The Scripted Metric Aggregation in Elasticsearch allows you to perform complex, multi-step metric calculations across documents in your index using custom scripts. This aggregation is highly flexible and can be used when built-in aggregations don't meet your specific requirements.
Syntax
{
"scripted_metric": {
"init_script": "...",
"map_script": "...",
"combine_script": "...",
"reduce_script": "..."
}
}
For detailed syntax and options, refer to the official Elasticsearch documentation.
Example Usage
Here's an example that calculates the average price of items in an order:
{
"aggs": {
"avg_price_per_order": {
"scripted_metric": {
"init_script": "state.transactions = []",
"map_script": "state.transactions.add(doc.price.value)",
"combine_script": "double sum = 0; for (t in state.transactions) { sum += t } return sum",
"reduce_script": "double sum = 0; for (a in states) { sum += a } return sum / states.size()"
}
}
}
}
Common Issues
- Performance impact: Scripted metric aggregations can be resource-intensive, especially with large datasets.
- Script errors: Syntax errors or logical mistakes in scripts can lead to execution failures.
- Security concerns: Ensure proper access controls are in place, as scripted aggregations can potentially access sensitive data.
Best Practices
- Use built-in aggregations when possible for better performance.
- Test scripts thoroughly on a subset of data before running on large datasets.
- Implement appropriate error handling and logging in your scripts.
- Consider using stored scripts for frequently used calculations to improve reusability and maintainability.
Frequently Asked Questions
Q: Can I use different programming languages for scripted metric aggregations?
A: Elasticsearch primarily supports Painless as the default scripting language. However, you can also use other languages like Groovy, JavaScript, or Python if they are enabled in your Elasticsearch configuration.
Q: How can I debug a scripted metric aggregation?
A: You can use Elasticsearch's explain API to get detailed information about how your aggregation is executed. Additionally, adding logging statements in your scripts can help identify issues.
Q: Are there any limitations to what I can do with scripted metric aggregations?
A: While scripted metric aggregations are very flexible, they are subject to the same security restrictions as other Elasticsearch scripts. There may also be performance limitations when dealing with very large datasets.
Q: Can I access external resources or make API calls within a scripted metric aggregation?
A: For security reasons, Elasticsearch scripts, including those in scripted metric aggregations, are sandboxed and cannot access external resources or make API calls directly.
Q: How do I optimize the performance of scripted metric aggregations?
A: To optimize performance, minimize the amount of data processed in each script phase, use efficient data structures, and consider using faster built-in aggregations where possible. Also, ensure your Elasticsearch cluster has sufficient resources to handle the computational load.
