VictoriaMetrics/docs/anomaly-detection/FAQ.md

---
sort: 2
weight: 4
title: FAQ
menu:
  docs:
    identifier: "vmanomaly-faq"
    parent: "anomaly-detection"
    weight: 4
aliases:
- /anomaly-detection/FAQ.html
---

# FAQ - VictoriaMetrics Anomaly Detection

## What is VictoriaMetrics Anomaly Detection (vmanomaly)?
VictoriaMetrics Anomaly Detection, also known as `vmanomaly`, is a service for detecting unexpected changes in time series data. Utilizing machine learning models, it computes and pushes back an ["anomaly score"](/anomaly-detection/components/models.html#vmanomaly-output) for user-specified metrics. This hands-off approach to anomaly detection reduces the need for manual alert setup and can adapt to various metrics, improving your observability experience.

Please refer to [our guide section](/anomaly-detection/#practical-guides-and-installation) to find out more.

> **Note: `vmanomaly` is a part of [enterprise package](https://docs.victoriametrics.com/enterprise/). You need to get a [free trial license](https://victoriametrics.com/products/enterprise/trial/) for evaluation.**

## What is anomaly score?
Among the metrics produced by `vmanomaly` (as detailed in [vmanomaly output metrics](/anomaly-detection/components/models.html#vmanomaly-output)), `anomaly_score` is a pivotal one. It is **a continuous score > 0**, calculated in such a way that **scores ranging from 0.0 to 1.0 usually represent normal data**, while **scores exceeding 1.0 are typically classified as anomalous**. However, it's important to note that the threshold for anomaly detection can be customized in the alert configuration settings.

The decision to set the changepoint at `1.0` is made to ensure consistency across various models and alerting configurations, such that a score above `1.0` consistently signifies an anomaly, thus, alerting rules are maintained more easily.

> Note: `anomaly_score` is a metric itself, which preserves all labels found in input data and (optionally) appends [custom labels, specified in writer](/anomaly-detection/components/writer.html#metrics-formatting) - follow the link for detailed output example.

## How is anomaly score calculated?
For most of the [univariate models](/anomaly-detection/components/models/#univariate-models) that can generate `yhat`, `yhat_lower`, and `yhat_upper` time series in [their output](/anomaly-detection/components/models/#vmanomaly-output) (such as [Prophet](/anomaly-detection/components/models/#prophet) or [Z-score](/anomaly-detection/components/models/#z-score)), the anomaly score is calculated as follows:
- If `yhat` (expected series behavior) equals `y` (actual value observed), then the anomaly score is 0.
- If `y` (actual value observed) falls within the `[yhat_lower, yhat_upper]` confidence interval, the anomaly score will gradually approach 1, the closer `y` is to the boundary.
- If `y` (actual value observed) strictly exceeds the `[yhat_lower, yhat_upper]` interval, the anomaly score will be greater than 1, increasing as the margin between the actual value and the expected range grows.

Please see example graph illustrating this logic below:

<img alt="anomaly-score-calculation-example" src="vmanomaly-prophet-example.webp">


## How does vmanomaly work?
`vmanomaly` applies built-in (or custom) [anomaly detection algorithms](/anomaly-detection/components/models.html), specified in a config file. Although a single config file supports one model, running multiple instances of `vmanomaly` with different configs is possible and encouraged for parallel processing or better support for your use case (i.e. simpler model for simple metrics, more sophisticated one for metrics with trends and seasonalities).

1. For more detailed information, please visit the [overview section](/anomaly-detection/Overview.html#about).
2. To view a diagram illustrating the interaction of components, please explore the [components section](/anomaly-detection/components/).

## What data does vmanomaly operate on?
`vmanomaly` operates on data fetched from VictoriaMetrics, where you can leverage full power of [MetricsQL](https://docs.victoriametrics.com/metricsql/) for data selection, sampling, and processing. Users can also [apply global filters](https://docs.victoriametrics.com/#prometheus-querying-api-enhancements) for more targeted data analysis, enhancing scope limitation and tenant visibility.

Respective config is defined in a [`reader`](/anomaly-detection/components/reader.html#vm-reader) section.

## Handling noisy input data
`vmanomaly` operates on data fetched from VictoriaMetrics using [MetricsQL](https://docs.victoriametrics.com/metricsql/) queries, so the initial data quality can be fine-tuned with aggregation, grouping, and filtering to reduce noise and improve anomaly detection accuracy.

