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EventGate takes in JSON events via HTTP POST, validates and then produces them to a pluggable destination (usually to Kafka). Valid events pass through the gate, invalid ones do not.

Throughout this codebase, an 'event' is a parsed JSON object with a strict JSONSchema, and a 'schema' refers to an instance of a JSONSchema for an event.


For configuration of the default EventGate implementation (see below), edit config.yaml and run npm start. The service will listen for HTTP POST requests with Content-Type set to application/json at /v1/events. The POST body should be an array of JSON event objects. (The default EventGate uses Kafka, so you must have a running Kafka instance to produce to.)


The EventGate service is a generic HTTP POST JSON event intake, event schema validation and event producing service. The schema validation and event produce implementation is left up to the user. This service ships with a schema URL based validator and Kafka produce implementation (using node-rdkafka), but you can plug in your own by implementing a factory module that returns an instantiated EventGate and use it via eventgate_factory_module application config. See documentation below for more on the default Kafka EventGate.

EventGate implementation configuration

The EventGate class is generic enough to be used with any type of validation and event production via function injection. The HTTP route that handles POSTing of events needs to have an instantiated EventGate instance. To make this configurable (without editing the route code), the route in routes/events.js will look for app.conf.eventgate_factory_module and require it. This module is expected to export a function named factory that takes a conf options object, a bunyan logger, and an optional metrics object that conforms to the node-statsd interface (This will actually be provided from service-runner app metrics, and the Express router used to handle the events (in case your EventGate implementation wants to add any extra routes at this time). E.g.

function myEventGateFactory(options, logger, metrics, router) { ... }

The factory function should return a Promise of an instantiated EventGate.

Once the EventGate Promise resolves, the /v1/events HTTP route will be added and will use the instantiated EventGate to validate and produce incoming events.

EventGate class

The EventGate class in lib/eventgate.js handles event validation and produce logic. It is instantiated with validate and a produce functions that each take a single event and extra context object. validate should either return a Promise of the validated event (possibly augmented, e.g. field defaults populated) or throw a ValidationError. produce is expected to return a Promise of the produce result, or throw an Error.

Once instantiated with these injected functions, an EventGate instance can be used by calling the process function with an array of events and any extra context. process will catch both validation and produce errors, and will return an object with errors for each event grouped by the type of error. E.g. if you passed in 3 events, and 1 was valid, 1 was invalid, and another encountered an unknown error, process will return an object like:

    'success': [{ ... }],
    'invalid': [{ ... }],
    'error':   [{ ... }]

with each array containing an EventStatus instance representing the event's process result.

Throughout the code, functions that are injected into to constructors are expected to take a single event object and a context object. E.g. validate(event, context), produce(event, context), etc. These functions should be able to do their job with only these arguments. Any additional logic or context should be bound into the function, either by partially applying or creating a closure. Example:

If you wanted to write all incoming events to a file based on some field in the event, you could write an EventGate produce function like this:

const _ require('lodash');
const writeFileAsync = promisify(fs.writeFile);
 * @return a function that writes event to the file at event[file_path_field]
function makeProduceFunction(options) {
    const file_path_field = options.file_path_field;
    return (event, context = {}) => {
        const path = _.get(event, file_path_field);
        return writeFileAsync(path, JSON.stringify(event));

// Instantiate a new EventGate using this configured produce function closure:
const eventGate = new EventGate({
    // ...,
    const options = {
        file_path_field: 'file_path'
    produce: makeProduceFunction(options);
    // ...,

// This eventGate will produce every event by calling the function returned by makeProduceFunction.

Error events

The EventGate constructor also takes an optional mapToErrorEvent function. mapToErrorEvent takes an event and an error and returns a new error event representing the error. This new error event should be suitable for producing through the same instantiated EventGate. If mapToErrorEvent is provided and event processing fails for any reason, those errors will be converted to event errors via this function, and then produced. There should be no difference between the kind of events that mapToErrorEvent returns and the kind of events that your instantiated EventGate can handle. eventGate.process([errorEvents]) should work. If your mapToErrorEvent implementation returns null for any given failed event, no error event for that error will be produced. This allows mapToErrorEvent implementations to decide what types of Errors should be produced.

Default EventGate - Schema URI validation & producing with Kafka

If eventgate_factory_module is not specified, this service will use provided configuration to instantiate and use an EventGate that validates events with JSONSchemas discovered via schema URLs. Depending on configuration, the default EventGate can write events to a file, and/or produce them to Kafka.

