Variables and conditions in input configurations

When running Elastic Agent in some environments, you might not know all the input configuration details up front. To solve this problem, the input configuration accepts variables and conditions that get evaluated at runtime using information from the running environment. Similar to autodiscovery, these capabilities allow you to apply configurations dynamically.

Let’s consider a unique agent policy that is deployed on two machines: a Linux machine named "linux-app" and a Windows machine named "winapp". Notice that the configuration has some variable references: ${host.name} and ${host.platform}:

		inputs:
 - id: unique-logfile-id
   type: logfile
   streams:
    - paths: /var/log/${host.name}/another.log
      condition: ${host.platform} == "linux"
    - path: c:/service/app.log
      condition: ${host.platform} == "windows"

	

At runtime, Elastic Agent resolves variables and evaluates the conditions based on values provided by the environment, generating two possible input configurations.

On the Windows machine:

		inputs:
 - id: unique-logfile-id
   type: logfile
   streams:
    - path: c:/service/app.log

	

On the Linux machine:

		inputs:
 - id: unique-logfile-id
   type: logfile
   streams:
    - paths: /var/log/linux-app/another.log

	

Using variable substitution along with conditions allows you to create concise, but flexible input configurations that adapt to their deployed environment.

Variable substitution

The syntax for variable substitution is ${var}, where var is the name of a variable defined by a provider. A provider defines key/value pairs that are used for variable substitution and conditions.

Elastic Agent supports a variety of providers, such as host and local, that supply variables to Elastic Agent. For example, earlier you saw ${host.name} used to resolve the path to the host’s log file based on the {host.platform} value. Both of these values were provided by the host provider.

All providers are enabled by default when Elastic Agent starts. If a provider cannot be configured, its variables are ignored.

Refer to Providers for more detail.

The following agent policy uses a custom key named foo to resolve a value defined by a local provider:

		inputs:
 - id: unique-logfile-id
   type: logfile
   streams:
    - paths: /var/log/${foo}/another.log

providers:
  local:
    vars:
      foo: bar

	

The policy generated by this configuration looks like this:

		inputs:
 - id: unique-logfile-id
   type: logfile
   streams:
    - paths: /var/log/bar/another.log

	

When an input uses a variable substitution that is not present in the current key/value mappings being evaluated, the input is removed in the result.

For example, this agent policy uses an unknown key:

		inputs:
  - id: logfile-foo
    type: logfile
    path: "/var/log/foo"
  - id: logfile-unknown
    type: logfile
    path: "${ unknown.key }"

	

The policy generated by this configuration looks like this:

		inputs:
  - id: logfile-foo
    type: logfile
    path: "/var/log/foo"

	

Alternative variables and constants

Variable substitution can also define alternative variables or a constant.

To define a constant, use either ' or ". When a constant is reached during variable evaluation, any remaining variables are ignored, so a constant should be the last entry in the substitution.

To define alternatives, use | followed by the next variable or constant. The power comes from allowing the input to define the preference order of the substitution by chaining multiple variables together.

For example, the following agent policy chains together multiple variables to set the log path based on information provided by the running container environment. The constant /var/log/other is used to end of the path, which is common to both providers:

		inputs:
  - id: logfile-foo
    type: logfile
    path: "/var/log/foo"
  - id: logfile-container
    type: logfile
    path: "${docker.paths.log|kubernetes.container.paths.log|'/var/log/other'}"

	

Escaping variables

In some cases the ${var} syntax causes an issue with using a value where the actually wanted variable is ${var}. In this case double $$ can be provided for the variable.

The double $$ causes the variable to be ignored and the extra $ is removed from the beginning.

For example, the following agent policy uses the escaped variable so the actual value is used instead.

		inputs:
  - id: logfile-foo
    type: logfile
    path: "/var/log/foo"
    processors:
      - add_tags:
          tags: [$${development}]
          target: "environment"

	

The policy generated by this configuration looks like this:

		inputs:
  - id: logfile-foo
    type: logfile
    path: "/var/log/foo"
    processors:
      - add_tags:
          tags: [${development}]
          target: "environment"

	

Conditions

A condition is a boolean expression that you can specify in your agent policy to control whether a configuration is applied to the running Elastic Agent. You can set a condition on inputs, streams, or even processors.

In this example, the input is applied if the host platform is Linux:

		inputs:
  - id: unique-logfile-id
    type: logfile
    streams:
      - paths:
         - /var/log/syslog
    condition: ${host.platform} == 'linux'

	

In this example, the stream is applied if the host platform is not Windows:

		inputs:
  - id: unique-system-metrics-id
    type: system/metrics
    streams:
      - metricset: load
        data_stream.dataset: system.cpu
        condition: ${host.platform} != 'windows'

	

In this example, the processor is applied if the host platform is not Windows:

		inputs:
  - id: unique-system-metrics-id
    type: system/metrics
    streams:
      - metricset: load
        data_stream.dataset: system.cpu
    processors:
      - add_fields:
          fields:
            platform: ${host.platform}
          to: host
        condition: ${host.platform} != 'windows'

	

Condition syntax

The conditions supported by Elastic Agent are based on EQL's boolean syntax, but add support for variables from providers and functions to manipulate the values.

