@JohnKoepi @ g on software engineering

Notes on Telegram (TG) instant messaging (IM) system data protocol

The protocol structure was reverse engineered from the prototypes of the TG protocol and it’s open source clients.

Telegram is an instance messaging (IM) platform. It consists of a users who talk to each other in a chat rooms which can be public or private. The problems in the design of the chat systems are fast and light clients (devices) synchronization over the same view of the chat histories and their states, distribution (routing) of the messages flows and reducing overall overhead of storage keeping operational latency low.

Telegram uses encapsulated append-only logs (AOL) of events to model robust replication machine. Multiple devices can agree on a single view of the data (snapshot of the chats state) consuming an event log and detect their synchronization state with O(1).

/------------------------------------------------------------------
|         |         |
| event 1 | event 2 | event 3 | ...         | event N |
|         |         |
\------------------------------------------------------------------
              ^                                  ^
              |                                  |
              read ptr                           last event ptr

Telegram protocol.
Replication machine based on ordered log of events.

This event log (EVL) is persistent (at least to some extent) and unique for each user (single log per multiple clients/devices). It’s implemented as a ring buffer with a 32 bit events space. Clients consume tail of the EVL from the known point to the end of the tail to get consistent data view. All events has an associated monotonically increasing ID. The volume of unsynced state may be expressed as a difference between known state and the latest event in the log: last_event_id - device_read_event_id.

This design allows to:

  • organize replication in a simple lightweight and reliable way.
  • synchronize data with a single RPC call (load the EVL tail) really quick;
  • survive disconnections and bad network with little overhead;
  • count unread events/messages for O(1);

Then, the EVL has layered structure it self:

/-----------------------------------------------------\/--------\
|  /----------------------\ /----------------------\  || Online |
|  | Public messaging log | | Secret messaging log |  || events |
|  \----------------------/ \----------------------/  || log    |
|            Application-level events log             ||        |
\-----------------------------------------------------/\--------/
|                                                               |
| In-house AES based encryption                                 |
| ------------------------------------------------------------- |
| Transport-level messages                                      |
\---------------------------------------------------------------/

Transport level (TL) has an output queue on the server side per client (device). All application level events (messages) are encapsulated into the transport level events (messages). On a transport level clients explicitly acknowledge (ACK) that they have successfully received every message in order. Application level ACK allows to use any of the available protocols for the networking (i.e. UDP).

TL network messages have ordered monotonically increasing IDs. TL queue is not persistent as serves the socket buffer purposes.

Transport-level message ID structure

/63<------------------------64 bits---------------------------0\
|63                          20                               0|
| sign bit = 0 | id = 43 bits |   full type mask = 17 bits 0 + |
|                             |       type flag = 3 lowers bit |
\--------------------------------------------------------------/

Flags:
- OK     = 000
- UNSENT = 001
- LOCAL  = 002

uint64_t ID >= 0

RPC

RPC is implemented as an asynchronous system of events running over a transport level. Each RPC query has an Id for which the client awaits a result with status. Status can be error or ok. Client registers waiting handlers for specific Ids when making a requests. For handling waiting handlers simple vector is used in TDLib, besides the actors model for all other running state machines.

RPC queries IDs are usual monotonically increasing sequence of natural numbers starting from 1.

RPC net layer uses gzip for compressing bodies of the events. Events are usually of type: request, response_ok, response_error.

Chats and messages

r There are chat containers and dialogs. Dialogs are about a chat container state with unreads and other specific data related to the caller.

Chat containers may be of personal chats or groups. Super groups are handled separately as well as read-only channels.

A chat message is identified globally with a pair of (chat_id, date). A chat message has also a message id which is a natural number monotonically increasing starting from 1. This message id exists outside of the chat container in which a message resides. For each user, a message has it’s own unique id.

Messages IDs define a total order of all messages received by the client/user from the beginning. This totally ordered stream of messages embed into an event journal of the user (EVL).

/--------------------------------------------------------------\
| msg(id=1, chat_id=100500)  ->
|   msg(id=2, chat_id=3)     ->
|     msg(id=3, chat_id=999) ->
|       msg(id=4, chat_id=3) -> ...

Message ID is a natural number counting all messages received by
the user starting from 1.

Unreads may be calculated for O(1).

QPS and PTS

These two are interesting. The doc mentions them as follows:

  • pts is a number of events occurred in a text box. That sounds misleading. It may stand for plain text sequence.
  • qts is a position in a sequence of updates in secret chats.

pts is an Id of the message received by a user. The user EVL contains all messages starting from 1. To synchronize state client downloads the tail of the EVL based on (pts, qts, date) tuple. This allows to seeing whether a client has seen all the messages since the last disconnection.

Both pts and qts are monotonically increasing sequences. qts starts from some unreasonable number. Messages are identified uniquely by pts or qts on a user level, and by a Peer(chat_id, user_id) + date on a global level.

Inbox and Outbox updates

Telegram uses separate entities to notify clients about what parts of the chats history are read. It tracks incoming and outcoming histories separately. For a client a set of incoming messages is a set of all messages of a chat history written by others. An outcoming set of messages is a subset of chat history which are written by a client. The client receives an event about what part of incoming history is read by others along the updated unread count:

// @description Incoming messages were read or number of unread messages has been
// changed @chat_id Chat identifier @last_read_inbox_message_id Identifier of the
// last read incoming message @unread_count The number of unread messages left in
// the chat

updateChatReadInbox
    - chat_id:int53
    - last_read_inbox_message_id:int53
    - unread_count:int32 = Update;

A separate event on what part of the messages being sent by the client are read by others:

// @description Outgoing messages were read @chat_id Chat identifier
// @last_read_outbox_message_id Identifier of last read outgoing message

updateChatReadOutbox
    - chat_id:int53
    - last_read_outbox_message_id:int53 = Update;

Inbox and outbox messages ids are user-level pts numbers. Thus evaluating these events for every chat participant takes some effort for the backend system at they are individual and relative. It’s interesting what advantage this organization provides over an absolute history updates. Having outbox updates separately requires read history full scan to detect to which users the updates must be sent (imho). Then the chat histories read ranges must be somehow mapped to the users events space. Or if it uses a scan of a user events journal for finding out whose messages are getting read when a user calls for readHistory, they still must be remapped to the others pts spaces.

Channels and supergroups

They are fully separate entities. Their messages are no part of the users events journal (EVL). Big problem for implementing such big groups of 100000 users is in organization of the scalable fan-out and lazy updates.

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