CLI output in plugins

Warning: the API documented on this page is available in Serverless Framework v3, which is not released yet. This documentation is published to help plugins upgrade to v3.

Plugins can integrate and extend the CLI output of the Serverless Framework in different ways.

Writing to the output

In Serverless Framework v2, plugins could write to the CLI output via serverless.cli.log():

// This approach is deprecated:
serverless.cli.log('Message');

The method above is deprecated. It should no longer be used in Serverless Framework v3.

Instead, plugins can log messages to the CLI output via a standard log interface:

class MyPlugin {
  constructor(serverless, cliOptions, { log }) {
    log.error('Error');
    log.warning('Warning');
    log.notice('Message');
    log.info('Verbose message'); // --verbose log
    log.debug('Debug message'); // --debug log
  }
}

Some aliases exist to make log levels more explicit:

log('Here is a message');
// is an alias to:
log.notice('Here is a message');

log.verbose('Here is a verbose message'); // displayed with --verbose
// is an alias to:
log.info('Here is a verbose message'); // displayed with --verbose

To write a formatted "success" message, use the following helper:

log.success('The task executed with success');

Log methods also support the printf format:

log.warning('Here is a %s log', 'formatted');

Best practices:

  • Keep the default CLI output minimal.
  • Log most information to the --verbose output.
  • Warnings should be used exceptionally. Consider whether the plugin should instead throw an exception, log a --verbose message or trigger a deprecation (see below).
  • Before using log.error(), consider throwing an exception: exceptions are automatically caught by the Serverless Framework and formatted with details.
  • Debugging logs should be logged to the --debug level. Debug logs can be namespaced following the debug convention via log.get('my-namespace').debug('Debug message'). Such logs can then be filtered in the CLI output via --debug=plugin-name:my-namespace.

By default, logs are written to stderr, which displays in terminals (humans cannot tell the difference). This is intentional: plugins can safely log extra messages to any command, even commands meant to be piped or parsed by another program. Read the next section to learn more.

Writing command output to stdout

By default, plugins should write messages to stderr via the log object. To write command output to stdout instead, use writeText():

class MyPlugin {
  constructor(serverless, cliOptions, { writeText }) {
    writeText('Command output');
    writeText(['Here is a', 'multi-line output']);
  }
}

Best practices:

  • stdout output is usually meant to be piped to/parsed by another program.
  • Plugins should only write to stdout in commands they define (to avoid breaking the output of other commands).
  • The only content written to stdout should be the main output of the command.

Take, for example, the serverless invoke command:

  • Its output is the result of the Lambda invocation: by writing that result (and only that) to stdout, it allows any script to parse the result of the Lambda invocation.
  • All other messages should be written to stderr: such logs are useful to humans, for example configuration warnings, upgrade notifications, Lambda logs… Since they are written to stderr, they do not break the parsable output of stdout.

If unsure, write to stderr (with the log object) instead of stdout. Why: human users will not see any difference, but the door will stay open to write a parsable output later in the future.

Colors and formatting

To format and color text output, use the chalk package. For example:

log.notice(chalk.gray('Here is a message'));

Best practices:

  • Write primary information in white, secondary information in gray.
    • Primary information is the direct outcome of a command (e.g. deployment result of the deploy command, or result of the invoke command). Secondary information is everything else.
  • Plugins should generally not use any other color, nor introduce any other custom formatting. Output formatting is meant to be minimalistic.
  • Plugins should use built-in formats documented in this page: success messages (log.success()), interactive progress…

The "Serverless red" color (#fd5750) is used to grab the user's attention:

  • It should be used minimally, and maximum once per command.
  • It should be used only to grab attention to the command's most important information.

Errors

The Serverless Framework differentiates between 2 errors:

  • user errors (wrong input, invalid configuration, etc.)
  • programmer errors (aka bugs)

To throw a user error and have it properly formatted, use Serverless' error class:

throw new serverless.classes.Error('Invalid configuration in X');

All other errors are considered programmer errors by default (and are properly formatted in the CLI output as well).

Best practices:

  • If an error should stop the execution of the command, use throw.
  • If an error should not stop the execution of the command (which should be exceptional), log it via log.error().
    • For example any execution error in serverless-offline should not stop the local server.

Interactive progress

Plugins can create an interactive progress:

class MyPlugin {
  constructor(serverless, cliOptions, { progress }) {
    const myProgress = progress.create({
      message: 'Doing extra work in my-plugin',
    });
    // ...
    myProgress.update('Almost finished');
    // ...
    myProgress.remove();
  }
}

In case of parallel processing (for example compiling multiple files in parallel), it is possible to create multiple progress items if that is useful to users.

Best practices:

  • Create a progress for tasks that usually take more than 2 seconds. Below that threshold, plugins can operate silently and log to --verbose only.
  • Users should know which plugin is working from the progress message:
    • Bad: "Compiling"
    • Bad: "[Webpack] Compiling" (avoid prefixes)
    • Good: "Compiling with webpack"
  • Displaying multiple progresses should be exceptional, and limited to 3-4 progresses at a time. It is better to keep the output minimal than too noisy.

Note that it is possible to give a unique name to a progress. That name can be used to retrieve the progress without having to pass the instance around:

// Progress without any name:
const myProgress = progress.create({
  message: 'Doing extra work in my-plugin',
});

// Progress with a unique name
progress.create({
  message: 'Doing extra work in my-plugin',
  name: 'my-plugin-progress', // Try to make the name unique across all plugins
});
// elsewhere...
progress.get('my-plugin-progress').update('Almost finished');
// elsewhere...
progress.get('my-plugin-progress').remove();

Service information

Plugins can add their own sections to the "Service information", i.e. the information displayed after serverless deploy or in serverless info.

To add a single item:

serverless.addServiceOutputSection('my section', 'content');

The example above will be displayed as:

$ serverless info
functions:
  ...
my section: content

To add a multi-line section:

serverless.addServiceOutputSection('my section', ['line 1', 'line 2']);

The example above will be displayed as:

$ serverless info
functions:
  ...
my section:
  line 1
  line 2

Deprecations

Plugins can signal deprecated features to users via logDeprecation():

serverless.logDeprecation(
  'DEPRECATION_CODE',
  'Feature X of my-plugin is deprecated. Please use Y instead.'
);

These deprecations will integrate with the deprecation system of the Serverless Framework.

Best practices:

  • Prefix the deprecation code with the plugin name, for example: OFFLINE_XXX.
  • Make the message actionable for users: if a feature is deprecated, what should users use instead? Feel free to add links if necessary.

Retrieving the I/O API

As shown in the examples above, the I/O API is injected in the constructor of plugins:

class MyPlugin {
  constructor(serverless, cliOptions, { writeText, log, progress }) {
    // ...
  }
}

However, it is also possible to retrieve it from any JavaScript file by requiring the @serverless/utils package:

const { writeText, log, progress } = require('@serverless/utils/log');
GitHub
Go to Github