@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
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: '22.13.0',
        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.