AppNet Element Specification Documentation

An AppNet element defines the logic for an application network function (ANF) in a modular, programmable way. Elements process RPC requests and responses using match-action rules and can maintain typed local or shared state.

Element Structure

Each element specification includes the following sections:

1. state

Defines local or shared, typed state variables. Maps and other types are supported.

state:
  firewall_table: Map<string, string>

2. init()

Optional initialization logic for state variables.

init():
  set(firewall_table, 'test', 'DENY')

3. req(rpc)

Match-action rules for processing RPC requests. Match expressions can be nested, with pattern matching on Some(...) and None for optional types.

req(rpc):
  match get(acl, get(rpc, 'username')):
    Some(permission) =>
      match permission:
        'ALLOWED' =>
          send(rpc, Down)
        'DENY' =>
          send(err('firewall'), Up)
    None =>
      send(rpc, Down)

4. resp(rpc)

Logic for processing RPC responses.

resp(rpc):
  send(rpc, Up)

Syntax Reference

Typed State Declaration

state:
  variable_name: Type

Examples:

• Map<string, string>
• int, float, bool

Pattern Matching with Option Types

AppNet supports optional value semantics:

match get(map, key):
  Some(value) => ...
  None => ...

Common Actions

• send(rpc, Down): Forward the RPC to the next element.
• send(rpc, Up): Return the RPC to the previous element.
• send(err("msg"), Up): Send an error upstream.
• set(map, key, value): Write to a map or state variable.
• get(map, key): Read a value from state; returns Some(v) or None.

Full Example: Application Firewall

state:
  firewall_table: Map<string, string>

init():
  set(firewall_table, 'test', 'No')

req(rpc):
  match get(acl, get(rpc, 'username')):
    Some(permission) =>
      match permission:
        'ALLOWED' =>
          send(rpc, Down)
        'DENY' =>
          send(err('firewall'), Up)
    None =>
      send(rpc, Down)

resp(rpc):
  send(rpc, Up)

This firewall element checks a body field of the RPC against an access control list (firewall_table). If the value is 'ALLOWED', it allows the request. If 'DENY', it blocks the request. If there's no entry (None), the request is allowed by default.

Runtime Behavior

• Down: sends the request to the next element or the microservice.
• Up: returns a message to the previous element.