Backends

A backend provides the communication infrastructure that network programs run on. After endpoint projection transforms a global Choreo into per-location Network programs, a backend interprets the network operations (send, receive, broadcast) using a concrete transport mechanism.

The Backend trait

trait Backend[B, M[_]]:
  extension (backend: B) def runNetwork[A](at: Loc)(network: Network[M, A]): M[A]
  extension (backend: B) def locs: Set[Loc]

Backend[B, M] is a type class parameterized by:

• B — the backend implementation type (e.g., LocalBackend[M] or TcpBackend[M])
• M[_] — the effect type (e.g., IO)

It provides two operations:

• runNetwork(at)(network) — Interprets a Network[M, A] program at the given location, translating Send, Recv, Broadcast, and Par operations into concrete effects.
• locs — Returns the set of all locations managed by this backend.

The Channel type

type Channel = (from: Loc, to: Loc)

A Channel is a named tuple identifying a directed communication path from one location to another. Backends use channels to route messages — for example, the local backend maintains one queue per channel, and the TCP backend maintains one socket connection per channel.

From Choreo to execution

The typical flow from choreography to execution is:

// 1. Define a global choreography
val choreo: Choreo[IO, A] = ...

// 2. Create a backend
val backend <- Backend.local(List(loc1, loc2))

// 3. Project and run at each location
val task1 = choreo.project(backend, loc1)
val task2 = choreo.project(backend, loc2)

// 4. Run all tasks concurrently
(task1, task2).parTupled

The .project(backend, loc) extension method on Choreo performs two steps internally:

1. Endpoint projection (EPP) — Transforms the global Choreo into a Network program specific to loc. Operations involving other locations become sends, receives, or no-ops as appropriate.

2. Interpretation — Passes the resulting Network program to backend.runNetwork(loc)(...), which maps each network operation to concrete effects in M.

Available backends

Backend	Transport	Use case
LocalBackend	In-memory queues	Testing, demos, single-process execution
TcpBackend	TCP sockets	Distributed execution across processes or machines