Tail - Fluent Bit: Official Manual

The tail input plugin allows to monitor one or several text files. It has a similar behavior like tail -f shell command.
The plugin reads every matched file in the Path pattern and for every new line found (separated by a ), it generates a new record. Optionally a database file can be used so the plugin can have a history of tracked files and a state of offsets, this is very useful to resume a state if the service is restarted.
Configuration Parameters
The plugin supports the following configuration parameters:
Key
Description
Default
Buffer_Chunk_Size
Set the initial buffer size to read files data. This value is used to increase buffer size. The value must be according to the Unit Size specification.
32k
Buffer_Max_Size
Set the limit of the buffer size per monitored file. When a buffer needs to be increased (e.g: very long lines), this value is used to restrict how much the memory buffer can grow. If reading a file exceeds this limit, the file is removed from the monitored file list. The value must be according to the Unit Size specification.
32k
Path
Pattern specifying a specific log file or multiple ones through the use of common wildcards. Multiple patterns separated by commas are also allowed.
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Path_Key
If enabled, it appends the name of the monitored file as part of the record. The value assigned becomes the key in the map.
​
Exclude_Path
Set one or multiple shell patterns separated by commas to exclude files matching certain criteria, e.g: Exclude_Path *.gz,*.zip
​
Offset_Key
If enabled, Fluent Bit appends the offset of the current monitored file as part of the record. The value assigned becomes the key in the map
​
Read_from_Head
For new discovered files on start (without a database offset/position), read the content from the head of the file, not tail.
False
Refresh_Interval
The interval of refreshing the list of watched files in seconds.
60
Rotate_Wait
Specify the number of extra time in seconds to monitor a file once is rotated in case some pending data is flushed.
5
Ignore_Older
Ignores files which modification date is older than this time in seconds. Supports m,h,d (minutes, hours, days) syntax.
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Skip_Long_Lines
When a monitored file reaches its buffer capacity due to a very long line (Buffer_Max_Size), the default behavior is to stop monitoring that file. Skip_Long_Lines alter that behavior and instruct Fluent Bit to skip long lines and continue processing other lines that fits into the buffer size.
Off
Skip_Empty_Lines
Skips empty lines in the log file from any further processing or output.
Off
DB
Specify the database file to keep track of monitored files and offsets.
​
DB.sync
Set a default synchronization (I/O) method. Values: Extra, Full, Normal, Off. This flag affects how the internal SQLite engine do synchronization to disk, for more details about each option please refer to this section. Most of workload scenarios will be fine with normal mode, but if you really need full synchronization after every write operation you should set full mode. Note that full has a high I/O performance cost.
normal
DB.locking
Specify that the database will be accessed only by Fluent Bit. Enabling this feature helps to increase performance when accessing the database but it restrict any external tool to query the content.
false
DB.journal_mode
sets the journal mode for databases (WAL). Enabling WAL provides higher performance. Note that WAL is not compatible with shared network file systems.
WAL
Mem_Buf_Limit
Set a limit of memory that Tail plugin can use when appending data to the Engine. If the limit is reach, it will be paused; when the data is flushed it resumes.
​
Exit_On_Eof
When reading a file will exit as soon as it reach the end of the file. Useful for bulk load and tests
false
Parser
Specify the name of a parser to interpret the entry as a structured message.
​
Key
When a message is unstructured (no parser applied), it's appended as a string under the key name log. This option allows to define an alternative name for that key.
log
Note that if the database parameter DB is not specified, by default the plugin will start reading each target file from the beginning. This also might cause some unwanted behavior, for example when a line is bigger that Buffer_Chunk_Size and Skip_Long_Lines is not turned on, the file will be read from the beginning of each Refresh_Interval until the file is rotated.
Multiline Support
Starting from Fluent Bit v1.8 we have introduced a new Multiline core functionality. For Tail input plugin, it means that now it supports the old configuration mechanism but also the new one. In order to avoid breaking changes, we will keep both but encourage our users to use the latest one. We will call the two mechanisms as:
Multiline Core
Old Multiline
Multiline Core (v1.8)
The new multiline core is exposed by the following configuration:
Key
Description
multiline.parser
Specify one or multiple Multiline Parser definitions to apply to the content.
