Alter filter plugin

Applies field-level alterations that do not fit the mutate filter: conditional replacement, fallback values, and coalesce operations across several fields.

• Package: logstash-filter-alter
• Coverage source: default/bundled, explicitly installed in the Logit image
• Official catalog entry: Yes

Plugin overview

alter is used in the Logstash filter stage. Performs field alterations that are not handled by mutate.

Typical use cases

• Transform fields before indexing to keep schema and naming consistent.
• Prepare high-quality fields for alerts, dashboards, and downstream pipelines.

Input and output behavior

• Flow: processes matching events and mutates fields/tags within the same event.
• Input: works on events that match your surrounding if conditions.
• Output: updates the current event in place unless configured otherwise.
• Important options: coalesce, condrewrite, condrewriteother.

Options

Required

• No required plugin-specific options.

Optional

• coalesce (type: array; default: none) — Takes the first non-null value from a list of fields and writes it to a destination field.
• condrewrite (type: array; default: none) — Rewrites a field value only when it matches a supplied condition.
• condrewriteother (type: array; default: none) — Conditionally rewrites a different field based on the source field's value.

Example configuration

filter {
  alter {
    coalesce => ["[user][email]", "%{[user][username]}", "%{[user][id]}", "unknown"]
    condrewrite => [
      "[response][status]", "ok", "success",
      "[response][status]", "fail", "failure"
    ]
  }
}

Common options configuration

All Logstash filter plugins support these shared options:

• add_field (type: hash; default: {}) — Adds fields when the filter succeeds. Supports dynamic field names and values.
• add_tag (type: array; default: []) — Adds one or more tags when the filter succeeds.
• enable_metric (type: boolean; default: true) — Enables or disables metric collection for this plugin instance.
• id (type: string; default: none) — Sets an explicit plugin instance ID for monitoring and troubleshooting.
• periodic_flush (type: boolean; default: false) — Calls the filter flush method at regular intervals.
• remove_field (type: array; default: []) — Removes fields when the filter succeeds. Supports dynamic field names.
• remove_tag (type: array; default: []) — Removes tags when the filter succeeds.

filter {
  alter {
    add_field => { "pipeline_stage" => "parsed" }
    add_tag => ["parsed", "logstash_filter"]
    enable_metric => true
    id => "my_filter_instance"
    periodic_flush => false
    remove_field => ["tmp_field"]
    remove_tag => ["temporary"]
  }
}

Apply in Logit.io

1. Open your stack in Logit.io and navigate to Logstash Pipelines.
2. In the filter { ... } section, add a alter block.
3. Save your pipeline changes, then restart the Logstash pipeline if prompted.
4. Send sample events and verify parsed/enriched fields in OpenSearch Dashboards.

Validation checklist

• Confirm the alter block compiles without syntax errors.
• Verify expected new/updated fields exist in sample documents.
• Verify unexpected fields are not removed unless explicitly configured.
• Confirm tags added on success/failure align with your alerting and routing rules.

Troubleshooting

• If events are unchanged, verify your filter condition (if ...) matches incoming events.
• If the pipeline fails to start, validate braces/quotes and retry with a minimal filter block.
• If throughput drops, reduce expensive operations and test with representative sample volume.

References

• GitHub package: logstash-filter-alter (opens in a new tab)
• Canonical catalog: /log-management/ingestion-pipeline/logstash-filters-reference