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Barn

Barn is a log aggregation and archival system.

Overview

Barn attempts to solve the following problems:

  • provide high durability guarantees
  • provide high throughput
  • provide real-time stream inspection
  • be easy to integrate with
  • be suave to operate
  • be smoothly fault-tolerant

The first three requirements are clearly competing with each other. In contrast to other systems, which attempt to treat log aggregation as a messaging problem, Barn separates it into two diffent problems: file transfer and messaging.

Producer

Let's look at the producer: we assume that a process is emitting newline- separated character data on standard output. Every line is considered a log record, and gets written to the local disk using the excellent svlogd program from the runit package. A separate agent watches svlogds output directory, and, upon log file rotation, attempts to ship non-current log files to a remote server.

# example snippet of a runit 'log/run' script

barn-harvester-agent farm.acme.org:1025 ./main my-service main &
svlogd -tt ./main

It is also trivial to intercept the standard output stream using a simple UNIX pipe and, for example, split it into multiple topic streams, each handled by a separate svlogd:

# (setup barn-harvester-agent ...)

# capture stdout and split into two streams
tee >( grep "^ERROR" | svlogd -tt ./errors  ) \
    >( grep -v "^ERROR" | svlogd -tt ./main )
    > /dev/null

Now, even though we can tweak svlogds rotation policy, we have to wait until a certain amount of data has been written, until it gets shipped to remote storage. To get real-time access to the stream, we need to insert rtail into the pipe:

# (setup barn-harvester-agent ...)

RTAILD_SOCK=./rtaild.sock
RTAILD_PORT=12345

rtaild $RTAILD_SOCK $RTAILD_PORT &

tee >( grep "^ERROR" | rtailp $RTAILD_SOCK myservice.errors.$(hostname) | svlogd -tt ./errors ) \
    >( grep -v "^ERROR" | rtailp $RTAILD_SOCK myservice.main.$(hostname) | svlogd -tt ./main )
    > /dev/null

We can now use the rtail program to connect to a set of machines, and subscribe to (a subset of) the log streams produced by services running there:

HOSTS="host1:12345 host2:12345 host3:12345"
# subscribe to all streams of 'myservice' on three machines:
rtail $HOSTS myservice

# subscribe to just the 'error' stream:
rtail $HOSTS myservice.error

# subscribe to the 'main' stream, and the 'error' stream of host3:
rtail $HOSTS myservice.main myservice.error.host3

Collector

The destination for barn-harvester-agent doesn't do a lot. In fact, the current implementation is just a rsync daemon (so barn-harvester-agent is only little more than a clever way to invoke rsync). We also provide some script to keep a slave rsync server in sync (failover of the producers is not automatic).

Barn Baler

The collector node is assumed to be just a single machine, thus with limited disk space. So ultimately we want our data to be archived in distributed storage -- we use HDFS. Since importing data into HDFS in a reliable, fault-tolerant way is a bit of a hairy thing, we provide baler, which exploits the Tai64 timestamps svlogd uses to name rotated logfiles to provide idempotent, block-optimizing writes to HDFS.

Related Work

  • syslog + Flume

This is the combination we've been running in production for about a year. The main problem was around appending to HDFS files, which is very sensitive to cluster availablity -- even restarting data nodes could confuse the DFSClient. We also had to write a lot of custom code (disk buffering, sequence files, syslog parsing to get the desired bucketing), which would need to be rewritten for flume-ng.

While syslog mainly does what you would expect, it is archaic to configure (we use rsyslogd), and doesn't tell you anything if it happens to not do what you expect.

  • AMQP

For a while, we were obsessed with the idea that we could use our existing RabbitMQ infrastructure and experience, and treat logging as just a very high throughput messaging problem. But we couldn't get anywhere near the desired performance.

TODO: wasn't there some OSS project which uses AMQP?

