VictoriaMetrics/docs/VictoriaLogs/keyConcepts.md

---
weight: 2
title: Key concepts
menu:
  docs:
    identifier: vl-key-concepts
    parent: victorialogs
    weight: 2
    title: Key concepts
aliases:
- /VictoriaLogs/keyConcepts.html
---
## Data model

[VictoriaLogs](https://docs.victoriametrics.com/victorialogs/) works with both structured and unstructured logs.
Every log entry must contain at least [log message field](#message-field) plus arbitrary number of additional `key=value` fields.
A single log entry can be expressed as a single-level [JSON](https://www.json.org/json-en.html) object with string keys and string values.
For example:

```json
{
  "job": "my-app",
  "instance": "host123:4567",
  "level": "error",
  "client_ip": "1.2.3.4",
  "trace_id": "1234-56789-abcdef",
  "_msg": "failed to serve the client request"
}
```

Empty values are treated the same as non-existing values. For example, the following log entries are equivalent,
since they have only one identical non-empty field - [`_msg`](#message-field):

```json
{
  "_msg": "foo bar",
  "some_field": "",
  "another_field": ""
}
```

```json
{
  "_msg": "foo bar",
  "third_field": "",
}
```

```json
{
  "_msg": "foo bar",
}
```

VictoriaLogs automatically transforms multi-level JSON (aka nested JSON) into single-level JSON
during [data ingestion](https://docs.victoriametrics.com/victorialogs/data-ingestion/) according to the following rules:

- Nested dictionaries are flattened by concatenating dictionary keys with `.` char. For example, the following multi-level JSON
  is transformed into the following single-level JSON:

  ```json
  {
    "host": {
      "name": "foobar"
      "os": {
        "version": "1.2.3"
      }
    }
  }
  ```

  ```json
  {
    "host.name": "foobar",
    "host.os.version": "1.2.3"
  }
  ```

- Arrays, numbers and boolean values are converted into strings. This simplifies [full-text search](https://docs.victoriametrics.com/victorialogs/logsql/) over such values.
  For example, the following JSON with an array, a number and a boolean value is converted into the following JSON with string values:

  ```json
  {
    "tags": ["foo", "bar"],
    "offset": 12345,
    "is_error": false
  }
  ```

  ```json
  {
    "tags": "[\"foo\", \"bar\"]",
    "offset": "12345",
    "is_error": "false"
  }
  ```

Both field name and field value may contain arbitrary chars. Such chars must be encoded
during [data ingestion](https://docs.victoriametrics.com/victorialogs/data-ingestion/)
according to [JSON string encoding](https://www.rfc-editor.org/rfc/rfc7159.html#section-7).
Unicode chars must be encoded with [UTF-8](https://en.wikipedia.org/wiki/UTF-8) encoding:

```json
{
  "field with whitespace": "value\nwith\nnewlines",
  "Поле": "价值"
}
```

VictoriaLogs automatically indexes all the fields in all the [ingested](https://docs.victoriametrics.com/victorialogs/data-ingestion/) logs.
This enables [full-text search](https://docs.victoriametrics.com/victorialogs/logsql/) across all the fields.

VictoriaLogs supports the following special fields additionally to arbitrary [other fields](#other-fields):

* [`_msg` field](#message-field)
* [`_time` field](#time-field)
* [`_stream` fields](#stream-fields)

### Message field

Every ingested [log entry](#data-model) must contain at least a `_msg` field with the actual log message. For example, this is the minimal
log entry, which can be ingested into VictoriaLogs:

```json
{
  "_msg": "some log message"
}
```

If the actual log message has other than `_msg` field name, then it is possible to specify the real log message field
via `_msg_field` query arg during [data ingestion](https://docs.victoriametrics.com/victorialogs/data-ingestion/).
For example, if log message is located in the `event.original` field, then specify `_msg_field=event.original` query arg
during [data ingestion](https://docs.victoriametrics.com/victorialogs/data-ingestion/).

### Time field

The ingested [log entries](#data-model) may contain `_time` field with the timestamp of the ingested log entry.
The timestamp must be in [RFC3339](https://www.rfc-editor.org/rfc/rfc3339) format. The most commonly used subset of [ISO8601](https://en.wikipedia.org/wiki/ISO_8601)
is also supported. It is allowed specifying seconds part of the timestamp with any precision up to nanoseconds.
For example, the following [log entry](#data-model) contains valid timestamp with millisecond precision in the `_time` field:

```json
{
  "_msg": "some log message",
  "_time": "2023-04-12T06:38:11.095Z"
}
```

If the actual timestamp has other than `_time` field name, then it is possible to specify the real timestamp
field via `_time_field` query arg during [data ingestion](https://docs.victoriametrics.com/victorialogs/data-ingestion/).
For example, if timestamp is located in the `event.created` field, then specify `_time_field=event.created` query arg
during [data ingestion](https://docs.victoriametrics.com/victorialogs/data-ingestion/).

If `_time` field is missing, then the data ingestion time is used as log entry timestamp.

The `_time` field is used in [time filter](https://docs.victoriametrics.com/victorialogs/logsql/#time-filter) for quickly narrowing down
the search to a particular time range.

