About Nodes

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A node is any device—physical, virtual, cloud, network device, etc.—that is under management by Chef Infra.

The types of nodes that can be managed by Chef include, but are not limited to, the following:

Node Type	Description
	A physical node is typically a server or a virtual machine, but it can be any active device attached to a network that is capable of sending, receiving, and forwarding information over a communications channel. In other words, a physical node is any active device attached to a network that can run a Chef Infra Client and also allow that Chef Infra Client to communicate with a Chef Infra Server.
	A cloud-based node is hosted in an external cloud-based service, such as Amazon Web Services (AWS), OpenStack, Rackspace, Google Compute Engine, or Microsoft Azure. Plugins are available for knife that provide support for external cloud-based services. knife can use these plugins to create instances on cloud-based services. Once created, Chef Infra Client can be used to deploy, configure, and maintain those instances.
	A virtual node is a machine that runs only as a software implementation, but otherwise behaves much like a physical machine.
	A network node is any networking device---a switch, a router---that is being managed by a Chef Infra Client, such as networking devices by Juniper Networks, Arista, Cisco, and F5. Use Chef to automate common network configurations, such as physical and logical Ethernet link properties and VLANs, on these devices.
	Containers are an approach to virtualization that allows a single operating system to host many working configurations, where each working configuration---a container---is assigned a single responsibility that is isolated from all other responsibilities. Containers are popular as a way to manage distributed and scalable applications and services.

The key components of nodes that are under management by Chef include:

Component	Description
	Chef Infra Client is an agent that runs locally on every node that is under management by Chef Infra Server. When Chef Infra Client runs, performs all of the steps required for bringing a node into the expected state, including: Registering and authenticating the node with Chef Infra Server Building the node object Synchronizing cookbooks Compiling the resource collection by loading each of the required cookbooks, including recipes, attributes, and all other dependencies Taking the appropriate and required actions to configure the node Looking for exceptions and notifications, handling each as required Chef Infra Client authenticates with the Chef Infra Server using RSA public key-pairs each time a Chef Infra Client needs access to data that is stored on the Chef Infra Server. This prevents any node from accessing data that it shouldn’t and it ensures that only nodes that are properly registered with the Chef Infra Server can be managed.
	Ohai is a tool that is used to collect system configuration data, which is provided to Chef Infra Client for use within cookbooks. Ohai is run by Chef Infra Client at the beginning of every Chef run to determine system state. Ohai includes many built-in plugins to detect common configuration details as well as a plugin model for writing custom plugins. The types of attributes Ohai collects include but are not limited to: Operating System Network Memory Disk CPU Kernel Host names Fully qualified domain names Virtualization Cloud provider metadata Attributes that are collected by Ohai are automatic level attributes, in that these attributes are used by Chef Infra Client to ensure that these attributes remain unchanged after Chef Infra Client is done configuring the node.

About Run-lists

A run-list defines all of the information necessary for Chef to configure a node into the desired state. A run-list is:

• An ordered list of roles and/or recipes that are run in the exact order defined in the run-list; if a recipe appears more than once in the run-list, Chef Infra Client will not run it twice
• Always specific to the node on which it runs; nodes may have a run-list that is identical to the run-list used by other nodes
• Stored as part of the node object on the Chef server
• Maintained using knife and then uploaded from the workstation to the Chef Infra Server, or maintained using Chef Automate

Run-list Format

A run-list must be in one of the following formats: fully qualified, cookbook, or default. Both roles and recipes must be in quotes, for example:

"role[NAME]"

or

"recipe[COOKBOOK::RECIPE]"

Use a comma to separate roles and recipes when adding more than one item the run-list:

"recipe[COOKBOOK::RECIPE],COOKBOOK::RECIPE,role[NAME]"

Empty Run-lists

Use an empty run-list to determine if a failed Chef Infra Client run has anything to do with the recipes that are defined within that run-list. This is a quick way to discover if the underlying cause of a Chef Infra Client run failure is a configuration issue. If a failure persists even if the run-list is empty, check the following:

• Configuration settings in the config.rb file
• Permissions for the user to both the Chef Infra Server and to the node on which a Chef Infra Client run is to take place

About Node Names

The name of a node is required as part of the authentication process to the Chef Infra Server. The name of each node must be unique within an organization, but otherwise can be any string that matches the following regular expression:

/^[\-[:alnum:]_:.]+$/

The name of a node can be obtained from the node_name attribute in the client.rb file or by allowing Ohai to collect this data during a Chef Infra Client run. When Ohai collects this data during a Chef Infra Client run, it uses the node’s FQDN, which is always unique within an organization, as the name of the node.

