ZeroMQ

Ephemeral Values

An ephemeral value is one that expires automatically unless regularly refreshed. If you think of Clone being used for a registration service, then ephemeral values would let you use dynamic values. A node joins the network, publishes its address, and refreshes this regularly. If the node dies, its address eventually gets removed.

The usual abstraction for ephemeral values is to attach them to a session and delete them when the session ends. In Clone, sessions would be defined by clients and would end if the client died. A simpler alternative is to attach a time to live (TTL) to ephemeral values, which the server uses to expire values that haven’t been refreshed in time.

A good design principle that I use whenever possible is to not invent concepts that are not absolutely essential. If we have a large quantity of ephemeral values, sessions offer better performance. If we use a handful of ephemeral values, it’s fine to set a TTL on each one. If we use masses of ephemeral values, it’s more efficient to attach them to sessions and expire them in bulk. This isn’t a problem we face at this stage, and we may never face it, so sessions go out the window here.

Now we will implement ephemeral values. First, we need a way to encode the TTL in the key-value message. We could add a frame, but the problem with using ØMQ frames for properties is that each time we want to add a new property, we have to change the message structure. It breaks compatibility. So, let’s add ...