A Replication Instance file maintains the current state of an instance. It also serves as a repository of the history of the journal sequence numbers that are generated locally or received from other instances.

It includes 3 sections -- File Header, Source Server slots, and History Records.

The File Header section records information about the current instance, such as semaphore and shared memory ids of the Journal and Receive Pool, journal sequence number of the current instance.

The Source Server slots store information for each replicating instance for which a Source Server is started. A slot stores the name of the replicating instance, the last transmitted sequence number, and the sequence number when the Source Server was last connected to the originating instance (Connect Sequence Number).

A Replication Instance file has 16 slots. Initially, all are unused. A Source Server replicating to a replicating instance for the first time utilizes an unused slot to store the information and any future Source Server process replicating to the same replicating instance updates this information.

If an unused slot is not available, the first time a Source Server is started to replicate to an instance, the slot for the least recently started replicating instance is reused, and the information that is previously stored in that slot is overwritten. Any subsequent mupip replic -source on the preempted replicating instance generates a REPLINSTSECNONE message.

Preemption of slots does not occur if an instance connects to no more than 16 different replicating instances throughout its lifetime.

In the History Records section, the history of on instance is maintained as a set of records. A new history records is added to the tail of the instance file whenever an instance changes from being an originating instance to replicating instance or vice versa --the only exception being when a history record is removed from the tail of the instance file when updates are rolled back from the database as part of a mupip journal -rollback. Every record identifies a range of journal sequence numbers and the name of the originating instance that generated those journal records. The first history record starts with the current journal sequence number of the instance.

When an originating instance transmits a sequence number to a replicating instance, the originating instance name is recorded as "Root Primary Instance Name" in the replication instance file history of both the instances. The same rule applies when a replicating instance is acting as an originating instance for another replicating instance downstream.

This history is crucial for determining the journal sequence numbers through which both instances are synchronized when two instances attempt to connect. This journal sequence number is determined by going back in the history of both the instance files and finding the earliest shared journal sequence number that was generated by the same originating instance. If the shared journal sequence number matches the current journal sequence number of the replicating instance, the Receiver Server on the replicating instance continues with normal replication. Otherwise, a mupip journal -rollback -fetchresync must be performed on the replicating instance to rollback to a common synchronization point from which the originating instance can transmit updates to allow it to catch up.

	It is necessary to take a backup of the Replication Instance file that is consistent with the snapshot of the database files in the backup. mupip backup -replinstance creates a backup of the Replication Instance file. If the replication instance file is damaged or deleted, a new instance file must be created, and all replicating instance downstream must be recreated from backups.