## Clusters

updated: 2020-10-17 18:54

# Clusters

Clusters are first-class citizens in Senlin service design. A cluster is defined as a collection of homogeneous objects. The “homogeneous” here means that the objects managed (aka. Nodes) have to be instantiated from the same “profile type”.

A cluster can contain zero or more nodes. Senlin provides REST APIs for users to create, retrieve, update, delete clusters. Using these APIs, a user can manage the node membership of a cluster.

A cluster is owned by a user (the owner), and it is accessible from within the Keystone project (tenant) which is the default project of the user.

A cluster has the following timestamps when instantiated:

- `init_at`: the timestamp when a cluster object is initialized in the Senlin database, but the actual cluster creation has not yet started;

- `created_at`: the timestamp when the cluster object is created, i.e. the `CLUSTER_CREATE` action has completed;

- `updated_at`: the timestamp when the cluster was last updated.

## Cluster Statuses

A cluster can have one of the following statuses during its lifecycle:

- `INIT`: the cluster object has been initialized, but not created yet;

- `ACTIVE`: the cluster is created and providing service;

- `CREATING`: the cluster creation action is still on going;

- `ERROR`: the cluster is still providing services, but there are things going wrong that needs human intervention;

- `CRITICAL`: the cluster is not operational, it may or may not be providing services as expected. Senlin cannot recover it from its current status. The best way to deal with this cluster is to delete it and then re-create it if needed.

- `DELETING`: the cluster deletion is ongoing;

- `WARNING`: the cluster is operational, but there are some warnings detected during past operations. In this case, human involvement is suggested but not required.

- `UPDATING`: the cluster is being updated.

Along with the `status` property, Senlin provides a `status_reason` property for users to check what is the cause of the cluster’s current status.

To avoid frequent databases accesses, a cluster object has a runtime data property named `rt` which is a Python dictionary. The property caches the profile referenced by the cluster, the list of nodes in the cluster and the policies attached to the cluster. The runtime data is not directly visible to users. It is merely a convenience for cluster operations.

## Creating A Cluster

When creating a cluster, the Senlin API will verify whether the request carries a body with valid, sufficient information for the engine to complete the creation job. The following fields are required in a map named `cluster` in the request JSON body:

- `name`: the name of the cluster to be created;

- `profile`: the name or ID or short-ID of a profile to be used;

- `desired_capacity`: the desired number of nodes in the cluster, which is treated also as the initial number of nodes to be created.

The following optional fields can be provided in the `cluster` map in the JSON request body:

- `min_size`: the minimum number of nodes inside the cluster, default value is 0;

- `max_size`: the maximum number of nodes inside the cluster, default value is -1, which means there is no upper limit on the number of nodes;

- `timeout`: the maximum number of seconds to wait for the cluster to become ready, i.e. `ACTIVE`.

- `metadata`: a list of key-value pairs to be associated with the cluster.

- `dependents`: A dict contains dependency information between nova server/ heat stack cluster and container cluster. The container node’s id will be stored in ‘dependents’ property of its host cluster.

The `max_size` and the `min_size` fields, when specified, will be checked against each other by the Senlin API. The API also checks if the specified `desired_capacity` falls out of the range [`min_size`, `max_size`]. If any verification failed, a `HTTPBadRequest` exception is thrown and the cluster creation request is rejected.

A cluster creation request is then forwarded to the Senlin RPC engine for processing, where the engine creates an Action for the request and queues it for any worker threads to execute. Once the action is queued, the RPC engine returns the current cluster properties in a map to the API. Along with these properties, the engine also returns the UUID of the Action that will do the real job of cluster creation. A user can check the status of the action to determine whether the cluster has been successfully completed or failed.

## Listing Clusters

Clusters in the current project can be queried using some query parameters. None of these parameters is required. By default, the Senlin API will return all clusters that are not deleted.

When listing clusters, the following query parameters can be specified, individually or combined:

- `filters`: a map containing key-value pairs for matching. Records that fail to match the criteria will be filtered out. The valid keys in this map include:

  - `name`: name of clusters to list, can be a string or a list of strings;

  - `status`: status of clusters, can be a string or a list of strings;

- `limit`: a number that restricts the maximum number of records to be returned from the query. It is useful for displaying the records in pages where the page size can be specified as the limit.

- `marker`: A string that represents the last seen UUID of clusters in previous queries. This query will only return results appearing after the specified UUID. This is useful for displaying records in pages.

- `sort`: A string to enforce sorting of the results. It accepts a list of known property names of a cluster as sorting keys separated by commas. Each sorting key can optionally have either `:asc` or `:desc` appended to the key for controlling the sorting direction.

