Fluentbit Loki Output Plugin

Fluent Bit is a Fast and Lightweight Data Forwarder, it can be configured with the Loki output plugin to ship logs to Loki. You can define which log files you want to collect using the Tail or Stdin input plugin. Additionally Fluent Bit supports multiple Filter and Parser plugins (Kubernetes, JSON, etc..) to structure and alter log lines.



You can run a Fluent Bit container with Loki output plugin pre-installed using our docker hub image:

docker run -v /var/log:/var/log \
    -e LOG_PATH="/var/log/*.log" -e LOKI_URL="http://localhost:3100/loki/api/v1/push" \


You can run Fluent Bit as a Daemonset to collect all your Kubernetes workload logs.

To do so you can use our Fluent Bit helm chart:

helm repo add grafana
helm repo update
helm upgrade --install fluent-bit grafana/fluent-bit \
    --set loki.serviceName=loki.svc.cluster.local

By default it will collect all containers logs and extract labels from Kubernetes API (container_name, namespace, etc..).

Alternatively you can install the Loki and Fluent Bit all together using:

helm upgrade --install loki-stack grafana/loki-stack \
    --set fluent-bit.enabled=true,promtail.enabled=false

AWS Elastic Container Service (ECS)

You can use fluent-bit Loki Docker image as a Firelens log router in AWS ECS. For more information about this see our AWS documentation


First, you need to follow the instructions in order to build the plugin dynamic library.

The assuming you have Fluent Bit installed in your $PATH you can run the plugin using:

fluent-bit -e /path/to/built/ -c fluent-bit.conf

You can also adapt your plugins.conf, removing the need to change the command line options:

    Path /path/to/built/

Configuration Options

Key Description Default
Url Url of loki server API endpoint. http://localhost:3100/loki/api/v1/push
TenantID The 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. ""
BatchWait Time to wait before send a log batch to Loki, full or not. 1s
BatchSize Log batch size to send a log batch to Loki (unit: Bytes). 10 KiB (10 * 1024 Bytes)
Timeout Maximum time to wait for loki server to respond to a request. 10s
MinBackoff Initial backoff time between retries. 500ms
MaxBackoff Maximum backoff time between retries. 5m
MaxRetries Maximum number of retries when sending batches. 10
Labels labels for API requests. {job=“fluent-bit”}
LogLevel LogLevel for plugin logger. “info”
RemoveKeys Specify removing keys. none
AutoKubernetesLabels If set to true, it will add all Kubernetes labels to Loki labels false
LabelKeys Comma separated list of keys to use as stream labels. All other keys will be placed into the log line. LabelKeys is deactivated when using LabelMapPath label mapping configuration. none
LineFormat 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 fluentd record (excluding any keys extracted out as labels) 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
DropSingleKey If set to true and after extracting label_keys a record only has a single key remaining, the log line sent to Loki will just be the value of the record key. true
LabelMapPath Path to a json file defining how to transform nested records. none
Buffer Enable buffering mechanism false
BufferType Specify the buffering mechanism to use (currently only dque is implemented). dque
DqueDir Path to the directory for queued logs /tmp/flb-storage/loki
DqueSegmentSize Segment size in terms of number of records per segment 500
DqueSync Whether to fsync each queue change false
DqueName Queue name, must be uniq per output dque


Labels are used to query logs {container_name="nginx", cluster="us-west1"}, they are usually metadata about the workload producing the log stream (instance, container_name, region, cluster, level). In Loki labels are indexed consequently you should be cautious when choosing them (high cardinality label values can have performance drastic impact).

You can use Labels, RemoveKeys , LabelKeys and LabelMapPath to how the output plugin will perform labels extraction.


If set to true, it will add all Kubernetes labels to Loki labels automatically and ignore parameters LabelKeys, LabelMapPath.


When using the Parser and Filter plugins Fluent Bit can extract and add data to the current record/log data. While Loki labels are key value pair, record data can be nested structures. You can pass a json file that defines how to extract labels from each record. Each json key from the file will be matched with the log record to find label values. Values from the configuration are used as label names.

Considering the record below :

  "kubernetes": {
    "container_name": "promtail",
    "pod_name": "promtail-xxx",
    "namespace_name": "prod",
    "labels" : {
        "team": "x-men",
  "HOSTNAME": "docker-desktop",
  "log" : "a log line",
  "time": "20190926T152206Z",

and a LabelMap file as follow :

  "kubernetes": {
    "container_name": "container",
    "pod_name": "pod",
    "namespace_name": "namespace",
    "labels" : {
        "team": "team",

The labels extracted will be {team="x-men", container="promtail", pod="promtail-xxx", namespace="prod"}.

If you don’t want the kubernetes and HOSTNAME fields to appear in the log line you can use the RemoveKeys configuration field. (e.g. RemoveKeys kubernetes,HOSTNAME).


Buffering refers to the ability to store the records somewhere, and while they are processed and delivered, still be able to store more. Loki output plugin in certain situation can be blocked by loki client because of its design:

  • BatchSize is over limit, output plugin pause receiving new records until the pending batch is successfully sent to the server
  • Loki server is unreachable (retry 429s, 500s and connection-level errors), output plugin blocks new records until loki server will be available again and the pending batch is successfully sent to the server or as long as the maximum number of attempts has been reached within configured back-off mechanism

The blocking state with some of the input plugins is not acceptable because it can have a undesirable side effects on the part that generates the logs. Fluent Bit implements buffering mechanism that is based on parallel processing and it cannot send logs in order which is loki requirement (loki logs must be in increasing time order per stream).

Loki output plugin has buffering mechanism based on dque which is compatible with loki server strict time ordering and can be set up by configuration flag:

    Name grafana-loki
    Match *
    Url http://localhost:3100/loki/api/v1/push
    Buffer true
    DqueSegmentSize 8096
    DqueDir /tmp/flb-storage/buffer
    DqueName loki.0

Configuration examples

To configure the Loki output plugin add this section to fluent-bit.conf

    Name grafana-loki
    Match *
    Url http://localhost:3100/loki/api/v1/push
    BatchWait 1s
    BatchSize 30720
    # (30KiB)
    Labels {test="fluent-bit-go", lang="Golang"}
    RemoveKeys key1,key2
    LabelKeys key3,key4
    LineFormat key_value
    Name grafana-loki
    Match *
    Url http://localhost:3100/loki/api/v1/push
    BatchWait 1s
    BatchSize 30720 # (30KiB)
    AutoKubernetesLabels true
    RemoveKeys key1,key2

A full example configuration file is also available in this repository.

Running multiple plugin instances

You can run multiple plugin instances in the same fluent-bit process, for example if you want to push to different Loki servers or route logs into different Loki tenant IDs. To do so, add additional [Output] sections.