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Troubleshooting

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Tap Functionality

Tap can be used to generate events or records detailing what messages pass through Fluent Bit, at what time and what filters affect them.

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Simple example

First, we will make sure that the container image we are going to use actually supports Fluent Bit Tap (available in Fluent Bit 2.0+):

$ docker run --rm -ti fluent/fluent-bit:latest --help | grep trace
  -Z, --enable-chunk-traceenable chunk tracing, it can be activated either through the http api or the command line
  --trace-input           input to start tracing on startup.
  --trace-output          output to use for tracing on startup.
  --trace-output-property set a property for output tracing on startup.
  --trace                 setup a trace pipeline on startup. Uses a single line, ie: "input=dummy.0 output=stdout output.format='json'"

If the --enable-chunk-trace option is present it means Fluent Bit has support for Fluent Bit Tap but it is disabled by default, so remember to enable it with this option.

You can start fluent-bit with tracing activated from the beginning by using the trace-input and trace-output properties, like so:

$ fluent-bit -Z -i dummy -o stdout -f 1 --trace-input=dummy.0 --trace-output=stdout
Fluent Bit v2.1.8
* Copyright (C) 2015-2022 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

[2023/07/21 16:27:01] [ info] [fluent bit] version=2.1.8, commit=824ba3dd08, pid=622937
[2023/07/21 16:27:01] [ info] [storage] ver=1.4.0, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2023/07/21 16:27:01] [ info] [cmetrics] version=0.6.3
[2023/07/21 16:27:01] [ info] [ctraces ] version=0.3.1
[2023/07/21 16:27:01] [ info] [input:dummy:dummy.0] initializing
[2023/07/21 16:27:01] [ info] [input:dummy:dummy.0] storage_strategy='memory' (memory only)
[2023/07/21 16:27:01] [ info] [sp] stream processor started
[2023/07/21 16:27:01] [ info] [output:stdout:stdout.0] worker #0 started
[2023/07/21 16:27:01] [ info] [fluent bit] version=2.1.8, commit=824ba3dd08, pid=622937
[2023/07/21 16:27:01] [ info] [storage] ver=1.4.0, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2023/07/21 16:27:01] [ info] [cmetrics] version=0.6.3
[2023/07/21 16:27:01] [ info] [ctraces ] version=0.3.1
[2023/07/21 16:27:01] [ info] [input:emitter:trace-emitter] initializing
[2023/07/21 16:27:01] [ info] [input:emitter:trace-emitter] storage_strategy='memory' (memory only)
[2023/07/21 16:27:01] [ info] [sp] stream processor started
[2023/07/21 16:27:01] [ info] [output:stdout:stdout.0] worker #0 started
.[0] dummy.0: [[1689971222.068537501, {}], {"message"=>"dummy"}]
[0] dummy.0: [[1689971223.068556121, {}], {"message"=>"dummy"}]
[0] trace: [[1689971222.068677045, {}], {"type"=>1, "trace_id"=>"0", "plugin_instance"=>"dummy.0", "records"=>[{"timestamp"=>1689971222, "record"=>{"message"=>"dummy"}}], "start_time"=>1689971222, "end_time"=>1689971222}]
[1] trace: [[1689971222.068735577, {}], {"type"=>3, "trace_id"=>"0", "plugin_instance"=>"dummy.0", "records"=>[{"timestamp"=>1689971222, "record"=>{"message"=>"dummy"}}], "start_time"=>1689971222, "end_time"=>1689971222}]
[0] dummy.0: [[1689971224.068586317, {}], {"message"=>"dummy"}]
[0] trace: [[1689971223.068626923, {}], {"type"=>1, "trace_id"=>"1", "plugin_instance"=>"dummy.0", "records"=>[{"timestamp"=>1689971223, "record"=>{"message"=>"dummy"}}], "start_time"=>1689971223, "end_time"=>1689971223}]
[1] trace: [[1689971223.068675735, {}], {"type"=>3, "trace_id"=>"1", "plugin_instance"=>"dummy.0", "records"=>[{"timestamp"=>1689971223, "record"=>{"message"=>"dummy"}}], "start_time"=>1689971223, "end_time"=>1689971223}]
[2] trace: [[1689971224.068689341, {}], {"type"=>1, "trace_id"=>"2", "plugin_instance"=>"dummy.0", "records"=>[{"timestamp"=>1689971224, "record"=>{"message"=>"dummy"}}], "start_time"=>1689971224, "end_time"=>1689971224}]
[3] trace: [[1689971224.068747182, {}], {"type"=>3, "trace_id"=>"2", "plugin_instance"=>"dummy.0", "records"=>[{"timestamp"=>1689971224, "record"=>{"message"=>"dummy"}}], "start_time"=>1689971224, "end_time"=>1689971224}]
^C[2023/07/21 16:27:05] [engine] caught signal (SIGINT)
[2023/07/21 16:27:05] [ warn] [engine] service will shutdown in max 5 seconds
[2023/07/21 16:27:05] [ info] [input] pausing dummy.0
[0] dummy.0: [[1689971225.068568875, {}], {"message"=>"dummy"}]
[2023/07/21 16:27:06] [ info] [engine] service has stopped (0 pending tasks)
[2023/07/21 16:27:06] [ info] [input] pausing dummy.0
[2023/07/21 16:27:06] [ warn] [engine] service will shutdown in max 1 seconds
[0] trace: [[1689971225.068654038, {}], {"type"=>1, "trace_id"=>"3", "plugin_instance"=>"dummy.0", "records"=>[{"timestamp"=>1689971225, "record"=>{"message"=>"dummy"}}], "start_time"=>1689971225, "end_time"=>1689971225}]
[1] trace: [[1689971225.068695829, {}], {"type"=>3, "trace_id"=>"3", "plugin_instance"=>"dummy.0", "records"=>[{"timestamp"=>1689971225, "record"=>{"message"=>"dummy"}}], "start_time"=>1689971225, "end_time"=>1689971225}]
[2023/07/21 16:27:07] [ info] [engine] service has stopped (0 pending tasks)
[2023/07/21 16:27:07] [ info] [output:stdout:stdout.0] thread worker #0 stopping...
[2023/07/21 16:27:07] [ info] [output:stdout:stdout.0] thread worker #0 stopped
[2023/07/21 16:27:07] [ info] [output:stdout:stdout.0] thread worker #0 stopping...
[2023/07/21 16:27:07] [ info] [output:stdout:stdout.0] thread worker #0 stopped

