Tail

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 \n), 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:

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 behaviour, 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 each Refresh_Interval until the file is rotated.

Multiline Configuration Parameters

Additionally the following options exists to configure the handling of multi-lines files:

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:

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:

$ fluent-bit -i tail -p path=/var/log/syslog -o stdout

Configuration File

In your main configuration file append the following Input & Output sections. An example visualization can be found here

[INPUT]
    Name        tail
    Path        /var/log/syslog
[OUTPUT]
    Name   stdout
    Match  *

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

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)

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

[PARSER]
    Name multiline
    Format regex
    Regex /(?<time>Dec \d+ \d+\:\d+\:\d+)(?<message>.*)/
    Time_Key  time
    Time_Format %b %d %H:%M:%S

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:

# Note this is generally added to parsers.conf and referenced in [SERVICE]
[PARSER]
    Name multiline
    Format regex
    Regex /(?<time>Dec \d+ \d+\:\d+\:\d+)(?<message>.*)/
    Time_Key  time
    Time_Format %b %d %H:%M:%S
[INPUT]
    Name             tail
    Multiline        On
    Parser_Multiline multiline
    Path             /var/log/java.log
[OUTPUT]
    Name             stdout
    Match            *

Our output will be as follows.

[0] tail.0: [1607928428.466041977, {"message"=>"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)", "message"=>"at com.myproject.module.MyProject.main(MyProject.java:6)"}]

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:

$ fluent-bit -i tail -p path=/var/log/syslog -p db=/path/to/logs.db -o stdout

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:

$ sqlite3 tail.db
-- Loading resources from /home/edsiper/.sqliterc
SQLite version 3.14.1 2016-08-11 18:53:32
Enter ".help" for usage hints.
sqlite> SELECT * FROM in_tail_files;
id     name                              offset        inode         created
-----  --------------------------------  ------------  ------------  ----------
1      /var/log/syslog                   73453145      23462108      1480371857
sqlite>

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:

.headers on
.mode column
.width 5 32 12 12 10

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.