Transport Adapter Guide

This guide covers the runtime side of adding a new API transport to Reventless (e.g., OpenAPI/REST, gRPC). It is the counterpart to api-protocol-integration.md, which covers the deploy-time schema generation layer.

A transport adapter is the in-memory (or Lambda handler) code that receives an incoming request, bridges it to the framework's internal types, calls the relevant interceptors, and dispatches the command or query.

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Architecture: Two Layers

Layer	Guide	Purpose
Schema generation (deploy-time)	api-protocol-integration.md	Generates protocol-specific schema (SDL, OpenAPI spec, MCP manifest) from entry types
Resolver / handler (runtime)	this guide	Receives requests, extracts identity, calls interceptors, dispatches commands and queries

These layers are independent. A new transport needs both, but they can be developed separately.

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Identity Propagation

Every command and query carries an Identity.t value. The transport adapter is responsible for extracting it from the incoming request and passing it into the framework.

Type

// Reventless.Identity.t — parsed from JSON
type t = {
  userId: string,
  // ... additional fields from your identity provider
}

let anonymous: t  // fallback when no identity is present
let schema: S.t<t> // sury schema for parsing

Convention

Transport adapters use a dedicated header (or equivalent transport-level mechanism) to carry the identity. The in-memory GraphQL transport reads X-Identity — a JSON-encoded Identity.t value set by the caller (e.g., an auth middleware layer).

Extraction pattern

// Example: HTTP transport
let extractIdentity = (request: myTransportRequest): Reventless.Identity.t => {
  try {
    switch request->getHeader("x-identity") {
    | Some(json) => json->JSON.parseOrThrow->S.parseOrThrow(Reventless.Identity.schema)
    | None => Reventless.Identity.anonymous
    }
  } catch {
  | _ => Reventless.Identity.anonymous
  }
}

Always fall back to Reventless.Identity.anonymous on any parse error — never throw from identity extraction.

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Command Interceptor

Hook location

// reventless-core/src/components/CommandGenerator/CommandGenerator_Callback.res

type interceptResult = Allow | Deny(string)

type commandComponentKind = Aggregate | StateChangeSlice

type commandInterceptor = (
  ~identity: Reventless.Identity.t,
  ~componentName: string,   // aggregate / slice name
  ~componentKind: commandComponentKind,
  ~tag: string,             // command variant name (e.g. "AddProduct")
  ~args: JSON.t,            // raw command arguments (includes entity ID)
) => promise<interceptResult>

let commandInterceptorHook: ref<option<commandInterceptor>> = ref(None)

The hook is a module-level ref set once at application startup. None means passthrough (default).

When to call it

Call commandInterceptorHook after identity extraction and before calling generateCommand. The framework's CommandGenerator_Callback.makeGenerateCommand already does this — transport adapters do not call the hook directly. Instead, they construct a CommandGenerator.payload with the extracted identity and pass it to generateCommand, which calls the hook internally.

Payload construction

// CommandGenerator.payload — the struct passed to generateCommand
type payload = {
  command: string,           // variant constructor name (e.g. "AddProduct")
  arguments: 'a,             // command args object (entity ID + command fields)
  meta: {
    ip: array<string>,
    user: string,            // identity.userId
    info: string,            // human-readable source (e.g. "Mutation.Catalog_AddProduct")
  },
  identity: Reventless.Identity.t,
}

The identity flows through the payload into makeGenerateCommand, where the hook intercepts it before publishJsons is called.

DCB StateChangeSlice specifics

For DCB mutations, the entity ID lives inside a tagged field (annotated with @s.matches(DcbTag.string)) rather than as a separate id parameter. Before constructing the payload, extract the tagged field value and inject it as "id" into the args dict:

// Find the DcbTag-annotated field in the command schema, extract its value,
// add it as "id" so generateCommand sees a consistent payload shape.
switch idFieldName {
| Some(idField) if idField != "id" =>
  let id = switch argsDict->Dict.get(idField) {
  | Some(JSON.String(s)) => s
  | _ => ""
  }
  argsDict->Dict.set("id", JSON.Encode.string(id))
| _ => ()
}

Error response on Deny

makeGenerateCommand throws a JsError with the Deny message. The transport adapter should catch this and return the appropriate protocol-level error (HTTP 403, GraphQL error, etc.).

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Query Interceptor

Hook location

// reventless-core/src/components/QueryDb/QueryDb_Callback.res

type interceptResult = Allow | Deny(string)

type queryInterceptor = (
  ~identity: Reventless.Identity.t,
  ~readModelName: string,  // the QueryDb / read model name
  ~args: JSON.t,           // raw query arguments (filters, pagination, etc.)
) => promise<interceptResult>

/** None = passthrough (the default). */
let queryInterceptorHook: ref<option<queryInterceptor>> = ref(None)

let registerQueryInterceptor: queryInterceptor => unit
let clearQueryInterceptor: unit => unit

The AWS provider already consults this hook from its query resolvers, on both the DynamoDB and Postgres read paths.

When to call it

Unlike the command interceptor (which is called inside makeGenerateCommand), the query interceptor must be called directly by each transport adapter resolver, because there is no equivalent to generateCommand on the query path.

