Handling a stateful workload

You can use instance groups to host applications with stateful, stateless, and combined workloads.

The difference between stateful and stateless workloads lies in whether the application saves data about its states and/or user interaction history:

• Stateful: State data is stored. Examples include RESTless services, databases, message queues, monitoring servers, and log servers.
• Stateless: State data is not stored. Examples include RESTful services and web application frontend.

Application states can be stored on secondary disks of the group instances or in the cloud resources independent of the instance group.

Storing states on group instance disksStoring states on group instance disks

You can store application states on secondary disks of the group instances if you:

• Do not decrease the group size.

• Do not decrease the instance disk size.

• Use the max_expansion parameter (the maximum number of instances by which you can expand the target size of the group) set to 0 in the deployment policy.

• Do not delete VM instances via instance group interfaces:

  • In the management console, on the Instance groups tab.
  • Using the CLI yc compute instance-group delete-instances command.
  • Using the InstanceGroupService/DeleteInstances gRPC API call.

  These actions will delete the secondary disk along with the VM. You can directly delete a VM in Compute Cloud without deleting its secondary disk.

If you meet these conditions, the instance group will not delete secondary disks, even if any VM needs to be recreated. The secondary disks will be saved and attached to the new VM.

Storing states outside the instance groupStoring states outside the instance group

To freely manage group instances with stateful workload applications running, save the application state to cloud resources independent of the instance group:

• Managed databases
• File storages (for more information, see Creating an instance group connected to a file storage)
• Yandex Object Storage buckets (for more information, see Creating an instance group connected to Yandex Object Storage)