  • Scribe

Facebook's Scribe essentially implements what barn-agent does. It does it well, or so we hear, but we would still have had to implement the other parts ourselves. Another issue is that Scribe speaks Thrift, over TCP, exclusively. Which, apart from more involved integration in heterogenous environments, would require the Haystacker to parse, inspect and transform log files -- we can't and don't want to use Thrift for long-term archival, and tailor all our workflows around that.

  • Kafka

LinkedIn's Kafka is a very interesting project, so we looked at it very hard. And from different angles. And with 1-6 eyes. Eventually, we decided to not use it, because it seemed too much of an operational overhead to keep it running reliably and with high throughput. Namely:

  • Kafka relies heavily on ZooKeeper, which is hard to operate
  • as we understand, a single Kafka broker shouldn't serve a large number of producers: due to the non-reliable messaging protocol Kafka implements, if a broker goes down, all messages currently in-flight will be lost (most likely more, depending on the client's network implementation). This might be fine, eg. if you're anyway planning to dedicate a machine per rack to collect and forward logs, and if you can afford to loose a couple of messages here and there. In our setup, however, it isn't.
  • Kafka uses a custom binary wire protocol, which is also used for disk persistence. That's nice, but we again would have to parse it before writing to HDFS.

That being said, barns design is heavily influenced by Kafka, except that we got rid of coordination (mostly) and that we push the "queue history" idea to the producer itself (as well as the collector).

  • Others

There are also a couple of other systems out there, eg. Heroku's logplex, or logstash. They seem to be targeted at very different scenarios though, and wouldn't satisfy all of our requirements. YMMV, as usual.

Build

Make sure you have the following installed on the build system:

  • a recent gcc
  • a recent JDK
  • a recent Haskell Platform
  • a working make

Then, in the top level of the repository, simply type:

make dist

Installation

We provide debian packages of all barn components (TODO), as well has zip and tar packages. The runtime dependencies are as follows:

Agent

runit, rsync, inotify, bash, (grep, awk), optional: rtail

Collector

rsync, bash

Haystacker

JRE >= 6, and a HDFS cluster to talk to

We recommend to run the Collector and Haystacker as runit services.

Configuration

All barn executables are configured through command line parameters. Where svlogd is used, refer to man svlogd for additional file based configuration options.

Some knobs to turn are:

Agent

  • you may want to adjust the rotation / history policy for svlogd to the amount of disk space you have available
  • for low-volume producers, you may want to set the rotation policy for svlogd to a low value
  • for high-volume producers, you may want to mount a RAM disk for svlogd to write to. This will allow the producer to continue functioning even if the machine's disk crashes, giving up a little bit of safety in case the system needs to be power-cycled.
  • a RAM disk may also be an option if you can't write to local disk at all
  • TODO rtail uses a 0mq PUB socket over TCP to publish log messages. This is because the assumption is that most of the time, data will be discarded anyway, and there are only a few consumers on only a few services at any time. As the number of consumers grows, this may cause network congestion because the log streams are duplicated many times. We'll look into providing an option to use UDP (PGM) instead for those scenarios (PGM is not available on all platforms, and would cause constant network traffic by default).

Collector / Haystacker

It's all about RAID and disk partitioning. Heavily depends on what you want to optimize for. Make sure you can use as much bandwidth as possible to talk to HDFS (possibly use two NICs on two switches for ingress and egress respectively).

Failover

At this point, we don't have any automatic failover mechanism for barn-agent in case the collector is not reachable. We recommend to use a DNS entry with a short TTL and perform manual failover.

FAQ

  • Why is rtail written in Haskell?! I thought that's for academic curmudgeons who wear no shoes, even in winter!

    We are curmudgeons, appreciate academia (from a safe distance), and we wear shoes (most of the time).

    There's also a C++ implementation (see the rtail++ branch), but it needs some love: rtailp currently consumes way too much CPU.

  • Why is rtail not written in Go?! I hear it's webscale!

    Yeah sure. We'll consider it when it's implementation reaches version 2.0, and someone writes a proper 0mq binding.

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Contributors