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Chapter 2 • Synchronizing at a Glance
36
Time Stamp, Identity Mappings, and Record Versions
The Synchronizer stores various data during the synchronization process.
During subsequent synchronization runs, the Synchronizer uses this data to
determine if a record needs to be synchronized. The stored data includes:
➤ Time stamp. Indicates when the last synchronization task started running
on a specific link.
➤ Identity mappings. For each link, the Synchronizer stores a table of identity
mappings between records in each endpoint. Each record in each endpoint
is identified by a unique ID, and this table records the correspondence
between each pair of records.
➤ Record versions. Indicates the version of each synchronized record in each
endpoint. During synchronization, each synchronized record receives a
unique version.
Cycle Redundancy Checks
After the Synchronizer has determined that a mapping is a candidate for
synchronization, a cycle redundancy check (CRC) is performed to determine
whether to synchronize the records in the mapping. A cycle redundancy
check is a mathematical operation used to indicate whether data in an
endpoint has changed.
The Synchronizer performs a cycle redundancy check for each endpoint on
the mapped fields only. This enables the Synchronizer to determine whether
the modifications made to the records in the mapping necessitate
synchronizing the records.
For example, suppose you are mapping defects in Quality Center with
defects in ClearQuest, and the only field mapping for the link is between the
Status field in Quality Center and the State field in ClearQuest. Suppose the
Priority field is updated for a defect in Quality Center. As the defect was
modified, the mapping that includes the defect is a candidate for
synchronization, based on the link time stamp. However, as the mapped
fields were not modified, the records should not be synchronized, even
though there were other modifications to the records. During
synchronization, the Synchronizer performs a cycle redundancy check on
the mapping. The check indicates that the mapped fields were not modified,
and therefore the records are not synchronized.