Chapter 7. Routing

Cloud native systems are dynamic; services come and go as demand requires. Services must be able to discover and communicate with other services, even if one instance of a service disappears or new capacity is added to the system. We can’t rely on fixed IP addresses; IP addresses couple clients to services. We need a bit of indirection to give us the desired flexibility to reroute dynamically.

We could use DNS, but DNS isn’t necessarily a great fit in a cloud environment. DNS benefits from caching in the clients that use the DNS services. The caching means that clients may “resolve” stale IP addresses that no longer exist. You can invalidate those caches (quickly) with low time-to-live (TTL) values, but you’ll then (necessarily) spend a lot of time re-resolving DNS records. In a cloud environment DNS requires extra time for resolution, because requests must leave the cloud and then reenter through the router. Many cloud providers support multihomed DNS resolution, with a private address and a public address. In this case, calls from one service to another within the cloud are fast and efficient and virtually free (in terms of provider-imposed costs for things like bandwidth), but it requires that your code base be aware of the complexity in the DNS scheme. It implies complexity of implementation that could also be pretty difficult to reproduce in a developer’s local environment.

DNS is also a pretty simple protocol: it doesn’t have the ability to answer basic ...

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