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Using load balancing for highly available applications | Compute Engine Documentation

Skip to main content Using load balancing for highly available applications

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This tutorial explains how to use load balancing with a regional managed instance group to redirect traffic away from busy or unavailable VM instances, allowing you to provide high availability even during a zonal outage.

A regional managed instance group distributes an application on multiple instances across multiple zones. A global load balancer directs traffic across multiple regions via a single IP address. By using both of these services to distribute your application across multiple zones, you can help ensure that your application is available even in extreme cases, like a zonal disruption.

Load balancers can be used to direct a variety of traffic types. This tutorial shows you how to create a global load balancer that directs external HTTP traffic, but much of the content of this tutorial is still relevant to other types of load balancers. To learn about other types of traffic that can be directed with a load balancer, see Types of Cloud Load Balancing.

This tutorial includes detailed steps for launching a web application on a regional managed instance group, configuring network access, creating a load balancer for directing traffic to the web application, and observing the load balancer by simulating a zonal outage. Depending on your experience with these features, this tutorial takes about 45 minutes to complete.

Objectives Costs

In this document, you use the following billable components of Google Cloud:

To generate a cost estimate based on your projected usage, use the pricing calculator.

New Google Cloud users might be eligible for a

free trial

.

Before you begin
  1. Sign in to your Google Cloud account. If you're new to Google Cloud, create an account to evaluate how our products perform in real-world scenarios. New customers also get $300 in free credits to run, test, and deploy workloads.
  2. In the Google Cloud console, on the project selector page, select or create a Google Cloud project.

    Note: If you don't plan to keep the resources that you create in this procedure, create a project instead of selecting an existing project. After you finish these steps, you can delete the project, removing all resources associated with the project.

    Go to project selector

  3. Verify that billing is enabled for your Google Cloud project.

  4. In the Google Cloud console, on the project selector page, select or create a Google Cloud project.

    Note: If you don't plan to keep the resources that you create in this procedure, create a project instead of selecting an existing project. After you finish these steps, you can delete the project, removing all resources associated with the project.

    Go to project selector

  5. Verify that billing is enabled for your Google Cloud project.

Application architecture

The application includes the following Compute Engine components:

Launching the web application

This tutorial uses a web application that is stored on GitHub. If you would like learn more about how the application was implemented, see the GoogleCloudPlatform/python-docs-samples repository on GitHub.

Launch the web application on every VM in an instance group by including a startup script in an instance template. Additionally, run the instance group in a dedicated VPC network to keep this tutorial's firewall rules from interfering with any existing resources running in your project.

Create a VPC network

Using a VPC network protects existing resources in your project from being affected by the resources that you will create for this tutorial. A VPC network is also required to restrict incoming traffic so that it must go through the load balancer.

Create a VPC network to encapsulate the firewall rules for the demo web application:

  1. In the Google Cloud console, go to the VPC networks page.

    Go to VPC networks

  2. Click Create VPC Network.

  3. Under Name, enter web-app-vpc.

  4. Set Subnet creation mode to Custom.

  5. Create a new subnet as follows:

    1. In the Subnets section, set the Name field, enter web-app-vpc-subnet.
    2. In the Region drop-down, select us-central1.
    3. Make sure that the IP stack type option is set to IPv4.
    4. In the Primary IPv4 range section, enter the IPv4 range 10.2.0.0/24.
  6. At the bottom of the page, click Create.

Wait until the VPC network is created before continuing.

Create a firewall rule

After the VPC network is created, set up a firewall rule to allow HTTP traffic to the VPC network:

Note: This example creates an ingress allow VPC firewall rule of which the target is all instances in the network. For production applications, consider using a more specific target. You can also use rules in a global network firewall policy, regional network firewall policy, or hierarchical firewall policy. For more information, see Firewall policies and best practices for network security.
  1. In the Google Cloud console, go to the Firewalls page.

    Go to Firewalls

  2. Click Create firewall rule.

  3. In the Name field, enter allow-web-app-http.

  4. Set Network to web-app-vpc.

  5. Make sure that the following options are set as given:

  6. In the Targets drop-down, select All instances in the network.

  7. Set Source filter to IPv4 ranges.

  8. In the Source IP ranges field, enter 130.211.0.0/22, 35.191.0.0/16 to allow for load balancer health checks.

    Note: Health check probes for the load balancer come from addresses in the ranges 130.211.0.0/22 and 35.191.0.0/16. For this tutorial, your health check uses the HTTP protocol, so the firewall rule should allow connections to port 80. For more information on firewall rules for health checks, see Probe IP ranges and firewall rules.
  9. Under Protocols and ports, do the following:

    1. Select Specified protocols and ports.
    2. Select TCP.
    3. In the Ports field, enter 80 to allow access for HTTP traffic.
  10. Click Create.

