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Loki

Loki is multi-tenant log aggregation system inspired by Prometheus. It is designed to be very cost effective and easy to operate.

The Fluent Bit loki built-in output plugin allows you to send your log or events to a Loki service. It supports data enrichment with Kubernetes labels, custom label keys and Tenant ID within others.

Be aware there is a separate Golang output plugin provided by Grafana with different configuration options.

Configuration Parameters

Key
Description
Default

host

Loki hostname or IP address. Do not include the subpath, i.e. loki/api/v1/push, but just the base hostname/URL.

127.0.0.1

uri

Specify a custom HTTP URI. It must start with forward slash.

/loki/api/v1/push

port

Loki TCP port

3100

tls

Use TLS authentication

off

http_user

Set HTTP basic authentication user name

http_passwd

Set HTTP basic authentication password

bearer_token

Set bearer token authentication token value.

header

Add additional arbitrary HTTP header key/value pair. Multiple headers can be set.

tenant_id

Tenant ID used by default to push logs to Loki. If omitted or empty it assumes Loki is running in single-tenant mode and no X-Scope-OrgID header is sent.

labels

Stream labels for API request. It can be multiple comma separated of strings specifying key=value pairs. In addition to fixed parameters, it also allows to add custom record keys (similar to label_keys property). More details in the Labels section.

job=fluent-bit

label_keys

Optional list of record keys that will be placed as stream labels. This configuration property is for records key only. More details in the Labels section.

label_map_path

Specify the label map file path. The file defines how to extract labels from each record. More details in the Labels section.

structured_metadata

remove_keys

Optional list of keys to remove.

drop_single_key

If set to true and after extracting labels only a single key remains, the log line sent to Loki will be the value of that key in line_format. If set to raw and the log line is a string, the log line will be sent unquoted.

off

line_format

Format to use when flattening the record to a log line. Valid values are json or key_value. If set to json, the log line sent to Loki will be the Fluent Bit record dumped as JSON. If set to key_value, the log line will be each item in the record concatenated together (separated by a single space) in the format.

json

auto_kubernetes_labels

If set to true, it will add all Kubernetes labels to the Stream labels

off

tenant_id_key

Specify the name of the key from the original record that contains the Tenant ID. The value of the key is set as X-Scope-OrgID of HTTP header. It is useful to set Tenant ID dynamically.

compress

Set payload compression mechanism. The only available option is gzip. Default = "", which means no compression.

workers

0

Labels

Loki store the record logs inside Streams, a stream is defined by a set of labels, at least one label is required.

Fluent Bit implements a flexible mechanism to set labels by using fixed key/value pairs of text but also allowing to set as labels certain keys that exists as part of the records that are being processed. Consider the following JSON record (pretty printed for readability):

{
    "key": 1,
    "sub": {
        "stream": "stdout",
        "id": "some id"
    },
    "kubernetes": {
        "labels": {
            "team": "Santiago Wanderers"
        }
    }
}

If you decide that your Loki Stream will be composed by two labels called job and the value of the record key called stream , your labels configuration properties might look as follows:

[OUTPUT]
    name   loki
    match  *
    labels job=fluentbit, $sub['stream']

As you can see the label job has the value fluentbit and the second label is configured to access the nested map called sub targeting the value of the key stream . Note that the second label name must starts with a $, that means that's a Record Accessor pattern so it provide you the ability to retrieve values from nested maps by using the key names.

When processing above's configuration, internally the ending labels for the stream in question becomes:

job="fluentbit", stream="stdout"

Another feature of Labels management is the ability to provide custom key names, using the same record accessor pattern we can specify the key name manually and let the value to be populated automatically at runtime, e.g:

[OUTPUT]
    name   loki
    match  *
    labels job=fluentbit, mystream=$sub['stream']

When processing that new configuration, the internal labels will be:

job="fluentbit", mystream="stdout"

Using the label_keys property

The additional configuration property called label_keys allow to specify multiple record keys that needs to be placed as part of the outgoing Stream Labels, yes, this is a similar feature than the one explained above in the labels property. Consider this as another way to set a record key in the Stream, but with the limitation that you cannot use a custom name for the key value.

The following configuration examples generate the same Stream Labels:

[OUTPUT]
    name       loki
    match      *
    labels     job=fluentbit
    label_keys $sub['stream']

the above configuration accomplish the same than this one:

[OUTPUT]
    name   loki
    match  *
    labels job=fluentbit, $sub['stream']

both will generate the following Streams label:

job="fluentbit", stream="stdout"

Using the label_map_path property

The configuration property label_map_path is to read a JSON file that defines how to extract labels from each record.

The file should contain a JSON object. Each keys define how to get label value from a nested record. Each values are used as label names.

