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description Concatenate multiline or stack trace log messages.

Multiline

{% hint style="info" %} Supported event types: logs {% endhint %}

The Multiline filter helps concatenate messages that originally belonged to one context but were split across multiple records or log lines. Common examples are stack traces or applications that print logs in multiple lines.

Along with multiline filters, you can enable one of the following built-in Fluent Bit parsers with auto-detection and multi-format support:

  • Go
  • Python
  • Ruby
  • Java (Google Cloud Platform Java stack trace format)

When using this filter:

  • The usage of this filter depends on a previous configuration of a multiline parser definition.
  • To concatenate messages read from a log file, it's highly recommended to use the multiline support in the Tail plugin itself. This is because performing concatenation while reading the log file is more performant. Concatenating messages that were originally one line, but split by Docker or CRI container engines because of their size, is supported in the Tail plugin in combination with the docker or cri parser. To concatenate application logs like stack traces on top of that, you can use this multiline filter.

{% hint style="warning" %}

This filter only performs buffering that persists across different Chunks when Buffer is enabled. Otherwise, the filter processes one chunk at a time and isn't suitable for most inputs which might send multiline messages in separate chunks.

When buffering is enabled, the filter doesn't immediately emit messages it receives. It uses the in_emitter plugin, similar to the Rewrite Tag filter, and emits messages once they're fully concatenated, or a timeout is reached.

{% endhint %}

{% hint style="warning" %}

Since concatenated records are re-emitted to the head of the Fluent Bit log pipeline, you can not configure multiple multiline filter definitions that match the same tags. This will cause an infinite loop in the Fluent Bit pipeline; to use multiple parsers on the same logs, configure a single filter definitions with a comma separated list of parsers for multiline.parser. For more, see issue #5235.

Secondly, for the same reason, the multiline filter should be the first filter. Logs will be re-emitted by the multiline filter to the head of the pipeline - the filter will ignore its own re-emitted records, but other filters won't. If there are filters before the multiline filter, they will be applied twice.

{% endhint %}

Configuration parameters

The plugin supports the following configuration parameters:

Key Description
buffer Enable buffered mode. In buffered mode, the filter can concatenate multiple lines from inputs that ingest records one by one (like Forward), rather than in chunks, re-emitting them into the beginning of the pipeline (with the same tag) using the in_emitter instance. With buffer off, this filter won't work with most inputs, except Tail.
debug_flush Enable debug logging for flush operations. Default: false.
emitter_mem_buf_limit Set a limit on the amount of memory the emitter can consume if the outputs provide backpressure. The default for this limit is 10M. The pipeline will pause once the buffer exceeds the value of this setting. For example, if the value is set to 10M then the pipeline pauses if the buffer exceeds 10M. The pipeline will remain paused until the output drains the buffer under the 10M limit.
emitter_name Name for the emitter input instance which re-emits the completed records at the beginning of the pipeline.
emitter_storage.type The storage type for the emitter input instance. This option supports the values memory (default) and filesystem.
flush_ms Flush time for pending multiline records. Default: 2000.
mode Mode can be parser for regular expression concatenation, or partial_message to concatenate split Docker logs.
multiline.key_content Key name that holds the content to process. A multiline parser definition can specify the key_content This option allows for overwriting that value for the purpose of the filter.
multiline.parser Specify one or multiple Multiline Parser definitions to apply to the content. You can specify multiple multiline parsers to detect different formats by separating them with a comma.

Configuration example

The following example aims to parse a log file called test.log that contains some full lines, a custom Java stack trace and a Go Stack Trace.

The following example files can be located in the Fluent Bit repository.

Example files content:

{% tabs %} {% tab title="fluent-bit.yaml" %}

This is the primary Fluent Bit YAML configuration file. It includes the parsers_multiline.yaml and tails the file test.log by applying the multiline parsers multiline-regex-test and go. Then it sends the processing to the standard output.

service:
  flush: 1
  log_level: info
  parsers_file: parsers_multiline.yaml

pipeline:
  inputs:
    - name: tail
      path: test.log
      read_from_head: true

  filters:
    - name: multiline
      match: '*'
      multiline.key_content: log
      multiline.parser: go,multiline-regex-test

  outputs:
    - name: stdout
      match: '*'

{% endtab %} {% tab title="fluent-bit.conf" %}

This is the primary Fluent Bit classic configuration file. It includes the parsers_multiline.conf and tails the file test.log by applying the multiline parsers multiline-regex-test and go. Then it sends the processing to the standard output.

