middleware-log-errors

@dotcom-reliability-kit/middleware-log-errors

Express middleware to consistently log errors. This module is part of FT.com Reliability Kit.

• Usage
  • createErrorLogger
  • Configuration options
    • options.filter
    • options.includeHeaders
    • options.logger
    • options.logUserErrorsAsWarnings
• Migrating
• Contributing
• License

Usage

Install @dotcom-reliability-kit/middleware-log-errors as a dependency:

npm install --save @dotcom-reliability-kit/middleware-log-errors

Include in your code:

import createErrorLogger from '@dotcom-reliability-kit/middleware-log-errors';
// or
const createErrorLogger = require('@dotcom-reliability-kit/middleware-log-errors');

createErrorLogger

The createErrorLogger function can be used to generate Express middleware which logs errors to the console and Splunk via Reliability Kit logger.

Caution

This middleware must be added to your Express app after all your application routes – you won't get error logs for any routes which are mounted after this middleware.

const app = express();
// App routes go here
app.use(createErrorLogger());

This will automatically serialize error objects and log them along with a serialized HTTP request which lead to the error being thrown. The information logged looks like this:

{
    event: 'HANDLED_ERROR',
    message: 'Error: something went wrong',

    error: {
        code: 'EXAMPLE_CODE',
        message: 'Something went wrong'
        // etc. (see `@dotcom-reliability-kit/serialize-error` linked above
        // for information about the logged properties
    },

    request: {
        id: 'abc123',
        method: 'GET',
        url: '/'
        // etc. (see `dotcom-reliability-kit/serialize-request` linked above
        // for information about the logged properties)
    },

    app: {
        commit: '137da65185397a7d699ed54c3052d10d83e82137',
        name: 'example-app',
        nodeVersion: '18.17.0',
        region: 'EU',
        releaseDate: '2022-07-25T01:37:00Z'
    }
}

Configuration options

Config options can be passed into the createErrorLogger function as an object with any of the keys below.

app.use(createErrorLogger({
    // Config options go here
}));

options.filter

A function used to determine whether a particular error or request should be logged. This must be a Function which returns a Boolean and accepts both an error object and an Express Request object:

type ErrorLoggingFilter = (error: any, request: express.Request) => boolean;

If the function returns true then the error and request details will be logged. Otherwise no logs will be output.

Warning

This option can be dangerous, misconfiguring it can result in a loss of log information. Consider whether you definitely need to filter logs before using, sometimes it's better to have a few too many logs than miss an important one.

Example of usage:

app.use(createErrorLogger({
    filter: (error, request) => {
        if (request.url === '/deliberate-erroring-endpoint') {
            return false;
        }
        if (error?.code === 'ERROR_WE_DO_NOT_CARE_ABOUT') {
            return false;
        }
        return true;
    }
}));

options.includeHeaders

An array of request headers to include in the serialized request object. This must be an Array of Strings, with each string being a header name. It's important that you do not include headers which include personally-identifiable-information, API keys, or other privileged information. This option gets passed directly into dotcom-reliability-kit/serialize-request which has further documentation.

This option defaults to:

[
    'accept',
    'accept-encoding',
    'accept-language',
    'content-type',
    'referer',
    'user-agent'
]

Example of usage:

app.use(createErrorLogger({
    includeHeaders: [
        'accept',
        'content-length',
        'content-type',
        'user-agent'
    ]
}));

The default set of headers is also available to use, so that you don't need to repeat them if you want to add new included headers. You'll need to import @dotcom-reliability-kit/serialize-request, then these headers are available:

const { DEFAULT_INCLUDED_HEADERS } = require('@dotcom-reliability-kit/serialize-request');

app.use(createErrorLogger({
    includeHeaders: [
        ...DEFAULT_INCLUDED_HEADERS,
        'my-custom-header'
    ]
}));

Note

There's no need to include the x-request-id header in this array, as this is automatically included as request.id in the logs.

options.logger

A logger object which implements two methods, error and warn, which have the following permissive signature:

type LogMethod = (...logData: any) => any;

This is passed directly onto the relevant log-error method, see the documentation for that package for more details.

options.logUserErrorsAsWarnings

A boolean indicating whether to log user errors (those with a 400–499 status property) with a level of warn rather than error. This helps to reduce the amount of error-level logs that you need to focus on.

This option defaults to false.

Migrating

Consult the Migration Guide if you're trying to migrate to a later major version of this package.

Contributing

See the central contributing guide for Reliability Kit.

License

Licensed under the MIT license.
Copyright © 2022, The Financial Times Ltd.