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+# CPP#3 - Private user messages
+
+## Introduction
+
+This proposal introduces the concept of chats (this proposal describes only private chats) and a mechanism for sending/receiving messages in the chat.
+
+![Message Mindmap](CPP3 - Message Mindmap.png)
+
+## Message type identifiers
+
+- `chat:create`: create chat action
+- `chat:created`: notification about creating new chat by some user
+- `message:send`: send message to chat
+- `message:receive`: receive message from chat
+- `message:read`: read a message
+- `message:receive:read`: receive notification about read message
+
+## Glossary
+
+- **Chat (the so-called "room")** - the entity in which communication occurs between users (participants).
+- **Chat history** - an ordered by time list of messages of chat
+
+## Use Cases
+
+Note: C - client, S - server
+
+### Creating private chat
+
+1. C1:
+
+```json
+{
+    "id": "abcd",
+    "type": "chat:create",
+    "from": "@user1@cadmium.im",
+    "to": "cadmium.im",
+    "payload": {
+        "chatType": "user",
+        "receiverUserId": "@user2@cadmium.im"
+    }
+}
+```
+
+2. S to C1:
+
+```json
+{
+    "id": "abcd",
+    "type": "chat:create",
+    "from": "cadmium.im",
+    "to": "@user1@cadmium.im",
+    "ok": true,
+    "payload": {
+        "roomId": "!22c5d942-22bc-45b3-9541-ec2b49afe5ec@cadmium.im"
+    }
+}
+```
+
+Error response (S):
+
+```json
+{
+    "id": "abcd",
+    "type": "chat:create",
+    "from": "cadmium.im",
+    "to": "@user1@cadmium.im",
+    "ok": false,
+    "payload": {
+        "errCode": "blacklistedUser",
+        "errText": "You are blacklisted for this user"
+    }
+}
+```
+
+3. S to C2:
+
+```json
+{
+    "id": "abcd",
+    "type": "chat:created",
+    "from": "cadmium.im",
+    "to": "@user2@cadmium.im",
+    "ok": true,
+    "payload": {
+        "roomId": "!22c5d942-22bc-45b3-9541-ec2b49afe5ec@cadmium.im",
+        "creatorUserId": "@user1@cadmium.im"
+    }
+}
+```
+
+### Sending message to chat
+
+1. C1:
+
+```json
+{
+    "id": "abcd",
+    "type": "message:send",
+    "from": "@user1@cadmium.im",
+    "to": "!22c5d942-22bc-45b3-9541-ec2b49afe5ec@cadmium.im",
+    "payload": {
+        "messageType": "text",
+        "messageText": "Hello world!"
+    }
+}
+```
+
+2. S to C1:
+
+```json
+{
+    "id": "abcd",
+    "type": "message:send",
+    "from": "cadmium.im",
+    "to": "@user1@cadmium.im",
+    "ok": true,
+    "payload": {
+        "messageTimestamp": 1577988125,
+        "messageId": "ee55b940-0f2f-46dc-8c8e-e6ea5874c3c2"
+    }
+}
+```
+
+3. S to C2:
+
+```json
+{
+    "id": "abcd",
+    "type": "message:receive",
+    "from": "cadmium.im",
+    "to": "@user2@cadmium.im",
+    "ok": true,
+    "payload": {
+        "messageType": "text",
+        "messageId": "ee55b940-0f2f-46dc-8c8e-e6ea5874c3c2",
+        "messageText": "Hello world!"
+    }
+}
+```
+
+### Reading message
+
+1. C2:
+
+```json
+{
+    "id": "abcd",
+    "type": "message:read",
+    "from": "@user2@cadmium.im",
+    "to": "!22c5d942-22bc-45b3-9541-ec2b49afe5ec@cadmium.im",
+    "payload": {
+        "messageId": "ee55b940-0f2f-46dc-8c8e-e6ea5874c3c2"
+    }
+}
+```
+
+2. S to C2:
+
+```json
+{
+    "id": "abcd",
+    "type": "message:read",
+    "from": "cadmium.im",
+    "to": "@user2@cadmium.im",
+    "ok": true,
+    "payload": {}
+}
+```
+
+3. S to C1:
+
+```json
+{
+    "id": "abcd",
+    "type": "message:receive:read",
+    "from": "cadmium.im",
+    "to": "@user1@cadmium.im",
+    "ok": true,
+    "payload": {
+        "messageId": "ee55b940-0f2f-46dc-8c8e-e6ea5874c3c2",
+        "readUserId": "@user2@cadmium.im"
+    }
+}
+```
+
+## Errors
+
+- Ratelimit system: enabled
+- Authentication required: true
+- `blacklistedUser`: user 1, who is trying to create a chat, is blacklisted by user 2
+
+## Business Rules
+
+- Every message MUST be recorded in chat message history
+- Timestamp MUST be in UTC timezone
+- Server MUST store read state of message as list of user IDs which read this message
+
+## Security Considerations
+
+None.
+
+## JSON Schema
+
+### Creating chat payload
+
+- Request for creating chat:
+
+```typescript
+interface CreatePrivateChatRequestPayload {
+    /**
+     * Type of chat which need to create
+     */
+    chatType: ChatType,
+
+    /**
+     * Second chat member ID (first member is who trying to create this chat) in
+     * case when chatType is "user"
+     */
+    receiverUserId: EntityID
+}
+```
+
+- Response for creating chat:
+
+```typescript
+interface CreatePrivateChatResponsePayload {
+    /**
+     * Identifier of newly created chat
+     */
+    roomId: EntityID
+}
+```
+
+- Notification for second member of newly created chat about the fact of creating this chat
+
+```typescript
+interface CreatePrivateChatNotificationPayload {
+    /**
+     * Identifier of newly created chat
+     */
+    roomId: EntityID,
+
+    /**
+     * Identifier of user which created this chat
+     */
+    creatorUserId: EntityID
+}
+```
+
+### Sending message to chat payload
+
+- Send message action request payload
+
+```typescript
+interface SendMessageRequestPayload {
+    /**
+     * Type of the message
+     */
+    messageType: MessageType,
+
+    /**
+     * Text of the message
+     */
+    messageText: string,
+
+
+    /**
+     * URL of file/image/voice which attached to this message (in case of
+     * appropriate message type)
+     */
+    fileUrl?: string
+}
+```
+
+- Send message action response payload
+
+```typescript
+interface SendMessageResponsePayload {
+    /**
+     * UNIX Timestamp of newly sent message
+     */
+    messageTimestamp: number,
+
+    /**
+     * Message identifier in chat history of newly sent message
+     */
+    messageId: string
+}
+```
+
+- Notification about new message in the chat
+
+```typescript
+interface NewMessageNotificationPayload {
+    /**
+     * Identifier of received message in chat history
+     */
+    messageId: string,
+
+    /**
+     * UNIX timestamp of received message
+     */
+    messageTimestamp: number,
+
+    /**
+     * Text of received message
+     */
+    messageText: string
+}
+```
+
+### Reading message payload
+
+- Read message action request payload
+
+```typescript
+interface ReadMessageRequesPayload {
+    /**
+     * Identifier of message which need mark as read
+     */
+    messageId: string,
+}
+```
+
+- Notification about reading message
+
+```typescript
+interface ReadMessageRequesPayload {
+    /**
+     * Identifier of message which was read
+     */
+    messageId: string,
+
+    /**
+     * Identifier of user which read this message
+     */
+    readUserId: EntityID
+}
+```
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