Sahara Cluster Statuses Overview

All Sahara Cluster operations are performed in multiple steps. A Cluster object has a Status attribute which changes when Sahara finishes one step of operations and starts another one. Also a Cluster object has a Status description attribute which changes whenever Cluster errors occur.

Sahara supports three types of Cluster operations:
  • Create a new Cluster
  • Scale/Shrink an existing Cluster
  • Delete an existing Cluster

Creating a new Cluster

1. Validating

Before performing any operations with OpenStack environment, Sahara validates user input.

There are two types of validations, that are done:
  • Check that a request contains all necessary fields and that the request does not violate
any constraints like unique naming, etc.
  • Plugin check (optional). The provisioning Plugin may also perform any specific checks like a Cluster topology validation check.

If any of the validations fails during creating, the Cluster object will still be kept in the database with an Error status. If any validations fails during scaling the Active Cluster, it will be kept with an Active status. In both cases status description will contain error messages about the reasons of failure.

2. InfraUpdating

This status means that the Provisioning plugin is performing some infrastructure updates.

3. Spawning

Sahara sends requests to OpenStack for all resources to be created:
  • VMs
  • Volumes
  • Floating IPs (if Sahara is configured to use Floating IPs)

It takes some time for OpenStack to schedule all the required VMs and Volumes, so sahara will wait until all of the VMs are in an Active state.

4. Waiting

Sahara waits while VMs’ operating systems boot up and all internal infrastructure components like networks and volumes are attached and ready to use.

5. Preparing

Sahara prepares a Cluster for starting. This step includes generating the /etc/hosts file, so that all instances can access each other by a hostname. Also Sahara updates the authorized_keys file on each VM, so that VMs can communicate without passwords.

6. Configuring

Sahara pushes service configurations to VMs. Both XML and JSON based configurations and environmental variables are set on this step.

7. Starting

Sahara is starting Hadoop services on Cluster’s VMs.

8. Active

Active status means that a Cluster has started successfully and is ready to run EDP Jobs.

Scaling/Shrinking an existing Cluster

1. Validating

Sahara checks the scale/shrink request for validity. The Plugin method called for performing Plugin specific checks is different from the validation method in creation.

2. Scaling

Sahara performs database operations updating all affected existing Node Groups and creating new ones to join the existing Node Groups.

3. Adding Instances

Status is similar to Spawning in Custer creation. Sahara adds required amount of VMs to the existing Node Groups and creates new Node Groups.

4. Configuring

Status is similar to Configuring in Cluster creation. New instances are being configured in the same manner as already existing ones. The VMs in the existing Cluster are also updated with a new /etc/hosts file.

5. Decommissioning

Sahara stops Hadoop services on VMs that will be deleted from a Cluster. Decommissioning a Data Node may take some time because Hadoop rearranges data replicas around the Cluster, so that no data will be lost after that Data Node is deleted.

6. Deleting Instances

Sahara sends requests to OpenStack to release unneeded resources:
  • VMs
  • Volumes
  • Floating IPs (if they are used)

7. Active

The same Active status as after Cluster creation.

Deleting an existing Cluster

1. Deleting

The only step, that releases all Cluster’s resources and removes it from the database.

Error State

If the Cluster creation fails, the Cluster will enter the Error state. This status means the Cluster may not be able to perform any operations normally. This cluster will stay in the database until it is manually deleted. The reason for failure may be found in the sahara logs. Also, the status description will contain information about the error.

If an error occurs during the Adding Instances operation, Sahara will first try to rollback this operation. If a rollback is impossible or fails itself, then the Cluster will also go into an Error state. If a rollback was successful, Cluster will get into an Active state and status description will contain a short message about the reason of Adding Instances failure.