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API written in node.js to facilitate publish/subscribe communication in Winding Tree
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WT Notification API

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API written in node.js to facilitate publish / subscribe communication within the WT platform.


We expect that some of the data that is available through the WT platform will be very short-lived in nature. This includes availability data but it might also include other information, such as price data. The crucial question is: how can a WT data consumer keep track of all the changes without resorting to polling all resources all the time?

The solution lies in a publish / subscribe mechanism that works as follows:

  1. There is an API specification for update notification publication / subscription together with a reference implementation.
  2. Any data structure (hotel, airline) within WT contains an optional reference to an instance of this service. It is assumed that update notifications will be pushed there actively by the actors representing the hotel/airline. (If wt-write-api is used for data publication, things should work out of the box.)
  3. Data consumers can subscribe to update notifications and receive them via webhooks.

This solution is decentralized in nature as it allows multiple independent publish / subscribe channel providers to coexist and be easily discovered via the WT index.

This repository contains both the API specification (in docs/swagger.yml) as well as the reference implementation.


  • Nodejs 10.x


In order to install and run tests, we must:

git clone
nvm install
npm install
npm test

Running in dev mode

With all the dependencies installed, you can start the dev server.

First step is to initialize the SQLite database used to store subscriptions. If you want to use a different database, feel free to change the connection settings in the appropriate configuration file in src/config/.

npm run createdb-dev

If you'd like to start afresh later, just delete the .dev.sqlite file.

Running this server


You can run the whole API in a docker container, and you can control which config will be used by passing an appropriate value to WT_CONFIG variable at runtime. Database will be setup during the container startup in the current setup. You can skip this with SKIP_DB_SETUP environment variable.

$ docker build -t windingtree/wt-notification-api .
$ docker run -p 8080:8080 -e WT_CONFIG=playground windingtree/wt-notification-api

After that you can access the wt-notification-api on local port 8080


You can install and run this from NPM as well:

$ npm install -g @windingtree/wt-notification-api
$ WT_CONFIG=playground wt-notification-api

This will also create a local SQLite instance in the directory where you run the wt-notification-api command. To prevent that, you can suppress DB creation with SKIP_DB_SETUP environment variable.

Running in production

You can customize the behaviour of the instance by many environment variables which get applied if you run the API with WT_CONFIG=envvar. These are:

  • WT_CONFIG - Which config will be used. Defaults to dev.
  • PORT - HTTP Port where the API will lsiten, defaults to 8080.
  • BASE_URL - Base URL of this API instance, for example
  • DB_CLIENT - Knex database client name, for example sqlite3.
  • DB_CLIENT_OPTIONS - Knex database client options as JSON string, for example {"filename": "./envvar.sqlite"}.
  • LOG_LEVEL - Log level, defaults to info.
  • SKIP_DB_SETUP - Whether to not setup new database upon startup.

We recommend to use a more robust database than sqlite3 for any serious deployment.

How tos

Publishing notifications

Currently, no authentication is needed when publishing update notifications.

When sending an update notification, you should specify:

  • WT Index address the update pertains to.
  • Type of WT resource ("hotel" or "airline").
  • WT Resource address (e.g. hotel/airline address).
  • Scope (what has changed).

The purpose of scope is twofold:

  1. Allow consumers to subscribe only to a subset of updates (e.g. updates of prices).
  2. Enable consumers to keep track of which remote resources can be kept in local cache. For example, if updates keep coming without the dataIndex subject, the consumer knows the old URL of, say, ratePlans is still valid and can be simply fetched again to get the newest data.


$ curl -X POST localhost:8080/notifications -H 'Content-Type: application/json' -d '
    "wtIndex": "0x3b476ac17ffea8dcf2dbd5ef787a5baeeebe9984",
    "resourceType": "hotel",
    "resourceAddress": "0x6a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a",
    "scope": {
        "action": "update",
        "subjects": ["dataIndex", "ratePlans"]

Consuming notifications

If you want to consume notifications, you need to prepare a publicly accessible http(s) endpoint where you can accept the notifications. The notifications will come as json-encoded data via POST HTTP requests. Notification data will be unchanged (i.e. what the publisher sends will be broadcast to consumers.) Make sure the endpoint responds to notifications with HTTP status 200 and the response body is the text notification accepted.

Once you have this endpoint ready, you can subscribe for notifications of interest:

$ curl -X POST localhost:8080/subscriptions -H 'Content-Type: application/json' -d '
    "wtIndex": "0x3b476ac17ffea8dcf2dbd5ef787a5baeeebe9984",
    "resourceType": "hotel",
    "resourceAddress": "0x6a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a",
    "scope": {
        "action": "update",
        "subjects": ["dataIndex", "ratePlans"]
    "url": ""

# ID of the created subscription will be returned.
{"subscriptionId": "63ccc93d66321f37a7203a26567fd1b0"}

resourceAddress as well as scope are optional. If you do not specify them, all notifications that fulfill the remaining criteria will be broadcast to you.

If possible, consider using unique webhook URLs for individual subscriptions to make eventual cancelling of selected subscriptions easier in case you lose / do not store subscription IDs (see the next section for more information).

Cancelling a subscription

If you want to cancel a subscription, you have two possibilities:

  1. Stop sending the notification accepted response from the endpoint. (To prevent abuse of our service, subscription is deactivated as soon as the recipient stops replying with confirmations.)
  2. Actively unsubscribe like this (using the correct subscription ID):
$ curl -X DELETE localhost:8080/subscriptions/63ccc93d66321f37a7203a26567fd1b0

Validating subscription status

Sometimes you might need to validate what the status of your subscription is, for instance when you are not sure whether the subscription has been cancelled according to rule 1 in the previous section or not. This is how you can retrieve the data related to your subscription:

$ curl localhost:8080/subscriptions/63ccc93d66321f37a7203a26567fd1b0 | python -m json.tool
# JSON representation of the subscription will be returned:
    "id": "63ccc93d66321f37a7203a26567fd1b0",
    "active": true,
    "wtIndex": "0x3b476ac17ffea8dcf2dbd5ef787a5baeeebe9984",
    "resourceType": "hotel",
    "scope": {
        "action": "update",
        "subjects": ["dataIndex", "ratePlans"]
    "url": ""

Note the active attribute denoting the subscription status.

Publicly available instances

For currently available public instances of wt-notification-api, please see this page.

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