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Build an Event-Driven Uptime Monitor in Go 🚀

marcuskohlberg

Marcus Kohlberg

Posted on October 20, 2023

Build an Event-Driven Uptime Monitor in Go 🚀

TL;DR

This guide shows you how to build an uptime monitoring tool using Go and Encore. Get your entire application running in the cloud in 30 minutes!

🚀 What's on deck:

  • Install Encore
  • Create your app from a starter branch
  • Run locally to try the frontend
  • Build the backend
  • Deploy to Encore's free development cloud

✨ Final result:

Uptime Monitor

Demo app: Try the app

When we're done, we'll have a backend with this event-driven architecture:

Uptime Monitor Architecture
In this diagram (automatically generated by Encore) you can see individual services as white boxes, and Pub/Sub topics as black boxes.

🏁 Let's go!

To make it easier to follow along, we've laid out a trail of croissants to guide your way.

Whenever you see a 🥐 it means there's something for you to do!

💽 Install Encore

Install the Encore CLI to run your local environment:

  • macOS: brew install encoredev/tap/encore
  • Linux: curl -L https://encore.dev/install.sh | bash
  • Windows: iwr https://encore.dev/install.ps1 | iex

Create your Encore application

🥐 Create your new app from this starter branch with a ready-to-go frontend to use:



encore app create uptime --example=github.com/encoredev/example-app-uptime/tree/starting-point
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💻 Run your app locally

🥐 Check that your frontend works by running your app locally. 



cd uptime
encore run
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You should see this:
Encore Run This means Encore has started your local environment and created local infrastructure for Pub/Sub and Databases.

Then visit http://localhost:4000/frontend/ to see the frontend.
The functionality won't work yet, since we haven't yet built the backend yet.

– Let's do that now!

🔨 Create the monitor service

Let's start by creating the functionality to check if a website is currently up or down.

Later we'll store this result in a database so we can detect when the status changes and
send alerts.

🥐 Create a service named monitor containing a file named ping.go. With Encore, you do this by creating a Go package:



mkdir monitor
touch monitor/ping.go
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🥐 Add an API endpoint named Ping that takes a URL as input and returns a response indicating whether the site is up or down.

With Encore you do this by creating a function and adding the //encore:api annotation to it.

Paste this into the ping.go file:



package monitor

import (
    "context"
    "net/http"
    "strings"
)

// PingResponse is the response from the Ping endpoint.
type PingResponse struct {
    Up bool `json:"up"`
}

// Ping pings a specific site and determines whether it's up or down right now.
//
//encore:api public path=/ping/*url
func Ping(ctx context.Context, url string) (*PingResponse, error) {
    // If the url does not start with "http:" or "https:", default to "https:".
    if !strings.HasPrefix(url, "http:") && !strings.HasPrefix(url, "https:") {
        url = "https://" + url
    }

    // Make an HTTP request to check if it's up.
    req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
    if err != nil {
        return nil, err
    }
    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        return &PingResponse{Up: false}, nil
    }
    resp.Body.Close()

    // 2xx and 3xx status codes are considered up
    up := resp.StatusCode < 400
    return &PingResponse{Up: up}, nil
}
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🥐 Let's try it! Make sure you have Docker installed and running, then run encore run in your terminal and you should see the service start up.

🥐 Now open up the Local Dev Dashboard running at http://localhost:9400 and try calling
the monitor.Ping endpoint, passing in google.com as the URL.

If you prefer to use the terminal instead run curl http://localhost:4000/ping/google.com in a new terminal instead. Either way, you should see the response:



{"up": true}
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You can also try with httpstat.us/400 and some-non-existing-url.com and it should respond with {"up": false}.
(It's always a good idea to test the negative case as well.)