Create an instance template

Create a template that you will use to create a group of VM instances. Each instance created from the template launches a demo web application by using a startup script.

  1. In the Google Cloud console, go to the Instance templates page.

    Go to Instance templates

  2. Click Create instance template.

  3. Under Name, enter load-balancing-web-app-template.

  4. Under Machine configuration, set the Machine type to e2-medium.

  5. Click the Advanced options section to expand.

  6. Click the Networking section and do the following:

    1. In the Network interfaces section, delete any existing network interfaces by clicking the delete icon next to them.
    2. Click Add a network interface, and then select the web-app-vpc network. This forces each instance created with this template to run on the previously created network.
    3. In the Subnetwork drop-down, select web-app-vpc-subnet.
    4. Click Done.
  7. Click the Management section and do the following:

    1. In the Automation section, enter the following startup script:

      apt-get update
      apt-get -y install git python3-pip python3-venv
      git clone https://github.com/GoogleCloudPlatform/python-docs-samples.git
      python3 -m venv venv
      ./venv/bin/pip3 install -Ur ./python-docs-samples/compute/managed-instances/demo/requirements.txt
      ./venv/bin/pip3 install gunicorn
      ./venv/bin/gunicorn --bind 0.0.0.0:80 app:app --daemon --chdir ./python-docs-samples/compute/managed-instances/demo
      

      The script gets, installs, and launches the web application when a VM instance starts up.

  8. Leave the default values for the other options.

  9. Click Create.

Wait until the template is created before continuing.

Create a regional managed instance group

To run the web application, use the instance template to create a regional managed instance group:

  1. In the Google Cloud console, go to the Instance groups page.

    Go to Instance groups

  2. Click Create instance group.

  3. For Name, enter load-balancing-web-app-group.

  4. For Instance template, select load-balancing-web-app-template.

  5. Set Number of instances to 6. If this field is disabled, turn off autoscaling first.

    To turn off autoscaling, go to the Autoscaling section. In the Autoscaling mode drop-down, select Off: do not autoscale.

    Pro Tip: When creating a regional managed instance group, Compute Engine recommends that you provision enough instances so that, if all of the instances in any one zone are unavailable, the remaining instances still meet the minimum number of instances that you require. However, provisioning more instances than you need might incur additional costs. For more information, see How to increase availability by overprovisioning.

  6. For Location, select Multiple zones.

    Pro Tip: To ensure your application is available during extreme events, like zonal outages, Compute Engine recommends that you distribute your application across multiple zones.

  7. For Region, select us-central1.

  8. For Zones, select the following zones from the drop-down list:

  9. Leave the default values for the other options.

  10. Click Create. This redirects you back to the Instance groups page.

    You might need to wait a few minutes until all of the instances in the group are running.

Configuring the load balancer

To use a load balancer to direct traffic to your web application, you must reserve an external IP address to receive all incoming traffic. Then, create a load balancer that accepts traffic from that IP address and redirects that traffic to the instance group.

Reserve a static IP address

Use a global static external IP address to provide the load balancer with a single point of entry for receiving all user traffic. Compute Engine preserves static IP addresses even if you change or delete any affiliated Google Cloud resources. This allows the web application to always have the same entry point, even if other parts of the web application might change.

  1. In the Google Cloud console, go to the IP addresses page.

    Go to IP addresses

  2. Click Reserve external static IP address.

  3. In the Name field, enter web-app-ipv4.

  4. Set IP version to IPv4.

  5. Set Type to Global.

  6. Click Reserve.

Create a load balancer

This section explains the steps required to create a global load balancer that directs HTTP traffic.

This load balancer uses a frontend to receive incoming traffic and a backend to distribute this traffic to healthy instances. Because the load balancer is made of multiple components, this task is divided into five parts:

Complete all the parts to create the load balancer.

