Publishes one price to many sessions, applying each venue transform once.

A subscription is a symbol and an ordered path of permission stages. Paths sharing a prefix share the computation of that prefix: the stages form a tree, a published price walks it from the root, each stage is applied once however many sessions lie beneath it, and every node hands its value to the sessions subscribed at exactly that node.

path = [{:stream, "SUPERFINE"}, {:spread, :tier_a}, {:markup, 1.2}]

FixAlchemy.Feed.subscribe("AUD/USD", path, md_req_id)
FixAlchemy.Feed.publish("AUD/USD", {:stream, "SUPERFINE"}, price_id, price, MyVenue.Prices)

A subscriber receives, for each node it is subscribed to:

{:feed, %{symbol: symbol, price_id: price_id, price: price, ref: ref}}

price is the value as it reached that node. price_id identifies the source price and is the same for every session the publish reaches, whatever transforms were applied. ref is the term given at subscribe time. The path is not delivered.

Stages

A stage is any comparable term. Two subscriptions share a node only when their stages compare equal, so a stage names a transform rather than being one: {:markup, 1.2} and not fn price -> price * 1.2 end, which produces an unequal term each time it is written and would split a node that should be shared.

The venue turns a stage into a computation through FixAlchemy.Feed.Pipeline.apply_stage/2.

Stages must be ordered consistently across subscriptions. [a, b] and [b, a] are different paths sharing no work beyond the root, which is correct — their values differ — but a venue that lets clients order stages freely fragments its own tree.

Lifetime

Subscribers are monitored; every subscription a process holds is dropped when it exits.