🧪 Add a test

🥐 Let's write an automated test so we don't break this endpoint over time. Create the file monitor/ping_test.go and add this code:



package monitor

import (
    "context"
    "testing"
)

func TestPing(t *testing.T) {
    ctx := context.Background()
    tests := []struct {
        URL string
        Up  bool
    }{
        {"encore.dev", true},
        {"google.com", true},
        // Test both with and without "https://"
        {"httpbin.org/status/200", true},
        {"https://httpbin.org/status/200", true},

        // 4xx and 5xx should considered down.
        {"httpbin.org/status/400", false},
        {"https://httpbin.org/status/500", false},
        // Invalid URLs should be considered down.
        {"invalid://scheme", false},
    }

    for _, test := range tests {
        resp, err := Ping(ctx, test.URL)
        if err != nil {
            t.Errorf("url %s: unexpected error: %v", test.URL, err)
        } else if resp.Up != test.Up {
            t.Errorf("url %s: got up=%v, want %v", test.URL, resp.Up, test.Up)
        }
    }
}
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🥐 Run encore test ./... to check that it all works as expected. You should see something like this:



$ encore test ./...
9:38AM INF starting request endpoint=Ping service=monitor test=TestPing
9:38AM INF request completed code=ok duration=71.861792 endpoint=Ping http_code=200 service=monitor test=TestPing
[... lots more lines ...]
PASS
ok      encore.app/monitor      1.660
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🎉 It works. Well done!

🔨 Create site service

Next, we want to keep track of a list of websites to monitor.

Since most of these APIs will be simple CRUD (Create/Read/Update/Delete) endpoints, let's build this service using GORM, an ORM library that makes building CRUD endpoints really simple.

🥐 Create a new service named site with a SQL database. To do so, create a new directory site in the application root with migrations folder inside that folder:



mkdir site
mkdir site/migrations
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🥐 Add a database migration file inside that folder, named 1_create_tables.up.sql. The file name is important (it must look something like 1_<name>.up.sql) as Encore uses the file name to automatically run migrations.

Add the following contents:



CREATE TABLE sites (
    id BIGSERIAL PRIMARY KEY,
    url TEXT NOT NULL
);
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🥐 Next, install the GORM library and PostgreSQL driver:



go get -u gorm.io/gorm gorm.io/driver/postgres
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Now let's create the site service itself. To do this we'll use Encore's support for dependency injection to inject the GORM database connection.

🥐 Create site/service.go and add this code:



package site

import (
    "encore.dev/storage/sqldb"
    "gorm.io/driver/postgres"
    "gorm.io/gorm"
)

//encore:service
type Service struct {
    db *gorm.DB
}

var siteDB = sqldb.Named("site").Stdlib()

// initService initializes the site service.
// It is automatically called by Encore on service startup.
func initService() (*Service, error) {
    db, err := gorm.Open(postgres.New(postgres.Config{
        Conn: siteDB,
    }))
    if err != nil {
        return nil, err
    }
    return &Service{db: db}, nil
}
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🥐 With that, we're now ready to create our CRUD endpoints.
Create the following files:

site/get.go:



package site

import "context"

// Site describes a monitored site.
type Site struct {
    // ID is a unique ID for the site.
    ID int `json:"id"`
    // URL is the site's URL.
    URL string `json:"url"`
}

// Get gets a site by id.
//
//encore:api public method=GET path=/site/:siteID
func (s *Service) Get(ctx context.Context, siteID int) (*Site, error) {
    var site Site
    if err := s.db.Where("id = $1", siteID).First(&site).Error; err != nil {
        return nil, err
    }
    return &site, nil
}
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site/add.go:



package site

import "context"

// AddParams are the parameters for adding a site to be monitored.
type AddParams struct {
    // URL is the URL of the site. If it doesn't contain a scheme
    // (like "http:" or "https:") it defaults to "https:".
    URL string `json:"url"`
}

// Add adds a new site to the list of monitored websites.
//
//encore:api public method=POST path=/site
func (s *Service) Add(ctx context.Context, p *AddParams) (*Site, error) {
    site := &Site{URL: p.URL}
    if err := s.db.Create(site).Error; err != nil {
        return nil, err
    }
    return site, nil
}
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site/list.go:



package site

import "context"

type ListResponse struct {
    // Sites is the list of monitored sites.
    Sites []*Site `json:"sites"`
}

// List lists the monitored websites.
//
//encore:api public method=GET path=/site
func (s *Service) List(ctx context.Context) (*ListResponse, error) {
    var sites []*Site
    if err := s.db.Find(&sites).Error; err != nil {
        return nil, err
    }
    return &ListResponse{Sites: sites}, nil
}
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site/delete.go:



package site

import "context"

// Delete deletes a site by id.
//
//encore:api public method=DELETE path=/site/:siteID
func (s *Service) Delete(ctx context.Context, siteID int) error {
    return s.db.Delete(&Site{ID: siteID}).Error
}
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🥐 Restart encore run to cause the site database to be created, and then call the site.Add endpoint:



curl -X POST 'http://localhost:4000/site' -d '{"url": "https://encore.dev"}'
{
  "id": 1,
  "url": "https://encore.dev"
}
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📝 Record uptime checks

In order to notify when a website goes down or comes back up, we need to track the previous state it was in.

