Logging in Clojure / JVM – Part 4

In the previous part of this series, we learnt how we could store log data so it’s easy to get insights from it later. For instance, we use the GELF log format on Zolodeck, our side project. In this part, we’ll look at how to actually get insights from our logs, using a bunch of open source tools.

Here’re the simple 3-steps to get insights from our logs:

1) Write Logs
2) Transport Logs
3) Process Logs

Simple enough!

Write Logs:

In part 3, we saw how logging in a standard JSON format is beneficial. Some of my readers asked me why not use Clojure or Ruby data structures instead of JSON. Here’s why it’s better to use JSON format:

  • JSON is accessible from all languages
  • There are already a bunch of tools available to transport and process logs that accept JSON

Transport Logs:

Always write logs to local disk. It is tempting to have a log4j appender that directly sends logs to a remote server using UDP or HTTP. Unfortunately, you can’t guarantee that you’ll consistently reach those servers. So it’s better to write to local disk first, and then transport your logs to wherever your logs are going to be processed. There are many open source tools available for transporting logs, and depending on what tool you end up using to process your logs, your choice for transporting logs will change. Some tools that you can use for transporting logs are:

  • Scribe – Facebook open-sourced this. This tool does more than just transporting logs.
  • Lumberjack
  • rsync
  • Logstash – This tool does a lot more than transporting logs.

Process Logs:

We need to be able to collect, index, and search through our log data for anything we care to find. Some open-source tools out there for processing logs are:

  • Logstash – Like I said, this tool does lot more than transporting logs:)
  • Graylog2

Both Logstash (Kibana) and Graylog2 provides web interfaces to make life easy when analyzing and searching the underlying logs.

As you can see there are many options for managing and analyzing logs. Here’s what we currently do in our project:

Logging in Clojure 4.001

It is simple for now and we’re hoping to keep it that way:)


Logs are useful. Logs that provide us with insights are more useful:), and if they do this easily, they’re even more so. When we start a new project, we need to spend some time thinking about logging up front, since this is crucial part of managing growth. Thanks to a variety of open-source tools and libraries, it is not that expensive to try out different logging strategies. A properly thought-out logging architecture will save you a lot of time later on. I hope this series has shed some light on logging and why it’s important in this age of distributed computing.

Please do share your experiences, how’re handling logging on your projects. What went well? What didn’t? I’d love to see what folks are doing out there, and document them here, to make this knowledge available for others. Onward!

Logging in Clojure / JVM – Part 3

In part 1 and 2, we looked at the history of logging and how to use SLF4J (a library I’m using with Zolodeck). In this part, we’re going to learn about different formats that we can use to log things in. We need to choose a correct log format so we can then get insights from our logs when we want them. If we cannot easily process the logs to get at the insights we need, then it doesn’t matter what logging framework we use and how many gigabytes of logs we collect everyday.

Purpose of logs:

  • Debugging issues
  • Historic analysis
  • Business and Operational Analysis

If the logs are not adequate for these purposes, it means we’re doing something wrong. Unfortunately, I’ve seen this happen on many projects.

Consumer of logs:

Before we look into what format to log in, we need to know who is going to consume our logs for insights. Most logging implementations assume that humans will be consuming log statements. So they’re essentially formatted string (think printf) so that humans can easily read them. In these situations,  what we’re really doing is creating too much log data for humans to consume to get any particularly useful insights. People then try to solve this overload problem by being cautious about what they log, the idea being that lesser information will be easier for humans to handle. Unfortunately, we can’t know beforehand what information we may need to debug an issue. What always ends up happening is that some important piece of information is missed out.

Remember how we can program machines to consume lots of data and provide better insights? So instead of creating log files for humans to consume, we need to create them for machines.

Format of logs:

Now that we know that machines are will be consuming our logs, we need to make a decision what format our logs should be in. Optimizing for machine readability makes sense, of course.