- `global_project`: A boolean indicating whether cluster listing should be done in a tenant-safe way. When this value is specified as False (the default), only clusters from the current project that match the other criteria will be returned. When this value is specified as True, clusters that matching all other criteria would be returned, no matter in which project a cluster was created. Only a user with admin privilege is permitted to do a global listing.

## Getting a Cluster

When a user wants to check the details about a specific cluster, he or she can specify one of the following keys for query:

- cluster UUID: Clusters are queried strictly based on the UUID given. This is the most precise query supported.

- cluster name: Senlin allows multiple clusters to have the same name. It is user’s responsibility to avoid name conflicts if needed. The output may be the details of a cluster if the cluster name is unique, or else Senlin will return a message telling users that multiple clusters found matching the specified name.

- short ID: Considering that UUID is a long string not so convenient to input, Senlin supports a short version of UUIDs for query. Senlin engine will use the provided string as a prefix to attempt a matching in the database. When the “ID” is long enough to be unique, the details of the matching cluster is returned, or else Senlin will return an error message indicating that more than one cluster matching the short ID have been found.

Senlin engine service will try the above three ways in order to find a match in database.

In the returned result, Senlin injects a list of node IDs for nodes in the cluster. It also injects the name of the profile used by the cluster. These are all for user’s convenience.

## Updating A Cluster

A cluster can be updated upon user’s requests. In theory, all properties of a cluster could be updated/changed. However, some update operations are light -weight ones, others are heavy weight ones. This is because the semantics of properties differ a lot from each other. Currently, cluster profile related changes and cluster size related changes are heavy weight because they may induce a chain of operations on the cluster. Updating other properties are light weight operations.

In the JSON body of a `cluster_update` request, users can specify new values for the following properties:

- `name`: new cluster name;

- `profile_id`: ID or name or short ID of a profile object to use;

- `metadata`: a list of key-value pairs to be associated with the cluster, this dict will be merged with the existing key-value pairs based on keys.

- `desired_capacity`: new desired size for the cluster;

- `min_size`: new lower bound for the cluster size;

- `max_size`: new upper bound for the cluster size.

- `timeout`: new timeout value for the specified cluster.

- `profile_only`: a boolean value indicating whether cluster will be only updated with profile.

### Update Cluster’s Profile

When `profile_id` is specified, the request will be interpreted as a wholistic update to all nodes across the cluster. The targeted use case is to do a cluster wide system upgrade. For example, replacing glance images used by the cluster nodes when new kernel patches have been applied or software defects have been fixed.

When receiving such an update request, the Senlin engine will check if the new profile referenced does exist and whether the new profile has the same profile type as that of the existing profile. Exceptions will be thrown if any verification has failed and thus the request is rejected.

After the engine has validated the request, an Action of `CLUSTER_UPDATE` is created and queued internally for execution. Later on, when a worker thread picks up the action for execution, it will first lock the whole cluster and mark the cluster status as `UPDATING`. It will then fork `NODE_UPDATE` actions per node inside the cluster, which are in turn queued for execution. Other worker threads will pick up the node level update action for execution and mark the action as completed/failed. When all these node level updates are completed, the `CLUSTER_UPDATE` operation continues and marks the cluster as `ACTIVE` again.

Senlin also provides a parameter `profile_only` for this action, so that any newly created nodes will use the new profile, but existing nodes should not be changed.

The cluster update operation may take a long time to complete, depending on the response time from the underlying profile operations. Note also, when there is a update policy is attached to the cluster and enabled, the update operation may be split into several batches so that 1) there is a minimum number of nodes remained in service at any time; 2) the pressure on the underlying service is controlled.

### Update Cluster Size Properties

When either one of the `desired_capacity`, `min_size` and `max_size` property is specified in the `CLUSTER_UPDATE` request, it may lead to a resize operation on the cluster.

The Senlin API will do a preliminary validation upon the new property values. For example, if both `min_size` and `max_size` are specified, they have to be integers and the value for `max_size` is greater than the value for `min_size`, unless the value of `max_size` is -1 which means the upper bound of cluster size is unlimited.

When the request is then received by the Senlin engine, the engine first retrieves the cluster properties from the database and do further cross-verifications between the new property values and the current values. For example, it is treated as an invalid request if a user has specified value for `min_size` but no value for `max_size`, however the new `min_size` is greater than the existing `max_size` of the cluster. In this case, the user has to provide a valid `max_size` to override the existing value, or he/she has to lower the `min_size` value so that the request becomes acceptable.

