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Faas-flow - Function Composition for OpenFaaS

Template CI License: MIT OpenTracing Badge OpenFaaS GoDoc

  • [x] Pure              FaaS with OpenFaaS
  • [x] Fast               Built with Go
  • [x] Secured        With HMAC
  • [x] Stateless      By design
  • [x] Tracing         With open-tracing
  • [x] Available      As faas-flow template

Faas-flow tower visualizes and monitors flow functions.


Faas-flow allows you to realize OpenFaaS function composition with ease. By defining a simple pipeline, you can orchestrate multiple functions without having to worry about the internals.

func Define(flow *faasflow.Workflow, context *faasflow.Context) (err error) {
    return nil

After building and deploying, it will give you an OpenFaaS function that orchestrates calling Func2 with the output of Func1.

Use Cases

Faas-flow as a function composure provides the back-bone for building complex solutions and promote automation.

Data Processing Pipeline

Faas-flow can orchestrate a pipeline with long and short running function performing ETL jobs without having to orchestrate them manually or maintaining a separate application. Faas-flow ensures the execution order of several functions running in parallel or dynamically and provides rich construct to aggregate results while maintaining the intermediate data.

Application Orchestration Workflow

Functions are great for isolating certain functionalities of an application. Although one still need to call the functions, write workflow logic, handle parallel processing and retries on failures. Using Faas-flow you can combine multiple OpenFaaS functions with little codes while your workflow will scale up/down automatically to handle the load.

Function Reusability

Fass-flow allows you to write function only focused on solving one problem without having to worry about the next. It makes function loosely coupled from the business logic promoting reusability. You can write the stateless function and use it across multiple applications, where Faas-flow maintains the execution state for individual workflow per requests.

Pipeline Definition

By supplying a number of pipeline operators, the complex composition can be achieved with little work: alt overview

The above pipelines can be achieved with little, but powerful code:

Sync chain

func Define(flow *faasflow.Workflow, context *faasflow.Context) (err error) {
        .Modify(func(data []byte) ([]byte, error) {
            // do something
            return data, nil
    return nil

Async chain

func Define(flow *faasflow.Workflow, context *faasflow.Context) (err error) {
    dag := flow.Dag()
        .Modify(func(data []byte) ([]byte, error) {
            // do something
            return data, nil
    dag.Edge("n1", "n2")
    dag.Edge("n2", "n3")
    return nil

Parallel branching

func Define(flow *faasflow.Workflow, context *faasflow.Context) (err error) {
    dag := flow.Dag()
    dag.Node("n1").Modify(func(data []byte) ([]byte, error) {
        // do something
        return data, nil
    dag.Node("n3").Apply("func2").Modify(func(data []byte) ([]byte, error) {
        // do something
        return data, nil
    dag.Node("n4", faasflow.Aggregator(func(data map[string][]byte) ([]byte, error) {
        // aggregate branch result data["n2"] and data["n3"]
        return []byte(""), nil

    dag.Edge("n1", "n2")
    dag.Edge("n1", "n3")
    dag.Edge("n2", "n4")
    dag.Edge("n3", "n4")
    return nil

Dynamic branching

func Define(flow *faasflow.Workflow, context *faasflow.Context) (err error) {
    dag := flow.Dag()
    dag.Node("n1").Modify(func(data []byte) ([]byte, error) {
        // do something
        return data, nil
    conditionalDags := dag.ConditionalBranch(
        []string{"c1", "c2"}, // possible conditions
        func(response []byte) []string {
            // for each returned condition the corresponding branch will execute
            // this function executes in the runtime of condition C
            return []string{"c1", "c2"}
        faasflow.Aggregator(func(data map[string][]byte) ([]byte, error) {
            // aggregate all dynamic branches results
            return []byte(""), nil

    conditionalDags["c2"].Node("n1").Apply("func1").Modify(func(data []byte) ([]byte, error) {
        // do something
        return data, nil
    foreachDag := conditionalDags["c1"].ForEachBranch(
        func(data []byte) map[string][]byte {
            // for each returned key in the hashmap a new branch will be executed
            // this function executes in the runtime of foreach F
            return map[string][]byte{"f1": data, "f2": data}
        faasflow.Aggregator(func(data map[string][]byte) ([]byte, error) {
            // aggregate all dynamic branches results
            return []byte(""), nil
    foreachDag.Node("n1").Modify(func(data []byte) ([]byte, error) {
        // do something
        return data, nil
    dag.Edge("n1", "C")
    dag.Edge("C", "n2")

    return nil

Full implementation of the above examples are available here.