To do so, let's add a database to the monitor service as well.

🥐 Create the directory monitor/migrations and the file monitor/migrations/1_create_tables.up.sql:



CREATE TABLE checks (
    id BIGSERIAL PRIMARY KEY,
    site_id BIGINT NOT NULL,
    up BOOLEAN NOT NULL,
    checked_at TIMESTAMP WITH TIME ZONE NOT NULL
);
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We'll insert a database row every time we check if a site is up.

🥐 Add a new endpoint Check to the monitor service, that takes in a Site ID, pings the site, and inserts a database row in the checks table.

For this service we'll use Encore's sqldb package instead of GORM (in order to showcase both approaches).

Add this to monitor/check.go:




package monitor

import (
    "context"

    "encore.app/site"
    "encore.dev/storage/sqldb"
)

// Check checks a single site.
//
//encore:api public method=POST path=/check/:siteID
func Check(ctx context.Context, siteID int) error {
    site, err := site.Get(ctx, siteID)
    if err != nil {
        return err
    }
    result, err := Ping(ctx, site.URL)
    if err != nil {
        return err
    }
    _, err = sqldb.Exec(ctx, `
        INSERT INTO checks (site_id, up, checked_at)
        VALUES ($1, $2, NOW())
    `, site.ID, result.Up)
    return err
}
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🥐 Restart encore run to cause the monitor database to be created, and then call the new monitor.Check endpoint:



curl -X POST 'http://localhost:4000/check/1'
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🥐 Inspect the database to make sure everything worked:



encore db shell monitor
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You should see this:




psql (14.4, server 14.2)
Type "help" for help.

monitor=> SELECT * FROM checks;
 id | site_id | up |          checked_at
----+---------+----+-------------------------------
  1 |       1 | t  | 2022-10-21 09:58:30.674265+00
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If that's what you see, everything's working great!🎉

⏰ Add a cron job to check all sites

We now want to regularly check all the tracked sites so we can
immediately respond in case any of them go down.

We'll create a new CheckAll API endpoint in the monitor service that will list all the tracked sites and check all of them.

🥐 Let's extract some of the functionality we wrote for the
Check endpoint into a separate function.

In monitor/check.go it should look like so:



// Check checks a single site.
//
//encore:api public method=POST path=/check/:siteID
func Check(ctx context.Context, siteID int) error {
    site, err := site.Get(ctx, siteID)
    if err != nil {
        return err
    }
    return check(ctx, site)
}

func check(ctx context.Context, site *site.Site) error {
    result, err := Ping(ctx, site.URL)
    if err != nil {
        return err
    }
    _, err = sqldb.Exec(ctx, `
        INSERT INTO checks (site_id, up, checked_at)
        VALUES ($1, $2, NOW())
    `, site.ID, result.Up)
    return err
}
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Now we're ready to create our new CheckAll endpoint.

🥐 Create the new CheckAll endpoint inside monitor/check.go:



import "golang.org/x/sync/errgroup"

// CheckAll checks all sites.
//
//encore:api public method=POST path=/checkall
func CheckAll(ctx context.Context) error {
    // Get all the tracked sites.
    resp, err := site.List(ctx)
    if err != nil {
        return err
    }

    // Check up to 8 sites concurrently.
    g, ctx := errgroup.WithContext(ctx)
    g.SetLimit(8)
    for _, site := range resp.Sites {
        site := site // capture for closure
        g.Go(func() error {
            return check(ctx, site)
        })
    }
    return g.Wait()
}
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This uses an errgroup to check up to 8 sites concurrently, aborting early if we encounter any error. (Note that a website being down is not treated as an error.)

🥐 Run go get golang.org/x/sync/errgroup to install that dependency.

