Google的服务响应也是一个多层处理架构。在广域网调度上使用GSLB进行最优路径寻址，在本地使用GSLB、BNS（名字服务）查找可用的服务器。应用服务包括前端和后端，两者异步运行，通过protobuf协议通信。这点同样与我们的升龙类似，升龙在广域网调度也使用了GSLB，而web专区的本地调度更为复杂一些，使用了OSPF、LVS、Nginx等协同作用。不过升龙没有使用名字服务，也没有将所有本地服务注册在GSLB里。
Figure 2-4shows how a user’s request is serviced: first, the user points their browser
to shakespeare.google.com. To obtain the corresponding IP address, the user’s device
resolves the address with its DNS server (1). This request ultimately ends up at Goo‐
gle’s DNS server, which talks to GSLB. As GSLB keeps track of traffic load among
frontend servers across regions, it picks which server IP address to send to this user.
The browser connects to the HTTP server on this IP. This server (named the Google
Frontend, or GFE) is a reverse proxy that terminates the TCP connection (2). The
GFE looks up which service is required (web search, maps, or—in this case—Shake‐
speare). Again using GSLB, the server finds an available Shakespeare frontend server,
and sends that server an RPC containing the HTML request (3).
The Shakespeare server analyzes the HTML request and constructs a protobuf con‐
taining the word to look up. The Shakespeare frontend server now needs to contact
the Shakespeare backend server: the frontend server contacts GSLB to obtain the BNS
address of a suitable and unloaded backend server (4). That Shakespeare backend
server now contacts a Bigtable server to obtain the requested data (5).
The answer is written to the reply protobuf and returned to the Shakespeare backend
server. The backend hands a protobuf containing the results to the Shakespeare
frontend server, which assembles the HTML and returns the answer to the user.
This entire chain of events is executed in the blink of an eye—just a few hundred mil‐
liseconds! Because many moving parts are involved, there are many potential points
of failure; in particular, a failing GSLB would wreak havoc. However, Google’s policies
of rigorous testing and careful rollout, in addition to our proactive error recovery methods such as graceful degradation, allow us to deliver the reliable service that our
users have come to expect. After all, people regularly use www.google.comto check if
their Internet connection is set up correctly.