OpenFaaS flow librray are available as

Faas-flow Design

The current design consideration is made based on the below goals:

  1. Leverage the OpenFaaS platform
  2. Not to violate the notions of function
  3. Provide flexibility, scalability, and adaptability

Just as function as any other

Faas-flow is deployed and provisioned just like any other OpenFaaS function. It allows Faas-flow to take advantage of rich functionalities available on OpenFaaS. Faas-flow provide an OpenFaaS template (faas-flow) and just like any other OpenFaaS function it can be deployed with faas-cli.

alt its a function

Adapter pattern for zero instrumentation in code

Faas-flow function follows the adapter pattern. Here the adaptee is the functions and the adapter is the flow. For each node execution, Faas-flow handle the calls to the functions. Once the execution is over, it forwards an event to itself. This way the arrangement logic is separated from the functions and is implemented in the adapter. Compositions need no code instrumentations, making functions completely independent of the details of the compositions.

alt function is independent of composition

Aggregate pattern as chaining

Aggregation of separate function calls is done as chaining. Multiple functions can be called from a single node with order maintained as per the chain. This way one execution node can be implemented as an aggregator function that invokes multiple functions collects the results, optionally applies business logic, and returns a consolidated response to the client or forward to next nodes. Faas-flow fuses the adapter pattern and aggregate pattern to support more complex use cases.

alt aggregation

Event driven iteration

OpenFaaS uses Nats for event delivery and Faas-flow leverages OpenFaaS platform. Node execution in Faas-flow starts by a completion event of one or more previous nodes. A completion event denotes that all the previous dependent nodes have completed. The event carries the execution state and identifies the next node to execute. With events Faas-flow asynchronously carry-on execution of nodes by iterating itself over and over till all nodes are executed.

alt iteration

3rd party KV store for coordination

When executing branches, one node is dependent on more than one predecessor nodes. In that scenario, the event for completion is generated by coordination of earlier nodes. Like any distributed system the coordination is achieved via a centralized service. Faas-flow keeps the logic of the coordination controller inside of Faas-flow implementation and lets the user use any external synchronous KV store by implementing StateStore.

alt coordination

3rd party Storage for intermediate data

Results from function execution and intermediate data can be handled by the user manually. Faas-flow provides data-store for intermediate result storage. It automatically initializes, store, retrieve and remove data between nodes. This fits great for data processing applications. Faas-flow keeps the logic of storage controller inside of Faas-flow implementation and lets the user use any external object storage by implementing DataStore.

alt storage

Faas-flow design is not fixed and like any good design, it is evolving. Please contribute to make it better.

Getting Started

Deploy OpenFaaS

FaasFlow requires the OpenFaaS to be deployed and the OpenFaaS Cli to be installed. You can either have your OpenFaaS deployed in Kubernets or in Swarm.

To deploy OpenFaaS and to install the OpenFaaS cli client follow this guide:

Deploy Faas-flow Components with Faas-flow Infra

Faas-Flow infra provides the kubernetes and swarm deployment resources for faas-flow dependencies. Follow the README to deploy Faas-Flow Infra in Kubernets or in Swarm

Deploy Faas-flow Tower

Faas-Flow tower provides the dashboard to visualise and monitor your flow. Follow the README to deploy Faas-Flow tower on OpenFaaS

Writing Flow

This example implements a very simple flow to Greet

Get template

Pull faas-flow template with the faas-cli

faas template pull

Create new flow function

Create a new function using faas-flow template

faas new greet --lang faas-flow

Edit stack.yml

Edit function stack file greet.yml

  lang: faas-flow
  handler: ./greet
  image: greet:latest
    faas-flow: 1
    faas-flow-desc: "test flow to greet"
    - flow.yml
    - s3-secret-key
    - s3-access-key

Add configuration

Add a separate configuration file flow.yml with faas-flow related configuration.

  gateway: "gateway.openfaas:8080" # The address of OpenFaaS gateway
  enable_tracing: true # tracing allows to monitor requests
  trace_server: "jaeger-agent.faasflow:5775" # The address of jaeger tracing agent
  consul_url: "consul.faasflow:8500" # The address of consul
  s3_url: "minio.faasflow:9000" # The address of minio

Edit flow definition

Edit greet/handler.go and Update Define()

func Define(flow *faasflow.Workflow, context *faasflow.Context) (err error) {
    flow.SyncNode().Modify(func(data []byte) ([]byte, error) {
        result := "Hello " + string(data)
        return []byte(result), nil
    return nil

Build and Deploy

Build and deploy

faas build -f greet.yml
faas deploy -f greet.yml

This function will generate one Synchronous node

Modify("name") -> Hello name

All calls will be performed in one single execution of the flow function and result will be returned to the callee.

Note: For flow that has more than one nodes, Faas-flow doesn't return any response. External storage or callback can be used to retrieve an async result.


echo "Adam" | faas invoke greet

Request Tracking by ID

For each new request, faas-flow generates a unique Request Id for the flow. The same Id is used when logging.

2018/08/13 07:51:59 [Request `bdojh7oi7u6bl8te4r0g`] Created
2018/08/13 07:52:03 [Request `bdojh7oi7u6bl8te4r0g`] Received

The assigned request Id is set on the response header X-Faas-Flow-Reqid One may provide custom request Id by setting X-Faas-Flow-Reqid in the request header.