🥐 Now that we have a CheckAll endpoint, define a cron job to automatically call it every 5 minutes.

Add this to monitor/check.go:



import "encore.dev/cron"

// Check all tracked sites every 5 minutes.
var _ = cron.NewJob("check-all", cron.JobConfig{
    Title:    "Check all sites",
    Endpoint: CheckAll,
    Every:    5 * cron.Minute,
})
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Note: For ease of development, cron jobs are not triggered when running the application locally, but work when deploying the application to a cloud environment.

🚀 Deploy to Encore's free development cloud

To try out your uptime monitor for real, let's deploy it to Encore's development cloud.

Encore comes with built-in CI/CD, and the deployment process is as simple as a git push encore.

(You can also integrate with GitHub to activate per Pull Request Preview Environments, learn more in the CI/CD docs.)

🥐 Deploy your app by running:



git add -A .
git commit -m 'Initial commit'
git push encore
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Encore will now build and test your app, provision the needed infrastructure, and deploy your application to the cloud.

After triggering the deployment, you will see a URL where you can view its progress in Encore's Cloud Dashboard. It will look something like: https://app.encore.dev/$APP_ID/deploys/...

From there you can also see metrics, traces, link your app to a GitHub repo to get automatic deploys on new commits, and connect your own AWS or GCP account to use for production deployment.

🥐 When the deploy has finished, you can try out your uptime monitor by going to:
https://staging-$APP_ID.encr.app/frontend.

You now have an Uptime Monitor running in the cloud, well done!✨

Publish Pub/Sub events when a site goes down

An uptime monitoring system isn't very useful if it doesn't
actually notify you when a site goes down.

To do so let's add a Pub/Sub topic
on which we'll publish a message every time a site transitions from being up to being down, or vice versa.

🥐 Define the topic using Encore's Pub/Sub package in a new file, monitor/alerts.go:



package monitor

import "encore.dev/pubsub"

// TransitionEvent describes a transition of a monitored site
// from up->down or from down->up.
type TransitionEvent struct {
    // Site is the monitored site in question.
    Site *site.Site `json:"site"`
    // Up specifies whether the site is now up or down (the new value).
    Up bool `json:"up"`
}

// TransitionTopic is a pubsub topic with transition events for when a monitored site
// transitions from up->down or from down->up.
var TransitionTopic = pubsub.NewTopic[*TransitionEvent]("uptime-transition", pubsub.TopicConfig{
    DeliveryGuarantee: pubsub.AtLeastOnce,
})
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Now let's publish a message on the TransitionTopic if a site's up/down state differs from the previous measurement.

🥐 Create a getPreviousMeasurement function in alerts.go to report the last up/down state:



import "encore.dev/storage/sqldb"

// getPreviousMeasurement reports whether the given site was
// up or down in the previous measurement.
func getPreviousMeasurement(ctx context.Context, siteID int) (up bool, err error) {
    err = sqldb.QueryRow(ctx, `
        SELECT up FROM checks
        WHERE site_id = $1
        ORDER BY checked_at DESC
        LIMIT 1
    `, siteID).Scan(&up)

    if errors.Is(err, sqldb.ErrNoRows) {
        // There was no previous ping; treat this as if the site was up before
        return true, nil
    } else if err != nil {
        return false, err
    }
    return up, nil
}
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🥐 Now add a function in alerts.go to conditionally publish a message if the up/down state differs:



import "encore.app/site"

func publishOnTransition(ctx context.Context, site *site.Site, isUp bool) error {
    wasUp, err := getPreviousMeasurement(ctx, site.ID)
    if err != nil {
        return err
    }
    if isUp == wasUp {
        // Nothing to do
        return nil
    }
    _, err = TransitionTopic.Publish(ctx, &TransitionEvent{
        Site: site,
        Up:   isUp,
    })
    return err
}
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🥐 Finally modify the check function in check.go to call the publishOnTransition function:



func check(ctx context.Context, site *site.Site) error {
    result, err := Ping(ctx, site.URL)
    if err != nil {
        return err
    }

    // Publish a Pub/Sub message if the site transitions
    // from up->down or from down->up.
    if err := publishOnTransition(ctx, site, result.Up); err != nil {
        return err
    }

    _, err = sqldb.Exec(ctx, `
        INSERT INTO checks (site_id, up, checked_at)
        VALUES ($1, $2, NOW())
    `, site.ID, result.Up)
    return err
}
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Now the monitoring system will publish messages on the TransitionTopic whenever a monitored site transitions from up->down or from down->up.

