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SAP Sybase SQL Anywhere 16.0 » SQL Anywhere Server - Database Administration » High availability and read-only scale-out systems » SQL Anywhere read-only scale-out » Maintaining a read-only scale-out system

 

How read-only scale-out systems handle the loss of a parent connection

When the parent server for a copy node becomes unavailable, the copy node's database remains available. The copy node continues to try to connect to its parent for a period defined by the max_retry_connect_time mirror option (by default, 120 seconds). If a connection cannot be established in the specified time, then the copy attempts to connect to its alternate parent (defined by using the OR SERVER clause of the CREATE MIRROR SERVER STATEMENT) if one was defined, for an additional max_retry_connect_time seconds.

If a connection cannot be made, the copy node tries to connect to the root database server and obtain log pages from it. If the promotion_time elapses since it could not connect to its original parent or alternate parent, then the copy node requests that the root database server replaces the copy node's former parent with the copy node itself, assigning any of the copy node's siblings as its children. This behavior can result in a situation where the copy node has more children than specified by the auto_add_fan_out setting.

If the copy node is unable to connect the parent, the alternate parent, or the root database server within max_disconnected_time seconds, then the database is shut down. The max_disconnected_time default is to never shut down.

During the time the copy node is trying to connect to another database server, it continues to try to connect to its original parent. If that connection attempt is successful, the copy node resumes obtaining log pages from the original parent database server.

When a copy node is first started, it makes its database available for read-only connections even if other nodes in the tree, including its parent, are not available.

 Determining the state of a server in a scale-out system
 See also