Macula SDK — RPC Guide

Request/response over the mesh: advertise a handler, call a procedure, get a result.

Audience: applications that need a request/response call to a specific procedure, as opposed to a broadcast (PubSub Guide) or an open-ended feed (Streaming Guide).

Overview

A provider advertises a procedure with macula:advertise/5. A consumer calls it with macula:call/5 — the simple, recommended default: it tries each of your own pool's connected stations in turn, and whichever one answers is responsible for finding a handler, locally or by forwarding to a peer.

Procedure = macula_topic:app_hope(Realm, Org, App, <<"math">>, <<"add">>, 1),

ok = macula:advertise(Pool, Realm, Procedure,
                      fun(#{<<"a">> := A, <<"b">> := B}) -> {ok, A + B} end,
                      #{}),

{ok, 5} = macula:call(Pool, Realm, Procedure, #{<<"a">> => 2, <<"b">> => 3}, 5_000),

ok = macula:unadvertise(Pool, Realm, Procedure).

call/5 doesn't resolve or choose a specific provider — it's "ask any of my stations to handle this." For direct-dial — resolving a specific provider's station in the DHT and dialing it in one hop, bypassing your own pool's seeds entirely — use call_station/6,7, below.

Advertising a procedure

-spec advertise(pool(), realm(), procedure(), Handler, opts()) -> ok | {error, term()}.

Build the procedure name via macula_topic — inline strings are rejected by the client validator:

ProcApp   = macula_topic:app_hope(Realm, Org, App, Domain, Verb, Version),
ProcOrg   = macula_topic:org_hope(Realm, Org, Domain, Verb, Version),
ProcRealm = macula_topic:realm_hope(Realm, Domain, Verb, Version),

Pick a tier by who owns the schema: app_hope for an app-specific procedure, org_hope for one an org exposes to itself, realm_hope for a realm authority's own procedure (e.g. check_health). See the Topic Naming Guide for the full naming convention — present-tense verbs, no CRUD, IDs in the payload never the name.

advertise/5 fans out to every link in the pool and registers the handler for replay on reconnect. Opts takes auth: open (default — serve any identified caller) or {ucan_required, Issuer} (gated — see Authorization Guide).

ok = macula:unadvertise(Pool, Realm, Procedure).

The handler contract

A handler is fun((term()) -> term()) or {Module, Function}, called as Handler(Payload). What it returns decides what the caller sees:

Keep handlers fast — there's no async-reply mechanism for unary RPC; a slow handler blocks the caller until it returns or the timeout fires. For multi-chunk or long-lived work, use the Streaming Guide instead.

Direct-dial: call_station/6,7

-spec call_station(pool(), seed(), realm(), procedure(), term(), timeout_ms()) ->
    {ok, term()} | {error, term()}.
-spec call_station(pool(), seed(), realm(), procedure(), term(), timeout_ms(), opts()) ->
    {ok, term()} | {error, term()}.  %% opts: #{ucan_token => Token}

call_station/6 dials a specific station URL directly — reusing an existing link or opening and monitoring a new one, waiting for the handshake, then calling through it. One hop, no dependency on your pool's own seed set. Use it once you've resolved which station serves the procedure:

%% 1. Resolve the procedure_advertisement from the DHT.
Key = macula_record:procedure_key(Procedure),
{ok, Records} = macula:find_records(Pool, Key),

%% 2. Read it, optionally verifying the provider's cert chain to the realm
%%    CA first -- drops advertisements from a squatter who doesn't chain.
[Advertisement | _] = Records,
#{serving_station := StationNodeId} = macula_record:read_procedure_advertisement(Advertisement),
ok = macula_record:verify_advertisement_cert_chain(RealmCaPem, Advertisement, ExpectedOrg),

%% 3. Resolve that station's dialable endpoint.
EndpointKey = macula_record:station_endpoint_key(StationNodeId),
{ok, EndpointRecord} = macula:find_record(Pool, EndpointKey),
#{quic_port := Port, host_advertised := [Host | _]} =
    macula_record:read_station_endpoint(EndpointRecord),
StationUrl = <<"quic://", Host/binary, ":", (integer_to_binary(Port))/binary>>,

%% 4. Dial it directly.
{ok, Result} = macula:call_station(Pool, StationUrl, Realm, Procedure, Payload, 5_000).

This is the same resolve done for content's find_content_providers/2 and streaming's call_stream_station/6 — one shape, three call sites. A squatter's advertisement — signed, but whose cert doesn't chain to the realm CA — is dropped by step 2 and never dialed.

call_station/7's Opts map takes ucan_token to present a capability token to a gated ({ucan_required, _}) procedure.

Errors

case macula:call(Pool, Realm, Procedure, Payload, Timeout) of
    {ok, Result} ->
        Result;
    {error, timeout} ->
        retry_later;
    {error, {disconnected, Reason}} ->
        %% the link went down mid-call; pending calls on it all fail this way
        retry_later;
    {error, {call_error, Code, Name}} ->
        %% wire-level BOLT#4 error -- see macula_bolt4:is_retryable/1
        maybe_retry(Code, Name);
    {error, Detail} ->
        %% the handler itself returned {error, Detail}
        logger:warning("RPC refused: ~p", [Detail])
end.

{error, no_healthy_station} (from call/5) or {error, not_connected} (from call_station/6) means no link has completed its handshake yet — the pool hasn't connected, or the direct-dial target hasn't finished handshaking within the deadline.

Wire-level errors carry a BOLT#4 code; macula_bolt4:is_retryable/1 tells you whether the same path is worth retrying after backoff, or whether you need a fresh resolve:

The full table, including codes not relevant to RPC, is in macula_bolt4's own moduledoc.

Procedure naming

See the Topic Naming Guide — RPC procedures and pub/sub topics share the same canonical format, built via macula_topic, never inline strings.

See also

• Streaming Guide — when one request/response isn't enough: a live feed, an upload, a duplex session.
• Authorization Guide — gating a procedure with {ucan_required, Issuer} and presenting a UCAN token to call it.
• Records Guide — the DHT record primitive procedure_advertisement is built on.