[SERVICE]
  Flush                 1
  Log_Level             info
  Parsers_File          parsers_multiline.conf

[INPUT]
  Name                  tail
  Path                  test.log
  Read_From_Head        true

[FILTER]
  Name                  multiline
  Match                 *
  multiline.key_content log
  multiline.parser      go, multiline-regex-test

[OUTPUT]
  Name                  stdout
  Match                 *

{% endtab %} {% tab title="parsers_multiline.yaml" %}

This file defines a multiline parser for the example. A second multiline parser called go is used in fluent-bit.yaml, but this one is a built-in parser.

multiline_parsers:
  - name: multiline-regex-test
    type: regex
    flush_timeout: 1000
    #
    # Regex rules for multiline parsing
    # ---------------------------------
    #
    # configuration hints:
    #
    #  - first state always has the name: start_state
    #  - every field in the rule must be inside double quotes
    #
    # rules |   state name  | regex pattern                  | next state
    # ------|---------------|--------------------------------------------
    rules:
      - state: start_state
        regex: '/([a-zA-Z]+ \d+ \d+\:\d+\:\d+)(.*)/'
        next_state:  cont
      - state: cont
        regex: '/^\s+at.*/'
        next_state: cont

{% endtab %} {% tab title="parsers_multiline.conf" %}

This file defines a multiline parser for the example. A second multiline parser called go is used in fluent-bit.conf, but this one is a built-in parser.

[MULTILINE_PARSER]
  Name          multiline-regex-test
  Type          regex
  Flush_Timeout 1000
  #
  # Regex rules for multiline parsing
  # ---------------------------------
  #
  # configuration hints:
  #
  #  - first state always has the name: start_state
  #  - every field in the rule must be inside double quotes
  #
  # rules |   state name  | regex pattern                  | next state
  # ------|---------------|--------------------------------------------
  Rule      "start_state"   "/([A-Za-z]+ \d+ \d+\:\d+\:\d+)(.*)/"  "cont"
  Rule      "cont"          "/^\s+at.*/"                     "cont"

{% endtab %} {% tab title="test.log" %}

An example file with multiline and multi-format content:

single line...
Dec 14 06:41:08 Exception in thread "main" java.lang.RuntimeException: Something has gone wrong, aborting!
    at com.myproject.module.MyProject.badMethod(MyProject.java:22)
    at com.myproject.module.MyProject.oneMoreMethod(MyProject.java:18)
    at com.myproject.module.MyProject.anotherMethod(MyProject.java:14)
    at com.myproject.module.MyProject.someMethod(MyProject.java:10)
    at com.myproject.module.MyProject.main(MyProject.java:6)
another line...
panic: my panic

goroutine 4 [running]:
panic(0x45cb40, 0x47ad70)
  /usr/local/go/src/runtime/panic.go:542 +0x46c fp=0xc42003f7b8 sp=0xc42003f710 pc=0x422f7c
main.main.func1(0xc420024120)
  foo.go:6 +0x39 fp=0xc42003f7d8 sp=0xc42003f7b8 pc=0x451339
runtime.goexit()
  /usr/local/go/src/runtime/asm_amd64.s:2337 +0x1 fp=0xc42003f7e0 sp=0xc42003f7d8 pc=0x44b4d1
created by main.main
  foo.go:5 +0x58

goroutine 1 [chan receive]:
runtime.gopark(0x4739b8, 0xc420024178, 0x46fcd7, 0xc, 0xc420028e17, 0x3)
  /usr/local/go/src/runtime/proc.go:280 +0x12c fp=0xc420053e30 sp=0xc420053e00 pc=0x42503c
runtime.goparkunlock(0xc420024178, 0x46fcd7, 0xc, 0x1000f010040c217, 0x3)
  /usr/local/go/src/runtime/proc.go:286 +0x5e fp=0xc420053e70 sp=0xc420053e30 pc=0x42512e
runtime.chanrecv(0xc420024120, 0x0, 0xc420053f01, 0x4512d8)
  /usr/local/go/src/runtime/chan.go:506 +0x304 fp=0xc420053f20 sp=0xc420053e70 pc=0x4046b4
runtime.chanrecv1(0xc420024120, 0x0)
  /usr/local/go/src/runtime/chan.go:388 +0x2b fp=0xc420053f50 sp=0xc420053f20 pc=0x40439b
main.main()
  foo.go:9 +0x6f fp=0xc420053f80 sp=0xc420053f50 pc=0x4512ef
runtime.main()
  /usr/local/go/src/runtime/proc.go:185 +0x20d fp=0xc420053fe0 sp=0xc420053f80 pc=0x424bad
runtime.goexit()
  /usr/local/go/src/runtime/asm_amd64.s:2337 +0x1 fp=0xc420053fe8 sp=0xc420053fe0 pc=0x44b4d1