As stated in the Multiline Parser documentation, now we provide built-in configuration modes. Note that when using a new multiline.parser definition, you must disable the old configuration from your tail section like:
parser
parser_firstline
parser_N
multiline
multiline_flush
docker_mode
Multiline and Containers (v1.8)
If you are running Fluent Bit to process logs coming from containers like Docker or CRI, you can use the new built-in modes for such purposes. This will help to reassembly multiline messages originally split by Docker or CRI:
fluent-bit.conf
fluent-bit.yaml
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[INPUT]
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    name              tail
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    path              /var/log/containers/*.log
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    multiline.parser  docker, cri
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pipeline:
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  inputs:
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    - tail:
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      path: /var/log/containers/*.log
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      multiline.parser: docker, cri
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The two options separated by a comma means multi-format: try docker and cri multiline formats.
We are still working on extending support to do multiline for nested stack traces and such. Over the Fluent Bit v1.8.x release cycle we will be updating the documentation.
Old Multiline Configuration Parameters
For the old multiline configuration, the following options exist to configure the handling of multilines logs:
Key
Description
Default
Multiline
If enabled, the plugin will try to discover multiline messages and use the proper parsers to compose the outgoing messages. Note that when this option is enabled the Parser option is not used.
Off
Multiline_Flush
Wait period time in seconds to process queued multiline messages
4
Parser_Firstline
Name of the parser that matches the beginning of a multiline message. Note that the regular expression defined in the parser must include a group name (named capture), and the value of the last match group must be a string
​
Parser_N
Optional-extra parser to interpret and structure multiline entries. This option can be used to define multiple parsers, e.g: Parser_1 ab1, Parser_2 ab2, Parser_N abN.
​
Old Docker Mode Configuration Parameters
Docker mode exists to recombine JSON log lines split by the Docker daemon due to its line length limit. To use this feature, configure the tail plugin with the corresponding parser and then enable Docker mode:
Key
Description
Default
Docker_Mode
If enabled, the plugin will recombine split Docker log lines before passing them to any parser as configured above. This mode cannot be used at the same time as Multiline.
Off
Docker_Mode_Flush
Wait period time in seconds to flush queued unfinished split lines.
4
Docker_Mode_Parser
Specify an optional parser for the first line of the docker multiline mode. The parser name to be specified must be registered in the parsers.conf file.
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Getting Started
In order to tail text or log files, you can run the plugin from the command line or through the configuration file:
Command Line
From the command line you can let Fluent Bit parse text files with the following options:
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$ fluent-bit -i tail -p path=/var/log/syslog -o stdout
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Configuration File
In your main configuration file append the following Input & Output sections. An example visualization can be found here​
fluent-bit.conf
fluent-bit.yaml
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[INPUT]
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    Name        tail
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    Path        /var/log/syslog
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​
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[OUTPUT]
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    Name   stdout
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    Match  *
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pipeline:
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  inputs:
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    - tail:
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      path: /var/log/syslog
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  outputs:
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    - stdout:
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      match: *
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Old Multi-line example
When using multi-line configuration you need to first specify Multiline On in the configuration and use the Parser_Firstline and additional parser parameters Parser_N if needed. If we are trying to read the following Java Stacktrace as a single event
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Dec 14 06:41:08 Exception in thread "main" java.lang.RuntimeException: Something has gone wrong, aborting!
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    at com.myproject.module.MyProject.badMethod(MyProject.java:22)
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    at com.myproject.module.MyProject.oneMoreMethod(MyProject.java:18)
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    at com.myproject.module.MyProject.anotherMethod(MyProject.java:14)
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    at com.myproject.module.MyProject.someMethod(MyProject.java:10)
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    at com.myproject.module.MyProject.main(MyProject.java:6)
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We need to specify a Parser_Firstline parameter that matches the first line of a multi-line event. Once a match is made Fluent Bit will read all future lines until another match with Parser_Firstline is made .
