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Promtail Scraping (Service Discovery)

File Target Discovery

Promtail discovers locations of log files and extract labels from them through the scrape_configs section in the config YAML. The syntax is identical to what Prometheus uses.

scrape_configs contains one or more entries which are executed for each discovered target (i.e., each container in each new pod running in the instance):

scrape_configs:
  - job_name: local
    static_configs:
      - ...

  - job_name: kubernetes
    kubernetes_sd_config:
      - ...

If more than one scrape config section matches your logs, you will get duplicate entries as the logs are sent in different streams likely with slightly different labels.

There are different types of labels present in Promtail:

  • Labels starting with __ (two underscores) are internal labels. They usually come from dynamic sources like service discovery. Once relabeling is done, they are removed from the label set. To persist internal labels so they're sent to Loki, rename them so they don't start with __. See Relabeling for more information.

  • Labels starting with __meta_kubernetes_pod_label_* are "meta labels" which are generated based on your Kubernetes pod's labels.

    For example, if your Kubernetes pod has a label name set to foobar, then the scrape_configs section will receive an internal label __meta_kubernetes_pod_label_name with a value set to foobar.

  • Other labels starting with __meta_kubernetes_* exist based on other Kubernetes metadata, such as the namespace of the pod (__meta_kubernetes_namespace) or the name of the container inside the pod (__meta_kubernetes_pod_container_name). Refer to the Prometheus docs for the full list of Kubernetes meta labels.

  • The __path__ label is a special label which Promtail uses after discovery to figure out where the file to read is located. Wildcards are allowed.

  • The label filename is added for every file found in __path__ to ensure the uniqueness of the streams. It is set to the absolute path of the file the line was read from.

Kubernetes Discovery

Note that while Promtail can utilize the Kubernetes API to discover pods as targets, it can only read log files from pods that are running on the same node as the one Promtail is running on. Promtail looks for a __host__ label on each target and validates that it is set to the same hostname as Promtail's (using either $HOSTNAME or the hostname reported by the kernel if the environment variable is not set).

This means that any time Kubernetes service discovery is used, there must be a relabel_config that creates the intermediate label __host__ from __meta_kubernetes_pod_node_name:

relabel_configs:
  - source_labels: ['__meta_kubernetes_pod_node_name']
    target_label: '__host__'

See Relabeling for more information.

Journal Scraping (Linux Only)

On systems with systemd, Promtail also supports reading from the journal. Unlike file scraping which is defined in the static_configs stanza, journal scraping is defined in a journal stanza:

scrape_configs:
  - job_name: journal
    journal:
      max_age: 12h
      path: /var/log/journal
      labels:
        job: systemd-journal
    relabel_configs:
      - source_labels: ['__journal__systemd_unit']
        target_label: 'unit'

All fields defined in the journal section are optional, and are just provided here for reference. The max_age field ensures that no older entry than the time specified will be sent to Loki; this circumvents "entry too old" errors. The path field tells Promtail where to read journal entries from. The labels map defines a constant list of labels to add to every journal entry that Promtail reads.

By default, Promtail reads from the journal by looking in the /var/log/journal and /run/log/journal paths. If running Promtail inside of a Docker container, the path appropriate to your distribution should be bind mounted inside of Promtail along with binding /etc/machine-id. Bind mounting /etc/machine-id to the path of the same name is required for the journal reader to know which specific journal to read from. For example:

docker run \
  -v /var/log/journal/:/var/log/journal/ \
  -v /run/log/journal/:/run/log/journal/ \
  -v /etc/machine-id:/etc/machine-id \
  grafana/promtail:latest \
  -config.file=/path/to/config/file.yaml

When Promtail reads from the journal, it brings in all fields prefixed with __journal_ as internal labels. Like in the example above, the _SYSTEMD_UNIT field from the journal was transformed into a label called unit through relabel_configs. See Relabeling for more information.

Relabeling

Each scrape_configs entry can contain a relabel_configs stanza. relabel_configs is a list of operations to transform the labels from discovery into another form.

A single entry in relabel_configs can also reject targets by doing an action: drop if a label value matches a specified regex. When a target is dropped, the owning scrape_config will not process logs from that particular source. Other scrape_configs without the drop action reading from the same target may still use and forward logs from it to Loki.

A common use case of relabel_configs is to transform an internal label such as __meta_kubernetes_* into an intermediate internal label such as __service__. The intermediate internal label may then be dropped based on value or transformed to a final external label, such as __job__.

Examples

  • Drop the target if a label (__service__ in the example) is empty:
  - action: drop
    regex: ''
    source_labels:
    - __service__
  • Drop the target if any of the source_labels contain a value:
  - action: drop
    regex: .+
    separator: ''
    source_labels:
    - __meta_kubernetes_pod_label_name
    - __meta_kubernetes_pod_label_app
  • Persist an internal label by renaming it so it will be sent to Loki:
  - action: replace
    source_labels:
    - __meta_kubernetes_namespace
    target_label: namespace
  • Persist all Kubernetes pod labels by mapping them, like by mapping __meta_kube__meta_kubernetes_pod_label_foo to foo.
  - action: labelmap
    regex: __meta_kubernetes_pod_label_(.+)

Additional reading:

HTTP client options

Promtail uses the Prometheus HTTP client implementation for all calls to Loki. Therefore it can be configured using the clients stanza, where one or more connections to Loki can be established:

clients:
  - [ <client_option> ]

Refer to client_config from the Promtail Configuration reference for all available options.