SocketShark

SocketShark is a WebSocket message router based on Python/Redis/asyncio.

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Summary

SocketShark makes it easy to build WebSocket-based services without requiring those services to be aware of WebSockets. Instead, services implement HTTP endpoints for receiving messages from WebSocket clients, and publish messages to WebSocket clients via Redis, while SocketShark takes care of long-running WebSocket connections and passing messages between clients and services.

Features

• Pub-sub messages

  SocketShark allows building applications relying on the publish-subscribe pattern without having to be aware of long-running WebSocket connections. Subscriptions and published messages from the WebSocket client are forwarded to the application via HTTP. Messages can be pushed from the application to the WebSocket client synchronously by pushing them to Redis.

• Flexible WebSocket backend

  SocketShark comes with Websockets for Python 3 (websockets) backend but can easily be adapted to other frameworks compatible with asyncio.

• Multiple services

  Through its configuration file, SocketShark can work with any number of services.

• Out-of-order message filtering

  If needed, an internal order can be supplied with messages from services, and SocketShark will automatically filter out out-of-order messages.

• Message throttling

  If needed, service messages can be throttled by SocketShark.

• Authentication

  SocketShark comes with ticket authentication built-in. To authenticate WebSocket connections, the client requests a temporary token from the app server and submits it when logging in. The token is then exchanged for a session/user ID that can be used to authenticate the user by the service backed by SocketShark.

• Authorization

  Pub-sub subscriptions can be authorized using a custom HTTP endpoint. SocketShark can periodically reauthorize subscriptions to ensure subscribers are unsubscribed if they're no longer authorized.

• Custom fields

  SocketShark supports custom application-specific fields for authentication and authorization purposes.

• Metrics

  SocketShark keeps track and reports metrics such as connection counts and successfully or unsuccessfully executed commands, with built-in Prometheus and logging backends.

• Connection maintenance

  SocketShark supports keeping the WebSocket connection alive and automatic discovery of its closure through automated server-side pings and handlers for client-side pings.

Quick start

See example_config.py for an example configuration file.

To start, install SocketShark (python setup.py install), create your own configuration file, and run SocketShark as follows:

PYTHONPATH=. socketshark -c my_config

Client Protocol

SocketShark uses WebSockets as the transport protocol for clients. This section describes the structure of the protocol between web clients and SocketShark.

Both clients and the server exchange JSON-messages. Each message is a JSON dict containing an event field which specifies the type of event. SocketShark supports the following events:

• auth: Authentication
• subscribe: Subscribe to a topic
• message: Send a message to a topic
• unsubscribe: Unsubscribe from a topic
• ping: Monitor connectivity

Responses usually contain a status field which can be ok or error. In case of an error, an error field is supplied containing the error description as a string.

Authentication

WebSockets clients can authenticate using the auth event using ticket authentication. For more information about ticket authentication see the Ticket-based authentication for session-based apps section.

The auth event takes an optional method (ticket is the only currently supported authentication method, and the default), and a ticket argument, containing the login ticket.

Example client request:

{"event": "auth", "method": "ticket", "ticket": "SECRET_AUTH_TICKET"}

Example server responses (successful and unsuccessful):

{"event": "auth", "status": "ok"}

{"event": "auth", "status": "error", "error": "Authentication failed."}

Subscriptions

WebSocket clients can subscribe to any number of topics. Messages can be passed from the client to the server, and pushed from the server to the client at any time while subscribed to a topic. For example, a client may subscribe to an object ID, and the server may send a message whenever the object is updated. The server may include extra data when subscribing or unsubscribing. For example, the server might send the current state of the object when subscribing.

Topics are unique, and a client can be subscribed to each topic at most once. Extra fields can be associated with a subscription which are passed along with all subscription commands. For example, a client could be required to indicate the organization ID for a particular object subscription so that the service can authorize and process the message properly.

Subscribe

The subscribe event subscribes to a topic given in the subscription argument, which is composed of the service name and the topic, separated by period. Extra fields can be defined by the service and directly specified in the subscription message.

