diff --git a/src/declarations.ts b/src/declarations.ts
index 9c138be7..16e620ed 100644
--- a/src/declarations.ts
+++ b/src/declarations.ts
@@ -19,6 +19,7 @@ import {
   PublicItemTaskManager,
   S3FileConfiguration,
 } from './services';
+import { WebsocketService } from './services/websockets/index';
 
 declare module 'fastify' {
   interface FastifyRequest {
@@ -135,5 +136,10 @@ declare module 'fastify' {
     h5p?: {
       taskManager: H5PTaskManager;
     };
+
+    /**
+     * Websockets service
+     */
+    websockets?: WebsocketService;
   }
 }
diff --git a/src/services/index.ts b/src/services/index.ts
index b02d7c28..528aeae8 100644
--- a/src/services/index.ts
+++ b/src/services/index.ts
@@ -1,14 +1,15 @@
 export * from './app';
+export * from './categories';
+export * from './chat';
 export * from './database';
+export * from './etherpad';
 export * from './file';
 export * from './flag';
 export * from './h5p';
-export * from './etherpad';
+export * from './invitation';
 export * from './item-memberships';
 export * from './item-tags';
 export * from './items';
-export * from './invitation';
 export * from './members';
 export * from './public';
-export * from './chat';
-export * from './categories';
+export * from './websockets';
diff --git a/src/services/websockets/api/client.ts b/src/services/websockets/api/client.ts
new file mode 100644
index 00000000..68233a01
--- /dev/null
+++ b/src/services/websockets/api/client.ts
@@ -0,0 +1,65 @@
+/**
+ * Client message types for the graasp websockets network API
+ * {@link https://github.com/graasp/graasp-plugin-websockets/blob/main/API.md}
+ */
+import { NotifMessage } from './message';
+
+/**
+ * Client actions: describe what the client requests from the server
+ */
+export enum ClientActions {
+  /** Subscribe to a channel */
+  Subscribe = 'subscribe',
+  /** Unsubscribe from a channel */
+  Unsubscribe = 'unsubscribe',
+  /** Subscribe only to a channel and unsubscribe from all others */
+  SubscribeOnly = 'subscribeOnly',
+  /** Disconnect from the websocket system */
+  Disconnect = 'disconnect',
+}
+
+export type ClientAction = ClientActions | `${ClientActions}`;
+
+/**
+ * Message sent by client to disconnect
+ */
+export interface ClientDisconnect extends NotifMessage {
+  action: `${ClientActions.Disconnect}`;
+}
+
+/**
+ * Message sent by client to subscribe to some channel
+ */
+export interface ClientSubscribe extends NotifMessage {
+  action: `${ClientActions.Subscribe}`;
+  topic: string;
+  channel: string;
+}
+
+/**
+ * Message sent by client to unsubscribe from some channel
+ */
+export interface ClientUnsubscribe extends NotifMessage {
+  action: `${ClientActions.Unsubscribe}`;
+  topic: string;
+  channel: string;
+}
+
+/**
+ * Message sent by client to subscribe to a single channel
+ * (i.e. it also unsubscribes it from any other channel)
+ */
+export interface ClientSubscribeOnly extends NotifMessage {
+  action: `${ClientActions.SubscribeOnly}`;
+  topic: string;
+  channel: string;
+}
+
+/**
+ * Client message type is union type of all client message subtypes
+ */
+export type ClientMessage =
+  | ClientDisconnect
+  | ClientSubscribe
+  | ClientUnsubscribe
+  | ClientSubscribeOnly;
diff --git a/src/services/websockets/api/index.ts b/src/services/websockets/api/index.ts
new file mode 100644
index 00000000..8dd0a7bd
--- /dev/null
+++ b/src/services/websockets/api/index.ts
@@ -0,0 +1,3 @@
+export * from './client';
+export * from './message';
+export * from './server';
diff --git a/src/services/websockets/api/message.ts b/src/services/websockets/api/message.ts
new file mode 100644
index 00000000..398aae03
--- /dev/null
+++ b/src/services/websockets/api/message.ts
@@ -0,0 +1,28 @@
+/**
+ * Message types for the graasp websockets network API
+ * {@link https://github.com/graasp/graasp-plugin-websockets/blob/main/API.md}
+ */
+
+/**
+ * Message realms: describe the universe in which the message belongs to
+ */
+export enum Realms {
+  /** Notifications realm for real-time updates in Graasp */
+  Notif = 'notif',
+}
+
+export type Realm = Realms | `${Realms}`;
+
+/**
+ * Base message shape
+ */
+export interface Message {
+  realm: Realm;
+}
+
+/**