Once the cross-verification has passed, Senlin engine will calculate the new `desired_capacity` and adjust the size of the cluster if deemed necessary. For example, when the cluster size is below the new `min_size`, new nodes will be created and added to the cluster; when the cluster size is above the new `max_size`, some nodes will be removed from the cluster. If the `desired_capacity` is set and the property value falls between the new range of cluster size, Senlin tries resize the cluster to the `desired_capacity`.

When the size of the cluster is adjusted, Senlin engine will check if there are relevant policies attached to the cluster so that the engine will add and/or remove nodes in a predictable way.

### Update Other Cluster Properties

The update to other cluster properties is relatively straightforward. Senlin engine simply verifies the data types when necessary and override the existing property values in the database.

Note that in the cases where multiple properties are specified in a single `CLUSTER_UPDATE` request, some will take a longer time to complete than others. Any mixes of update properties are acceptable to the Senlin API and the engine.

## Cluster Actions

A cluster object supports the following asynchronous actions:

- `add_nodes`: add a list of nodes into the target cluster;

- `del_nodes`: remove the specified list of nodes from the cluster;

- `replace_nodes`: replace the specified list of nodes in the cluster;

- `resize`: adjust the size of the cluster;

- `scale_in`: explicitly shrink the size of the cluster;

- `scale_out`: explicitly enlarge the size of the cluster.

- `policy_attach`: attach a policy object to the cluster;

- `policy_detach`: detach a policy object from the cluster;

- `policy_update`: modify the settings of a policy that is attached to the cluster.

The `scale_in` and the `scale_out` actions are subject to change in future. We recommend using the unified `CLUSTER_RESIZE` action for cluster size adjustments.

Software or a user can trigger a `cluster_action` API to issue an action for Senlin to perform. In the JSON body of these requests, Senlin will verify if the top-level key contains one of the above actions. When no valid action name is found or more than one action is specified, the API will return error messages to the caller and reject the request.

### Adding Nodes to a Cluster

Senlin API provides the `add_nodes` action for user to add some existing nodes into the specified cluster. The parameter for this action is interpreted as a list in which each item is the UUID, name or short ID of a node.

When receiving an `add_nodes` action request, the Senlin API only validates if the parameter is a list and if the list is empty. After this validation, the request is forwarded to the Senlin engine for processing.

The Senlin engine will examine nodes in the list one by one and see if any of the following conditions is true. Senlin engine rejects the request if so.

- Any node from the list is not in `ACTIVE` state?

- Any node from the list is still member of another cluster?

- Any node from the list is not found in the database?

- Number of nodes to add is zero?

When this phase of validation succeeds, the request is translated into a `CLUSTER_ADD_NODES` builtin action and queued for execution. The engine returns to the user an action UUID for checking.

When the action is picked up by a worker thread for execution, Senlin checks if the profile type of the nodes to be added matches that of the cluster. Finally, a number of `NODE_JOIN` action is forked and executed from the `CLUSTER_ADD_NODES` action. When `NODE_JOIN` actions complete, the `CLUSTER_ADD_NODES` action returns with success.

In the cases where there are load-balancing policies attached to the cluster, the `CLUSTER_ADD_NODES` action will save the list of UUIDs of the new nodes into the action’s `data` field so that those policies could update the associated resources.

### Deleting Nodes from a Cluster

Senlin API provides the `del_nodes` action for user to delete some existing nodes from the specified cluster. The parameter for this action is interpreted as a list in which each item is the UUID, name or short ID of a node.

When receiving a `del_nodes` action request, the Senlin API only validates if the parameter is a list and if the list is empty. After this validation, the request is forwarded to the Senlin engine for processing.

The Senlin engine will examine nodes in the list one by one and see if any of the following conditions is true. Senlin engine rejects the request if so.

- Any node from the list cannot be found from the database?

- Any node from the list is not member of the specified cluster?

- Number of nodes to delete is zero?

When this phase of validation succeeds, the request is translated into a `CLUSTER_DEL_NODES` builtin action and queued for execution. The engine returns to the user an action UUID for checking.

When the action is picked up by a worker thread for execution, Senlin forks a number of `NODE_DELETE` actions and execute them asynchronously. When all forked actions complete, the `CLUSTER_DEL_NODES` returns with a success.

In the cases where there are load-balancing policies attached to the cluster, the `CLUSTER_DEL_NODES` action will save the list of UUIDs of the deleted nodes into the action’s `data` field so that those policies could update the associated resources.

