Keep-alive communication sequence

A first general overview, of the possible communication sequences between a data warehouse and update agent.

Review the attached communication-sequence diagram and the issue's data warehouse/update agent context first. Clarify which keep-alive sequences the overview should cover and where the resulting documentation belongs; done means the possible communication sequences are documented clearly enough for readers to understand the interactions.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.