Note: For simplicity, this tutorial uses an HTTP load balancer. To learn how to support HTTPS and HTTP/2, see Creating content-based HTTP(S) load balancing. For other types of traffic, see Choosing a load balancer. Select the load balancer type
  1. In the Google Cloud console, go to the Load balancing page.

    Go to Load balancing

  2. Click Create load balancer.
  3. For Type of load balancer, select Application Load Balancer (HTTP/HTTPS) and click Next.
  4. For Public facing or internal, select Public facing (external) and click Next.
  5. For Global or single region deployment, select Best for global workloads and click Next.
  6. For Load balancer generation, select Global external Application Load Balancer and click Next.
  7. Click Configure.
Name the load balancer
  1. In the left panel, for Load balancer name, enter web-app-load-balancer.
Configure the frontend
  1. On the Frontend configuration page, under Name, enter web-app-ipv4-frontend.
  2. Set the Protocol to HTTP.
  3. Set the IP version to IPv4.
  4. Set the IP address to web-app-ipv4.
  5. Set the Port to 80.
  6. Click Done to create the frontend.
Configure the backend
  1. In the left panel, click Backend configuration.
  2. Click Backend services & backend buckets drop-down to open a menu, and then click Create a backend service.
  3. In the new window, for the Name of the backend service, enter web-app-backend.
  4. In the Backends section, do the following:
    1. Set Instance group to load-balancing-web-app-group.
    2. Set Port numbers to 80. This allows HTTP traffic between the load balancer and the instance group.
    3. Under Balancing mode, select Utilization.
    4. Click Done.
  5. Create the health check for the backend of the load balancer as follows:

    Pro Tip: Health checks are used for both load balancing and autohealing, but for different purposes:

    Use separate health checks for load balancing and for autohealing. Using the same health check for these services would remove the distinction between unresponsive instances and failed instances, causing unnecessary latency and/or unavailability for your users. For more information, see Health check concepts.

    1. Click the Health check drop-down, and then click Create a health check. A new window opens.
    2. In the new window under Name, enter web-app-load-balancer-check.
    3. Set the Protocol to HTTP.
    4. Under Port, enter 80.
    5. For this tutorial, set the Request path to /health, which is a path that the demo web application is set up to respond to.
    6. Set the following Health criteria:

      1. Set Check interval to 3 seconds. This defines the amount of time from the start of one probe to the start of the next one.
      2. Set Timeout to 3 seconds. This defines the amount of time that Google Cloud waits for a response to a probe. Its value must be less than or equal to the check interval.
      3. Set Healthy Threshold to 2 consecutive successes. This defines the number of sequential probes that must succeed in order for the instance to be considered healthy.
      4. Set Unhealthy Threshold to 2consecutive failures. This defines the number of sequential probes that must fail in order for the instance to be considered unhealthy.

      Pro Tip: For information about refining the Check interval and Timeout values for your own application, see How health checks work. For detailed information about optimizing and measuring latency, see Optimizing Application Latency with Load Balancing

    7. Click Create to create the health check.

  6. Leave the default values for the other options.

  7. Click Create to create the backend service.

Review and finalize

Verify your load balancing settings before creating the load balancer:

  1. In the left panel of the Create global external Application Load Balancer page, click Review and finalize.
  2. On the Review and finalize page, verify that Frontend uses an IP address with a Protocol of HTTP.

  3. On the same page, verify the following Backend settings:

  4. Click Create to finish creating the load balancer.

You might need to wait a few minutes for the load balancer to finish being created.

Test the load Balancer

Verify that you can connect to the web application by using the load balancer as follows:

  1. In the Google Cloud console, go to the Load balancing page.

    Go to Load balancing

  2. In the Name column, click web-app-load-balancer to expand the load balancer you just created.

  3. To connect to the web-app using the external static IP addresses, do the following:

    1. In the Frontend section, copy the IP address shown in the IP:Port column.
    2. Open a new browser tab and paste the IP address into the address bar. This should display the demo web application:

    Notice that, whenever you refresh the page, the load balancer connects to different instances in different zones. This happens because you are not connecting to an instance directly; you are connecting to the load balancer, which selects the instance you are redirected to.

    When you are done, close the browser tab for the demo web application.

Simulating a zonal outage

You can observe the functionality of the load balancer by simulating the widespread unavailability of a zonal outage. This simulation works by forcing all of the instances located in a specified zone to report an unhealthy status on the /health request path. When these instances report an unhealthy status, they fail the load balancing health check, prompting the load balancer to stop directing traffic to these instances.