goroutine 2 [force gc (idle)]:
runtime.gopark(0x4739b8, 0x4ad720, 0x47001e, 0xf, 0x14, 0x1)
  /usr/local/go/src/runtime/proc.go:280 +0x12c fp=0xc42003e768 sp=0xc42003e738 pc=0x42503c
runtime.goparkunlock(0x4ad720, 0x47001e, 0xf, 0xc420000114, 0x1)
  /usr/local/go/src/runtime/proc.go:286 +0x5e fp=0xc42003e7a8 sp=0xc42003e768 pc=0x42512e
runtime.forcegchelper()
  /usr/local/go/src/runtime/proc.go:238 +0xcc fp=0xc42003e7e0 sp=0xc42003e7a8 pc=0x424e5c
runtime.goexit()
  /usr/local/go/src/runtime/asm_amd64.s:2337 +0x1 fp=0xc42003e7e8 sp=0xc42003e7e0 pc=0x44b4d1
created by runtime.init.4
  /usr/local/go/src/runtime/proc.go:227 +0x35

goroutine 3 [GC sweep wait]:
runtime.gopark(0x4739b8, 0x4ad7e0, 0x46fdd2, 0xd, 0x419914, 0x1)
  /usr/local/go/src/runtime/proc.go:280 +0x12c fp=0xc42003ef60 sp=0xc42003ef30 pc=0x42503c
runtime.goparkunlock(0x4ad7e0, 0x46fdd2, 0xd, 0x14, 0x1)
  /usr/local/go/src/runtime/proc.go:286 +0x5e fp=0xc42003efa0 sp=0xc42003ef60 pc=0x42512e
runtime.bgsweep(0xc42001e150)
  /usr/local/go/src/runtime/mgcsweep.go:52 +0xa3 fp=0xc42003efd8 sp=0xc42003efa0 pc=0x419973
runtime.goexit()
  /usr/local/go/src/runtime/asm_amd64.s:2337 +0x1 fp=0xc42003efe0 sp=0xc42003efd8 pc=0x44b4d1
created by runtime.gcenable
  /usr/local/go/src/runtime/mgc.go:216 +0x58
one more line, no multiline

{% endtab %} {% endtabs %}

Running Fluent Bit with the given configuration file:

fluent-bit -c fluent-bit.conf

Should return something like the following:

[0] tail.0: [1626736433.143567481, {"log"=>"single line..."}]
[1] tail.0: [1626736433.143570538, {"log"=>"Dec 14 06:41:08 Exception in thread "main" java.lang.RuntimeException: Something has gone wrong, aborting!
    at com.myproject.module.MyProject.badMethod(MyProject.java:22)
    at com.myproject.module.MyProject.oneMoreMethod(MyProject.java:18)
    at com.myproject.module.MyProject.anotherMethod(MyProject.java:14)
    at com.myproject.module.MyProject.someMethod(MyProject.java:10)
    at com.myproject.module.MyProject.main(MyProject.java:6)"}]
[2] tail.0: [1626736433.143572538, {"log"=>"another line..."}]
[3] tail.0: [1626736433.143572894, {"log"=>"panic: my panic

goroutine 4 [running]:
panic(0x45cb40, 0x47ad70)
  /usr/local/go/src/runtime/panic.go:542 +0x46c fp=0xc42003f7b8 sp=0xc42003f710 pc=0x422f7c
main.main.func1(0xc420024120)
  foo.go:6 +0x39 fp=0xc42003f7d8 sp=0xc42003f7b8 pc=0x451339
runtime.goexit()
  /usr/local/go/src/runtime/asm_amd64.s:2337 +0x1 fp=0xc42003f7e0 sp=0xc42003f7d8 pc=0x44b4d1
created by main.main
  foo.go:5 +0x58

