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JSON Logging: What, Why, How, & Tips

  • Coralogix
  • February 1, 2022
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JSON logging

When you’re working with large data sets, having that data structured in a way that means you can use software to process and understand it will enable you to derive insights far more quickly than any manual approach. Logfile data is no exception.

As increasing numbers of organizations embrace the idea that log files can offer far more than an aid to troubleshooting or a regulatory requirement, the importance of log file monitoring and structuring the data in those log files so that it can be extracted, manipulated, and analyzed efficiently is quickly moving up the priority list. In this article, we’re going to explore one of the most popular formats for structuring log files: JSON.

What is JSON?

JSON, or JavaScript Object Notation to give it its full form, is a machine-readable syntax for encoding and exchanging data. Despite the name, you’ll find JSON in use far outside its original realm of web servers and browsers. With all major computing languages supporting it, JSON is one of the most widely used formats for exchanging data in machine-to-machine communications.

One of the advantages of JSON over other data exchange formats, such as XML, is that it’s easy for us humans to both read and write. Unlike XML, JSON doesn’t rely on a complex schema and completely avoids the forest of angle brackets that results from requiring everything to be enclosed within tags. This makes it much easier for first-time users to get started with JSON.

A JSON document is made up of a simple syntax of key-value pairs ordered and nested within arrays. For example, a key called “status” might have values “success,” “warning,” and “error.” Keys are defined within the document and are always quoted, meaning there are no reserved words to avoid, and arrays can be nested to create hierarchies. 

That means you can create whatever keys make sense for your context, and structure them however you need. The keys and how they are nested (the JSON specification) need to be agreed upon between the sender and the recipient, which can then read the file and extract the data as required.

The simplicity and flexibility of JSON make it an ideal candidate for generating structured log statements; log data can be extracted and analyzed programmatically, while the messages remain easy for individuals to understand. JSON logging is supported by all major programming languages, either natively or via libraries.

Benefits of JSON logging

Given that log messages are always generated by software, you might expect that they are always structured and be wondering what JSON can add. While it’s true that log messages will always follow a particular syntax (in accordance with how the software has been programmed to output logs), that syntax could be one long string of characters, multiple lines of obscure codes and statuses, or a series of values delimited by a character of the programmer’s choice.

In order to make sense of these logs, you first need to decipher their syntax and then write logic to parse the messages and extract the data you need. Unfortunately, that logic is often quite brittle, so if something changes in the log format – perhaps a new piece of data is included, or the order of items is changed – then the parser will break. 

If you’re only dealing with logs from a single system that you have control over, that might be manageable. But the reality in many enterprises is that you’re working with multiple systems and services, some developed in-house and others that are open-source or commercial, and all of them are generating log messages.

Those log messages are a potential mine of information that can be used to gain insights into how your systems – and therefore your business – are performing. However, before you can derive those insights, you first need to make sense of the information that is being provided. Writing and maintaining custom logic to parse logs for dozens of pieces of software is no small task. 

That’s where a structured format such as JSON can help. The key-value pairs make it easy to extract specific values and to filter and search across a data set. If new key-value pairs are added, the software parsing the log messages will just ignore those keys it doesn’t expect, rather than failing completely.

Writing logs in JSON format

So what does a log message written in JSON look like? The following is an example log line generated by an Nginx web server and formatted in JSON:

"time": "17/May/2015:08:05:24 +0000",
"remote_ip": "",
"remote_user": "-",
"request": "GET /downloads/product_1 HTTP/1.1",
"response": 304,
"bytes": 0,
"referrer": "-",
"agent": "Debian APT-HTTP/1.3 (0.8.16~exp12ubuntu10.16)"

The same data in combined log format would look like this: - - 17/May/2015:08:05:24 +0000 "GET /downloads/product_1 HTTP/1.1" 304 0 "-" "Debian APT-HTTP/1.3 (0.8.16~exp12ubuntu10.16)"

With the JSON format, it’s easy for someone unfamiliar with web server logs to understand what the message contains, as each field is labeled. With a common log format, you need to know what you’re looking at.