## Output produced by vmanomaly
`vmanomaly` models generate [metrics](/anomaly-detection/components/models.html#vmanomaly-output) like `anomaly_score`, `yhat`, `yhat_lower`, `yhat_upper`, and `y`. These metrics provide a comprehensive view of the detected anomalies. The service also produces [health check metrics](/anomaly-detection/components/monitoring.html#metrics-generated-by-vmanomaly) for monitoring its performance.

## Choosing the right model for vmanomaly
Selecting the best model for `vmanomaly` depends on the data's nature and the [types of anomalies](https://victoriametrics.com/blog/victoriametrics-anomaly-detection-handbook-chapter-2/#categories-of-anomalies) to detect. For instance, [Z-score](anomaly-detection/components/models.html#z-score) is suitable for data without trends or seasonality, while more complex patterns might require models like [Prophet](anomaly-detection/components/models.html#prophet).

Also, starting from [v1.12.0](/anomaly-detection/changelog/#v1120) it's possible to auto-tune the most important params of selected model class, find [the details here](https://docs.victoriametrics.com/anomaly-detection/components/models/#autotuned).

Please refer to [respective blogpost on anomaly types and alerting heuristics](https://victoriametrics.com/blog/victoriametrics-anomaly-detection-handbook-chapter-2/) for more details.

Still not 100% sure what to use? We are [here to help](/anomaly-detection/#get-in-touch).

## Alert generation in vmanomaly
While `vmanomaly` detects anomalies and produces scores, it *does not directly generate alerts*. The anomaly scores are written back to VictoriaMetrics, where an external alerting tool, like [`vmalert`](/vmalert.html), can be used to create alerts based on these scores for integrating it with your alerting management system.

## Preventing alert fatigue
Produced anomaly scores are designed in such a way that values from 0.0 to 1.0 indicate non-anomalous data, while a value greater than 1.0 is generally classified as an anomaly. However, there are no perfect models for anomaly detection, that's why reasonable defaults expressions like `anomaly_score > 1` may not work 100% of the time. However, anomaly scores, produced by `vmanomaly` are written back as metrics to VictoriaMetrics, where tools like [`vmalert`](/vmalert.html) can use [MetricsQL](https://docs.victoriametrics.com/metricsql/) expressions to fine-tune alerting thresholds and conditions, balancing between avoiding [false negatives](https://victoriametrics.com/blog/victoriametrics-anomaly-detection-handbook-chapter-1/#false-negative) and reducing [false positives](https://victoriametrics.com/blog/victoriametrics-anomaly-detection-handbook-chapter-1/#false-positive).

## How to backtest particular configuration on historical data?
Starting from [v1.7.2](/anomaly-detection/changelog/#v172) you can produce (and write back to VictoriaMetrics TSDB) anomaly scores for historical (backtesting) period, using `BacktestingScheduler` [component](/anomaly-detection/components/scheduler/#backtesting-scheduler) to imitate consecutive "production runs" of `PeriodicScheduler` [component](/anomaly-detection/components/scheduler/#periodic-scheduler). Please find an example config below:

```yaml
schedulers:
  scheduler_alias:
    class: 'backtesting' # or "scheduler.backtesting.BacktestingScheduler" until v1.13.0
    # define historical period to backtest on
    # should be bigger than at least (fit_window + fit_every) time range
    from_iso: '2024-01-01T00:00:00Z'
    to_iso: '2024-01-15T00:00:00Z'
    # copy these from your PeriodicScheduler args
    fit_window: 'P14D'
    fit_every: 'PT1H'

models:
  model_alias1:
    # ...
    schedulers: ['scheduler_alias']  # if omitted, all the defined schedulers will be attached
    queries: ['query_alias1']  # if omitted, all the defined queries will be attached
    # https://docs.victoriametrics.com/anomaly-detection/components/models/#provide-series
    provide_series: ['anomaly_score']  
  # ... other models

reader:
  datasource_url: 'some_url_to_read_data_from'
  queries:
    query_alias1: 'some_metricsql_query'
  sampling_frequency: '1m'  # change to whatever you need in data granularity
  # other params if needed
  # https://docs.victoriametrics.com/anomaly-detection/components/reader/#vm-reader

writer:
  datasource_url: 'some_url_to_write_produced_data_to'
  # other params if needed
  # https://docs.victoriametrics.com/anomaly-detection/components/writer/#vm-writer

# optional monitoring section if needed
# https://docs.victoriametrics.com/anomaly-detection/components/monitoring/
```

