My work on the flow compiler for sending campaigns and auto reply

At work, I’m working on the flow compiler, which compile from flow documents into ETA flow descriptors. The use cases are to provide our users the ability to automatically response to their customers' messages. Users can define their own flow and customize auto reply behaviours. This article gives a look into my work. Also see related article.

Package flow implements the flow document compiler and runtime controller. The compiler is responsible for parsing the flow document and compiling it into a specific format for that flow document type, and can be executed by a runtime engine. Currently, there are two compiler engines: auto reply and campaign. The compilation process is divided into five phases: scanner, parser, analyzer, generating ir, and generating output.

Package Structure

List of compiler engines:

  • AutoReply: Compile flow documents with type AUTO_REPLY to *etapb.FlowDescriptor. Use case: to reply automatically to customer messages.
  • Campaign: Compile flow document with type SENDOUT to *etapb.FlowDescriptor and campaign configuration. Use case: to send out messages to customers and reply to their responses.

The general structure:

  • compiler: Contains the generic compiler infrastructure implementation, which can be shared by different compiler engines.
  • compilers: Contains the compiler engines implementation, which is responsible for compiling each type of flow documents into specific formats.
  • registry: Contains the registry implementation, which is responsible for registering different type of objects for the purposes of scanner, parser, analyzer, and different compiler engines.
  • objects: Provides the objects which are used by the compiler engines, and their customized behaviors. Each document type has its own list of available objects. They are grouped by document type and registered in the registry.

The structure of shared packages:

  • compiler/diagnostics: The diagnostic implementation, which is a list of errors, warnings, and notes. These diagnostics can be produced by any step of the compilers.
  • compiler/syntax: Provide the tokens in form of syntax.TypedFlowObject and ast in form of syntax.Node and syntax.Edge interfaces.
  • compiler/scanner: The lexer implementation, which receives a flow document and produces syntax.TypedFlowDocument.
  • compiler/parser: The parser implementation, which receives a syntax.TypedFlowDocument and produces syntax.ParsedFlowDocument.
  • compiler/dependency: The dependency resolver implementation, dependency.Resolver, which is an interface that can be used to resolve dependencies in compiler engines and mocked in tests.
  • compiler/analyzer: The static and dependency analyzer implementation, which executes on syntax.ParsedFlowDocument and dependency.Resolver.

The structure of each compiler engine:

  • ir: The intermediate representation of the flow document, which is used by the compiler engine during the compilation. engine: The engine implementation, which is responsible for compiling the flow document into a specific format for that flow document type and runtime engine.
  • cctx: The context and state using during the compilation, which includes the logger, the diagnostic, dependency resolver, and the intermediate representation.

Importing constraints:

  • compiler: The innermost layer. Can NOT import any other layers.
  • registry: Can import compiler.
  • compilers: Can import registry and compiler packages. Each compiler engine must NOT import other compiler engines’ code.
  • objects: The outtermost layer. Can import registry, compiler, and compilers packages.


The flow document protobuf is first parsed into a syntax.TypedFlowDocument, which is a list of syntax.TypedFlowObject. Each syntax.TypedFlowObject is a token, which is a struct with id, type and value for that specific object type. The value is parsed into a specific typed protobuf message grouping under an oneof. From that value, we can access the property and data for that specific object type.

package syntax

type TypedFlowObject struct {
	ID    uuid.ID
	Type  FlowObjectType
	Model struct { // oneof
		SimpleEdge struct {
			Source string
			Target string
			Data   SimpleEdgeData
		CustomAutoReply   struct{}
		IncomingRoomEvent struct{}
		SendWaMessage     struct{}
		// ...

Package scanner implement the Scan function, which receives a flow document and produces a syntax.TypedFlowDocument:

package scanner

func Scan(diag *diagx.Result, doc *flowpb.FlowDocument) (out *syntax.TypedFlowDocument)
package syntax

type TypedFlowDocument struct {
	FlowDoc      *flowpb.FlowDocument
	ObjByID      map[uuid.UUID]*flowpb.FlowObject
	TypedObjByID map[uuid.UUID]*TypedFlowObject
	PropByObjID  map[uuid.UUID][]*flowpb.FlowObjectProperty
	PropByKey    map[Position]*flowpb.FlowObjectProperty


The syntax.TypedFlowDocument is then parsed into a syntax.ParsedFlowDocument, which is a list of syntax.Node and syntax.Edge interface. They are the abstract syntax tree (AST) of the flow document. The syntax.Node interface represents a node in the flow document. It contains the id, type, and the list of incoming and outgoing edges. It also reports whether it is visited or not so the compiler engine can decide to create a new event/action or trigger a previous action.

package syntax

type Node interface {
	GetID() uuid.UUID
	GetType() flowpb.FlowObjectType
	GetCore() *NodeCore
	GetTypedFlowObject() *TypedFlowObject
	GetVisited() bool
	GetIncomingEdges() []Edge
	GetOutgoingEdges() []Edge

type NodeCore struct {

	Visited       bool
	IncomingEdges []Edge
	OutgoingEdges []Edge

The syntax.Edge interface represents an edge in the flow document. It connects two nodes in a direction from source node to target node. A syntax.Node can have multiple incoming and outgoing edges. Each outgoing syntax.Edge can have different source handle and target handle to indicate what should happen by that edge. For example, an edge trigger after a SendWaMessage action node, with the source handle is button-click:1 to indicate that this edge should be followed when the customer clicks the first button in that template or interaction message.