Formatted strings:

We can easily write a program using regex to parse our formatted strings log messages. Formatted strings, however, are still not a good fit, because of these following reasons:

  • Logging Java stack traces can break our parser thanks to new line characters
  • Developers can’t remove or add fields without breaking the parser.

What is a better way then?

JSON Objects:

JSON objects aren’t particularly human readable, but machines love them. We can use any JSON library to parse our logs. Developers can add and remove fields, our parser will still work fine. We can also log Java stacktraces without breaking our parser by just treating it as a field of data.

JSON log object fields:

Now that it makes sense to use JSON objects as logs, the question is  what basic fields ought to be included. Obviously, this will depend on the application and business requirements. But at a minimum, we’d need the following fields:

  • Host
  • Message
  • Timestamp
  • Log Level
  • Module / Facility
  • File
  • Line Number
  • Trace ID
  • Environment

Standard JSON log format:

Instead of coming up with a custom JSON log format, we ought to just use a standard JSON log format. One option is to use GELF, which is used by many log analysis tools. GELF stands for Graylog Extended Log Format. There are lot of open source log appenders that create logs in GELF format. I’m using it on my side project Zolodeck, and we use logback-gelf.

In this part of this blog series, we learnt why we need to think about machine readable logs, and why we ought to use JSON as the log format. In the next part, we will look at how to get insights from logs, using a bunch of open source tools.

Logging in Clojure / JVM – Part 2

In part 1 , we looked at the history of logging in Java. This time, we’ll learn more about SLF4J (Simple Logging Facade for Java).

From part 1, we saw that SLF4J is not a proxy to other logging frameworks, rather it is an API that allows end users to inject their desired logging framework at deployment time. SLF4J comes with adapters for many commonly used logging frameworks.

For my side project, I’m using SLF4J API. As many other projects, my project is dependent on many libraries. Unfortunately, not all libraries use SLF4J; and indeed, some of them use log4j API. You’ll be surprised to see how many newly written libraries use log4j (even though log4j is old and horrid). Even if they see the benefits to changing to SLF4J, it probably won’t happen soon. SLF4J comes with bridging modules for JCL, JUL and log4j to consolidate logging. These bridging modules redirect calls made to log4j, JCL and JUL to SLF4J instead. The image below explains the idea.


[From http://www.slf4j.org/legacy.html]

Mapped Diagnostic Context

Another awesome feature of SLF4J is MDC ( Mapped Diagnostic Context). Even though it sounds very complicated, it is simple to use and yet so powerful. MDC is essentially a hash map maintained by the logging framework that can be inserted into log messages. Applications can update this hash map using SLF4J. Currently only log4j and logback offer MDC functionality. SLF4J will simply delegate to log4j or logback. If you use some other logging framework, then SLF4J will maintain the hash map, but you’ll need to write some custom code to retrieve the information from the map.

What’s the use of MDC?

One of the main goals of logging is to audit and debug complex real-world, distributed systems. These systems handle multiple clients simultaneously. So log messages are going to be interleaved. So, it’s very important to consolidate log messages of a single client or a single API call. The simplest way is to tag all log messages with client-info and trace-id (we’ll discuss more about this in the next part of this series). Without MDC, we’d need to put this information in every logging call. With MDC, all we have to do is setup the context (client-info , trace-id etc) and all our log messages will automatically have this information. This transforms our log messages into an amazing resource to learn about the system and its users.

Using MDC in Clojure:

Unfortunately, as of now, clojure.tools.logging does not support MDC. I think this a big hole in clojure.tools.logging, as Clojure is known for building complex systems. Luckily the Clojure community is vibrant, and there’s an open source project by Malcolm Sparks called clj-logging-config. The main purpose of this library is to programmatically setup logging config files. But there is one function, with-logging-context that allows us to setup MDC. Even though I’m not programmatically setting up logging config in my project right now, I am using this library just for this with-logging-context function. I strongly believe this library (or at least the with-logging-context function) should be part of clojure.tools.logging.

So there! In this part we learnt more about SLF4J and MDC. In the next part we will learn more about structured logging.