If you see the following warning then the -Z or --enable-chunk-tracing option is missing:

Properties can be set for the output using the --trace-output-property option:

With that options set the stdout plugin is now emitting traces in json_lines format:

All three options can also be defined using the much more flexible --trace option:

We defined the entire tap pipeline using this configuration: input=dummy.0 output=stdout output.format=json_lines which defines the following:

  • input: dummy.0 (listens to the tag and/or alias dummy.0)

  • output: stdout (outputs to a stdout plugin)

  • output.format: json_lines (sets the stdout format o json_lines)

Tap support can also be activated and deactivated via the embedded web server:

In another terminal we can activate Tap by either using the instance id of the input; dummy.0 or its alias.

Since the alias is more predictable that is what we will use:

This response means we have activated Tap, the terminal with Fluent Bit running should now look like this:

All the records that now appear are those emitted by the activities of the dummy plugin.

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Complex example

This example takes the same steps but demonstrates the same mechanism works with more complicated configurations. In this example we will follow a single input of many which passes through several filters.

To make sure the window is not cluttered by the actual records generated by the input plugins we send all of it to null.

We activate with the following 'curl' command:

Now we should start seeing output similar to the following:

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Parameters for the output in Tap

When activating Tap, any plugin parameter can be given. These can be used to modify, for example, the output format, the name of the time key, the format of the date, etc.

In the next example we will use the parameter "format": "json" to demonstrate how in Tap, stdout can be shown in Json format.

First, run Fluent Bit enabling Tap:

Next, in another terminal, we activate Tap including the output, in this case stdout, and the parameters wanted, in this case "format": "json":

In the first terminal, we should be seeing the output similar to the following:

This parameter shows stdout in Json format, however, as mentioned before, parameters can be passed to any plugin.

