Logstash alter Filter Plugin

The alter filter plugin in Logstash allows you to selectively modify fields in events based on conditions. It's particularly useful when you need to change field values, rename fields, or remove fields based on specific criteria.

Syntax

filter {
  alter {
    condrewrite => [
      "field_name", "pattern", "replacement",
      "field_name", "pattern", "replacement"
    ]
    coalesce => [
      "field_name", "field1", "field2", "field3"
    ]
    rename => {
      "old_field" => "new_field"
    }
    remove => ["field1", "field2"]
  }
}

For detailed options and parameters, refer to the official Logstash alter filter documentation.

Example Use Case

Suppose you want to standardize the "status" field in your logs, rename a field, and remove sensitive information:

filter {
  alter {
    condrewrite => [
      "status", "ERROR", "error",
      "status", "WARNING", "warning",
      "status", "INFO", "info"
    ]
    rename => { "old_timestamp" => "timestamp" }
    remove => ["credit_card_number"]
  }
}

This configuration will:

Standardize the "status" field values
Rename "old_timestamp" to "timestamp"
Remove the "credit_card_number" field

Common Issues and Best Practices

Ensure that the fields you're trying to alter exist in your events to avoid errors.
Use the coalesce option when you want to set a field's value from the first non-null field in a list.
Be cautious when removing fields, as it might impact downstream processors or outputs.
Consider using conditional statements (if blocks) for more complex alterations.

Frequently Asked Questions

Q: Can I use regular expressions in the condrewrite patterns?
A: Yes, the patterns in condrewrite support regular expressions, allowing for more flexible matching.

Q: How does the alter filter handle non-existent fields?
A: If a field specified in the alter filter doesn't exist in an event, the operation for that field is simply skipped without raising an error.

Q: Can I use the alter filter to create new fields?
A: The alter filter is primarily designed to modify existing fields. To create new fields, consider using the mutate filter instead.

Q: Is it possible to alter nested fields with this plugin?
A: Yes, you can access nested fields using dot notation, e.g., "parent.child" in your field names.

Q: How does the performance of the alter filter compare to using multiple mutate filters?
A: The alter filter can be more efficient than using multiple mutate filters for simple operations, as it processes all alterations in a single step.