The following configuration examples generate the same Stream Labels:

map.json:

{
    "sub": {
           "stream": "stream"
    }
}

The following configuration examples generate the same Stream Labels:

[OUTPUT]
    name   loki
    match  *
    label_map_path /path/to/map.json

the above configuration accomplish the same than this one:

[OUTPUT]
    name   loki
    match  *
    labels job=fluentbit, $sub['stream']

both will generate the following Streams label:

job="fluentbit", stream="stdout"

Kubernetes & Labels

Note that if you are running in a Kubernetes environment, you might want to enable the option auto_kubernetes_labels which will auto-populate the streams with the Pod labels for you. Consider the following configuration:

[OUTPUT]
    name                   loki
    match                  *
    labels                 job=fluentbit
    auto_kubernetes_labels on

Based in the JSON example provided above, the internal stream labels will be:

job="fluentbit", team="Santiago Wanderers"

Drop Single Key

If there is only one key remaining after removing keys, you can use the drop_single_key property to send its value to Loki, rather than a single key=value pair.

Consider this simple JSON example:

{"key":"value"}

If the value is a string, line_format is json, and drop_single_key is true, it will be sent as a quoted string.

[OUTPUT]
    name            loki
    match           *
    drop_single_key on
    line_format     json

The outputted line would show in Loki as:

"value"

If drop_single_key is raw, or line_format is key_value, it will show in Loki as:

value

If you want both structured JSON and plain-text logs in Loki, you should set drop_single_key to raw and line_format to json. Loki does not interpret a quoted string as valid JSON, and so to remove the quotes without drop_single_key set to raw, you would need to use a query like this:

{"job"="fluent-bit"} | regexp `^"?(?P<log>.*?)"?$` | line_format "{{.log}}"

If drop_single_key is off, it will show in Loki as:

{"key":"value"}

You can get the same behavior this flag provides in Loki with drop_single_key set to off with this query:

{"job"="fluent-bit"} | json | line_format "{{.log}}"

Structured metadata

Structured metadata lets you attach custom fields to individual log lines without embedding the information in the content of the log line. This capability works well for high cardinality data that isn't suited for using labels. While not a label, the structured_metadata configuration parameter operates similarly to the labels parameter. Both parameters are comma-delimited key=value lists, and both can use record accessors to reference keys within the record being processed.

The following configuration:

  • Defines fixed values for the cluster and region labels.

  • Uses the record accessor pattern to set the namespace label to the namespace name as determined by the Kubernetes metadata filter (not shown).

  • Uses a structured metadata field to hold the Kubernetes pod name.

[OUTPUT]
    name                loki
    match               *
    labels              cluster=my-k8s-cluster, region=us-east-1, namespace=$kubernetes['namespace_name']
    structured_metadata pod=$kubernetes['pod_name']

Other common uses for structured metadata include trace and span IDs, process and thread IDs, and log levels.

Structured metadata is officially supported starting with Loki 3.0, and shouldn't be used with Loki deployments prior to Loki 3.0.

Networking and TLS Configuration

This plugin inherit core Fluent Bit features to customize the network behavior and optionally enable TLS in the communication channel. For more details about the specific options available refer to the following articles:

Note that all options mentioned in the articles above must be enabled in the plugin configuration in question.

Fluent Bit + Grafana Cloud

Fluent Bit supports sending logs (and metrics) to Grafana Cloud by providing the appropriate URL and ensuring TLS is enabled.

An example configuration - make sure to set the credentials and ensure the host URL matches the correct one for your deployment:

    [OUTPUT]
        Name        loki
        Match       *
        Host        logs-prod-eu-west-0.grafana.net
        port        443
        tls         on
        tls.verify  on
        http_user   XXX
        http_passwd XXX

Getting Started

The following configuration example, will emit a dummy example record and ingest it on Loki . Copy and paste the following content into a file called out_loki.conf:

[SERVICE]
    flush     1
    log_level info

[INPUT]
    name      dummy
    dummy     {"key": 1, "sub": {"stream": "stdout", "id": "some id"}, "kubernetes": {"labels": {"team": "Santiago Wanderers"}}}
    samples   1

[OUTPUT]
    name                   loki
    match                  *
    host                   127.0.0.1
    port                   3100
    labels                 job=fluentbit
    label_keys             $sub['stream']
    auto_kubernetes_labels on

run Fluent Bit with the new configuration file:

$ fluent-bit -c out_loki.conf

Fluent Bit output:

Fluent Bit v1.7.0
* Copyright (C) 2019-2020 The Fluent Bit Authors
* Copyright (C) 2015-2018 Treasure Data
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2020/10/14 20:57:45] [ info] [engine] started (pid=809736)
[2020/10/14 20:57:45] [ info] [storage] version=1.0.6, initializing...
[2020/10/14 20:57:45] [ info] [storage] in-memory
[2020/10/14 20:57:45] [ info] [storage] normal synchronization mode, checksum disabled, max_chunks_up=128
[2020/10/14 20:57:45] [ info] [output:loki:loki.0] configured, hostname=127.0.0.1:3100
[2020/10/14 20:57:45] [ info] [sp] stream processor started
[2020/10/14 20:57:46] [debug] [http] request payload (272 bytes)
[2020/10/14 20:57:46] [ info] [output:loki:loki.0] 127.0.0.1:3100, HTTP status=204

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