Request Tracing with Faas-Flow-Tower

FaasFlow Tower enables the real time monitoring for each requests. Request traces are visible when enable_tracing is enabled. FaaSFlow is the best way to monitor flows and execution status of each node for each request.

Below is an example of tracing page for a request of faas-flow-example.

alt monitoring

Use of Callback

To receive a result of long running FaaSFlow request, you can specify the X-Faas-Flow-Callback-Url. FaaSFlow will invoked the callback URL with the final result and with the request ID set as X-Faas-Flow-Reqid in request Header.

Note: X-Callback-Url from OpenFaaS is not supported in FaaSFlow.

Pause, Resume or Stop Request

A request in faas-flow has three states:

  1. Running
  2. Paused
  3. Stopped

Faas-flow doesn't keep the state of a finished request

To pause a running request:

faas invoke <workflow_name> --query pause-flow=<request_id>

To resume a paused request

faas invoke <workflow_name> --query resume-flow=<request_id>

To stop an active (paused/running) request

faas invoke <workflow_name> --query stop-flow=<request_id>

Use of context

Context can be used inside definition for different use cases. Context provide various information such as:

  • HttpQuery to retrieve original request queries
  • State to get flow state
  • Node to get current node along with that it wraps the DataStore to store data

Store data in context with DataStore

Context uses DataStore to store/retrieve data. User can do the same by calling Get(), Set(), and Del() from context:

    Modify(func(data []byte) {
        // parse data and set to be used later
        // json.Unmarshal(&req, data)
        context.Set("commitsha", req.Sha)
    .Modify(func(data []byte) {
        // retrieve the data that was set in the context
        commitsha, _ = context.GetString("commitsha")
        // use the query

Getting Http Query to Workflow

Http Query to flow can be used retrieved from context using context.Query

    .Apply("myfunc", Query("auth-token", context.Query.Get("token"))) // pass as a function query
    .Modify(func(data []byte) {
        token = context.Query.Get("token") // get query inside modifier

Use of request context

Node, requestId, State is provided by the context

currentNode := context.GetNode()
requestId := context.GetRequestId()
state := context.State

for more details check Faas-flow GoDoc.

External StateStore for coordination controller

Faas-flow implements coordination controller and store the intermediate request with StateStore. By default Faas-flow uses consul as default state-store, although user can define custom state-store with StateStore interface and use any external Synchronous KV store as backend.

type StateStore interface {
    // Configure the StateStore with flow name and request ID
    Configure(flowName string, requestId string)
    // Initialize the StateStore (called only once in a request span)
    Init() error
    // Set a value (override existing, or create one)
    Set(key string, value string) error
    // Get a value
    Get(key string) (string, error)
    // Compare and Update a value
    Update(key string, oldValue string, newValue string) error
    // Cleanup all the resorces in StateStore (called only once in a request span)
    Cleanup() error

The custom StateStore can be set with OverrideStateStore() at function/handler.go:

// OverrideStateStore provides the override of the default StateStore
func OverrideStateStore() (faasflow.StateStore, error) {
    myss, err := myStateStore.Init()
    return myss, err

StateStore is mandatory for a FaaSFlow to operate.

Official state-stores

External DataStore for storage controller

Faas-flow uses the DataStore to store partially completed data between nodes and request context data. By default Faas-flow uses minio as default data-store, although user can define custom data-store with DataStore interface and use any external storage as backend.

 type DataStore interface {
    // Configure the DaraStore with flow name and request ID
    Configure(flowName string, requestId string)
    // Initialize the DataStore (called only once in a request span)
    Init() error
    // Set store a value for key, in failure returns error
    Set(key string, value string) error
    // Get retrives a value by key, if failure returns error
    Get(key string) (string, error)
    // Del delets a value by a key
    Del(key string) error
    // Cleanup all the resorces in DataStore
    Cleanup() error

Data Store can be implemented and set by user at the OverrideDataStore() at function/handler.go:

// OverrideDataStore provides the override of the default DataStore
func OverrideDataStore() (faasflow.DataStore, error) {
    myds, err := myDs.Init()
    return myds, err

DataStore is mandatory for a FaaSFlow to operate.

Available data-stores

  • MinioDataStore: allows to store data in amazon s3 or local minio DB (default).

Cleanup with Finally()

Finally provides an efficient way to perform post-execution steps of the flow. If specified Finally() invokes in case of both failure and success of the flow. A Finally method can be set as:

func Define(flow *faasflow.Workflow, context *faasflow.Context) (err error) {
    // Define flow
    flow.SyncNode().Modify(func(data []byte) {
        // parse data and set to be used later
        // json.Unmarshal(&req, data)
        context.Set("commitsha", req.Sha)
    Apply("myfunc").Modify(func(data []byte) {
        // retrieve the data in different node from context
        commitsha, _ = context.GetString("commitsha")
    flow.OnFailure(func(err error) {
        // failure handler
    flow.Finally(func() {
        // delete the state resource


Join Faasflow Slack for more.

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