Leader (Master)

It is the crawler instance who gives order to Follower to do something. In SmoothCrawler-Cluster, although it may have multiple Leader instance, only one instance has responsible for being Leader, we call it Primary Leader, and the rest would be the backup one to standby for being Primary Leader, we call it Secondary Leader.

Responsibility

Here it would explain the details what things it should do as each different roles.

Primary leader

If it has only one crawler instance as Leader, it would be Primary Leader and it is responsible of giving order to Follower and monitoring them. But if it has multiple crawler instances as Leader, the Primary Leader instance only need to give order to Follower.

Note

If it has multiple crawler instances as Leader, it also uses election to decides to who is the Primary Leader.

Secondary Leader

It has Secondary Leader if it has multiple Leader crawler instances; otherwise, it doesn’t. Secondary Leader is responsible of 2 things: monitor all Follower instances and stand by for being Primary Leader.