Please visit the following link for more information on other output plugins: https://docs.fluentbit.io/manual/pipeline/outputs

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Analysis of a single Tap record

Here we analyze a single record from a filter event to explain the meaning of each field in detail. We chose a filter record since it includes the most details of all the record types.

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type

The type defines at what stage the event is generated:

  • type=1: input record

    • this is the unadulterated input record

  • type=2: filtered record

    • this is a record once it has been filtered. One record is generated per filter.

  • type=3: pre-output record

    • this is the record right before it is sent for output.

Since this is a record generated by the manipulation of a record by a filter is has the type 2.

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start_time and end_time

This records the start and end of an event, it is a bit different for each event type:

  • type 1: when the input is received, both the start and end time.

  • type 2: the time when filtering is matched until it has finished processing.

  • type 3: the time when the input is received and when it is finally slated for output.

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trace_id

This is a string composed of a prefix and a number which is incremented with each record received by the input during the Tap session.

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plugin_instance

This is the plugin instance name as it is generated by Fluent Bit at runtime.

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plugin_alias

If an alias is set this field will contain the alias set for a plugin.

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records

This is an array of all the records being sent. Since Fluent Bit handles records in chunks of multiple records and chunks are indivisible the same is done in the Tap output. Each record consists of its timestamp followed by the actual data which is a composite type of keys and values.

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Dump Internals / Signal

When the service is running we can export metrics to see the overall status of the data flow of the service. But there are other use cases where we would like to know the current status of the internals of the service, specifically to answer questions like what's the current status of the internal buffers ? , the Dump Internals feature is the answer.

Fluent Bit v1.4 introduces the Dump Internals feature that can be triggered easily from the command line triggering the CONT Unix signal.

circle-info

note: this feature is only available on Linux and BSD family operating systems

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Usage

Run the following kill command to signal Fluent Bit:

The command pidof aims to lookup the Process ID of Fluent Bit. You can replace the

Fluent Bit will dump the following information to the standard output interface (stdout):

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Input Plugins Dump

The dump provides insights for every input instance configured.

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Status

Overall ingestion status of the plugin.

Entry
Sub-entry
Description

overlimit

If the plugin has been configured with Mem_Buf_Limit, this entry will report if the plugin is over the limit or not at the moment of the dump. If it is overlimit, it will print yes, otherwise no.

mem_size

Current memory size in use by the input plugin in-memory.

mem_limit

Limit set by Mem_Buf_Limit.

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Tasks

When an input plugin ingest data into the engine, a Chunk is created. A Chunk can contains multiple records. Upon flush time, the engine creates a Task that contains the routes for the Chunk associated in question.

The Task dump describes the tasks associated to the input plugin:

Entry
Description

total_tasks

Total number of active tasks associated to data generated by the input plugin.

new

Number of tasks not assigned yet to an output plugin. Tasks are in new status for a very short period of time (most of the time this value is very low or zero).

running

Number of active tasks being processed by output plugins.

size

Amount of memory used by the Chunks being processed (Total chunks size).

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Chunks

The Chunks dump tells more details about all the chunks that the input plugin has generated and are still being processed.

Depending of the buffering strategy and limits imposed by configuration, some Chunks might be up (in memory) or down (filesystem).

Entry
Sub-entry
Description

total_chunks

Total number of Chunks generated by the input plugin that are still being processed by the engine.

up_chunks

Total number of Chunks that are loaded in memory.

down_chunks

Total number of Chunks that are stored in the filesystem but not loaded in memory yet.

busy_chunks

Chunks marked as busy (being flushed) or locked. Busy Chunks are immutable and likely are ready to (or being) processed.

size

Amount of bytes used by the Chunk.

size err

Number of Chunks in an error state where it size could not be retrieved.

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Storage Layer Dump

Fluent Bit relies on a custom storage layer interface designed for hybrid buffering. The Storage Layer entry contains a total summary of Chunks registered by Fluent Bit:

Entry
Sub-Entry
Description

total chunks

Total number of Chunks

mem chunks

Total number of Chunks memory-based

fs chunks

Total number of Chunks filesystem based

up

Total number of filesystem chunks up in memory

down

Total number of filesystem chunks down (not loaded in memory)

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