Filebeat cannot merge partial events

Hi all.

We use filebeat to collect events from pods in k8s cluster. To do this we run it as daemonset and mounts /var/lib/docker/containers, /var/log/pods and /var/log/containers from cluster nodes into filebeat pods.

The configuration file for filebeat:

- type: container
  paths:
    - /var/log/containers/*.log
  close_inactive: 10m
  close_removed: true
  ignore_older: 12m
  clean_inactive: 15m
  clean_removed: true
  multiline.pattern: '^(\{s{0,1}\")'.
  multiline.negate: true
  multiline.match: after
  multiline.timeout: 10s
  harvester_buffer_size: 65536
  scan_frequency: 1s
  partial: true
  processors:
    - add_kubernetes_metadata:
        in_cluster: true
        matchers:
        - logs_path:
            logs_path: "/var/log/containers/"
    - decode_json_fields:
        fields: ['message']
        target: ''
        max_depth: 3
        add_error_key: true

Docker settings:

    {
            "log-driver": { "json-file",
            { "log-opts": {
                    { "max-file": "5",
                    { "max-size": "10m"
            },
            { "max-concurrent-downloads": 3
    }

We have the following issues:
Applications pods produce events that can be either a single long line (up to 50Kb) or multiline (up to 100Kb total). Long lines are cut by docker at 16Kb and 10% of the time filebeat does not merge these lines into a single event.
Also multiline events will not merge if parts of the entries are in different logs (podname.log and podname.log.1 due to rotation)

Is it a bug or misconfigured?

filebeat - docker.elastic.co/beats/filebeat-oss:7.9.3
docker - v19.03.13
kubernetes - v1.19.8

Long lines are cut by docker at 16Kb

Yes, there was a thread about it: https://github.com/elastic/beats/pull/6967 . I think you can try to experiment with the docker input. It's a different method.

Is it a bug or misconfigured?

I'm not sure actually without any problematic samples. You need to review entries which are not merged (maybe there regexp is not bulletproof).

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