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Documentation Grafana Loki Clients Promtail Stages json

json stage

The json stage is a parsing stage that reads the log line as JSON and accepts JMESPath expressions to extract data.

Schema

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json:
  # Set of key/value pairs of JMESPath expressions. The key will be
  # the key in the extracted data while the expression will be the value,
  # evaluated as a JMESPath from the source data.
  #
  # Literal JMESPath expressions can be done by wrapping a key in
  # double quotes, which then must be wrapped in single quotes in
  # YAML so they get passed to the JMESPath parser.
  expressions:
    [ <string>: <string> ... ]

  # Name from extracted data to parse. If empty, uses the log message.
  [source: <string>]

  # When true, then any lines that cannot be successfully parsed as valid JSON objects
  # will be dropped instead of being sent to Loki.
  [drop_malformed: <bool> | default = false]

This stage uses the Go JSON unmarshaler, which means non-string types like numbers or booleans will be unmarshaled into those types. The extracted data can hold non-string values and this stage does not do any type conversions; downstream stages will need to perform correct type conversion of these values as necessary. Please refer to the the template stage for how to do this.

If the value extracted is a complex type, such as an array or a JSON object, it will be converted back into a JSON string before being inserted into the extracted data.

Examples

Using log line

For the given pipeline:

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- json:
    expressions:
      output: log
      stream: stream
      timestamp: time

Given the following log line:

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{"log":"log message\n","stream":"stderr","time":"2019-04-30T02:12:41.8443515Z"}

The following key-value pairs would be created in the set of extracted data:

• output: log message\n
• stream: stderr
• timestamp: 2019-04-30T02:12:41.8443515

Using extracted data

For the given pipeline:

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- json:
    expressions:
      output:    log
      stream:    stream
      timestamp: time
      extra:
- json:
    expressions:
      user:
    source: extra

And the given log line:

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{"log":"log message\n","stream":"stderr","time":"2019-04-30T02:12:41.8443515Z","extra":"{\"user\":\"marco\"}"}

The first stage would create the following key-value pairs in the set of extracted data:

• output: log message\n
• stream: stderr
• timestamp: 2019-04-30T02:12:41.8443515
• extra: {"user": "marco"}

The second stage will parse the value of extra from the extracted data as JSON and append the following key-value pairs to the set of extracted data:

• user: marco

Using a JMESPath Literal

This pipeline uses a literal JMESPath expression to parse JSON fields with special characters in the name, like @ or .

For the given pipeline:

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- json:
    expressions:
      output: log
      stream: '"grpc.stream"'
      timestamp: time

Given the following log line:

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{"log":"log message\n","grpc.stream":"stderr","time":"2019-04-30T02:12:41.8443515Z"}

The following key-value pairs would be created in the set of extracted data:

• output: log message\n
• stream: stderr
• timestamp: 2019-04-30T02:12:41.8443515

Note that referring to grpc.stream without the combination of double quotes wrapped in single quotes will not work properly.