Reliable transactions/operations tracking is one of the most common and important building blocks for complex Stellar-powered applications. Whether you want to build an anchor, payment processor, or trading tool, automatic payments tracking will be an essential part of it. I personally created two payment tracker modules for different projects. Naturally, a multipurpose customizable solution that covers all major use-cases is a next step. So, here it is...

Stellar Notifier is a standalone service that tracks Stellar Network operations and streams notifications to the subscribers. The notifications are sent as JSON-encoded HTTP POST requests. It's open source, use it at your sole discretion.

Highlights

Highly configurable.
Supports operations filtering by account, asset, transaction memo, and operation type.
Guaranteed delivery, even if the notification recipient or the Notifier itself is down for some.
Supports public and private Stellar networks.
Can be used as a shared microservice that streams events to multiple endpoints.
High performance (tested with thousands of subscriptions).
Reliable tracking and consistency (tracking is resumed from the same transaction after restart).
Does not require Stellar Core or Horizon node deployment.
Notifications are signed with an ED25519 secret key to protect the recipient from spoofing.

So... Where can I use it?

Payment processing.
Never miss a payment, even if a user decided to pay one week later, or you have some troubles with your server.
Anchors and ICOs.
Implement complex scenarios for your assets and track all user accounts.
Track all operations involving your asset to gather statistics or calculate dividends.
Trading platforms.
Observe trading activity, aggregate market data, gather statistics, build market-making applications.
Monitoring tools.
Keep an eye on specific accounts or your favorite assets. Without pooling and other inefficient techniques.
Inflation pools.
Subscribe to inflation operation and distribute rewards when the subscription was triggered.
And more...

⚠ Note: The project is at the early alpha stage. Battle-test it in your local environment before using in production.

System requirements

Stellar Horizon server

Notifier relies on an event stream from Horizon server, so you'll need to choose a public Horizon instance
(like https://horizon.stellar.org and https://horizon-testnet.stellar.org/), or you own hosted Horizon instance.
The latter option is preferable for production cases.
Nevertheless, public Horizon instances supported by SDF work just fine as well.

NodeJS 8.6.0+

The service requires NodeJS 8.6.0 or later.

(Optional) MongoDB 3.2+ or other storage provider

For the production deployment all notification queues are stored in the database.

MongoDB was selected as the most convenient option in terms of fast read/writes alongside with minimum memory/IO overhead.

Supported platforms

Linux, Windows, MacOS.

Effectively all platforms where you can run NodeJS and MongoDB.

Configuration

All configuration parameters are located in app.config.json file.

storageProvider
Storage provider for persistence layer (see details in Storage providers section).
Default value: "memory"
storageConnectionString
Connection string for storage provider.
Default value: ""
apiPort
API exposed by the notifier.
Default value: 4021
authorization
Server authorization mode.
disabled mode turns off the authentication (recommended for local installations).
token authentication expects access_token with user requests (see API Authentication).
Default value: "disabled".
horizon
Horizon server URL.
Default value: "https://horizon.stellar.org".
reactionResponseTimeout
Maximum expected HTTP response timeout (in seconds).
If the reaction URL takes more time to respond, the request is aborted and notification is marked as failed.
Default value: 10.
adminAuthenticationToken
API authentication token for admin user (for "authentication"="token" mode).
Leave empty to use a randomly generated token.
Default Value: "".
signatureSecret
Secret key used to sign the notifications. Do not forget to set your own secret.
It can be easily generated using Stellar Laboratory.
Do not use the default value or a secret key from the funded account.
Default value: "".
maxActiveSubscriptions
Maximum simultaneously tracked subscriptions.
Default value: 10000.
notificationConcurrency
Maximum concurrent notification threads (effectively, it equals the maximum parallel pending HTTP requests).
Default value: 100.

Storage providers

Notifier can store notification queues and operational information in the database or in memory.
The latter case is suited only for testing or cases without guaranteed delivery.

There are two built-in storage providers: mongodb and memory.
Other storage providers can be easily connected through the standard interface.

Config example:

{
    "authorization": "disabled",
    "storageProvider": "mongodb",
    "storageConnectionString": "mongodb://127.0.0.1:27017/stellar-notifier",
    "apiPort": 4021,
    "horizon": "https://horizon.stellar.org",
    "signatureSecret": "SDBT736EJIIRDC3RSN544NO6OSNMZAWAKRARLOMRP2XJAOGKTQLFFR3V",
    "maxActiveSubscriptions": 10000,
    "notificationConcurrency": 100,
    "reactionResponseTimeout": 10,
    "adminAuthenticationToken": "98c12910bf35c79a800e9ea893a93b078ea92fc7a26ca76c0cd2f6003464d781"
}

API

Authentication

With authorization config parameter set to disabled, there is no need to bother about authentication.
But it only works in cases where the service is not exposed to public networks (i.e. behind the firewall).

