2. Slon daemons

The programs that actually perform Slony-I replication are the slon daemons.

You need to run one slon instance for each node in a Slony-I cluster, whether you consider that node a “master” or a “slave”. On Windows when running as a service things are slightly different. One slon service is installed, and a seperate configuration file registered for each node to be serviced by that machine. The main service then manages the individual slons itself. Since a MOVE SET or FAILOVER can switch the roles of nodes, slon needs to be able to function for both providers and subscribers. It is not essential that these daemons run on any particular host, but there are some principles worth considering:

There are two “watchdog” scripts currently available:

The slon_watchdog2 script is probably usually the preferable thing to run. It was at one point not preferable to run it whilst subscribing a very large replication set where it is expected to take many hours to do the initial COPY SET. The problem that came up in that case was that it figured that since it hasn't done a SYNC in 2 hours, something was broken requiring restarting slon, thereby restarting the COPY SET event. More recently, the script has been changed to detect COPY SET in progress.