goroutine 1 [chan receive]:
runtime.gopark(0x4739b8, 0xc420024178, 0x46fcd7, 0xc, 0xc420028e17, 0x3)
  /usr/local/go/src/runtime/proc.go:280 +0x12c fp=0xc420053e30 sp=0xc420053e00 pc=0x42503c
runtime.goparkunlock(0xc420024178, 0x46fcd7, 0xc, 0x1000f010040c217, 0x3)
  /usr/local/go/src/runtime/proc.go:286 +0x5e fp=0xc420053e70 sp=0xc420053e30 pc=0x42512e
runtime.chanrecv(0xc420024120, 0x0, 0xc420053f01, 0x4512d8)
  /usr/local/go/src/runtime/chan.go:506 +0x304 fp=0xc420053f20 sp=0xc420053e70 pc=0x4046b4
runtime.chanrecv1(0xc420024120, 0x0)
  /usr/local/go/src/runtime/chan.go:388 +0x2b fp=0xc420053f50 sp=0xc420053f20 pc=0x40439b
main.main()
  foo.go:9 +0x6f fp=0xc420053f80 sp=0xc420053f50 pc=0x4512ef
runtime.main()
  /usr/local/go/src/runtime/proc.go:185 +0x20d fp=0xc420053fe0 sp=0xc420053f80 pc=0x424bad
runtime.goexit()
  /usr/local/go/src/runtime/asm_amd64.s:2337 +0x1 fp=0xc420053fe8 sp=0xc420053fe0 pc=0x44b4d1

goroutine 2 [force gc (idle)]:
runtime.gopark(0x4739b8, 0x4ad720, 0x47001e, 0xf, 0x14, 0x1)
  /usr/local/go/src/runtime/proc.go:280 +0x12c fp=0xc42003e768 sp=0xc42003e738 pc=0x42503c
runtime.goparkunlock(0x4ad720, 0x47001e, 0xf, 0xc420000114, 0x1)
  /usr/local/go/src/runtime/proc.go:286 +0x5e fp=0xc42003e7a8 sp=0xc42003e768 pc=0x42512e
runtime.forcegchelper()
  /usr/local/go/src/runtime/proc.go:238 +0xcc fp=0xc42003e7e0 sp=0xc42003e7a8 pc=0x424e5c
runtime.goexit()
  /usr/local/go/src/runtime/asm_amd64.s:2337 +0x1 fp=0xc42003e7e8 sp=0xc42003e7e0 pc=0x44b4d1
created by runtime.init.4
  /usr/local/go/src/runtime/proc.go:227 +0x35

goroutine 3 [GC sweep wait]:
runtime.gopark(0x4739b8, 0x4ad7e0, 0x46fdd2, 0xd, 0x419914, 0x1)
  /usr/local/go/src/runtime/proc.go:280 +0x12c fp=0xc42003ef60 sp=0xc42003ef30 pc=0x42503c
runtime.goparkunlock(0x4ad7e0, 0x46fdd2, 0xd, 0x14, 0x1)
  /usr/local/go/src/runtime/proc.go:286 +0x5e fp=0xc42003efa0 sp=0xc42003ef60 pc=0x42512e
runtime.bgsweep(0xc42001e150)
  /usr/local/go/src/runtime/mgcsweep.go:52 +0xa3 fp=0xc42003efd8 sp=0xc42003efa0 pc=0x419973
runtime.goexit()
  /usr/local/go/src/runtime/asm_amd64.s:2337 +0x1 fp=0xc42003efe0 sp=0xc42003efd8 pc=0x44b4d1
created by runtime.gcenable
  /usr/local/go/src/runtime/mgc.go:216 +0x58"}]
[4] tail.0: [1626736433.143585473, {"log"=>"one more line, no multiline"}]

Lines that don't match a pattern aren't considered as part of the multiline message, while the ones that matched the rules were concatenated properly.

Docker partial message use case

When Fluent Bit is consuming logs from a container runtime, such as Docker, these logs will be split when larger than a certain limit, usually 16&nbspKB. If your application emits a 100K log line, it will be split into seven partial messages. The docker parser will merge these back to one line. If instead you are using the Fluentd Docker Log Driver to send the logs to Fluent Bit, they might look like this:

{"source": "stdout", "log": "... omitted for brevity...", "partial_message": "true", "partial_id": "dc37eb08b4242c41757d4cd995d983d1cdda4589193755a22fcf47a638317da0", "partial_ordinal": "1", "partial_last": "false", "container_id": "a96998303938eab6087a7f8487ca40350f2c252559bc6047569a0b11b936f0f2", "container_name": "/hopeful_taussig"}]

Fluent Bit can re-combine these logs that were split by the runtime and remove the partial message fields. The following filter example is for this use case.

{% tabs %} {% tab title="fluent-bit.yaml" %}

pipeline:

  filters:
    - name: multiline
      match: '*'
      multiline.key_content: log
      mode: partial_message

{% endtab %} {% tab title="fluent-bit.conf" %}

[FILTER]
  Name                  multiline
  Match                 *
  multiline.key_content log
  Mode                  partial_message

{% endtab %} {% endtabs %}

The two options for mode are mutually exclusive in the filter. If you set the mode to partial_message then the multiline.parser option isn't allowed.