template stage

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The template stage is a transform stage that lets use manipulate the values in the extracted map using Go’s template syntax.

The template stage is primarily useful for manipulating data from other stages before setting them as labels, such as to replace spaces with underscores or converting an uppercase string into a lowercase one. template can also be used to construct messages with multiple keys.

The template stage can also create new keys in the extracted map.

Schema

template:
  # Name from extracted data to parse. If key in extract data doesn't exist, an
  # entry for it will be created.
  source: <string>

  # Go template string to use. In additional to normal template
  # functions, ToLower, ToUpper, Replace, Trim, TrimLeft, TrimRight,
  # TrimPrefix, TrimSuffix, and TrimSpace are available as functions.
  template: <string>

Examples

- template:
    source: new_key
    template: 'hello world!'

Assuming no data has been added to the extracted map yet, this stage will first add new_key with a blank value into the extracted map. Then its value will be set to hello world!.

- template:
    source: app
    template: '{{ .Value }}_some_suffix'

This pipeline takes the value of the app key in the existing extracted map and appends _some_suffix to its value. For example, if the extracted map had a key of app and a value of loki, this stage would modify the value from loki to loki_some_suffix.

- template:
    source: app
    template: '{{ ToLower .Value }}'

This pipeline takes the current value of app from the extracted map and converts its value to be all lowercase. For example, if the extracted map contained app with a value of LOKI, this pipeline would change its value to loki.

- template:
    source: output_msg
    template: '{{ .level }} for app {{ ToUpper .app }}'

This pipeline takes the current value of level and app from the extracted map and a new key output_msg will be added to extracted map with evaluated template.

For example, if the extracted map contained app with a value of loki, this pipeline would change its value to LOKI. Assuming value of level is warn. A new key output_msg will be added to extracted map with value warn for app LOKI.

Any previously extracted keys can be used in template. All extracted keys are available for template to expand.

- template:
    source: app
    template: '{{ .level }} for app {{ ToUpper .Value }} in module {{.module}}'

This pipeline takes the current value of level, app and module from the extracted map and converts value of app to the evaluated template.

For example, if the extracted map contained app with a value of loki, this pipeline would change its value to LOKI. Assuming value of level is warn and value of module is test. Pipeline will change the value of app to warn for app LOKI in module test.

Any previously extracted keys can be used in template. All extracted keys are available for template to expand. Also, if source is available it can be referred as .Value in template. Here, app is provided as source. So, it can be referred as .Value in template.

- template:
    source: app
    template: '{{ Replace .Value "loki" "blokey" 1 }}'

The template here uses Go’s string.Replace function. When the template executes, the entire contents of the app key from the extracted map will have at most 1 instance of loki changed to blokey.

A special key named Entry can be used to reference the current line, this can be useful when you need to append/prepend the log line.

- template:
    source: message
    template: '{{.app }}: {{ .Entry }}'
- output:
    source: message

The snippet above will for instance prepend the log line with the application name.

Supported Functions

ToLower & ToUpper

ToLower and ToUpper convert the entire string respectively to lowercase and uppercase.

Examples:

- template:
    source: out
    template: '{{ ToLower .app }}'
- template:
    source: out
    template: '{{ .app | ToUpper }}'

Replace

Replace returns a copy of the string s with the first n non-overlapping instances of old replaced by new. If old is empty, it matches at the beginning of the string and after each UTF-8 sequence, yielding up to k+1 replacements for a k-rune string. If n < 0, there is no limit on the number of replacements.

The example below will replace the first two words loki by Loki.

- template:
    source: output
    template: '{{ Replace .Value "loki" "Loki" 2 }}'

Trim

Trim returns a slice of the string s with all leading and trailing Unicode code points contained in cutset removed.

TrimLeft and TrimRight are the same as Trim except that it respectively trim only leading and trailing characters.

- template:
    source: output
    template: '{{ Trim .Value ",. " }}'

TrimSpace TrimSpace returns a slice of the string s, with all leading and trailing white space removed, as defined by Unicode.

- template:
    source: output
    template: '{{ TrimSpace .Value }}'

TrimPrefix and TrimSuffix will trim respectively the prefix or suffix supplied.

- template:
    source: output
    template: '{{ TrimPrefix .Value "--" }}'

Regex

regexReplaceAll returns a copy of the input string, replacing matches of the Regexp with the replacement string replacement. Inside string replacement, $ signs are interpreted as in Expand, so for instance $1 represents the text of the first submatch

- template:
    source: output
    template: '{{ regexReplaceAllLiteral "(a*)bc" .Value "{1}a" }}'

regexReplaceAllLiteral returns a copy of the input string, replacing matches of the Regexp with the replacement string replacement The replacement string is substituted directly, without using Expand.

- template:
    source: output
    template: '{{ regexReplaceAllLiteral "(ts=)" .Value "timestamp=" }}'

Hash and Sha2Hash

Hash returns a Sha3_256 hash of the string, represented as a hexadecimal number of 64 digits. You can use it to obfuscate sensitive data / PII in the logs. It requires a (fixed) salt value, to add complexity to low input domains (e.g. all possible Social Security Numbers).

- template:
    source: output
    template: '{{ Hash .Value "salt" }}'

Alternatively, you can use Sha2Hash for calculating the Sha2_256 of the string. Sha2_256 is faster and requires less CPU than Sha3_256, however it is less secure.

We recommend using Hash as it has a stronger hashing algorithm which we plan to keep strong over time without requiring client config changes.

- template:
    source: output
    template: '{{ Sha2Hash .Value "salt" }}'