Of course, common log format is widely used by web servers, and most log analysis platforms can parse it natively, without further manual configuration. But what about log files generated by other software, such as a custom-built application or third-party software? Looking at this unstructured log file from an iOS application you’d be forgiven for wondering what it’s telling you:

08:51:08 [DataLogger:27]: G L 35.76 +/- 12m <+52.55497710,-0.38856690> +/- 15.27m 0.60

A JSON formatted version of the same log returns:

  "eventTime": "08:51:08",
      "file": "DataLogger",
      "line": 27
  "eventType": "GPS update",
      "altitude": 35.76,
    "verticalAccuracy": 12,
      "latitude": 52.55497710,
      "longitude": -0.38856690,
      "horizontalAccuracy": 15.27,
      "speed": 0.60

With this format, it’s easy to understand the values and see how the fields are related.

JSON logging best practices

Now that we’ve covered the what, why, and how of JSON logging, let’s discuss some tips to help you get the most out of your JSON logs. Most of these apply whether you’re writing software in-house or are using third-party or open-source tools that allow you to configure the format of the logs they output.

Invest time in designing your logs

Just as you wouldn’t jump in and start writing code for the next ticket on the backlog without first thinking through the problem you’re trying to solve and how it fits into the wider solution, it’s important to take the time to design your log format. There is no standard format for JSON logs – just the JSON syntax – so you can decide on a structure to serve your needs.

When defining keys, think about what level of granularity makes sense. For example, do you need a dedicated “error” key, or is it more useful to have a key labeled “message” that is used for any type of message, and another labeled “status” that will record whether the message was an error, warning, or just information? With the latter approach, you can filter log data by status to view only error messages while reducing the number of columns and filter options.

Add log lines as you code

If you’re developing software in-house, make adding log lines as much a part of your code hygiene as writing unit tests. It’s much easier to decide what information would be useful to output, and at what level (for example, is this a critical error or just useful to know when debugging) when you’re writing that particular piece of functionality, than after you’ve finished the development work.

Capture as much detail as possible

When you’re thinking about what data to capture, it’s easy to focus on the parameters you want to be able to filter, sort, and query by, while losing sight of what you might want to learn from your logs when drilling down into more detail. 

Log files provide value on both a macro and micro level: by aggregating and analyzing log files, we can identify patterns of behavior and spot changes that might indicate a problem. Once we know where to look, we can zoom into the individual log files to find out more about what’s going on. This is where capturing as much detail as possible pays dividends.

For application logs, details such as the module name and line number will help you identify the cause of the problem quickly. In the case of server access logs, details such as the requester’s IP, their time zone, logged-in username, and authentication method can be invaluable when investigating a potential security breach.

Keep in mind that not all data needs to be broken down into separate fields; you can create a key to capture more verbose information that you wouldn’t want to filter by, but which is useful when reading individual log messages.

Consistency is king

Being consistent in the way you name keys and the way you record values will help you to analyze logs more efficiently. This applies both within a single piece of software and when designing logs across multiple systems and services. 

For example, using the same set of log levels across applications means you can easily filter by a particular type while being consistent in writing status codes, as either strings or numbers will ensure you can manipulate the data effectively.

Unstructured logs – parsing to JSON

Although structuring logs with JSON offers many advantages, in some cases it’s not possible to output logs in this format. For some third-party software, you may not have the ability to configure the format or content of log messages. 

If you’re dealing with a legacy system plagued with technical debt, the effort involved in updating the logging mechanism might not be justified – particularly if work on a replacement is underway.

When you’re stuck with an existing unstructured log format, the next best thing is to parse those logs into a JSON format after the fact. This involves identifying the individual values within each message (using a regular expression, for example) and mapping them to particular keys. 

Many log analysis platforms allow you to configure rules for parsing unstructured logs to JSON so that they can be processed automatically. You can then analyze the data programmatically alongside your structured log data. Transforming logs to JSON also renders the individual log files more readable to humans, ready for when you want to drill down in more detail.  

JSON log analytics with Coralogix

By structuring your logs in JSON format you can more effectively analyze log files from multiple sources and leverage machine learning techniques to identify trends and anomalies in your data. Because JSON is easy to read, you can still view and understand individual log entries when investigating and troubleshooting issues.

As a log analysis and observability platform, Coralogix automatically extracts fields from your JSON logs so that you can filter, sort, query, and visualize according to your JSON log file viewer. With custom views, you can configure reports based on the fields you’re interested in. For unstructured logs, you can set up log parsing rules to extract values and append the JSON to the log entry, or replace the entire entry with structured JSON, according to your needs. Using the Coralogix log analytics platform you can collate and aggregate logs from multiple sources and use sophisticated data analysis tools to improve your understanding of your systems and unlock valuable insights.

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