EventValidator class & event schema URLs

While the EventGate class that handles event validation and production can be configured to do validation in any way, the default EventGate uses the EventValidator class to validate events with schemas obtained from schema URIs in the events themselves. Every event should contain a URI to the JSONSchema for the event. EventValidator will extract and use those URIs to look up (and cache) those schemas to use for event validation. The EventValidator instance used by the default EventGate can request schema URIs from the local filesystem with file:// or remote ones via http:// based URIs. The field in the each event where the schema URI is located is configured by the schema_uri_field config, which defaults to $schema. ($schema is a standard field used by JSONSchemas to locate their metaschemas, so it makes sense to store an event's schema in this field as well.) When an event is received, the schema URI will be extracted from the schema_uri_field. The JSONSchema at the URI will be downloaded and used to validate the event. The extracted schema URI can optionally be prefixed with schema_base_uri and suffixed with schema_file_extension. Setting a schema_base_uri will allow set hostname relative URIs, e.g. '/path/to/event-schema/1.0.0' in the events, while configuring the base location of your schema repositories, e.g. 'http://my.schema-repo.org/schemas' If you use the defaults, all of your events should have a $schema field set to a resolvable schema URI.

EventValidator always supports draft-07 JSONSchemas (via AJV). It also supports additional JSONSchemas, defaulting to also supporting draft-04. If you need to modify or change this, you can override the metaSchemas array option to the EventValidator constructor.

By default, EventValidator will validate all (non-meta) schemas with AJV's json-schema-secure schema. This prevents schemas from including potentially risky features that could facilitate DOS attacks. See: https://github.com/epoberezkin/ajv#security-considerations To allow insecure schemas, you can set allowInsecureSchemas: true in the EventValidator constructor options.


A 'stream' here refers to the destination name of an event. It is closely related to Kafka's concept of a topic. Much of the time a stream might correspond 1:1 with a Kafka topic. If you don't care about the topic name that is used for a any given event, you don't need to configure a stream_field. The default behavior is to sanitize an event's schema URI and use it for the Kafka topic name. E.g. if an event's schema URI is /ui/element/button-push, the topic name will end up being ui_element_button-push. However, if stream_field is configured and present in an event, its value will be used as the destination Kafka topic of that event. If you need finer control over event -> Kafka topic mapping, you should implement your own Kafka produce function (see eventgate-wikimedia)
for an example.)


Configuration is passed to the service via the config.yaml file, which has a services object. This object contains a service named eventgate. The conf object of this service will be passed to the eventgate_factory_module. To use a custom EventGate implementation, set eventgate_factory_module to your javascript module that exports a factory function that instantiate an EventGate with options. See the section above entitled 'EventGate implementation configuration'.

Default EventGate configuration

The following values in conf will be used to instantiate a a default EventGate that extracts JSONSchemas from schema URIs, validates events using those schemas, and then produces them to an output file and or Kafka. If kafka.conf is set, than Kafka will be used. Since node-rdkafka-factory is an optional dependency, please make sure it (and node-rdkafka) is properly installed if you use this.

All *_field configs point to a field in an event, and use dotted path notation to access sub-objects. E.g. "value" will be extracted from { meta: { stream: "value" } } if stream_field is set to 'meta.stream'.

Property Default Description
port 6927 port on which the service will listen
interface localhost hostname on which to listen
user_agent eventgate The UserAgent seen when making remote http requests (e.g. for remote schemas)
schema_uri_field $schema The value extracted from this field will be used (with schema_base_uris and schema_file_extension) to download the event's JSONSchema for validation.
schema_base_uris undefined If given, a relative schema URI will be prepended with each of these base URIs to build schema URLs. The resulting URLs will each be requested, and the first existent schema found at that URL will be used. This allows you to configure multiple schema repositories/registries where your schema might be located. E.g. you could use this if you wanted to have some schemas locally for reliability, but remote for resolvability.
schema_file_extension undefined If given, this will be appended to every extracted schema URI unless the filename in the URI already has an extension.
stream_field undefined The name of the stream this event belongs to. If not set, schema_uri_field will be used (and sanitized) instead.
output_path stdout Path to file to write valid events to, or 'stdout'. If undefined, events will not be output to a file.
kafka.conf undefined node-rdkafka / librdkafka configuration. This will be passed directly to the node-rdkafka kafka.Producer constructor. Make sure you set kafka.conf['metadata.broker.list']. If undefined, events will not be produced to Kafka.
kafka.topic_conf undefined node-rdkafka (and librdkafka) topic specific configuration. This will be passed directly to the node-rdkafka kafka.Producer constructor.

eventgate-wikimedia implementation and use as a dependency

The Wikimedia Foundation runs this EventGate service as a dependency of eventgate-wikimedia. WMF implements a custom EventGate factory in eventgate-wikimedia.js.

If you are using EventGate as an npm dependency in a custom implementation repository, you will need to configure service-runner in your config.yaml file to run the EventGate express app. Set:

  - name: EventGate-mycustom-implementation
    # Load the eventgate module which is installed as an npm dependency
    module: eventgate
    # Make service-runner start running via tha exported app function.
    entrypoint: app
    # ...

/v1/_test/events route

If you are using EventGate as a service, and if test_events is configured, a GET /v1/_test/events route will be added. When requested, the test_events will be produced as if they were POSTed to /v1/events. This is useful for readiness probes that want to make sure the service can produce events end to end.


This service is based on Wikimedia's service-template-node. It is a fork of that 'template' repository. See also the ServiceTemplateNode documentation.

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