package syntax

type Edge interface {
	GetID() uuid.UUID
	GetType() flowpb.FlowObjectType
	GetCore() *EdgeCore
	GetTypedFlowObject() *TypedFlowObject
	GetSourceID() uuid.UUID
	GetTargetID() uuid.UUID
	GetSrcHandle() string
	GetTgtHandle() string
	GetStartNode() Node
	GetEndNode() Node

type EdgeCore struct {
	*TypedFlowObject // simple edge

	SourceID  uuid.UUID
	TargetID  uuid.UUID
	SrcHandle string // prefix trimmed
	TgtHandle string // prefix trimmed

	StartNode Node
	EndNode   Node

The syntax.Node interface are extended by registry.Node interface, which adds a few additional interfaces to help scanner initialize the node or analyzer validate the node type. It is extended further by autoreply.Node interface which packs the AutoReplyKind() method together to indicate the kind of the node. It is then extended again by autoreply.EventNode and autoreply.ActionNode interfaces to provide additional behavior code for each kind of node: start, event, or action.

package registry

type Object interface {

type Node interface {
package autoreply

type Node interface {
	AutoReplyKind() ir.AutoReplyKind

type EventNode interface {
	ProcessEvent(cc *cctx.CompilationContext, branch *ir.Branch) (*etapb.EventDescriptor, *ir.AvailableBranches)

type ActionNode interface {
	ProcessAction(cc *cctx.CompilationContext, branch *ir.Branch) (*etapb.ActionDescriptor, *ir.AvailableBranches)

Package parser implements the Parse function, which receives a syntax.TypedFlowDocument and produces a syntax.ParsedFlowDocument, which is a list of syntax.Node and syntax.Edge:

package parser

func Parse(diag *diagx.Result, in *syntax.TypedFlowDocument) *syntax.ParsedFlowDocument
package syntax

type ParsedFlowDocument struct {
	Root  Node
	Nodes []Node
	Edges []Edge


Each compiler engine implements its own set of intermediate representation (IR) for supporting the compilation process. The auto reply engine implements the following IR in the package compilers/autoreply/ir:

package ir

type Document struct {
	Root *Branch

type Branch struct {
	Edge syntax.Edge

	Event         Event
	Trigger       Trigger
	Action        Action
	GotoBranch    *Branch
	TriggerBranch *Branch
	Branches      []*Branch

The ir.Branch type is the main IR type for the auto reply engine. It represents a branch in the output ETA. Each branch may be a single edge or multiple edge starting from a trigger (start node or template/interactive send wa message action node) going through an event node and arriving at another action node. The main job of the compilation engine is to convert the syntax.ParsedFlowDocument into a ir.Document and then convert the ir.Document into an ETA descriptor, the final output.

Object registry

Each phase of the compilation process produces a list of various types of objects implements a specific interface. For example, the scanner produces a list of syntax.TypedFlowObject which can be SimpleEdge, CustomAutoReply, or IncomingRoomEvent, etc. The parser produces a list of syntax.Node which can be CustomAutoReply, IncomingRoomEvent, or SendWaMessage, etc. And each type of document has a different set of available building blocks. For example, the auto reply document uses CustomAutoReply while the campaign document uses CustomSendCampaign. Therefore, the registry package is required to keep track of all the available building blocks for each type of document and provide a way to initialize and validate them.

The scanner.ObjectInitializer interface is used to initialize the syntax.TypedFlowObject with various types. The initializer prepares a container proto.Message of the correct type to be fed to the scanner:

package registry

type ObjectInitializer interface {
	InitTypedFlowObject(out *syntax.TypedFlowObject) proto.Message

func RegisterObjectInitializer(typ flowpb.FlowObjectType, initializer ObjectInitializer) {
	objectInitializers[typ] = initializer

The parser.DocumentClassifier interface is used to provide each the flow document type a set of available building blocks. And each building block implements parser.ObjectConstructor interface to be used by the parser to create the specific syntax.Node and syntax.Edge implementation. For example, the NewNode() method of the incoming room event implementation of parser.ObjectConstructor may receive a syntax.NodeCore with type FLOW_OBJECT_TYPE_INCOMING_ROOM_EVENT and return an incoming_room_event.IncomingRoomEvent event node wrapping around that core:

package registry

type DocumentClassifier interface {
	Category() flowpb.FlowDocumentCategory
	DocumentType() flowpb.FlowDocumentType
	StartNodeType() flowpb.FlowObjectType
	AcceptNodeType(flowpb.FlowObjectType) bool

type ObjectConstructor interface {
	IsEdge() bool
	NewEdge(*syntax.EdgeCore) syntax.Edge
	NewNode(*syntax.NodeCore) syntax.Node

Each compiler engine provides its own node classifier to further separate the kind of its own node types. For example, the auto reply engine provides the following node classifier:

package autoreply

func ValidateNodeClassifier(sample syntax.Node) error {
  kindGetter, ok := sample.(Node)
  if !ok {
    return errors.Newf("node %T does not implement autoreply.KindGetter", sample)
  switch kindGetter.AutoReplyKind() {
  case ir.KindStart:
    // nothing
  case ir.KindEvent:
    _, ok = sample.(EventNode)
    if !ok {
      return errors.Newf("event node %T does not implement autoreply.EventNode", sample)
  case ir.KindAction:
    _, ok = sample.(ActionNode)
    if !ok {
      return errors.Newf("action node %T does not implement autoreply.ActionNode", sample)

The registry packages take care of registering the available building blocks for each type of document, feeding the scanner and parser package the corresponding initializers and constructors they need. This help keep the scanner and parser code clean without worrying about the details of each type of document or object. The autoreply engine can cast the syntax.Node into autoreply.EventNode or autoreply.ActionNode to access the specific methods it needs for that kind of object.


I'm Oliver Nguyen. A software maker working mostly in Go and JavaScript. I enjoy learning and seeing a better version of myself each day. Occasionally spin off new open source projects. Share knowledge and thoughts during my journey. Connect with me on , , , and .

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