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LogSplit with Akka Cluster

This is an example of using Akka Clustering to perform log distribution between distributed servers.

I was hitching to test the clustering mode in Akka and wanted to find a "real world" example that wasn't too big either. That's when I stumbled over this technical assignment and I thought it would be a fun week end project.

Checkout the annotated code.

While the code works, I am still working on the literal description of what's going on.

Problem Statement

You have a number of web servers (let's assume three) that receive load balanced traffic for your web site. On each server, you log each connection, including a cookie containing a userid. The logs you collect are ordered by time.

Write a program that re-groups all the connection logs for one user on the same server. You can place the grouped output on any one server or on many servers, as long as all the log entries of one user are together on the same physical server.

Example of a log file: - - [15/Aug/2013:13:54:38 -0300] "GET /meme.jpg HTTP/1.1" 200 2148 "-" "userid=5352b590-05ac-11e3-9923-c3e7d8408f3a" - - [15/Aug/2013:13:54:38 -0300] "GET /lolcats.jpg HTTP/1.1" 200 5143 "-" "userid=f85f124a-05cd-11e3-8a11-a8206608c529" - - [15/Aug/2013:13:57:48 -0300] "GET /lolcats.jpg HTTP/1.1" 200 5143 "-" "userid=5352b590-05ac-11e3-9923-c3e7d8408f3a"

Parsing wasn't the point of this exercise, the distribution of the file is the interesting part. While this could be accomplished with hadoop, I wanted to play with a lighter setup.

Possible Solution

The idea behind my approach is to use Akka+scala clustering to distribute the logs between the servers. The code is extensively documented, but please let me know if you have questions.

Once you are done reading this, you might want to start exploring from the main class: LogSplitApp.

Design and Data Flow

Akka deals with the communication between servers and once all nodes are registered on the cluster, the actors talk to each other transparently, without having to know if an actor is on a remote server or in the same JVM.

The data flow is divided in two main steps:

  • data distribution between server
  • local data sorting

The distribution of work assumes the following:

  • logs are already ordered in descending date order on each server
  • each server has a set of buckets of logs, numbered from 0 (with the newest date at the top) to N (with the oldest date at the bottom)

This is a common pattern in log systems and the assumptions help in distributing work to separate workers (by just giving different files to the workers). If we were dealing with a big giant file, we could always chunk it between actors by use of nio channels.

(note that the descending/ascending order of logs in each file is not very important, as long as they are sorted in one way or the other)

Data Distribution

The data distribution step relies on two main type of actors:

  • the Reader actor is responsible for loading a bucket from the local disk system, assigning a partition (server where all this user' logs will eventually live) to each log line and distributing these lines to the right Writer. One server will use a number of reader worker to serve lines from bucket files in parallel.
  • the Writer actor is responsible from pulling log lines from Readers and writing them to disk in a file part assigned to a particular user. To help running the process in parallel, we run a set of WriterWorker on each server that serve a set of Readers.

The reader and writer do not need to know on which node the actor they are talking to are located. They could be on the local node or the remote nodes.

Each reader is assigned one writer per node (including the local node) and a set of local part files to read from.

Each writer can deal with multiple readers and usually manages a fixed set of parts/bucket.

The reader is mostly passive, it will fill up a buffer and then wait for the writers to pull log lines. Writers are either blocked writing to the local disk or pulling work from the assigned readers.


Because all part files are sorted in the original log files, when we send them over to a single server, we keep them in separate sorted part files for one user. At the end of the data distribution step, the output directory will contain something like:


where we have for each userid a set of part files per server (first id) and per bucket from that server.

The sort step is responsible to merge the partial sorted files in one single file for each user. The sorter actor doesn't need to know it's running in a cluster and is only responsible for distributing parallel sorting tasks to local actors.


This is an sbt/scala project, so you will need java, scala and sbt installed.


you can build a package jar with

sbt clean assembly

you can also run the following commands directly with sbt 'run-main...' without running the assembly.

Generating a Sample

You can generate a sample of log files with the utils.LogGenerator.

> java -cp LogSplit-assembly-0.1-SNAPSHOT.jar net.pierreandrews.utils.LogGenerator --output /tmp/logsplittest/servers --linePerFile 10000 --numUsers 1000 --numFiles 50

This will generate three folders in /tmp/logsplittest/servers with 50 files each containing 10000 lines. The userids will be selected from a random pool of 1000 users.

You can then either transfer each serverN file to a separate machine, or just run three separate JVMs pointing to these separate folders.

Running the Code

The app is currently configured to run with three separate JVMs on the same machine (localhost) on the ports 2550, 2551 and 2552. You can start the three JVMs with:

> java -cp LogSplit-assembly-0.1-SNAPSHOT.jar net.pierreandrews.LogSplitApp --port 2550 --output /tmp/logsplittest/outputs/server0 --input /tmp/logsplittest/servers/server0 --serverID 0

> java -cp LogSplit-assembly-0.1-SNAPSHOT.jar net.pierreandrews.LogSplitApp --port 2551 --output /tmp/logsplittest/outputs/server1 --input /tmp/logsplittest/servers/server1 --serverID 1

> java -cp LogSplit-assembly-0.1-SNAPSHOT.jar net.pierreandrews.LogSplitApp --port 2552 --output /tmp/logsplittest/outputs/server2 --input /tmp/logsplittest/servers/server2 --serverID 2

If you are going to run each JVM on separate machines, you need to change the seeds, either with the --seeds arguments, e.g. for one server:

> java -cp LogSplit-assembly-0.1-SNAPSHOT.jar net.pierreandrews.LogSplitApp --port 2550 --output /tmp/logsplittest/outputs/server0 --input /tmp/logsplittest/servers/server0 --serverID 0 --seeds,,

or by updating the application.conf settings. Given the limited time and resources I had, I couldn't test this extensively on distributed servers but it should work transparently.


You can tune the number of workers, cache/buffer sizes, etc. by changing the command line arguments. Please see LogsplitAppArgs for documentation. The current settings work "OK" on my laptop, but are not optimal.

Assumption and alternative solution

The major assumption is that user-ids are evenly distributed between each server, that is, there no one server where a id would appear a lot more than another server. We assume some fair load-balancing between servers etc. that would create such balanced logs.

This assumption is important as the architecture of the system evenly distributes each users logs to each server. That means that with three servers, around 2/3 of the logs of a node will be transferred over to the other nodes in the cluster. If users are evenly distributed, this is as good a solution as any; however, if logs are unbalanced to start with the current data flow will create more data transfers than required.

If we were to deal with unbalanced logs, it would be better to split logs per user on each server first, then negotiate between each node which one contains the most logs for each user and send the data over from the smallest nodes to the bigger node.

Running this on an hadoop or spark cluster would definitely require a lot less code scaffolding, even if it would require a larger server architecture. Apache Samza (LinkedIn) also seemed to be a good solution but required to setup kafka, zookeeper and YARN, which was quite some overhead.