cop running on a system, which is required for some SharePlex configurations. For more information, see Run multiple instances of SharePlex.
Some suggestions for identifier are:
Every session of sp_cop has a process ID number. The ID is returned after startup and then the command prompt reappears. If a configuration was activated during a former session of sp_cop, replication begins immediately. Without an active configuration, sp_cop runs passively in the background.
On Unix and Linux systems, you can use the ps -ef | grep sp_ command to view the SharePlex processes that are running.
The following child processes are spawned by sp_cop on a source system:
The following child processes are spawned by sp_cop on a target system:
Command and Control process (sp_cnc)
Each child process has the same -uidentifier as its parent sp_cop process. This makes it easier to identify related processes when multiple session of sp_cop are running.
To stop SharePlex, issue the shutdown command in sp_ctrl. This is a graceful shutdown that saves the state of each process, performs a checkpoint to disk, read/releases buffered data, and removes child processes. Data in the queues remains safely in place, ready for processing when sp_cop starts again. The shutdown process can take some time if SharePlex is processing large operations.
You can use the force option with the shutdown command to forcefully shut down replication if necessary. It terminates sp_cop immediately, bypassing normal shutdown procedures. See the SharePlex Reference Guide for more information about this command.
You can safely shut down SharePlex for a short time while there is still transactional activity. The next time you start SharePlex, replication resumes at the correct place in the redo logs or the archive logs, if needed. However, the best practice is to leave SharePlex running while there is transactional activity. Otherwise, SharePlex may need to process a large volume of redo backlog when you start it again, and there will be latency between the source and target data.
If the redo logs wrap and the archive logs cannot be accessed, resynchronization of the source and target data may be the only option. Take this possibility into account whenever you stop SharePlex while redo is still being generated.
Note: If you want to shut down both SharePlex and the database, shut down SharePlex first. Otherwise, SharePlex will interpret that the database is failing and generate a warning message.
As an alternative to stopping SharePlex, you can use the stop command in sp_ctrl to stop individual SharePlex replication processes as needed. See the SharePlex Reference Guide for more information about this command.