  1. Monitor which zones the load balancer is directing traffic to.

    1. In the Google Cloud console, go to Cloud Shell.

      Open Cloud Shell

      Cloud Shell opens in a pane of the Google Cloud console. It can take a few seconds for the session to initialize.

      Pro Tip: You can open Cloud Shell from any Google Cloud console page by using the Activate Cloud Shell button.

    2. Save the static external IP address of your load balancer as follows:

      1. Get the external IP address from the frontend forwarding rule of the load balancer by entering the following command in your terminal:

        gcloud compute forwarding-rules describe web-app-ipv4-frontend --global
        

        The output looks as follows. Copy the EXTERNAl_IP_ADDRESS from the output.

        IPAddress: EXTERNAl_IP_ADDRESS
        ...
        
      2. Create a local bash variable:

        export LOAD_BALANCER_IP=EXTERNAl_IP_ADDRESS
        

        Replace EXTERNAl_IP_ADDRESS with the external IP address that you copied.

    3. To monitor which zones the load balancer is directing traffic to, run the following bash script:

      while true
      do
          BODY=$(curl -s "$LOAD_BALANCER_IP")
          NAME=$(echo -n "$BODY" | grep "load-balancing-web-app-group" | perl -pe 's/.+?load-balancing-web-app-group-(.+?)<.+/\1/')
          ZONE=$(echo -n "$BODY" | grep "us-" | perl -pe 's/.+?(us-.+?)<.+/\1/')
          echo $ZONE
          sleep 2 # Wait for 2 seconds
      done
      

      This script continuously attempts to connect to the web application by using the IP address for the frontend of the load balancer, and outputs which zone the web application is running from for each connection.

      The resulting output should include zones us-central1-b, us-central1-c, and us-central1-f:

      us-central1-f
      us-central1-b
      us-central1-c
      us-central1-f
      us-central1-f
      us-central1-c
      us-central1-f
      us-central1-c
      us-central1-c
      

      Keep this terminal open.

      Note: This monitor should run continuously. But, you can stop it at any time by pressing Control+C in the terminal.
  2. While your monitor is running, begin simulating the zonal outage.

    1. In Cloud Shell, open a second terminal session by clicking the add Add button.
    2. Create a local bash variable for the project ID:

      export PROJECT_ID=PROJECT_ID
      

      where PROJECT_ID is the project ID for your current project, which is displayed on each new line in the Cloud Shell:

      user@cloudshell:~ (PROJECT_ID)$
      
    3. Create a local bash variable for the zone that you want to disable. To simulate a failure of zone us-central1-f, use the following command:

      export DISABLE_ZONE=us-central1-f
      

      Then, run the following bash script. This script causes the demo web application instances in the disabled zone to output unhealthy responses to the load balancer health check. Unhealthy responses prompt the load balancer to direct traffic away from these instances.

      export MACHINES=$(gcloud --project=$PROJECT_ID compute instances list --filter="zone:($DISABLE_ZONE)" --format="csv(name,networkInterfaces[0].accessConfigs[0].natIP)" | grep "load-balancing-web-app-group")
      for i in $MACHINES;
      do
        NAME=$(echo "$i" | cut -f1 -d,)
        IP=$(echo "$i" | cut -f2 -d,)
        echo "Simulating zonal failure for zone $DISABLE_ZONE, instance $NAME"
        curl -q -s "http://$IP/makeUnhealthy" >/dev/null --retry 2
      done
      

      After a short delay, the load balancer stops directing traffic to the unhealthy zones, so the output from the first terminal window stops listing zone us-central1-f:

      us-central1-c
      us-central1-c
      us-central1-c
      us-central1-b
      us-central1-b
      us-central1-c
      us-central1-b
      us-central1-c
      us-central1-c
      

      This indicates that the load balancer is directing traffic only to the healthy, responsive instances.

      Note: Optionally, you can repeat this step to simulate failures of zones us-central1-b and us-central1-c.

      Keep both terminals open.

