When you configure a GTM property, you can create liveness tests to determine whether the data centers associated with the property are up. Once a liveness test is configured, GTM will answer queries for the property with the A or AAAA records of servers in your data centers which it considers to be live (up).
Liveness tests can be configured to use one of several popular protocols: HTTP, HTTPS, DNS, FTP, POP, POPS, SMTP, and SMTPS. You can also construct custom liveness tests by specifying the protocol as TCP or TCPS. In the simplest mode, TCP/TCPS liveness testing will succeed if it can open a TCP connection on the specified port. For additional validation, you can send an optional Request String (which must include any applicable control characters, such as CR and LF), and look for a Response String. The Response string can occur in the first 8KB of the response. If the connection succeeds, but the response string doesn't match, the liveness test will fail and the server will be marked down
Liveness testing is done by systems called liveness testing agents, also known as servermonitors. GTM allocates a set of seven agents for each of your data centers. Servers in data centers are considered up if their liveness tests are deemed successful by a majority of the agents. The sets of liveness testing agents assigned to each of your data centers might overlap with one another. Using multiple agents to conduct liveness tests minimizes the possibility of falsely declaring your data center down due to local network issues.
- Agents are in the same continent as your data center
- Agents are near your data center
- No two agents are in the same ISP (network diversity)
- Only live agents are assigned (sometimes agents are down for maintenance)
It is important that you enter accurate geographic information when configuring your data centers, to enable the best allocation of liveness testing agents.
A data center is considered up if any of the servers in it are considered up. You can configure the Minimum Live Percentage for a property to adjust this behavior.
If all the servers for a property are failing their liveness tests and there is no backup CNAME, GTM considers all the data centers to be up, as it has no basis for preferring any of them.