+ * Base message for real-time updates and notifications in Graasp
+ */
+export interface NotifMessage extends Message {
+  realm: `${Realms.Notif}`;
+}
diff --git a/src/services/websockets/api/server.ts b/src/services/websockets/api/server.ts
new file mode 100644
index 00000000..6f36aa2c
--- /dev/null
+++ b/src/services/websockets/api/server.ts
@@ -0,0 +1,76 @@
+/**
+ * Server message types for the graasp websockets network API
+ * {@link https://github.com/graasp/graasp-plugin-websockets/blob/main/API.md}
+ */
+import { WebsocketError } from '../errors';
+import { ClientMessage } from './client';
+import { NotifMessage } from './message';
+
+/**
+ * Server message types: describe what the server can send to the clients
+ */
+export enum ServerMessageTypes {
+  /** Response to a previous client request */
+  Response = 'response',
+  /** Update with new data on a given channel */
+  Update = 'update',
+  /** Global broadcasts unrelated to any specific channel */
+  Info = 'info',
+}
+
+export type ServerMessageType = ServerMessageTypes | `${ServerMessageTypes}`;
+
+/**
+ * Server response status
+ */
+export enum ResponseStatuses {
+  Success = 'success',
+  Error = 'error',
+}
+
+export type ResponseStatus = ResponseStatuses | `${ResponseStatuses}`;
+
+/**
+ * Message sent by server as a response to a {@link ClientMessage} on success
+ */
+export interface SuccessServerResponse extends NotifMessage {
+  type: `${ServerMessageTypes.Response}`;
+  status: `${ResponseStatuses.Success}`;
+  request: ClientMessage;
+}
+
+/**
+ * Error message sent by server as a response to a {@link ClientMessage}
+ */
+export interface ErrorServerResponse extends NotifMessage {
+  type: `${ServerMessageTypes.Response}`;
+  status: `${ResponseStatuses.Error}`;
+  request?: ClientMessage;
+  error: WebsocketError;
+}
+
+export type ServerResponse = ErrorServerResponse | SuccessServerResponse;
+
+/**
+ * Message sent by server for misc broadcasts unrelated to a channel
+ */
+export interface ServerInfo<T = unknown> extends NotifMessage {
+  type: `${ServerMessageTypes.Info}`;
+  message: string;
+  extra?: T;
+}
+
+/**
+ * Message sent by server for update notifications sent over a channel
+ */
+export interface ServerUpdate<T = unknown> extends NotifMessage {
+  type: `${ServerMessageTypes.Update}`;
+  topic: string;
+  channel: string;
+  body: T;
+}
+
+/**
+ * Server message type is union type of all server message subtypes
+ */
+export type ServerMessage = ServerResponse | ServerInfo | ServerUpdate;
diff --git a/src/services/websockets/errors.ts b/src/services/websockets/errors.ts
new file mode 100644
index 00000000..d5fcb6a2
--- /dev/null
+++ b/src/services/websockets/errors.ts
@@ -0,0 +1,47 @@
+/**
+ * Top-level error class for websockets
+ * (useful e.g. to check instanceof)
+ */
+export abstract class WebsocketError extends Error {}
+
+/**
+ * Available websocket error names
+ *
+ * @example
+ *  ```
+ *    if (err.name === Websocket.ErrorNames.AccessDenied) {
+ *      // handle access denied
+ *    }
+ *  ```
+ */
+export enum ErrorNames {
+  AccessDenied = 'ACCESS_DENIED',
+  BadRequest = 'BAD_REQUEST',
+  NotFound = 'NOT_FOUND',
+  ServerError = 'SERVER_ERROR',
+}
+
+export type ErrorName = ErrorNames | `${ErrorNames}`;
+
+export class AccessDeniedError extends WebsocketError {
+  name: `${ErrorNames.AccessDenied}` = ErrorNames.AccessDenied;
+  message = 'Websocket: Access denied for the requested resource';
+}
+
+export class BadRequestError extends WebsocketError {
+  name: `${ErrorNames.BadRequest}` = ErrorNames.BadRequest;
+  message =
+    'Websocket: Request message format was not understood by the server';
+}
+
+export class NotFoundError extends WebsocketError {
+  name: `${ErrorNames.NotFound}` = ErrorNames.NotFound;
+  message = 'Websocket: Requested resource not found';
+}
+
+export class ServerError extends WebsocketError {
+  name: `${ErrorNames.ServerError}` = ErrorNames.ServerError;
+  constructor(message: string) {
+    super((message = `Websocket: Internal server error: ${message}`));
+  }
+}
diff --git a/src/services/websockets/index.ts b/src/services/websockets/index.ts
new file mode 100644
index 00000000..18645f6d
--- /dev/null
+++ b/src/services/websockets/index.ts
