Real-Time Log Collection with Fluentd and MongoDB

Real-Time Log Collection with Fluentd and MongoDB

Let’s walk through how to use Fluentd‘s MongoDB plugin to aggregate semi-structured logs in real-time. For a more detailed version, visit the documentation.

Fluentd is an advanced open-source log collector developed at Treasure Data, Inc (see previous post). Because Fluentd handles logs as semi-structured data streams, the ideal database should have strong support for semi-structured data. There are several databases that meet this criterion, but we believe MongoDB is the market leader.

For those of you who do not know what MongoDB is, it is an open-source, document-oriented database developed at 10gen, Inc. It is schema-free and uses a JSON-like format to manage semi-structured data.

Let’s talk about how to import Apache logs into MongoDB with Fluentd, using really small configurations.


The figure below shows how it works.

Fluentd does 3 things:

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  • It continuously “tails” the access log.
  • It parses the incoming log entries into meaningful fields (such as ippath, etc) and buffers them.
  • It writes the buffered data to MongoDB periodically.


For simplicity, this post shows the one-node configuration. You should have the following software installed on the same node.

  • Fluentd with MongoDB Plugin
  • MongoDB
  • Apache (with the Combined Log Format)

Fluentd’s most recent version of deb/rpm package includes the MongoDB plugin. If you want to use Ruby Gems to install the plugin, gem install fluent-plugin-mongo does the job.

For MongoDB, please refer to the downloads page.


Let’s start the actual configurations. If you use deb/rpm, the Fluentd’s config file is located at /etc/td-agent/td-agent.conf. Otherwise, it is located at /etc/fluentd/fluentd.conf.

Tail Input

For input, let’s set up Fluentd to track the recent Apache logs (usually at /var/log/apache2/access_log). This is what the Fluentd configuration looks like.

  type tail
  format apache
  path /var/log/apache2/access_log
  tag mongo.apache

Let’s go through the configuration line by line.

  • type tail: The tail plugin continuously tracks the log file. This handy plugin is part of Fluentd’s core plugins.
  • format apache: Use Fluentd’s built-in Apache log parser.
  • path /var/log/apache2/access_log: Assuming the Apache log is in /var/log/apache2/access_log.
  • tag mongo.apachemongo.apach tells Fluentd to parse the log entry into meaningtful fields.

That’s it. You should be able to output a JSON-formatted data stream for MongoDB to consume.

MongoDB Output

The output configuration should look like this:

  # plugin type
  type mongo

  # mongodb db + collection
  database apache
  collection access

  # mongodb host + port
  host localhost
  port 27017

  # interval
  flush_interval 10s

The match section specifies the regexp to match the tags. If the tag is matched, then the config inside the ... is used. In this example, the mongo.apache tag (generated by tail) is always used.

The in match. matches zero or more period-delimited tag elements (e.g. match/match.a/match.a.b). flush_internal indicates how often the data is written to the database (MongoDB in this case). Other options specify MongoDB’s host, port, db, and collection.


To test the configuration, just ping the Apache server however you want. This example uses ab (Apache Bench) program.

$ ab -n 100 -c 10 http://localhost/

Then, let’s access MongoDB and see the stored data.

$ mongo
> use apache
> db.access.find()
{ "_id" : ObjectId("4ed1ed3a340765ce73000001"), "host" : "", "user" : "-", "method" : "GET", "path" : "/", "code" : "200", "size" : "44", "time" : ISODate("2011-11-27T07:56:27Z") }
{ "_id" : ObjectId("4ed1ed3a340765ce73000002"), "host" : "", "user" : "-", "method" : "GET", "path" : "/", "code" : "200", "size" : "44", "time" : ISODate("2011-11-27T07:56:34Z") }
{ "_id" : ObjectId("4ed1ed3a340765ce73000003"), "host" : "", "user" : "-", "method" : "GET", "path" : "/", "code" : "200", "size" : "44", "time" : ISODate("2011-11-27T07:56:34Z") }


Fluentd + MongoDB make real-time log collection simple, easy and robust.

Further Reading:


Masahiro Nakagawa contributed the MongoDB plugin for Fluentd. Thanks Masahiro!

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