Specification: Ballerina Log Library

Owners: @daneshk @MadhukaHarith92
Reviewers: @daneshk
Created: 2021/11/15
Updated: 2022/02/08
Edition: Swan Lake


This is the specification for the Log standard library of Ballerina language, which provides APIs to log information when running applications.

The Log library specification has evolved and may continue to evolve in the future. The released versions of the specification can be found under the relevant Github tag.

If you have any feedback or suggestions about the library, start a discussion via a GitHub issue or in the Discord server. Based on the outcome of the discussion, the specification and implementation can be updated. Community feedback is always welcome. Any accepted proposal, which affects the specification is stored under /docs/proposals. Proposals under discussion can be found with the label type/proposal in GitHub.

The conforming implementation of the specification is released and included in the distribution. Any deviation from the specification is considered a bug.


  1. Overview
  2. Logging
  3. Configure Logging
  4. Writing Logs to a File

1. Overview

This specification elaborates on the functionalities available in the Log library. The Ballerina log module has four log levels with their priority in descending order as follows.


2. Logging

The Ballerina log module has 4 functions to log at the 4 levels; printDebug(), printError(), printInfo(), and printWarn().

log:printDebug("debug log");
log:printError("error log");
log:printInfo("info log");
log:printWarn("warn log");

Optionally, an error can be passed to the functions.

error e = error("something went wrong!");
log:printError("error log with cause", 'error = e);

This will print the error message. In order to print the complete error stack traces, users need to pass the error stack trace as a key-value pair which will be discussed in the next section.

Users can pass any number of key/value pairs, which need to be displayed in the log message. The value can be of anydata type, a function pointer or an error stack trace.

log:printInfo("info log", id = 845315, name = "foo", successful = true);
log:printInfo("info log", current_time = isolated function() returns string { return time:utcToString(time:utcNow());});
error e = error("bad sad");
log:printError("error log", stackTrace = e.stackTrace());

3. Configure Logging

Only the INFO and higher level logs are logged by default. The log level can be configured via a Ballerina configuration file. To set the global log level to a different level (eg: DEBUG), place the entry given below in the Config.toml file.

level = "DEBUG"

Each module can also be assigned its own log level. To assign a log level to a module, provide the following entry in the Config.toml file.

level = "[LOG_LEVEL]"

By default, log messages are logged to the console in the LogFmt format. To set the output format to JSON, place the entry given below in the Config.toml file.

format = "json"

4. Writing Logs to a File

By default, logs are printed to the console. Users can set the output to a log file by providing an output file (with .log extension) to log:setOutputFile() function. Note that all the subsequent logs of the entire application will be written to this file.

var result = log:setOutputFile("./resources/myfile.log");

There are 2 options when writing to a file.

OVERWRITE - Truncate the existing content
APPEND - Append to the existing content

The following overwrites an existing log file.

var result = log:setOutputFile("./resources/myfile.log", log:OVERWRITE);