Logstash json_encode Filter Plugin

The json_encode filter plugin in Logstash is used to serialize a field to JSON. This is particularly useful when you need to convert a complex data structure, such as a hash or an array, into a JSON string. It's commonly used to prepare data for output plugins that expect JSON-formatted strings or to create a single JSON field from multiple nested fields.

Syntax

json_encode {
  source => "field_to_encode"
  target => "destination_field"
}

For more details, refer to the official Logstash json_encode filter plugin documentation.

Example Use Case

Suppose you have a log event with nested fields that you want to encode into a single JSON string:

filter {
  json_encode {
    source => "nested_field"
    target => "encoded_json"
  }
}

This configuration will take the content of nested_field, encode it as JSON, and store the result in encoded_json.

Common Issues and Best Practices

1. Data Types: Ensure that the source field contains data that can be properly serialized to JSON. Some complex Ruby objects may not be directly serializable.

2. Performance: Be mindful of the performance impact when encoding large or deeply nested structures, as it can be CPU-intensive.

3. Overwriting: If the target field already exists, it will be overwritten. Use conditional statements if you need to preserve existing data.

4. Encoding Options: The plugin supports various encoding options like pretty for formatted output and exclude_keys to omit specific keys from the encoded result.

Frequently Asked Questions

Q: Can I use json_encode to combine multiple fields into a single JSON object?
A: Yes, you can create a new field with multiple values and then encode that field. For example:

mutate {
  add_field => { "combined" => { "field1" => "%{field1}", "field2" => "%{field2}" } }
}
json_encode {
  source => "combined"
  target => "json_output"
}

Q: How can I pretty-print the JSON output?
A: Use the pretty option:

json_encode {
  source => "field_to_encode"
  target => "pretty_json"
  pretty => true
}

Q: Is it possible to exclude certain keys when encoding to JSON?
A: Yes, use the exclude_keys option:

json_encode {
  source => "field_to_encode"
  target => "filtered_json"
  exclude_keys => ["sensitive_key", "another_key"]
}

Q: Can json_encode handle nested JSON structures?
A: Yes, json_encode can handle nested structures. It will maintain the hierarchy when encoding.

Q: What happens if the source field is not a valid JSON structure?
A: If the source field is not a valid JSON structure, the plugin will attempt to convert it to a string representation. If this fails, it may result in an error or unexpected output.