    4. In the second terminal, create a local bash variable for the zone that you want to restore. To restore traffic to zone us-central1-f, use the following command:

      export ENABLE_ZONE=us-central1-f
      

      Then, run the following bash script. This script causes the demo web application instances in the enabled zone to output healthy responses to the load balancer health check. Healthy responses prompt the load balancer to begin distributing traffic back toward these instances.

      export MACHINES=$(gcloud --project=$PROJECT_ID compute instances list --filter="zone:($ENABLE_ZONE)" --format="csv(name,networkInterfaces[0].accessConfigs[0].natIP)" | grep "load-balancing-web-app-group")
      for i in $MACHINES;
      do
        NAME=$(echo "$i" | cut -f1 -d,)
        IP=$(echo "$i" | cut -f2 -d,)
        echo "Simulating zonal restoration for zone $ENABLE_ZONE, instance $NAME"
        curl -q -s "http://$IP/makeHealthy" >/dev/null --retry 2
      done
      

      After a few minutes, the output from the first terminal window gradually lists zone us-central1-fagain:

      us-central1-b
      us-central1-b
      us-central1-c
      us-central1-f
      us-central1-c
      us-central1-c
      us-central1-b
      us-central1-c
      us-central1-f
      

      This indicates that the load balancer is directing incoming traffic to all zones again.

      Note: If you also disabled zone us-central1-b or zone us-central1-c, you can repeat this step to restore traffic to them.

      Close both terminals when you have finished.

Clean up

After you finish the tutorial, you can clean up the resources that you created so that they stop using quota and incurring charges. The following sections describe how to delete or turn off these resources.

If you created a separate project for this tutorial, delete the entire project. Otherwise, if the project has resources that you want to keep, only delete the resources created in this tutorial.

Deleting the project
    Caution: Deleting a project has the following effects:

    If you plan to explore multiple architectures, tutorials, or quickstarts, reusing projects can help you avoid exceeding project quota limits.

  1. In the Google Cloud console, go to the Manage resources page.

    Go to Manage resources

  2. In the project list, select the project that you want to delete, and then click Delete.
  3. In the dialog, type the project ID, and then click Shut down to delete the project.
Deleting specific resources

The following sections describe how to delete the specific resources that you created during this tutorial.

Deleting the load balancer
  1. In the Google Cloud console, go to the Load balancing page.

    Go to Load balancing

  2. Click the checkbox next to web-app-load-balancer.

  3. Click delete Delete at the top of the page.

  4. In the new window, select all checkboxes. Then, click Delete load balancer and selected resources to confirm the deletion.

Deleting the static external IP address

Wait until the load balancer is deleted before deleting the static external IP address.

  1. In the Google Cloud console, go to the External IP addresses page.

    Go to External IP addresses

  2. Click the checkbox next to web-app-ipv4.

  3. Click Release static address at the top of the page. In the new window, click Release to confirm the release.

Deleting the instance group

Wait until the load balancer is deleted before deleting the instance group.

  1. In the Google Cloud console, go to the Instance groups page.

    Go to Instance groups

  2. Select the checkbox for your load-balancing-web-app-group instance group.
  3. To delete the instance group, click delete Delete.
Deleting the instance template

You must finish deleting the instance group before deleting the instance template. You cannot delete an instance template if a managed instance group is using it.

  1. In the Google Cloud console, go to the Instance Templates page.

    Go to Instance templates

  2. Click the checkbox next to load-balancing-web-app-template.

  3. Click delete Delete at the top of the page. In the new window, click Delete to confirm the deletion.

Deleting the VPC network

You must finish deleting the instance group before deleting the VPC network. You cannot delete a VPC network if other resources still uses it.

  1. In the Google Cloud console, go to the VPC networks page.

    Go to VPC networks

  2. Click web-app-vpc.

  3. Click delete Delete VPC network at the top of the page. In the new window, click Delete to confirm the deletion.

What's next

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.

Last updated 2025-08-07 UTC.

[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-07 UTC."],[[["This tutorial demonstrates how to use a regional managed instance group and a global load balancer to maintain high availability for web applications, even during zonal outages."],["A global load balancer directs HTTP traffic across multiple zones to ensure continuous application availability by redirecting traffic away from busy or unavailable VM instances."],["The process includes launching a demo web application on a regional managed instance group, configuring a global load balancer, and simulating a zonal outage to observe the load balancer's behavior."],["The tutorial outlines how to create necessary components such as a VPC network, firewall rules, instance templates, and health checks to support the load-balanced web application."],["By following this guide, users can learn how to set up their infrastructure to continue functioning by redirecting traffic to healthy zones in the event of an outage."]]],[]]


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