Call the hook before fetching from the QueryDb:

let runQueryInterceptor = async (
  ~identity: Reventless.Identity.t,
  ~readModelName: string,
  ~args: JSON.t,
): QueryDb_Callback.interceptResult => {
  switch QueryDb_Callback.queryInterceptorHook.contents {
  | None => Allow
  | Some(interceptor) =>
    await interceptor(~identity, ~readModelName, ~args)
  }
}

// In each resolver:
switch await runQueryInterceptor(~identity, ~readModelName=name, ~args) {
| Deny(_) => /* return empty / 403 */
| Allow => /* proceed with QueryDb fetch */
}

Response on Deny

Return the transport-appropriate empty or error response:

Query type	On Deny
Single item (byId)	null / 404
List / paginated	Empty list, zero count
Index query	Empty list

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Two-Phase Mutation Registration (in-memory)

The in-memory transport must register resolvers in two phases because generateCommand is produced inside an Output.apply chain, which fires asynchronously after the synchronous Plugin_Builder.construct() call.

Phase 1 — synchronous (during construct())

Register the protocol schema (SDL fields, OpenAPI paths, etc.) and resolver stubs with empty handler refs:

let handlerRefs: dict<ref<option<CommandGenerator.commandGenerator>>> = Dict.make()

let register = (~fields: array<string>, ~commandSchema: S.t<unknown>) => {
  fields->Array.forEach(field => {
    let handlerRef = ref(None)
    handlerRefs->Dict.set(field, handlerRef)
    let resolver = async (request) => {
      switch handlerRef.contents {
      | Some(generateCommand) => /* dispatch */
      | None => /* not yet bound — return error or queue */
      }
    }
    // register resolver stub with your transport server
  })
}

Phase 1 is called synchronously via the aggregateMutationResolverHook (aggregates) or dcbMutationResolverHook (DCB slices).

Phase 2 — async (inside Output.apply)

Bind the real generateCommand to the pre-registered stubs:

let make: CommandGenerator_Adapter.resolversMaker<...> = (~name as _, ~fields, ...) => {
  let generateCommand = /* retrieved from pending slot or passed directly */
  fields->Array.forEach(field => {
    switch handlerRefs->Dict.get(field) {
    | Some(handlerRef) => handlerRef.contents = Some(generateCommand)
    | None => ()
    }
  })
  {resources: []}
}

The CommandGenerator_Adapter.resolversMaker type signature is the contract between the framework and the transport adapter for Phase 2.

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Inbound Translation

InboundTranslationSlice transports follow a simpler, single-phase pattern. The resolver receives an external payload (e.g., a webhook body) and calls a receive function that validates and translates it into a command.

// Register a receive handler for an inbound translation endpoint
let receiveRegistry: dict<JSON.t => promise<result<string, string>>> = Dict.make()

let registerInbound = (~fieldName: string) => {
  // register your route/resolver stub, storing into receiveRegistry
}

let bindReceiveHandler = (
  ~fieldName: string,
  ~receive: JSON.t => promise<result<string, string>>,
) => {
  receiveRegistry->Dict.set(fieldName, receive)
}

The receive function returns Ok(msgId) on success or Error(message) on validation/translation failure.

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Query Resolver Types

Four standard query patterns map to QueryDb operations. Each transport adapter implements all four:

Pattern	Parameters	QueryDb call
byId	id: string, optional subId: string	getById
list	nextToken?: string, limit?: int	scan (paginated)
byIdList	id: string	getByIdList
byIndex	indexValue: string (one per index)	getByIndex

The GraphQL transport generates resolver field names from QueryDb_Adapter.resolversMaker and Plugin_Helpers.queryFieldNamesRegistry. Use the same registry for your transport to ensure consistent naming across all protocols.

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Hooks Reference

Hook	Location	Called by	Purpose
commandInterceptorHook	CommandGenerator_Callback	makeGenerateCommand (internal)	Intercept all commands before publish
queryInterceptorHook	QueryDb_Callback	Transport adapter query resolvers	Intercept all queries before QueryDb fetch
aggregateMutationResolverHook	Plugin_Builder	Plugin_Builder.construct	Register aggregate mutation resolvers (Phase 1)
dcbMutationResolverHook	Dcb_Builder	Dcb_Builder.construct	Register DCB mutation resolvers (Phase 1)
dcbMutationBindHook	Dcb_Builder	Output.apply in Dcb_Builder	Bind DCB mutation handlers (Phase 2)

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Checklist

• extractIdentity reads from the transport's auth mechanism, falls back to Identity.anonymous
• Command payload includes identity field populated from extractIdentity
• DCB commands: tagged ID field extracted and injected as "id" in args before payload construction
• Phase 1 registration is synchronous (called during construct())
• Phase 2 binding is inside Output.apply or equivalent async chain
• Deny from command interceptor mapped to transport-appropriate error (e.g., HTTP 403)
• Query resolvers call queryInterceptorHook before any QueryDb operation
• Deny from query interceptor returns empty/null — not a hard error (no data exposed)
• All four query patterns implemented: byId, list, byIdList, byIndex
• CommandGenerator_Adapter.resolversMaker type satisfied for Phase 2
• Tests cover: identity extraction, Allow passthrough, Deny response shape