Tables in spider with backend server information (e.g. spider_xa_member) only have host in the primary key.
That causes problems when a table has two backends with the same host value (but different ports, for example), and the errors it causes can be quite confusing.
For example, you may get "this xid already exists" due to conflicting entries for two participants of the same XA.
Adding port/socket to the primary key of these spider tables (in install_spider.sql) should resolve this issue.