Concepts

Product：
- The product equals "tenant" in BFE, which has its own configuration, such as routing policy, permission, etc.
- Field "host" in HTTP header is used to identify product that the incoming message belongs to.
Cluster：
- Cluster means a set of backend servers which provide same functionality and process similar task.
- Usually，cluster span multiple IDC/region in scope。
Sub Cluster：
- A cluster may be composed of multiple sub clusters conceptually。
- Usally, backend servers within one IDC/region is defiend as a sub cluster.
Instance：
- Instance is backend server. A sub cluster would contain multiple instances.
- To BFE，each backend instance expose IP address + port to accept request.

Typical message routing flow

Step1-2：DNS query
- request host name：demo.example.com
- get IP address 6.6.6.6（example）
Step3：IP diagram is routed to POP of IDC1，and processed by layer 4 load balancer L4LB.
Step4：L4LB forward diagram to BFE
Step5：BFE receive HTTP request, identify which product the request belongs to:
- BFE uses field "host" in HTTP header to decide product.
- In this scenario, assume demo.example.com belongs to product "demo".
Step6：Identify a cluster of product "demo", which would handle this message.
- Based on configured distribution rule of product "demo", identify cluster "demo-static" in this product to handle the message .
Step7：Identify a sub cluster within "demo-static"
- Based on routing rule in "demo-static", identify sub cluster "demo-static.idc1" to handle this message.
Step8：Identify instance
- based on configured load balancing policy, identify instance as "a-demo-static-1.idc1".
Step9：request is sent to instance "a-demo-static-1.idc1".
Step10：BFE receive response message from "a-demo-static-1.idc1".
Step11-12：BFE send back response to client, via L4LB.