Merge replication is when you allow multiple sites to make changes or publish into a database, track changes, and then merge any changes into a target database. It is used to allow each site to be able to work independently without a full time connection. Snapshot replication, on the other hand, creates an image of the data in a database at a point in time and then replicates the snapshot to subscribers. A snapshot, once the original data image is established, is differential, and essentially refreshes any tables in its definition. Snapshots are a fast means of keeping data sets synchronized. People use snapshot replication when the latency involving table refresh is within allowable limits of your application.
Both snapshot and merge replications can be monitored using the Replication Monitor tool. That tool posts an error message or alert when a replication fails, as well as providing information about which processes have succeeded and which have failed. Thus the Replication Monitor is a good place to go to troubleshoot your replications. This tool allows you to set up agents that generate alerts, and specify where the alert is to be sent. To set up an alert, right click on the Agent and in the Agent Properties dialog box go to the Notification tab and fill in the behavior you need.
You'll find that most often replications fail because either connectivity was lost or because the process didn't have adequate security privileges to complete the replication. Both of these problems, once detected, can be detected using a remote stored procedure in a subscriber database. Replications use the security settings of your SQL Server agent. If you have a security issue, then check that you have sufficient privileges to access the working directory on the distribution server, and that you can create new data files and schema. Also, for any snapshot replication, check that you can properly access the databases that are part of your snapshot replication definition.
About the author
Barrie Sosinsky is president of consulting company Sosinsky and Associates (Medfield MA). He has written extensively on a variety of computer topics. His company specializes in custom software (database and Web related), training and technical documentation.
This was first published in March 2003