An instance group is a collection of VM instances that you can manage as a single entity.
Compute Engine offers two kinds of VM instance groups:
- Managed instance groups (MIGs) allow you to operate applications on multiple identical VMs. You can make your workloads scalable and highly available by taking advantage of automated MIG services, including: autoscaling, autohealing, regional (multi-zone) deployment, and auto-updating.
- Unmanaged instance groups allow you to load balance across a fleet of VMs that you manage yourself.
Managed instance groups (MIGs)
Managed instance groups (MIGs) are suitable for stateless serving workloads (such as a website frontend) and for batch, high-performance, or high-throughput compute workloads (such as image processing from a queue).
Each VM instance in a MIG is created from an instance template.
To create a managed instance group, see Creating groups of managed instances.
Benefits
MIGs offer the following advantages:
- High availability
- Keeping instances running — If an instance in the group stops, crashes, or is deleted by an action other than an instance group management command (for example, an intentional scale-down), the MIG automatically recreates that instance in accordance with the original instance's specification (same instance name, same template) so that the instance can resume its work.
- Autohealing — You can also set up an autohealing policy that relies on an application-based health check, which periodically verifies that your application is responding as expected on each of the MIG's instances. If an application is not responding on an instance, that instance is automatically recreated. Checking that an application responds is more precise than simply verifying that an instance is up and running.
- Regional (multi-zone) coverage — Regional managed instance groups allow you to spread application load across multiple zones. This replication protects against zonal failures. If that happens, your application can continue serving traffic from instances running in the remaining available zones in the same region.
- Load balancing — Managed instance groups work with load balancing services to distribute traffic across all of the instances in the group.
- Scalability — When your applications require additional compute resources, autoscaled MIGs can automatically grow the number of instances in the group to meet demand. If demand drops, autoscaled MIGs can automatically shrink to reduce your costs.
- Automated updates — The managed instance group auto-updater allows you to easily and safely deploy new versions of software to instances in your MIG, supporting a flexible range of rollout scenarios such as rolling updates and canary updates. You can control the speed and scope of deployment as well as the level of disruption to your service.
Autohealing
Managed instance groups maintain high availability of your applications by
proactively keeping your instances available, which means in RUNNING state. A
managed instance group will automatically recreate an instance that is not
RUNNING. However, relying only on instance state may not be sufficient. You
may want to recreate instances when an application freezes, crashes, or runs
out of memory.
Application-based autohealing improves application availability by relying on a health checking signal that detects application-specific issues such as freezing, crashing, or overloading. If a health check determines that an application has failed on an instance, the group automatically recreates that instance.
Health checking
The health checks used to monitor managed instance groups are similar to the
health checks used for load balancing, with some differences in behavior. Load
balancing health checks help direct
traffic away from non-responsive instances and toward healthy instances; these
health checks do not cause Compute Engine to recreate instances. On
the other hand,
managed instance group health checks
proactively signal to delete and recreate instances that become UNHEALTHY.
For the majority of scenarios, use separate health checks for load balancing and for autohealing. Health checking for load balancing can and should be more aggressive since these health checks determine whether an instance receives user traffic. Since customers might rely on your services, you want to catch non-responsive instances quickly so you can redirect traffic if necessary. In contrast, health checking for autohealing will cause MIGs to proactively replace failing instances so this health check should be more conservative than a load balancing health check.
See Setting up health checking and autohealing for managed instance groups for more information.
Regional or zonal groups
You can create two types of managed instance groups:
- A zonal managed instance group, which deploys instances to a single zone.
- A regional managed instance group, which deploys instances to multiple zones across the same region.
Both types offer all of the advantages of managed instance groups. Regional MIGs add higher availability by spreading application load across multiple zones, which protects your workload against zonal failure, and regional MIGs offer more capacity, with a maximum of 2000 instances per regional group.
Load balancing
GCP load balancing can use instance groups to serve traffic. Depending on the type of load balancer you choose, you can add instance groups to a target pool or to a backend service.
See Adding a managed instance group to a load balancer for more information.
Autoscaling
Managed instance groups support autoscaling that dynamically adds or removes instances from a managed instance group in response to increases or decreases in load. You turn on autoscaling and configure an autoscaling policy to specify how you want the group to scale. Autoscaling policies include scaling based on CPU utilization, load balancing capacity, Stackdriver monitoring metrics, or, for zonal MIGs, by a queue-based workload like Google Cloud Pub/Sub.
For more information, read Autoscaling Groups of Instances.
Auto-updating
You can easily and safely deploy new versions of software to instances in a managed instance group. The rollout of an update happens automatically based on your specifications: you can control the speed and scope of the update rollout in order to minimize disruptions to your application. You can optionally perform partial rollouts which allows for canary testing.
See Updating managed instance groups.
Groups of preemptible instances
For workloads where minimal costs are more important than speed of execution, you can reduce the cost of your workload by using preemptible VM instances in your instance group. Preemptible instances last up to 24 hours, and are preempted gracefully - your application will have 30 seconds to exit correctly. Preemptible instances can be deleted at any time, but autohealing will bring the instances back when preemptible capacity becomes available again.
Containers
You can simplify application deployment by deploying containers to instances in managed instance groups. When you specify a container image in an instance template and then use that template to create a managed instance group, each instance will be created with a container-optimized OS that includes Docker, and your container will start automatically on each instance in the group. See Deploying containers on VMs and managed instance groups.
Network
By default, instances in the group will be placed in the default network and
randomly assigned IP addresses from the regional range.
Alternatively, you can restrict the IP range of the group by creating a
custom mode VPC network and
subnet that uses a smaller IP range, then specifying this subnet in the instance
template.
Demo of MIG capabilities
The following 45-minute video presentation, recorded at GCP NEXT '18, contains demos and best practices for setting up, running, and updating scalable and highly-available deployments using Compute Engine managed instance groups.
You will see how to deploy a container to a managed instance group, set up an autohealing policy, use a regional group to protect against a zonal failure, configure autoscaling to meet CPU targets and queue-based demands, and manage canary and rolling updates.
Unmanaged instance groups
Unmanaged instance groups can contain heterogeneous instances that you can arbitrarily add and remove from the group. Unmanaged instance groups do not offer autoscaling, auto-healing, rolling update support, or the use of instance templates and are not a good fit for deploying highly available and scalable workloads. Use unmanaged instance groups if you need to apply load balancing to groups of heterogeneous instances or if you need to manage the instances yourself.
If you must create unmanaged instance groups, see Unmanaged Instance Groups.
What's next
- Learn more about instance templates.
- Create an instance template.
- Create a zonal or regional managed instance group.
- Enable autohealing for your managed instance group.
- Enable load balancing for your managed instance group.
- Enable autoscaling for your managed instance group.
- Learn about updating managed instance groups.
