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Examples
========
This page demonstrates the primary usage of DTail. Please also see ``dtail --help`` for more available options.
# How to use ``dtail``
## Tailing logs
The following example demonstrates how to follow logs of multiple servers at once. The server list is provided as a flat text file. The example filters all records containing the string ``STAT``. Any other Go compatible regular expression can be used instead of ``STAT``.
```shell
% dtail --servers serverlist.txt --files "/var/log/service/*.log" --regex STAT
```
You can also use the shorthand version:
```shell
% dtail --servers serverlist.txt --regex STAT "/var/log/service/*.log"
```
## Aggregating logs
To run ad-hoc MapReduce aggregations on newly written log lines, you also must add a query. The following example follows all remote log lines and prints out every 5 seconds the top 10 servers with the most average free memory. To run a MapReduce query across log lines written in the past, please use the ``dmap`` command instead.
```shell
% dtail --servers serverlist.txt \
--query 'select avg(memfree), $hostname from MCVMSTATS group by $hostname order by avg(memfree) limit 10 interval 5' \
--files '/var/log/service/*.log'
```
For MapReduce queries to work, you have to ensure that DTail supports your log format. You can either use the ones already defined in ``internal/mapr/log format`` or add an extension to support a custom log format.
You can also use the shorthand version:
```shell
% dtail --servers serverlist.txt \
'select avg(memfree), $hostname from MCVMSTATS group by $hostname order by avg(memfree) limit 10 interval 5' \
'/var/log/service/*.log'
```
# How to use ``dcat``
The following example demonstrates how to cat files (display the full content of the files) of multiple servers at once. The servers are provided as a comma-separated list this time.
```shell
% dcat --servers serv-011.lan.example.org,serv-012.lan.example.org,serv-013.lan.example.org \
--files /etc/hostname
```
You can also use the shorthand version:
```shell
% dcat --servers serv-011.lan.example.org,serv-012.lan.example.org,serv-013.lan.example.org \
/etc/hostname
```
# How to use ``dgrep``
The following example demonstrates how to grep files (display only the lines which match a given regular expression) of multiple servers at once. In this example, we look after the swap partition in ``/etc/fstab``. We do that only on the first 20 servers from ``serverlist.txt``. ``dgrep`` is also very useful for searching log files of the past.
```shell
% dgrep --servers <(head -n 20 serverlist.txt) \
--files /etc/fstab \
--regex swap
```
You can also use the shorthand version:
TODO: Auto detect that swap is a regex.
```shell
% dgrep --servers <(head -n 20 serverlist.txt) \
/etc/fstab swap
```
# How to use ``dmap``
To run a MapReduce aggregation over logs written in the past, the ``dmap`` command can be used. For example, the following command aggregates all MapReduce fields of all the records and calculates the average memory free grouped by day of the month, hour, minute and the server hostname. ``dmap`` will print interim results every few seconds. The final product, however, will be written to file ``mapreduce.csv``.
```shell
% dmap --servers serv-011.lan.example.org,serv-012.lan.example.org,serv-013.lan.example.org,serv-021.lan.example.org,serv-022.lan.example.org,serv-023.lan.example.org \
--query 'select avg(memfree), $day, $hour, $minute, $hostname from MCVMSTATS group by $day, $hour, $minute, $hostname order by avg(memfree) limit 10 outfile mapreduce.csv' \
--files "/var/log/service/*.log"
```
Remember: For that to work, you have to make sure that DTail supports your log format. You can either use the ones already defined in ``internal/mapr/log format`` or add an extension to support a custom log format.
You can also use the shorthand version:
```shell
% dmap --servers serv-011.lan.example.org,serv-012.lan.example.org,serv-013.lan.example.org,serv-021.lan.example.org,serv-022.lan.example.org,serv-023.lan.example.org \
'select avg(memfree), $day, $hour, $minute, $hostname from MCVMSTATS group by $day, $hour, $minute, $hostname order by avg(memfree) limit 10 outfile mapreduce.csv' \
"/var/log/service/*.log"
```
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