If a deletion policy with hooks property is attached to the cluster, the `CLUSTER_DEL_NODES` action will create the `CLUSTER_DEL_NODES` actions in `WAITING_LIFECYCLE_COMPLETION` status which does not execute them. It also sends the lifecycle hook message to the target specified in the deletion policy. If the complete lifecylcle API is called for a `CLUSTER_DEL_NODES` action, it will be executed. If all the `CLUSTER_DEL_NODES` actions are not executed before the hook timeout specified in the deletion policy is reached, the remaining `CLUSTER_DEL_NODES` actions are moved into `READY` status and scheduled for execution. When all actions complete, the `CLUSTER_DEL_NODES` returns with a success.

Note also that by default Senlin won’t destroy the nodes that are deleted from the cluster. It simply removes the nodes from the cluster so that they become orphan nodes. Senlin also provides a parameter `destroy_after_deletion` for this action so that a user can request the deleted node(s) to be destroyed right away, instead of becoming orphan nodes.

### Replacing Nodes in a Cluster

Senlin API provides the `replace_nodes` action for user to replace some existing nodes in the specified cluster. The parameter for this action is interpreted as a dict in which each item is the node-pair{OLD_NODE:NEW_NODE}. The key OLD_NODE is the UUID, name or short ID of a node to be replaced, and the value NEW_NODE is the UUID, name or short ID of a node as replacement.

When receiving a `replace_nodes` action request, the Senlin API only validates if the parameter is a dict and if the dict is empty. After this validation, the request is forwarded to the Senlin engine for processing.

The Senlin engine will examine nodes in the dict one by one and see if all of the following conditions is true. Senlin engine accepts the request if so.

- All nodes from the list can be found from the database.

- All replaced nodes from the list are the members of the specified cluster.

- All replacement nodes from the list are not the members of any cluster.

- The profile types of all replacement nodes match that of the specified cluster.

- The statuses of all replacement nodes are ACTIVE.

When this phase of validation succeeds, the request is translated into a `CLUSTER_REPLACE_NODES` builtin action and queued for execution. The engine returns to the user an action UUID for checking.

When the action is picked up by a worker thread for execution, Senlin forks a number of `NODE_LEAVE` and related `NODE_JOIN` actions, and execute them asynchronously. When all forked actions complete, the `CLUSTER_REPLACE_NODES` returns with a success.

### Resizing a Cluster

In addition to the `cluster_update` request, Senlin provides a dedicated API for adjusting the size of a cluster, i.e. `cluster_resize`. This operation is designed for the auto-scaling and manual-scaling use cases.

Below is a list of API parameters recognizable by the Senlin API when parsing the JSON body of a `cluster_resize` request:

- `adjustment_type`: type of adjustment to be performed where the value should be one of the followings:

  - `EXACT_CAPACITY`: the adjustment is about the targeted size of the cluster;

  - `CHANGE_IN_CAPACITY`: the adjustment is about the number of nodes to be added or removed from the cluster and this is the default setting;

  - `CHANGE_IN_PERCENTAGE`: the adjustment is about a relative percentage of the targeted cluster.

  This field is mandatory.

- `number`: adjustment number whose value will be interpreted base on the value of `adjustment_type`. This field is mandatory.

- `min_size`: the new lower bound for the cluster size;

- `max_size`: the new upper bound for the cluster size;

- `min_step`: the minimum number of nodes to be added or removed when the `adjustment_type` is set to `CHANGE_IN_PERCENTAGE` and the absolute value computed is less than 1;

- `strict`: a boolean value indicating whether the service should do a best-effort resizing operation even if the request cannot be fully met.

For example, the following request is about increasing the size of the cluster by 20% and Senlin can try a best-effort if the calculated size is greater than the upper limit of the cluster size:

    {
      "adj_type": "CHANGE_IN_PERCENTAGE",
      "number": "20",
      "strict": False,
    }

When Senlin API receives a `cluster_resize` request, it first validates the data type of the values and the sanity of the value collection. For example, you cannot specify a `min_size` greater than the current upper bound (i.e. the `max_size` property of the cluster) if you are not providing a new `max_size` that is greater than the `min_size`.

After the request is forwarded to the Senlin engine, the engine will further validates the parameter values against the targeted cluster. When all validations pass, the request is converted into a `CLUSTER_RESIZE` action and queued for execution. The API returns the cluster properties and the UUID of the action at this moment.

When executing the action, Senlin will analyze the request parameters and determine the operations to be performed to meet user’s requirement. The corresponding cluster properties are updated before the resize operation is started.

### Scaling in/out a Cluster

As a convenience method, Senlin provides the `scale_out` and the `scale_in` action API for clusters. With these two APIs, a user can request a cluster to be resized by the specified number of nodes.