To enable token-based authentication, set authorization to token. In this mode, any interaction with protected API will require an authentication token.
The token can be passed as a POST parameter (access_token=<your_token>), GET query parameter (?access_token=<your_token>), or Authorization header (Authorization: Token <your_token>).

Create subscription

POST /api/subscription

Parameters:

reaction_url [string] (mandatory) - a full URL that should be requested by notification
account [string] (optional) - an account address to track
memo [string] (optional) - expected transaction memo
account [string] (optional) - an account address to track
asset_code [string] (optional) - asset code to track (specify XLM and no issuer to track operations involving XLM)
asset_issuer [string] (optional) - asset issuer to track
operation_types [Array<Number>] (optional) - operation types to track

Note: at least one operation filtering criteria should be specified.

Response example:

{
  "id": "5ae88ef89a9e5d4a589cf27d",
  "user": "5ae87f6bc6073d3e6c9c7d1d",
  "status": 0,
  "delivery_failures": 0,
  "sent": 8,
  "reaction_url": "http://localhost:4022/test/reaction",
  "operation_types": [
    0,
    1
  ],
  "created": "2018-05-01T15:59:52.475Z",
  "updated": "2018-05-01T16:01:19.742Z"
}

The id value from response can be used to delete the subscription.

Remove subscription

DELETE /api/subscription/:subscription_id

The endpoint returns 200 in case of successful unsubscription, and an error code otherwise.

Parameters:

subscription_id [string] (mandatory) - the subscription id to remove

Get all active user subscriptions

GET /api/subscription

Response example:

[
  {
    "id": "5ae88ef89a9e5d4a589cf27d",
    "user": "5ae87f6bc6073d3e6c9c7d1d",
    "status": 0,
    "delivery_failures": 0,
    "sent": 8,
    "reaction_url": "http://localhost:4022/test/reaction",
    "operation_types": [
      0,
      1
    ],
    "created": "2018-05-01T15:59:52.475Z",
    "updated": "2018-05-01T16:01:19.742Z"
  },
  {
    "id": "5ae88f54397cbf2474ef27e1",
    "user": "5ae87f6bc6073d3e6c9c7d1d",
    "status": 0,
    "operation_types": [],
    "delivery_failures": 4,
    "sent": 0,
    "reaction_url": "http://localhost:4022/test/reaction2",
    "asset_type": 0,
    "created": "2018-05-01T16:01:24.336Z",
    "updated": "2018-05-01T16:02:01.036Z"
  }
]

Notifications Format

Notifications are sent as JSON-encoded POST requests with the following format:

{
  "id": "5ae88ef89a9e5d4a589cf27d-757838947790641a6",
  "subscription": "5ae88ef89a9e5d4a589cf29d",
  "type": "operation",
  "created": "2018-05-01T16:45:37.529Z",
  "sent": "2018-05-01T16:45:37.991Z",
  "operation": {
    "id": "7578389477906432555",
    "type_i": 1,
    "type": "payment",
    "destination": "GC3UA6FHLDD7IFG3MXA7JCNC2YPW62AKMKV5H2CEEBWCLUNJR3OFJSNV",
    "asset": {
      "asset_code": "ETH",
      "asset_issuer": "GCNSGHUCG5VMGLT5RIYYZSO7VQULQKAJ62QA33DBC5PPBSO57LFWVV6P",
      "asset_type": "1"
    },
    "amount": "0.0012",
    "account": "GCUZSLQYXDXSOXC4DURSK3AJNAQEBDSYVGYE7BC7IXGTS32MJBW35QOU"
  },
  "transaction": {
    "hash": "7580614b4703704496173f578092a0b6a56e5906d7ad5f83e529af14bab2cdd3",
    "fee": 600,
    "source": "GCUZSLQYXDXSOXC4DURSK3AJNAQEBDSYVGYE7BC7IXGTS32MJBW35QOU",
    "paging_token": "757838947790643264",
    "source_account_sequence": "7196037745321120376",
    "created_at": "2018-05-01T16:45:37Z",
    "memo": { 
      "type": "text", 
      "value": "105683157"
    }
  }
}

Request headers:

X-Request-ED25519-Signature – the ED25519 signature for the current notification body.
X-Subscription – corresponding subscription id.

Roadmap and Further Improvements

The following features will be added in the nearest future:

Response validation for the flooding attacks prevention.
Implement other notification channels: email, messengers, etc.
Include not only transaction info, but also all operation effects.
Implement a multi-account support and basic acquiring to turn the service into a SAAS platform, so anyone could offer public SAAS notification services for some fee.
Add the ability to use environment variables instead of the configuration file.
Comprehensive comments and test suites.
Bulletproof errors processing and graceful SIGTERM handling.

Have a suggestion? Want to submit a bug or feature request? Visit the issue tracker.

Special Thanks and Acknowledgements

Many thanks to @MikeFair for his prominent ideas.
Kudos to Stellar Developer Foundation for the state-of-the-art software and contemporary approach.
And, as always, thanks to awesome Stellar Community for the support.