Database Replication - Agents

■ Snapshot Agent (snapshot.exe) This agent prepares the schemas and the initial


copy of the data files. All replication types use the Snapshot Agent as the starting

point for the synchronization process. Generally, the Snapshot Agent is run on a

regular basis to keep the data files updated. These files are required if the replication

synchronization process finds anomalies in the data that cannot be fixed.

The Snapshot Agent runs in the Distributor server.



■ Log Reader Agent (logread.exe) This agent monitors the database’s transaction

log and copies each transaction that affects the publication to the distribution

database, in which the transactions are stored until applied to the Subscribers. It

is important to note that multiple publications of the same database share the

same Log Reader Agent. Log Reader Agents are used only in transactional replication

configurations and run in the Publisher server.



■ Distribution Agent (distrib.exe) This agent performs two tasks: it delivers the initial

snapshot to Subscribers and applies transactions stored in the distribution

database to Subscribers. The Distribution Agent is used in snapshot and transactional

replication. The Distribution Agent runs in the Distributor when you

configure the publication as a push subscription and in the Subscriber when you

configure the publication as a pull subscription.





■ Merge Agent (replmerg.exe) This agent delivers the initial snapshot from the Distributor

to the Subscribers. It also merges data changes that occur in the Publisher

to the Subscribers, and vice versa. When two servers modify the same information

at the same time, a conflict occurs; the Merge Agent reconciles the conflict by using

a set of rules that you define during replication configuration. The Merge Agent

runs in the Distributor when you configure the publication as a push subscription

and in the Subscriber when you configure the publication as a pull subscription.



■ Queue Reader Agent (replmerg.exe) This agent reads messages stored in queues


(SQL Server queues or Microsoft Message Queues) and applies transactions sent

to the queue to the Publisher database. The Queue Reader Agent is used only

when snapshot or transactional replication is set with the option for queued

updating subscriptions.