Using the FQDN as the node name, and then allowing Ohai to collect this information during each Chef Infra Client run, is the recommended approach and the easiest way to ensure that the names of all nodes across the organization are unique.

Manage Nodes

There are several ways to manage nodes directly: via knife, Chef Automate, or by using command-line tools that are specific to Chef Infra Client.

• knife can be used to create, edit, view, list, tag, and delete nodes.
• knife plug-ins can be used to create, edit, and manage nodes that are located on cloud providers.
• Chef Automate can be used to create, edit, view, list, tag, and delete nodes. In addition, node attributes can be modified and nodes can be moved between environments.
• Chef Infra Client can be used to manage node data using the command line and JSON files. Each JSON file contains a hash, the elements of which are added as node attributes. In addition, the run_list setting allows roles and/or recipes to be added to the node.
• chef-solo can be used to manage node data using the command line and JSON files. Each JSON file contains a hash, the elements of which are added as node attributes. In addition, the run_list setting allows roles and/or recipes to be added to the node.
• The command line can also be used to edit JSON files and files that are related to third-party services, such as Amazon EC2, where the JSON files can contain per-instance metadata that is stored in a file on-disk and then read by chef-solo or Chef Infra Client as required.

Node Objects

For Chef Infra Client, two important aspects of nodes are groups of attributes and run-lists. An attribute is a specific piece of data about the node, such as a network interface, a file system, the number of clients a service running on a node is capable of accepting, and so on. A run-list is an ordered list of recipes and/or roles that are run in an exact order. The node object consists of the run-list and node attributes, which is a JSON file that is stored on the Chef Infra Server. Chef Infra Client gets a copy of the node object from the Chef Infra Server during each Chef Infra Client run and places an updated copy on the Chef Infra Server at the end of each Chef Infra Client run.

An attribute is a specific detail about a node. Attributes are used by Chef Infra Client to understand:

• The current state of the node
• What the state of the node was at the end of the previous Chef Infra Client run
• What the state of the node should be at the end of the current Chef Infra Client run

Attributes are defined by:

• The state of the node itself
• Attributes passed via JSON on the CLI
• Cookbooks (in attribute files and/or recipes)
• Roles
• Environments
• Policyfiles

During every Chef Infra Client run, Chef Infra Client builds the attribute list using:

• Attributes passed via JSON on the CLI
• Data about the node collected by [Ohai].
• The node object that was saved to the Chef Infra Server at the end of the previous Chef Infra Client run.
• The rebuilt node object from the current Chef Infra Client run, after it is updated for changes to cookbooks (attribute files and/or recipes), roles, and/or environments, and updated for any changes to the state of the node itself.

After the node object is rebuilt, all of the attributes are compared, and then the node is updated based on attribute precedence. At the end of every Chef Infra Client run, the node object that defines the current state of the node is uploaded to the Chef Infra Server so that it can be indexed for search.

Attributes

An attribute is a specific detail about a node, such as an IP address, a host name, a list of loaded kernel modules, the version(s) of available programming languages that are available, and so on. An attribute may be unique to a specific node or it can be identical across every node in the organization. Attributes are most commonly set from a cookbook, by using knife, or are retrieved by Ohai from each node prior to every Chef Infra Client run. All attributes are indexed for search on the Chef Infra Server. Good candidates for attributes include:

• any cross-platform abstraction for an application, such as the path to a configuration file
• default values for tunable settings, such as the amount of memory assigned to a process or the number of workers to spawn
• anything that may need to be persisted in node data between Chef Infra Client runs

In general, attribute precedence is set to enable cookbooks and roles to define attribute defaults, for normal attributes to define the values that should be specific for a node, and for override attributes to force a certain value, even when a node already has that value specified.

One approach is to set attributes at the same precedence level by setting attributes in a cookbook’s attribute files, and then also setting the same default attributes (but with different values) using a role. The attributes set in the role will be deep merged on top of the attributes from the attribute file, and the attributes set by the role will take precedence over the attributes specified in the cookbook’s attribute files.

Another (much less common) approach is to set a value only if an attribute has no value. This can be done by using the _unless variants of the attribute priority methods:

• default_unless

• set_unless (normal_unless is an alias of set_unless; use either alias to set an attribute with a normal attribute precedence.)

  Note

  This method was removed in Chef Client 14. Please use default_unless or override_unless instead.

