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.