Example client request:

{"event": "subscribe", "subscription": "books.book_1"}

Example server responses (successful and unsuccessful):

{"event": "subscribe", "subscription": "books.book_1", "status": "ok"}

{
  "event": "subscribe",
  "subscription": "books.book_1",
  "status": "error",
  "error": "Book does not exist."
}

Example server response with extra data:

{
  "event": "subscribe",
  "subscription": "books.book_1",
  "status": "ok",
  "data": {
    "title": "Everyone poops"
  }
}

Example client request with extra fields:

{"event": "subscribe", "subscription": "books.book_1", "author_id": "author_1"}

Example successful server responses with extra fields:

{
  "event": "subscribe",
  "subscription": "books.book_1",
  "author_id": "author_1",
  "status": "ok"
}

Note that the subscription name is unique for the subscription. When subscribed to books.book_1 we can't subscribe to another subcription with the same name even if the author_id is different. However, the server could use the author_id to ensure the book matches the given author ID.

Message

Once subscribed, the message event can be used to pass messages. Message data is contained in the data field, and should be dicts. The structure of the data is up to the application protocol, and the service decides whether messages are confirmed (successfully or unsuccessfully).

Example message (either client-to-server or server-to-client):

{
  "event": "message",
  "subscription": "books.book_1",
  "data": {
     "action": "update",
     "title": "New book title"
  }
}

Example (optional) server-side message confirmation of a successful message with extra data:

{
  "event": "message",
  "subscription": "books.book_1",
  "status": "ok",
  "data": {"status": "Book was updated."}
}

Example (optional) server-side message confirmation of a failed message:

{
  "event": "message",
  "subscription": "books.book_1",
  "status": "error",
  "error": "Book could not be updated."
}

If extra fields are passed with the subscription, they are included in all message events.

Note that a service may send messages limited to particular authentication fields (e.g. limited to a specific user ID), so multiple sessions subscribed to the same topic may not necessarily receive the same messages.

Unsubscribe

Clients can unsubscribe from a topic using the unsubscribe event.

Example client request:

{"event": "unsubscribe", "subscription": "books.book_1"}

Example server responses (successful and unsuccessful):

{"event": "unsubscribe", "subscription": "books.book_1", "status": "ok"}

{
  "event": "unsubscribe",
  "subscription": "books.book_1",
  "status": "error",
  "error": "Subscription does not exist."
}

Ping

Clients can send a ping message and Socketshark will send a pong back immediately, without contacting any services. Clients may choose to send pings and monitor for pongs to e.g. detect failed WebSocket connections, display latency metrics, etc. Furthermore, the ping message may contain some data, which the pong message should repeat back.

Example client request:

{"event": "ping", "data": "foobar"}

Example server response:

{"event": "pong", "data": "foobar"}

Service Protocol

SocketShark uses HTTP to send events to services, and Redis PUBSUB to receive messages from services that are published to subscribed clients. This section describes the structure of the protocol between services and SocketShark.

HTTP callbacks

An optional HTTP endpoint can be configured to authenticate a WebSocket session. The authentication endpoint can return authentication-related fields that can be configured (e.g. a user ID and/or session ID).

The following optional HTTP endpoints can be configured for each SocketShark service:

• authorizer: URL to call to authorize a new subscription.
• before_subscribe: URL to call when a client attempts to subscribe.
• on_subscribe: URL to call after a client subscribed to a topic.
• on_message: URL to call when a client sends a message to a topic.
• before_unsubscribe: URL to call when a client attempts to unsubscribe.
• on_unsubscribe: URL to call after a client unsubscribed from a topic.
• on_authorization_change: URL to call after if any authorizer fields change during periodic authorization.

Each HTTP endpoint is accessed via a POST request containing a JSON body.

Service-specific endpoints receive any client-supplied extra fields that are configured for the particular service, as well as authentication-related fields returned by the authentication endpoint.

HTTP endpoints should return a JSON dict containing a status field with the value ok or error. In case of an error, an error text may be specified in the error field.

Authentication

The authentication URL receives JSON dict with the client's ticket supplied in the ticket field. Only a successful response authenticates the user.

Example request body:

{"ticket": "SECRET_AUTH_TICKET"}

Example server responses (successful with auth fields and unsuccessful):

{"status": "ok", "user_id": "user_1", "session_id": "session_1"}

{"status": "error", "error": "Authentication failed."}

Authorization

If an authorizer URL is supplied for a service, it is invoked each time a user attempts to subscribe to a topic. Only a successful response authorizes the subscription, triggering the before_subscribe callback (if specified).