Configuration above will produce N intervals of full length (`fit_window`=14d + `fit_every`=1h) until `to_iso` timestamp is reached to run N consecutive `fit` calls to train models; Then these models will be used to produce `M = [fit_every / sampling_frequency]` infer datapoints for `fit_every` range at the end of each such interval, imitating M consecutive calls of `infer_every` in `PeriodicScheduler` [config](/anomaly-detection/components/scheduler/#periodic-scheduler). These datapoints then will be written back to VictoriaMetrics TSDB, defined in `writer` [section](/anomaly-detection/components/writer/#vm-writer) for further visualization (i.e. in VMUI or Grafana)

## Resource consumption of vmanomaly
`vmanomaly` itself is a lightweight service, resource usage is primarily dependent on [scheduling](/anomaly-detection/components/scheduler.html) (how often and on what data to fit/infer your models), [# and size of timeseries returned by your queries](/anomaly-detection/components/reader.html#vm-reader), and the complexity of the employed [models](anomaly-detection/components/models.html). Its resource usage is directly related to these factors, making it adaptable to various operational scales.

> **Note**: Starting from [v1.13.0](/anomaly-detection/changelog/#v1130), there is a mode to save anomaly detection models on host filesystem after `fit` stage (instead of keeping them in-memory by default). **Resource-intensive setups** (many models, many metrics, bigger [`fit_window` arg](/anomaly-detection/components/scheduler/#periodic-scheduler-config-example)) and/or 3rd-party models that store fit data (like [ProphetModel](/anomaly-detection/components/models/index.html#prophet) or [HoltWinters](/anomaly-detection/components/models/index.html#holt-winters)) will have RAM consumption greatly reduced at a cost of slightly slower `infer` stage. To enable it, you need to set environment variable `VMANOMALY_MODEL_DUMPS_DIR` to desired location. [Helm charts](https://github.com/VictoriaMetrics/helm-charts/blob/master/charts/victoria-metrics-anomaly/README.md) are being updated accordingly ([`StatefulSet`](https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/) for persistent storage starting from chart version `1.3.0`).

Here's an example of how to set it up in docker-compose using volumes:
```yaml
services:
  # ...
  vmanomaly:
    container_name: vmanomaly
    image: victoriametrics/vmanomaly:latest
    # ...
    ports:
      - "8490:8490"
    restart: always
    volumes:
      - ./vmanomaly_config.yml:/config.yaml
      - ./vmanomaly_license:/license
      # map the host directory to the container directory
      - vmanomaly_model_dump_dir:/vmanomaly/tmp/models
    environment:
      # set the environment variable for the model dump directory
      - VMANOMALY_MODEL_DUMPS_DIR=/vmanomaly/tmp/models/
    platform: "linux/amd64"
    command:
      - "/config.yaml"
      - "--license-file=/license"

volumes:
  # ...
  vmanomaly_model_dump_dir: {}
```

## Scaling vmanomaly
> **Note:** As of latest release we don't support cluster or auto-scaled version yet (though, it's in our roadmap for - better backends, more parallelization, etc.), so proposed workarounds should be addressed manually.

`vmanomaly` can be scaled horizontally by launching multiple independent instances, each with its own [MetricsQL](https://docs.victoriametrics.com/metricsql/) queries and [configurations](/anomaly-detection/components/):

- By splitting **queries**, [defined in reader section](/anomaly-detection/components/reader/?highlight=queries#vm-reader) and spawn separate service around it. Also in case you have *only 1 query returning huge amount of timeseries*, you can further split it by applying MetricsQL filters, i.e. using "extra_filters" [param in reader](/anomaly-detection/components/reader/?highlight=extra_filters#vm-reader)

- or **models** (in case you decide to run several models for each timeseries received i.e. for averaging anomaly scores in your alerting rules of `vmalert` or using a vote approach to reduce false positives) - see `queries` arg in [model config](/anomaly-detection/components/models/#queries)

- or **schedulers** (in case you want the same models to be trained under several schedules) - see `schedulers` arg [model section](/anomaly-detection/components/models/#schedulers) and `scheduler` [component itself](/anomaly-detection/components/scheduler/)


Here's an example of how to split on `extra_filters` param
```yaml
# config file #1, for 1st vmanomaly instance
# ...
reader:
  # ...
  queries:
    extra_big_query: metricsql_expression_returning_too_many_timeseries
    extra_filters:
      # suppose you have a label `region` with values to deterministically define such subsets
      - '{region="region_name_1"}'
      # ...
```

```yaml
# config file #2, for 2nd vmanomaly instance
# ...
reader:
  # ...
  queries:
    extra_big_query: metricsql_expression_returning_too_many_timeseries
    extra_filters:
      # suppose you have a label `region` with values to deterministically define such subsets
      - '{region="region_name_2"}'
      # ...
```