The `scale_out` and the `scale_in` APIs both take a parameter named `count` which is a positive integer. The integer parameter is optional, and it specifies the number of nodes to be added or removed if provided. When it is omitted from the request JSON body, Senlin engine will check if the cluster has any relevant policies attached that will decide the number of nodes to be added or removed respectively. The Senlin engine will use the outputs from these policies as the number of nodes to create (or delete) if such policies exist. When the request does contain a `count` parameter and there are policies governing the scaling arguments, the `count` parameter value may be overridden/ignored.

When a `scale_out` or a `scale_in` request is received by the Senlin engine, a `CLUSTER_SCALE_OUT` or a `CLUSTER_SCALE_IN` action is then created and queued for execution after some validation of the parameter value.

A worker thread picks up the action and execute it. The worker will check if there are outputs from policy checkings. For `CLUSTER_SCALE_OUT` actions, the worker checks if the policies checked has left a `count` key in the dictionary named `creation` from the action’s runtime `data` attribute. The worker will use such a `count` value for node creation. For a `CLUSTER_SCALE_OUT` action, the worker checks if the policies checked has left a `count` key in the dictionary named `deletion` from the action’s runtime `data` attribute. The worker will use such a `count` value for node deletion.

Note that both `scale_out` and `scale_in` actions will adjust the `desired_capacity` property of the target cluster.

## Cluster Policy Bindings

Senlin API provides the following action APIs for managing the binding relationship between a cluster and a policy:

- `policy_attach`: attach a policy to a cluster;

- `policy_detach`: detach a policy from a cluster;

- `policy_update`: update the properties of the binding between a cluster and a policy.

### Attaching a Policy to a Cluster

Once a policy is attached (bound) to a cluster, it will be enforced when related actions are performed on that cluster, unless the policy is (temporarily) disabled on the cluster.

When attaching a policy to a cluster, the following properties can be specified:

- `enabled`: a boolean indicating whether the policy should be enabled on the cluster once attached. Default is True. When specified, it will override the default setting for the policy.

Upon receiving the `policy_attach` request, the Senlin engine will perform some validations then translate the request into a `CLUSTER_ATTACH_POLICY` action and queue the action for execution. The action’s UUID is then returned to Senlin API and finally the requestor.

When the engine executes the action, it will try find if the policy is already attached to the cluster. This checking was not done previously because the engine must ensure that the cluster has been locked before this checking, or else there might be race conditions.

The engine calls the policy’s `attach` method when attaching the policy and record the binding into database if the `attach` method returns a positive response.

Currently, Senlin does not allow two policies of the same type to be attached to the same cluster. This constraint may be relaxed in future, but for now, it is checked and enforced before a policy gets attached to a cluster.

Policies attached to a cluster are cached at the target cluster as part of its runtime `rt` data structure. This is an optimization regarding DB queries.

### Detaching a Policy from a Cluster

Once a policy is attached to a cluster, it can be detached from the cluster at user’s request. The only parameter required for the `policy_detach` action API is `policy_id`, which can be the UUID, the name or the short ID of the policy.

Upon receiving a `policy_detach` request, the Senlin engine will perform some validations then translate the request into a `CLUSTER_DETACH_POLICY` action and queue the action for execution. The action’s UUID is then returned to Senlin API and finally the requestor.

When the Senlin engine executes the `CLUSTER_DETACH_POLICY` action, it will try find if the policy is already attached to the cluster. This checking was not done previously because the engine must ensure that the cluster has been locked before this checking, or else there might be race conditions.

The engine calls the policy’s `detach` method when detaching the policy from the cluster and then removes the binding record from database if the `detach` method returns a True value.

Policies attached to a cluster are cached at the target cluster as part of its runtime `rt` data structure. This is an optimization regarding DB queries. The `CLUSTER_DETACH_POLICY` action will invalidate the cache when detaching a policy from a cluster.

### Updating a Policy on a Cluster

When a policy is attached to a cluster, there are some properties pertaining to the binding. These properties can be updated as long as the policy is still attached to the cluster. The properties that can be updated include:

- `enabled`: a boolean value indicating whether the policy should be enabled or disabled. There are cases where some policies have to be temporarily disabled when other manual operations going on.

Upon receiving the `policy_update` request, Senlin API performs some basic validations on the parameters passed.

Senlin engine translates the `policy_update` request into an action `CLUSTER_UPDATE_POLICY` and queue it for execution. The UUID of the action is then returned to Senlin API and eventually the requestor.

During execution of the `CLUSTER_UPDATE_POLICY` action, Senlin engine simply updates the binding record in the database and returns.

updated: 2020-10-17 18:54

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