Logging Made Easy: Exploring Apple’s OSLogs

Evangelist Apps
Evangelist Apps Blog
4 min readNov 16


Photo by Ilya Pavlov on Unsplash


Logging is a critical aspect of software development, aiding developers in understanding the behavior of their applications. Apple’s OSLogs, introduced with iOS 14, represent a significant evolution in logging for macOS, iOS, watchOS, and tvOS. In this article, we’ll explore the ins and outs of OSLogs, understanding their architecture, benefits, and practical usage.

Understanding OSLogs Architecture

Apple’s OSLogs are part of the Unified Logging System (ULS), a system-wide logging infrastructure designed to streamline and centralize log messages across Apple platforms. ULS aggregates logs from various subsystems, including the kernel, system daemons, and user applications, providing a unified and comprehensive view of the system’s activity.

Key Benefits of OSLogs

1. Unified Logging System:
OSLogs are integral to the Unified Logging System, allowing developers to access a unified stream of log messages from different sources, leading to a more coherent and holistic understanding of system behavior.

2. Low-Overhead Logging:
OSLogs are engineered to be low-overhead, ensuring that logging operations have minimal impact on the performance of the system and applications. This makes them suitable for production environments and release builds.

3. Privacy and Security:
OSLogs introduce different log levels, such as Default, Info, Debug, Error, and Fault, enabling developers to log sensitive information at an appropriate level. This ensures that certain log entries are only visible when explicitly requested.

Getting Started with OSLogs

Let’s dive into the essential steps to start utilizing OSLogs within your Apple applications:

  1. Import the os Module:
    Begin by importing the os module in your Swift or Objective-C code.
import os

2. Create an OSLog Object:
Instantiate an OSLog object with a subsystem identifier, typically a reverse DNS-style string unique to your app or framework.

let log = OSLog(subsystem: "com.yourcompany.yourapp", category: "general")

3. Logging Messages:
Utilize various logging functions provided by OSLog to log messages at different levels.

os_log("This is a default log message", log: log)
os_log("An error occurred: %@", log: log, type: .error, error.localizedDescription)

Advanced OSLogs Usage

Log Levels and Severity:

Log levels in OSLogs represent the severity or importance of a log message. They help developers categorize and prioritize log entries based on their significance. In Apple’s Unified Logging System (ULS), which includes OSLogs, there are several standard log levels. Here’s a brief overview:

  1. Default:

The default log level is used for general messages and is typically informational in nature.


os_log("Application started successfully", log: log)

2. Info:

Info level is used for more detailed information that might be useful for tracking application flow.


os_log("Processing data: %@", log: log, type: .info, data)

3. Debug:

Debug level is used for messages that are helpful during development and debugging but may be too detailed for production.


os_log("Debugging information: %@", log: log, type: .debug, debugInfo)

4. Error:

Error level indicates that an error has occurred but may not be critical to the application’s operation.


os_log("Error occurred: %@", log: log, type: .error, error.localizedDescription)

5. Fault:

Fault level is used for critical errors that require immediate attention. These are severe issues that might lead to application instability.


os_log("Critical fault: %@", log: log, type: .fault, criticalError.localizedDescription)

We can choose the appropriate log level based on the nature and severity of the log message. This flexibility allows for effective logging practices, especially in different environments such as development, testing, and production. It also aids in dynamic log filtering, enabling users to adjust the level of logging based on their needs.

Dynamic Log Filtering:

The Unified Logging System supports dynamic log filtering, allowing users to adjust the level of logging for specific subsystems and categories. This feature aids in fine-tuning the amount of log data collected, especially in production environments.

Custom Log Metadata:

Enhance log entries with custom metadata, making it easier to trace the context of log messages during debugging and analysis.

os_log("Processing request", log: log, type: .debug, metadata: .dictionary(["RequestID": requestID]))

Best Practices for OSLogs

1. Descriptive Subsystems and Categories:
Choose meaningful subsystem and category identifiers to make it easier to identify and filter log entries related to your application.

2. Mindful Handling of Sensitive Data:
Exercise caution when logging sensitive information. Leverage different log levels to control the visibility of such data and avoid exposing sensitive information in production builds.

3. Performance Considerations:
Balance the amount of detail in your logs with the impact on performance. Aim for a comprehensive logging strategy without compromising the efficiency of your application.


Apple’s OSLogs, as part of the Unified Logging System, offer a sophisticated and efficient logging solution for developers across the Apple ecosystem. By embracing OSLogs, developers can gain valuable insights into the behaviour of their applications, enhance debugging processes, and maintain a balance between log detail and system performance. As you integrate OSLogs into your development workflow, you empower yourself to build more robust and maintainable Apple applications.

Hope you enjoyed reading this article.




Evangelist Apps
Evangelist Apps Blog

Evangelist Software is UK based mobile apps development agency that specializes in developing and testing iOS apps.