### Stream fields

Some [structured logging](#data-model) fields may uniquely identify the application instance, which generates logs.
This may be either a single field such as `instance="host123:456"` or a set of fields such as
`{datacenter="...", env="...", job="...", instance="..."}` or
`{kubernetes.namespace="...", kubernetes.node.name="...", kubernetes.pod.name="...", kubernetes.container.name="..."}`.

Log entries received from a single application instance form a **log stream** in VictoriaLogs.
VictoriaLogs optimizes storing and [querying](https://docs.victoriametrics.com/victorialogs/logsql/#stream-filter) of individual log streams.
This provides the following benefits:

- Reduced disk space usage, since a log stream from a single application instance is usually compressed better
  than a mixed log stream from multiple distinct applications.

- Increased query performance, since VictoriaLogs needs to scan lower amounts of data
  when [searching by stream fields](https://docs.victoriametrics.com/victorialogs/logsql/#stream-filter).

Every ingested log entry is associated with a log stream. Every log stream consists of two fields:

- `_stream_id` - this is an unique identifier for the log stream. All the logs for the particular stream can be selected
  via [`_stream_id:...` filter](https://docs.victoriametrics.com/victorialogs/logsql/#_stream_id-filter).

- `_stream` - this field contains stream labels in the format similar to [labels in Prometheus metrics](https://docs.victoriametrics.com/keyconcepts/#labels):
  ```
  {field1="value1", ..., fieldN="valueN"}
  ```
  For example, if `host` and `app` fields are associated with the stream, then the `_stream` field will have `{host="host-123",app="my-app"}` value
  for the log entry with `host="host-123"` and `app="my-app"` fields. The `_stream` field can be searched
  with [stream filters](https://docs.victoriametrics.com/victorialogs/logsql/#stream-filter).

By default the value of `_stream` field is `{}`, since VictoriaLogs cannot determine automatically,
which fields uniquely identify every log stream. This may lead to not-so-optimal resource usage and query performance.
Therefore it is recommended specifying stream-level fields via `_stream_fields` query arg
during [data ingestion](https://docs.victoriametrics.com/victorialogs/data-ingestion/).
For example, if logs from Kubernetes containers have the following fields:

```json
{
  "kubernetes.namespace": "some-namespace",
  "kubernetes.node.name": "some-node",
  "kubernetes.pod.name": "some-pod",
  "kubernetes.container.name": "some-container",
  "_msg": "some log message"
}
```

then specify `_stream_fields=kubernetes.namespace,kubernetes.node.name,kubernetes.pod.name,kubernetes.container.name`
query arg during [data ingestion](https://docs.victoriametrics.com/victorialogs/data-ingestion/) in order to properly store
per-container logs into distinct streams.

#### How to determine which fields must be associated with log streams?

[Log streams](#stream-fields) must contain [fields](#data-model), which uniquely identify the application instance, which generates logs.
For example, `container`, `instance` and `host` are good candidates for stream fields.

Additional fields may be added to log streams if they **remain constant during application instance lifetime**.
For example, `namespace`, `node`, `pod` and `job` are good candidates for additional stream fields. Adding such fields to log streams
makes sense if you are going to use these fields during search and want speeding up it with [stream filters](https://docs.victoriametrics.com/victorialogs/logsql/#stream-filter).

There is **no need to add all the constant fields to log streams**, since this may increase resource usage during data ingestion and querying.

**Never add non-constant fields to streams if these fields may change with every log entry of the same stream**.
For example, `ip`, `user_id` and `trace_id` **must never be associated with log streams**, since this may lead to [high cardinality issues](#high-cardinality).

#### High cardinality

Some fields in the [ingested logs](#data-model) may contain big number of unique values across log entries.
For example, fields with names such as `ip`, `user_id` or `trace_id` tend to contain big number of unique values.
VictoriaLogs works perfectly with such fields unless they are associated with [log streams](#stream-fields).

**Never** associate high-cardinality fields with [log streams](#stream-fields), since this may lead to the following issues:

- Performance degradation during [data ingestion](https://docs.victoriametrics.com/victorialogs/data-ingestion/)
  and [querying](https://docs.victoriametrics.com/victorialogs/querying/)
- Increased memory usage
- Increased CPU usage
- Increased disk space usage
- Increased disk read / write IO

VictoriaLogs exposes `vl_streams_created_total` [metric](https://docs.victoriametrics.com/victorialogs/#monitoring),
which shows the number of created streams since the last VictoriaLogs restart. If this metric grows at a rapid rate
during long period of time, then there are high chances of high cardinality issues mentioned above.
VictoriaLogs can log all the newly registered streams when `-logNewStreams` command-line flag is passed to it.
This can help narrowing down and eliminating high-cardinality fields from [log streams](#stream-fields).

### Other fields

Every ingested log entry may contain arbitrary number of [fields](#data-model) additionally to [`_msg`](#message-field) and [`_time`](#time-field).
For example, `level`, `ip`, `user_id`, `trace_id`, etc. Such fields can be used for simplifying and optimizing [search queries](https://docs.victoriametrics.com/victorialogs/logsql/).
It is usually faster to search over a dedicated `trace_id` field instead of searching for the `trace_id` inside long [log message](#message-field).
E.g. the `trace_id:="XXXX-YYYY-ZZZZ"` query usually works faster than the `_msg:"trace_id=XXXX-YYYY-ZZZZ"` query.

See [LogsQL docs](https://docs.victoriametrics.com/victorialogs/logsql/) for more details.