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.

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" 

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'}" 

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" 

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' 

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

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') 

The condition syntax supports the following functions.

add(Number, Number) Number

Usage:

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

arrayContains(Array, String) Boolean

Usage:

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

concat(String, String) String

Usage:

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

divide(Number, Number) Number

Usage:

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

endsWith(String, String) Boolean

Usage:

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

hasKey(Dictionary, String) Boolean

Usage:

hasKey(${host}, "platform") 

indexOf(String, String, Number?) Number

Usage:

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

length(Array|Dictionary|string)

Usage:

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

match(String, Regexp) boolean

Usage:

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

modulo(number, number) Number

Usage:

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

multiply(Number, Number) Number

Usage:

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

number(String) Integer

Usage:

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

startsWith(String, String) Boolean

Usage:

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

string(Number) String

Usage:

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

stringContains(String, String) Boolean

Usage:

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

subtract(Number, Number) Number

Usage:

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

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 ---