However, it doesn't know or care who actually listens to these messages. The truth is right now nobody does. So let's fix that by adding a Pub/Sub subscriber that posts these events to Slack.

Send Slack notifications when a site goes down

🥐 Start by creating a Slack service slack/slack.go containing the following:



package slack

import (
    "bytes"
    "context"
    "encoding/json"
    "fmt"
    "io"
    "net/http"
)

type NotifyParams struct {
    // Text is the Slack message text to send.
    Text string `json:"text"`
}

// Notify sends a Slack message to a pre-configured channel using a
// Slack Incoming Webhook (see https://api.slack.com/messaging/webhooks).
//
//encore:api private
func Notify(ctx context.Context, p *NotifyParams) error {
    reqBody, err := json.Marshal(p)
    if err != nil {
        return err
    }
    req, err := http.NewRequestWithContext(ctx, "POST", secrets.SlackWebhookURL, bytes.NewReader(reqBody))
    if err != nil {
        return err
    }
    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        return err
    }
    defer resp.Body.Close()

    if resp.StatusCode >= 400 {
        body, _ := io.ReadAll(resp.Body)
        return fmt.Errorf("notify slack: %s: %s", resp.Status, body)
    }
    return nil
}

var secrets struct {
    // SlackWebhookURL defines the Slack webhook URL to send
    // uptime notifications to.
    SlackWebhookURL string
}
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🥐 Now go to a Slack community of your choice (where you have permission to create a new Incoming Webhook). If you don't have any, join the Encore Slack and ask in #help and we're happy to help out.

🥐 Once you have the Webhook URL, save it as a secret using Encore's built-in secrets manager:



encore secret set --local,dev,prod SlackWebhookURL
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🥐 Test the slack.Notify endpoint by calling it via cURL:



curl 'http://localhost:4000/slack.Notify' -d '{"Text": "Testing Slack webhook"}'
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You should see the Testing Slack webhook message appear in the Slack channel you designated for the webhook.

🥐 It's now time to add a Pub/Sub subscriber to automatically notify Slack when a monitored site goes up or down. Add the following to slack/slack.go:



import (
    "encore.dev/pubsub"
    "encore.app/monitor"
)

var _ = pubsub.NewSubscription(monitor.TransitionTopic, "slack-notification", pubsub.SubscriptionConfig[*monitor.TransitionEvent]{
    Handler: func(ctx context.Context, event *monitor.TransitionEvent) error {
        // Compose our message.
        msg := fmt.Sprintf("*%s is down!*", event.Site.URL)
        if event.Up {
            msg = fmt.Sprintf("*%s is back up.*", event.Site.URL)
        }

        // Send the Slack notification.
        return Notify(ctx, &NotifyParams{Text: msg})
    },
})
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🎉 Deploy your finished Uptime Monitor

You're now ready to deploy your finished Uptime Monitor, complete with a Slack integration!

🥐 As before, deploying your app to the cloud is as simple as running:



git add -A .
git commit -m 'Add slack integration'
git push encore
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You now have a fully featured, production-ready, Uptime Monitoring system running in the cloud. Well done! ✨

🤯 Wrapping up: All of this came in at just over 300 lines of code

You've now built a fully functioning uptime monitoring system, accomplishing a remarkable amount with very little code:

  • You've built three different services (site, monitor, and slack)
  • You've added two databases (to the site and monitor services) for tracking monitored sites and the monitoring results
  • You've added a cron job for automatically checking the sites every 5 minutes
  • You've set up a Pub/Sub topic to decouple the monitoring system from the Slack notifications
  • You've added a Slack integration, using secrets to securely store the webhook URL, listening to a Pub/Sub subscription for up/down transition events

All of this in just a bit over 300 lines of code!🤯

If you want to build something more, check out Encore's templates library for inspiration.

If you need help or want to chat, come join the developer's hangout on Slack.

💖 💪 🙅 🚩
marcuskohlberg
Marcus Kohlberg

Posted on October 20, 2023

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