Context and message types in ADC

Two things are similar, but not the same in ADC. It is the context and type of messages.

The message types are stated in section 3.3. in the spec, and “specifies how messages should be routed and thus which additional fields can be found in the message header”.

Context, stated in section 5., are an abstraction of how messages should be sent.

Usually, action are stated to be used in context rather than by their message type to allow the client and hub decide for themselves what type should be used. (This can also be future proof.)

For example, the action MSG allow contexts F and T, which are “F – From hub (hub-client TCP)” and “T – To hub (hub-client TCP)”. What the F type mean is that a MSG message will originate from a hub and the T type will originate from a client (“and go to the hub”). That is, a client may want to send a message as a broadcast message to all users in the hub, or send them a direct (or echo) message. So instead of having the action MSG allow “B, D, E type for clients”, the context (abstraction) T is used instead.

The wording “hub-client TCP” does not mean the message always originates from the hub. It’s simply a choice of words to denote that it’s a hub and client TCP session, not who initiated the session or message.

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