If a service has no authorizer, all topics are authorized.

Example request body (for an authenticated session with auth fields as well as extra client fields):

{
  "subscription": "books.book_1",
  "user_id": "user_1",
  "session_id": "session_1",
  "author_id": "author_1"
}

Example server responses (successful and unsuccessful):

{"status": "ok"}

{"status": "error", "error": "Author ID does not match book ID."}

During an active subscription, SocketShark will periodically query the authorizer endpoint if authorization_renewal_period is set to the number of seconds. The user will be unsubscribed by SocketShark if the authorization is no longer valid and an unsubscribe message will be sent to the client, e.g.:

{
  "event": "unsubscribe",
  "subscription": "books.book_1",
  "error": "Unauthorized."
}

Before subscribe

After a subscription is authorized, the before_subscribe callback is invoked with the same arguments as the authorizer. Only a successful response confirms the subscription, triggering the on_subscribe callback (if specified).

Extra data can be returned in this callback using the data field which is forwarded to the client. If returned, the data field should be a dict.

On subscribe

After a subscription is confirmed, the on_subscribe callback is invoked with the same arguments as the authorizer. An unsuccessful response doesn't affect the client's subscription.

On message

When a client sends a message to the service, the on_message callback is invoked with the same arguments as the authorizer, plus the message data in the data field.

A successful response with a data field, or an unsuccessful response sends a confirmation to the client.

Example request body (for an authenticated session with auth fields as well as extra client fields supplied during the subscription):

{
  "subscription": "books.book_1",
  "user_id": "user_1",
  "session_id": "session_1",
  "author_id": "author_1",
  "data": {
    "action": "update",
    "title": "New book title"
  }
}

Example server response (successful, triggers no response):

{"status": "ok"}

Example server response (successful, triggers a response):

{"status": "ok", "data": {"status": "Book was updated."}

Example server response (unsuccessful, triggers a response):

{"status": "error", "error": "Book could not be updated."}

Before unsubscribe

When a client issues an unsubscribe event, the before_unsubscribe callback is invoked with the same arguments as the authorizer. Only a successful response confirms the unsubscription, triggering the on_unsubscribe callback (if specified).

Extra data can be returned in this callback using the data field which is forwarded to the client. If returned, the data field should be a dict.

On unsubscribe

After an unsubscription is confirmed, the on_unsubscribe callback is invoked with the same arguments as the authorizer. An unsuccessful response doesn't affect the client's unsubscription.

Publishing messages to clients

To publish a message, a service needs to publish a Redis message to the appropriate subscription. The message must be JSON-formatted, and contain the subscription field, a free-form data dict and any optional filters (if the service has configured filter fields). The channel name corresponds to the subscription (service.topic), but a Redis channel prefix may be optionally configured.

When a filter field is specified, the message is only published to sessions that match the filter. For example, a message could only be sent to sessions matching a specific user ID.

Example Redis PUBLISH command:

PUBLISH books.book_1 {
  "subscription": "books.book_1",
  "data": {
    "action": "update",
    "title": "New title"
  }
}

Out-of-order message filtering

Since messages published by services may not necessarily arrive in the desired order, SocketShark supports message filtering. For example, you might be publishing updates for a versioned object to Redis but they may arrive out-of-order due to network latency. Messages can be tagged with an order, and SocketShark will filter out older messages if a newer message arrives first. A float order can be supplied both in the before_subscribe callback's return value and in any published message using the order option in the options dict. Incoming messages with an order that is lower or equal to the last received highest order will be filtered out. Multiple independent orders can be specified using the optional order_key option.