In the case above we can use the following parser, that extracts the Time as time and the remaining portion of the multiline as log
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[PARSER]
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    Name multiline
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    Format regex
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    Regex /(?<time>Dec \d+ \d+\:\d+\:\d+)(?<message>.*)/
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    Time_Key  time
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    Time_Format %b %d %H:%M:%S
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If we want to further parse the entire event we can add additional parsers with Parser_N where N is an integer. The final Fluent Bit configuration looks like the following:
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# Note this is generally added to parsers.conf and referenced in [SERVICE]
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[PARSER]
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    Name multiline
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    Format regex
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    Regex /(?<time>Dec \d+ \d+\:\d+\:\d+)(?<message>.*)/
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    Time_Key  time
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    Time_Format %b %d %H:%M:%S
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​
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[INPUT]
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    Name             tail
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    Multiline        On
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    Parser_Firstline multiline
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    Path             /var/log/java.log
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​
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[OUTPUT]
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    Name             stdout
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    Match            *
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Our output will be as follows.
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[0] tail.0: [1607928428.466041977, {"message"=>"Exception in thread "main" java.lang.RuntimeException: Something has gone wrong, aborting!
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    at com.myproject.module.MyProject.badMethod(MyProject.java:22)
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    at com.myproject.module.MyProject.oneMoreMethod(MyProject.java:18)
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    at com.myproject.module.MyProject.anotherMethod(MyProject.java:14)
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    at com.myproject.module.MyProject.someMethod(MyProject.java:10)", "message"=>"at com.myproject.module.MyProject.main(MyProject.java:6)"}]
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Tailing files keeping state
The tail input plugin a feature to save the state of the tracked files, is strongly suggested you enabled this. For this purpose the db property is available, e.g:
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$ fluent-bit -i tail -p path=/var/log/syslog -p db=/path/to/logs.db -o stdout
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When running, the database file /path/to/logs.db will be created, this database is backed by SQLite3 so if you are interested into explore the content, you can open it with the SQLite client tool, e.g:
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$ sqlite3 tail.db
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-- Loading resources from /home/edsiper/.sqliterc
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​
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SQLite version 3.14.1 2016-08-11 18:53:32
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Enter ".help" for usage hints.
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sqlite> SELECT * FROM in_tail_files;
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id     name                              offset        inode         created
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-----  --------------------------------  ------------  ------------  ----------
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1      /var/log/syslog                   73453145      23462108      1480371857
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sqlite>
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Make sure to explore when Fluent Bit is not hard working on the database file, otherwise you will see some Error: database is locked messages.
Formatting SQLite
By default SQLite client tool do not format the columns in a human read-way, so to explore in_tail_files table you can create a config file in ~/.sqliterc with the following content:
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.headers on
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.mode column
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.width 5 32 12 12 10
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SQLite and Write Ahead Logging
Fluent Bit keep the state or checkpoint of each file through using a SQLite database file, so if the service is restarted, it can continue consuming files from it last checkpoint position (offset). The default options set are enabled for high performance and corruption-safe.
The SQLite journaling mode enabled is Write Ahead Log or WAL. This allows to improve performance of read and write operations to disk. When enabled, you will see in your file system additional files being created, consider the following configuration statement:
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[INPUT]
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    name    tail
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    path    /var/log/containers/*.log
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    db      test.db
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The above configuration enables a database file called test.db and in the same path for that file SQLite will create two additional files:
test.db-shm
test.db-wal
Those two files aims to support the WAL mechanism that helps to improve performance and reduce the number system calls required. The -wal file refers to the file that stores the new changes to be committed, at some point the WAL file transactions are moved back to the real database file. The -shm file is a shared-memory type to allow concurrent-users to the WAL file.
WAL and Memory Usage
The WAL mechanism give us higher performance but also might increase the memory usage by Fluent Bit. Most of this usage comes from the memory mapped and cached pages. In some cases you might see that memory usage keeps a bit high giving the impression of a memory leak, but actually is not relevant unless you want your memory metrics back to normal. Starting from Fluent Bit v1.7.3 we introduced the new option db.journal_mode mode that sets the journal mode for databases, by default it will be WAL (Write-Ahead Logging), currently allowed configurations for db.journal_mode are DELETE | TRUNCATE | PERSIST | MEMORY | WAL | OFF .
File Rotation
File rotation is properly handled, including logrotate's copytruncate mode.
Note that the Path patterns cannot match the rotated files. Otherwise, the rotated file would be read again and lead to duplicate records.