@@ -0,0 +1,45 @@
+import * as API from './api';
+import * as Errors from './errors';
+import * as Requests from './interfaces/request';
+
+export * from './interfaces/service';
+
+/**
+ * Re-export types and utility classes within the Websocket namespace
+ */
+/* eslint-disable @typescript-eslint/no-unused-vars */
+/* eslint-disable @typescript-eslint/no-namespace */
+export namespace Websocket {
+  /* Errors */
+  export import Error = Errors.WebsocketError;
+  export import ErrorNames = Errors.ErrorNames;
+  export type ErrorName = Errors.ErrorName;
+  export import AccessDeniedError = Errors.AccessDeniedError;
+  export import BadRequestError = Errors.BadRequestError;
+  export import NotFoundError = Errors.NotFoundError;
+  export import ServerError = Errors.ServerError;
+  /* Requests */
+  export type SubscriptionRequest = Requests.SubscriptionRequest;
+  /* API */
+  export import Realms = API.Realms;
+  export type Realm = API.Realm;
+  export type Message = API.Message;
+  export type NotifMessage = API.NotifMessage;
+  export import ClientActions = API.ClientActions;
+  export type ClientAction = API.ClientAction;
+  export type ClientDisconnect = API.ClientDisconnect;
+  export type ClientSubscribe = API.ClientSubscribe;
+  export type ClientUnsubscribe = API.ClientUnsubscribe;
+  export type ClientSubscribeOnly = API.ClientSubscribeOnly;
+  export type ClientMessage = API.ClientMessage;
+  export import ServerMessageTypes = API.ServerMessageTypes;
+  export type ServerMessageType = API.ServerMessageType;
+  export import ResponseStatuses = API.ResponseStatuses;
+  export type ResponseStatus = API.ResponseStatus;
+  export type SuccessServerResponse = API.SuccessServerResponse;
+  export type ErrorServerResponse = API.ErrorServerResponse;
+  export type ServerResponse = API.ServerResponse;
+  export type ServerInfo = API.ServerInfo;
+  export type ServerUpdate = API.ServerUpdate;
+  export type ServerMessage = API.ServerMessage;
+}
diff --git a/src/services/websockets/interfaces/request.ts b/src/services/websockets/interfaces/request.ts
new file mode 100644
index 00000000..5d287fe6
--- /dev/null
+++ b/src/services/websockets/interfaces/request.ts
@@ -0,0 +1,20 @@
+import { Member, UnknownExtra } from '@/index';
+
+/**
+ * Internal request type when clients attempt to subscribe to some channel
+ * Allows consumers to operate on the subscription attempt in the server
+ */
+export interface SubscriptionRequest {
+  /**
+   * Subscription target channel name
+   */
+  channel: string;
+  /**
+   * Member requesting a subscription
+   */
+  member: Member<UnknownExtra>;
+  /**
+   * Rejects the subscription request with a specified error
+   */
+  reject(error: Error): void;
+}
diff --git a/src/services/websockets/interfaces/service.ts b/src/services/websockets/interfaces/service.ts
new file mode 100644
index 00000000..b9b09b3d
--- /dev/null
+++ b/src/services/websockets/interfaces/service.ts
@@ -0,0 +1,35 @@
+import { SubscriptionRequest } from './request';
+
+/**
+ * Public WebSocket service exposed to other consumers on the server
+ * (e.g. other services that want to publish messages using websockets)
+ */
+export interface WebsocketService {
+  /**
+   * Registers a topic (a group of related channels) dedicated to the caller
+   * @param topic           topic name, must be unique across server
+   * @param validateClient  async function called when a client attempts to
+   *                        subscribe to a channel from this topic
+   */
+  register(
+    topic: string,
+    validateClient: (request: SubscriptionRequest) => Promise<void>,
+  ): this;
+
+  /**
+   * Publishes a message on a channel globally (incl. across server instances)
+   * @param topic topic name
+   * @param channel channel name
+   * @param message message to publish
+   */
+  publish<Message>(topic: string, channel: string, message: Message): void;
+
+  /**
+   * Publishes a message on a channel locally (i.e. on this specific server
+   * instance only)
+   * @param topic topic name
+   * @param channel channel name
+   * @param message message to publish
+   */
+  publishLocal<Message>(topic: string, channel: string, message: Message): void;
+}