Supported operators:

Full PEMDAS math support for + - * / %.
Relational operators < <= >= > == !=
Logical operators and and or

Functions:

Array functions arrayContains
Dict functions hasKey (not in EQL)
Length functions length
Math functions add, subtract, multiply, divide, modulo
String functions concat, endsWith, indexOf, match, number, startsWith, string, stringContains

Types:

Booleans true and false

Condition examples

Run only when a specific label is included.

arrayContains(${docker.labels}, 'monitor')

Skip on Linux platform or macOS.

${host.platform} != "linux" and ${host.platform} != "darwin"

Run only for specific labels.

		arrayContains(${docker.labels}, 'monitor') or arrayContains(${docker.label}, 'production')

	

Function reference

The condition syntax supports the following functions.

`add`

add(Number, Number) Number

Usage:

		add(1, 2) == 3
add(5, ${foo}) >= 5

`arrayContains`

arrayContains(Array, String) Boolean

Usage:

arrayContains(${docker.labels}, 'monitor')

`concat`

concat(String, String) String

Note

Parameters are coerced into strings before the concatenation.

Usage:

		concat("foo", "bar") == "foobar"
concat(${var1}, ${var2}) != "foobar"

`divide`

divide(Number, Number) Number

Usage:

		divide(25, 5) > 0
divide(${var1}, ${var2}) > 7

`endsWith`

endsWith(String, String) Boolean

Usage:

		endsWith("hello world", "hello") == true
endsWith(${var1}, "hello") != true

`hasKey`

hasKey(Dictionary, String) Boolean

Usage:

hasKey(${host}, "platform")

`indexOf`

indexOf(String, String, Number?) Number

Note

Returns -1 if the string is not found.

Usage:

		indexOf("hello", "llo") == 2
indexOf(${var1}, "hello") >= 0

`length`

length(Array|Dictionary|string)

Usage:

		length("foobar") > 2
length(${docker.labels}) > 0
length(${host}) > 2

	

`match`

match(String, Regexp) boolean

Note

Regexp supports Go’s regular expression syntax. Conditions that use regular expressions are more expensive to run. If speed is critical, consider using endWiths or startsWith.

Usage:

		match("hello world", "^hello") == true
match(${var1}, "world$") == true

`modulo`

modulo(number, number) Number

Usage:

		modulo(25, 5) > 0
modulo(${var1}, ${var2}) == 0

`multiply`

multiply(Number, Number) Number

Usage:

		multiply(5, 5) == 25
multiple(${var1}, ${var2}) > x

`number`

number(String) Integer

Usage:

		number("42") == 42
number(${var1}) == 42

`startsWith`

startsWith(String, String) Boolean

Usage:

		startsWith("hello world", "hello") == true
startsWith(${var1}, "hello") != true

`string`

string(Number) String

Usage:

		string(42) == "42"
string(${var1}) == "42"

`stringContains`

stringContains(String, String) Boolean

Usage:

		stringContains("hello world", "hello") == true
stringContains(${var1}, "hello") != true

`subtract`

subtract(Number, Number) Number

Usage:

		subtract(5, 1) == 4
subtract(${foo}, 2) != 2

Debugging

To debug configurations that include variable substitution and conditions, use the inspect command. This command shows the configuration that’s generated after variables are replaced and conditions are applied.

First run the Elastic Agent. For this example, we’ll use the following agent policy:

		outputs:
  default:
    type: elasticsearch
    hosts: [127.0.0.1:9200]
    apikey: <my-api-key>

providers:
  local_dynamic:
    items:
      - vars:
          key: value1
        processors:
          - add_fields:
              fields:
                custom: match1
              target: dynamic
      - vars:
          key: value2
        processors:
          - add_fields:
              fields:
                custom: match2
              target: dynamic
      - vars:
          key: value3
        processors:
          - add_fields:
              fields:
                custom: match3
              target: dynamic

inputs:
  - id: unique-logfile-id
    type: logfile
    enabled: true
    streams:
      - paths:
          - /var/log/${local_dynamic.key}

	

Then run elastic-agent inspect --variables to see the generated configuration. For example:

		$ ./elastic-agent inspect --variables
inputs:
- enabled: true
  id: unique-logfile-id-local_dynamic-0
  original_id: unique-logfile-id
  processors:
  - add_fields:
      fields:
        custom: match1
      target: dynamic
  streams:
  - paths:
    - /var/log/value1
  type: logfile
- enabled: true
  id: unique-logfile-id-local_dynamic-1
  original_id: unique-logfile-id
  processors:
  - add_fields:
      fields:
        custom: match2
      target: dynamic
  streams:
  - paths:
    - /var/log/value2
  type: logfile
- enabled: true
  id: unique-logfile-id-local_dynamic-2
  original_id: unique-logfile-id
  processors:
  - add_fields:
      fields:
        custom: match3
      target: dynamic
  streams:
  - paths:
    - /var/log/value3
  type: logfile
outputs:
  default:
    apikey: <my-api-key>
    hosts:
    - 127.0.0.1:9200
    type: elasticsearch
providers:
  local_dynamic:
    items:
    - processors:
      - add_fields:
          fields:
            custom: match1
          target: dynamic
      vars:
        key: value1
    - processors:
      - add_fields:
          fields:
            custom: match2
          target: dynamic
      vars:
        key: value2
    - processors:
      - add_fields:
          fields:
            custom: match3
          target: dynamic
      vars:
        key: value3

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