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Building an eCommerce web app with Temporal, Part 4: REST API

Valeri Karpov

Valeri Karpov


In Part 1, Part 2, and Part 3, you built and tested a shopping cart with an abandoned cart email notification using long-lived Workflows. Workflows, Activities, and Temporal's testing utilities make it easy to build and maintain features that involve external services and time, like sending an email reminder when a user hasn't touched their cart in a while.

Thus far, you've only worked with the Temporal SDK via starters and unit tests, which invoke the Temporal SDK directly. In this blog post, I'll demonstrate how you can build a RESTful API on top of Temporal Workflows, so you can create web apps and mobile apps that store data in Temporal.

API Setup#

For this tutorial, I'll be using httpx along with mux for routing and handlers for CORS.

package main
import (    "context"    ""    ""    ""    "net/http"    "os")
func main() {    var err error
    // Set up CORS for frontend    var cors = handlers.CORS(handlers.AllowedHeaders([]string{"X-Requested-With", "Content-Type", "Authorization"}), handlers.AllowedMethods([]string{"GET", "POST", "PUT", "HEAD", "OPTIONS"}), handlers.AllowedOrigins([]string{"*"}))
    http.Handle("/", cors(r))    server := httpx.NewServer(":"+HTTPPort, http.DefaultServeMux)    server.WriteTimeout = time.Second * 240
    err = server.Start()    if err != nil {        log.Fatal(err)    }}

The API endpoints will use Temporal Client methods to create Workflows, and execute Signals and Queries. For the purposes of this app, HTTP GET requests execute Queries, HTTP PUT or PATCH requests send Signals, and HTTP POST requests create new Workflows.

// Create a new cartr.Handle("/cart", http.HandlerFunc(CreateCartHandler)).Methods("POST")// Get the state of an existing cartr.Handle("/cart/{workflowID}", http.HandlerFunc(GetCartHandler)).Methods("GET")
// Add a new item to the cartr.Handle("/cart/{workflowID}/add", http.HandlerFunc(AddToCartHandler)).Methods("PUT")// Remove an item from the cartr.Handle("/cart/{workflowID}/remove", http.HandlerFunc(RemoveFromCartHandler)).Methods("PUT")// Update the cart's associated email addressr.Handle("/cart/{workflowID}/email", http.HandlerFunc(UpdateEmailHandler)).Methods("PUT")// Check outr.Handle("/cart/{workflowID}/checkout", http.HandlerFunc(CheckoutHandler)).Methods("PUT")

In this case, the API server and the Worker are separate processes. The API server is just an intermediary between the Temporal server and your API server's clients. The event history representing the cart is stored in the Temporal server.

Handler Functions#

First, let's take a look at the POST /cart endpoint. Since we've chosen to represent an individual shopping cart as a Workflow, the CreateCartHandler() function will create a new Workflow using ExecuteWorkflow(). For the purposes of this app, we need to make sure each POST /cart call creates a Workflow creates a unique workflowID.

func CreateCartHandler(w http.ResponseWriter, r *http.Request) {  // In production you should use uuids or something similar, but the  // current time is enough for this example. Make sure the Workflow ID  // is unique every time the user creates a new cart!    workflowID := "CART-" + fmt.Sprintf("%d", time.Now().Unix())
    options := client.StartWorkflowOptions{        ID:        workflowID,        TaskQueue: "CART_TASK_QUEUE",    }
    cart := app.CartState{Items: make([]app.CartItem, 0)}    we, err := temporal.ExecuteWorkflow(context.Background(), options, app.CartWorkflow, cart)    if err != nil {        WriteError(w, err)        return    }
  // Return the `workflowID` so clients can use it with other endpoints    res := make(map[string]interface{})    res["cart"] = cart    res["workflowID"] = we.GetID()
    w.WriteHeader(http.StatusCreated)    json.NewEncoder(w).Encode(res)}

Now you have a POST /cart endpoint that creates a new empty cart, and returns the workflowID that uniquely identifies this Workflow.

The next endpoint is GET /cart/{workflowID}, which returns the current state of the cart with the given WorkflowID. Below is the GetCartHandler() function, which gets the workflowID from the URL and executes a Query for the current state of the cart.

func GetCartHandler(w http.ResponseWriter, r *http.Request) {    vars := mux.Vars(r)    response, err := temporal.QueryWorkflow(context.Background(), vars["workflowID"], "", "getCart")    if err != nil {        WriteError(w, err)        return    }    var res interface{}    if err := response.Get(&res); err != nil {        WriteError(w, err)        return    }
    w.WriteHeader(http.StatusOK)    json.NewEncoder(w).Encode(res)}

PUT Requests and Signals#

For this app, HTTP PUT requests correspond to Temporal Signals. That means, in addition to the workflowID, you need to send Signal arguments. Remember that shared.go contains an AddToCartSignal struct which is what the cart Workflow's Signal handler expects:

type AddToCartSignal struct {    Route string    Item  CartItem}

The PUT /cart/{workflowID}/add handler needs to convert the HTTP request body into an AddToCartSignal as shown below.

func AddToCartHandler(w http.ResponseWriter, r *http.Request) {    vars := mux.Vars(r)    var item app.CartItem    err := json.NewDecoder(r.Body).Decode(&item)    if err != nil {        WriteError(w, err)        return    }
    update := app.AddToCartSignal{Route: app.RouteTypes.ADD_TO_CART, Item: item}
    err = temporal.SignalWorkflow(context.Background(), vars["workflowID"], "", "ADD_TO_CART_CHANNEL", update)    if err != nil {        WriteError(w, err)        return    }
    w.WriteHeader(http.StatusOK)    res := make(map[string]interface{})    res["ok"] = 1    json.NewEncoder(w).Encode(res)}

The PUT /cart/{workflowID}/remove and PUT /cart/{workflowID}/email handlers are almost identical, except they send RemoveFromCartSignal and UpdateEmailSignal, not AddToCartSignal.

func UpdateEmailHandler(w http.ResponseWriter, r *http.Request) {    vars := mux.Vars(r)
    var body UpdateEmailRequest    err := json.NewDecoder(r.Body).Decode(&body)    if err != nil {        WriteError(w, err)        return    }
    updateEmail := app.UpdateEmailSignal{Route: app.RouteTypes.UPDATE_EMAIL, Email: body.Email}
    err = temporal.SignalWorkflow(context.Background(), vars["workflowID"], "", "UPDATE_CART_CHANNEL", updateEmail)    if err != nil {        WriteError(w, err)        return    }
    w.WriteHeader(http.StatusOK)    res := make(map[string]interface{})    res["ok"] = 1    json.NewEncoder(w).Encode(res)}

Moving On#

You can build a RESTful API on top of Temporal by making HTTP POST requests create Workflows, GET requests execute Queries, and PUT requests execute Signals. This isn't the only way you can build a RESTful API with Temporal, but this pattern works well if you use long-lived Workflows to store user data. Because all of the work of updating your shopping cart happens in the Worker process, you can scale your API servers independently of your Worker processes, and rely on the Temporal server to handle the distributed computing.