In the following example, the "initiating" and "completed" messages, as well as the "h" and "hello" messages will be delivered to subscribers:

PUBLISH calls.call_1 {
  "subscription": "calls.call_1",
  "options": {
      "order": 1,
      "order_key": "call_1.status",
  },
  "data": {
    "status": "initiating",
  }
}

PUBLISH calls.call_1 {
  "subscription": "calls.call_1",
  "options": {
      "order": 3,
      "order_key": "call_1.status",
  },
  "data": {
    "status": "completed",
  }
}

PUBLISH calls.call_1 {
  "subscription": "calls.call_1",
  "options": {
      "order": 2,
      "order_key": "call_1.status",
  },
  "data": {
    "status": "ringing",
  }
}

PUBLISH calls.call_1 {
  "subscription": "calls.call_1",
  "options": {
      "order": 1,
      "order_key": "call_1.note",
  },
  "data": {
    "note": "h",
  }
}

PUBLISH calls.call_1 {
  "subscription": "calls.call_1",
  "options": {
      "order": 3,
      "order_key": "call_1.note",
  },
  "data": {
    "note": "hello",
  }
}

PUBLISH calls.call_1 {
  "subscription": "calls.call_1",
  "options": {
      "order": 2,
      "order_key": "call_1.note",
  },
  "data": {
    "note": "hell",
  }
}

Message throttling

Messages published by services can be throttled by specifying the time in seconds using the throttle option in the options dict in the published message.

For a constant stream of messages that are coming in shorter than the throttle period, the client will receive the first message immediately, then a message every throttle period until the stream ends, and then the final message will be sent after another throttle period elapses.

Multiple independent throttles can be specified using the optional throttle_key option. Throttling is performed per subscription per session.

In the example below, if the three messages are published at the same time, the first one will be delivered to subscribers immediately, the second one will be ignored, and the third message will be delivered to subscribers after 100ms pass.

PUBLISH calls.stats {
  "subscription": "calls.stats",
  "options": {
      "throttle" 0.1,
  },
  "data": {
    "n_calls": 1,
  }
}

PUBLISH calls.stats {
  "subscription": "calls.stats",
  "options": {
      "throttle" 0.1,
  },
  "data": {
    "n_calls": 2,
  }
}

PUBLISH calls.stats {
  "subscription": "calls.stats",
  "options": {
      "throttle" 0.1,
  },
  "data": {
    "n_calls": 3,
  }
}

Use patterns

This section illustrates how to implement common use patterns when building a service with SocketShark.

Ticket-based authentication for session-based apps

Most web applications use an HTTP-only cookie that stores a session ID for authentication. Since WebSocket connections are initiated via JavaScript, there is no access to the session ID via the cookie. To facilitate authentication of WebSocket connections, authentication with single-use tickets should be used:

• Implement a public "ticket" endpoint in your application. The endpoint should validate the user's session and return a random-generated short-lived ticket associated to the user's session ID. For example, a UUID4 ticket may be computed and stored in Redis with a 30 second expiration using the SETEX command, where the key name corresponds to the ticket (the UUID4), and the key value is the user's session ID.
• Implement an internal ticket validation in your application. The endpoint should be configured as the auth endpoint in SocketShark. It should retrieve and return the user's session ID, and at the same time invalidate the ticket. Any other user information (e.g. user ID) may also be returned. A Redis pipeline should be used to retrieve and delete the ticket.
• When the JavaScript code connects to SocketShark, it should first request a ticket via the public ticket endpoint, then connect to SocketShark and issue the authentication event with the obtained ticket.

Authorization with microservices

Suppose a user can access products from a set of authorized organization IDs. The auth service stores a list of users and corresponding organization IDs that users have access to. The product service stores a list of products with corresponding organization IDs but is not aware whether a user is authorized to access a specific organization (and therefore product). Subscriptions are per product and should only be authorized if the user can access the product's organization. To solve this problem without requiring the services to directly talk to each other, extra fields can be used in SocketShark:

• Add the user ID in SocketShark's authentication configuration under auth_fields, and the organization ID as under the product service's extra_fields.
• Return the user ID in the auth service's authorization endpoint. SocketShark will supply it in all subsequent requests to service endpoints.
• When the client subscribes to the product service (subscription example: product.PROD_ID), it must also supply the product's organization ID as an extra field.
• Set up an authorizer URL for the product service that points to the auth service. The auth service should authorize a subscription if the given user has access to the given organization. Since the authorizer doesn't have access to the product database, it doesn't validate the product ID.
• Set up a before_subscribe URL for the product service that points to the product service. The product service should allow a subscription if the subscription's product ID matches the given organization ID. Since the organization ID is already validated by the authorizer, no further validation is necessary.