• override_unless

Attribute Types

Chef Infra Client uses six types of attributes to determine the value that is applied to a node during a Chef Infra Client run. In addition, Chef Infra Client gathers attribute values from up to five locations. The combination of attribute types and sources makes up to 15 different competing values available during a Chef Infra Client run:

Attribute Persistence

All attributes except for normal attributes are reset at the beginning of a Chef Infra Client run. Attributes set via chef-client -j with a JSON file have normal precedence and are persisted between Chef Infra Client runs. Chef Infra Client rebuilds these attributes using automatic attributes collected by Ohai at the beginning of each Chef Infra Client run, and then uses default and override attributes that are specified in cookbooks, roles, environments, and Policyfiles. All attributes are then merged and applied to the node according to attribute precedence. The attributes that were applied to the node are saved to the Chef Infra Server as part of the node object at the conclusion of each Chef Infra Client run.

Attribute Precedence

Attributes are always applied by Chef Infra Client in the following order:

1. A default attribute located in a cookbook attribute file
2. A default attribute located in a recipe
3. A default attribute located in an environment
4. A default attribute located in a role
5. A force_default attribute located in a cookbook attribute file
6. A force_default attribute located in a recipe
7. A normal attribute located in a JSON file passed via chef-client -j
8. A normal attribute located in a cookbook attribute file
9. A normal attribute located in a recipe
10. An override attribute located in a cookbook attribute file
11. An override attribute located in a recipe
12. An override attribute located in a role
13. An override attribute located in an environment
14. A force_override attribute located in a cookbook attribute file
15. A force_override attribute located in a recipe
16. An automatic attribute identified by Ohai at the start of a Chef Infra Client run

where the last attribute in the list is the one that is applied to the node.

Attribute precedence, viewed from the same perspective as the overview diagram, where the numbers in the diagram match the order of attribute precedence:

Attribute precedence, when viewed as a table:

Blacklist Attributes

Attributes that should not be saved by a node may be blacklisted in the client.rb file. The blacklist is a Hash of keys that specify each attribute to be filtered out.

Attributes are blacklisted by attribute type, with each attribute type being blacklisted independently. Each attribute type—automatic, default, normal, and override—may define blacklists by using the following settings in the client.rb file:

For example, automatic attribute data similar to:

{
  "filesystem" => {
    "/dev/disk0s2" => {
      "size" => "10mb"
    },
    "map - autohome" => {
      "size" => "10mb"
    }
  },
  "network" => {
    "interfaces" => {
      "eth0" => {...},
      "eth1" => {...},
    }
  }
}

To blacklist the filesystem attributes and allow the other attributes to be saved, update the client.rb file:

automatic_attribute_blacklist ['filesystem']

When a blacklist is defined, any attribute of that type that is not specified in that attribute blacklist will be saved. So based on the previous blacklist for automatic attributes, the filesystem and map - autohome attributes will not be saved, but the network attributes will.

For attributes that contain slashes (/) within the attribute value, such as the filesystem attribute '/dev/diskos2', use an array. For example:

automatic_attribute_blacklist [['filesystem','/dev/diskos2']]

Whitelist Attributes

Attributes that should be saved by a node may be whitelisted in the client.rb file. The whitelist is a hash of keys that specifies each attribute to be saved.

Attributes are whitelisted by attribute type, with each attribute type being whitelisted independently. Each attribute type—automatic, default, normal, and override—may define whitelists by using the following settings in the client.rb file:

For example, automatic attribute data similar to:

{
  "filesystem" => {
    "/dev/disk0s2" => {
      "size" => "10mb"
    },
    "map - autohome" => {
      "size" => "10mb"
    }
  },
  "network" => {
    "interfaces" => {
      "eth0" => {...},
      "eth1" => {...},
    }
  }
}

To whitelist the network attributes and prevent the other attributes from being saved, update the client.rb file:

automatic_attribute_whitelist ['network/interfaces/']

When a whitelist is defined, any attribute of that type that is not specified in that attribute whitelist will not be saved. So based on the previous whitelist for automatic attributes, the filesystem and map - autohome attributes will not be saved, but the network attributes will.

Leave the value empty to prevent all attributes of that attribute type from being saved:

automatic_attribute_whitelist []

For attributes that contain slashes (/) within the attribute value, such as the filesystem attribute '/dev/diskos2', use an array. For example:

automatic_attribute_whitelist [['filesystem','/dev/diskos2']]