Advanced Log Parsing Patterns

Once you have mastered the basic parsing techniques for different formats, you can add more sophisticated logic for error handling and data enrichment.

Pattern 1: Graceful Error Handling

When a processor like parse_json or grok fails, it stops the processing of that message. For a robust pipeline, you should wrap parsing attempts in a try/catch block. This allows you to catch the error, route the malformed message to a Dead Letter Queue (DLQ), and continue processing other messages.

Add Error Handling to a Parser

- try:
    # Attempt to parse the log
    - grok:
        expressions: [ '%{COMMONAPACHELOG}' ]
        named_captures_only: true

    # If it succeeds, add a success status
    - mapping: |
        root = this
        meta parse_status = "success"

  # If the grok processor fails, this block is executed
  catch:
    - mapping: |
        root = this
        meta parse_status = "failed"
        root.parse_error = error() # Store the error message

# Now you can use a 'switch' in your output to route based on meta("parse_status")

Pattern 2: Data Enrichment

Parsing is often just the first step. After you have structured data, you typically want to enrich it with more context.

Use Case: After parsing an access log, you want to perform a GeoIP lookup on the client_ip and parse the user_agent string to identify the browser and OS.

Enriching Parsed Log Data

pipeline:
  processors:
    # 1. PARSE: Start with the grok processor from Step 3
    - grok:
        expressions: [ '%{COMBINEDAPACHELOG}' ]
        named_captures_only: true

    # 2. ENRICH: Add processors to enrich the parsed data
    
    # Enrich with GeoIP data
    - geoip:
        field: root.client_ip
        target_field: root.client_geo

    # Enrich with User Agent data
    - user_agent:
        field: root.user_agent
        target_field: root.client_ua

    # Enrich with custom business logic
    - mapping: |
        root = this
        root.request_type = if this.request.starts_with("/api/") { "api" } else { "web" }

This multi-stage process of parsing and then enriching is a very common and powerful pattern in data processing pipelines.