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SDS

Abstract​

This specification introduces the Scalable Data Sync (SDS) protocol to achieve end-to-end reliability when consolidating distributed logs in a decentralized manner. The protocol is designed for a peer-to-peer (p2p) topology where an append-only log is maintained by each member of a group of nodes who may individually append new entries to their local log at any time and is interested in merging new entries from other nodes in real-time or close to real-time while maintaining a consistent order. The outcome of the log consolidation procedure is that all nodes in the group eventually reflect in their own logs the same entries in the same order. The protocol aims to scale to very large groups.

Motivation​

A common application that fits this model is a p2p group chat (or group communication), where the participants act as log nodes and the group conversation is modelled as the consolidated logs maintained on each node. The problem of end-to-end reliability can then be stated as ensuring that all participants eventually see the same sequence of messages in the same causal order, despite the challenges of network latency, message loss, and scalability present in any communications transport layer. The rest of this document will assume the terminology of a group communication: log nodes being the participants in the group chat and the logged entries being the messages exchanged between participants.

Design Assumptions​

We make the following simplifying assumptions for a proposed reliability protocol:

  • Broadcast routing: Messages are broadcast disseminated by the underlying transport. The selected transport takes care of routing messages to all participants of the communication.
  • Store nodes: There are high-availability caches (a.k.a. Store nodes) from which missed messages can be retrieved. These caches maintain the full history of all messages that have been broadcast. This is an optional element in the protocol design, but improves scalability by reducing direct interactions between participants.
  • Message ID: Each message has a globally unique, immutable ID (or hash). Messages can be requested from the high-availability caches or other participants using the corresponding message ID.

Wire protocol​

The keywords “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in 2119.

Message​

Messages MUST adhere to the following meta structure:

syntax = "proto3";

message Message {
  // 1 Reserved for sender/participant id
  string message_id = 2;          // Unique identifier of the message
  string channel_id = 3;          // Identifier of the channel to which the message belongs
  optional int32 lamport_timestamp = 10;    // Logical timestamp for causal ordering in channel
  optional repeated string causal_history = 11;  // List of preceding message IDs that this message causally depends on. Generally 2 or 3 message IDs are included.
  optional bytes bloom_filter = 12;         // Bloom filter representing received message IDs in channel
  optional bytes content = 20;             // Actual content of the message
}

Each message MUST include its globally unique identifier in the message_id field, likely based on a message hash. The channel_id field MUST be set to the identifier of the channel of group communication that is being synchronized. For simple group communications without individual channels, the channel_id SHOULD be set to 0. The lamport_timestamp, causal_history and bloom_filter fields MUST be set according to the protocol steps set out below. These fields MAY be left unset in the case of ephemeral messages. The message content MAY be left empty for periodic sync messages, otherwise it MUST contain the application-level content

Participant state​

Each participant MUST maintain:

  • A Lamport timestamp for each channel of communication, initialized to current epoch time in nanosecond resolution.
  • A bloom filter for received message IDs per channel. The bloom filter SHOULD be rolled over and recomputed once it reaches a predefined capacity of message IDs. Furthermore, it SHOULD be designed to minimize false positives through an optimal selection of size and hash functions.
  • A buffer for unacknowledged outgoing messages
  • A buffer for incoming messages with unmet causal dependencies
  • A local log (or history) for each channel, containing all message IDs in the communication channel, ordered by Lamport timestamp.

Messages in the unacknowledged outgoing buffer can be in one of three states:

  1. Unacknowledged - there has been no acknowledgement of message receipt by any participant in the channel
  2. Possibly acknowledged - there has been ambiguous indication that the message has been possibly received by at least one participant in the channel
  3. Acknowledged - there has been sufficient indication that the message has been received by at least some of the participants in the channel. This state will also remove the message from the outgoing buffer.

Protocol Steps​

For each channel of communication, participants MUST follow these protocol steps to populate and interpret the lamport_timestamp, causal_history and bloom_filter fields.

Send Message​

Before broadcasting a message:

  • the participant MUST increase its local Lamport timestamp by 1 and include this in the lamport_timestamp field.
  • the participant MUST determine the preceding few message IDs in the local history and include these in an ordered list in the causal_history field. The number of message IDs to include in the causal_history depends on the application. We recommend a causal history of two message IDs.
  • the participant MUST include the current bloom_filter state in the broadcast message.

After broadcasting a message, the message MUST be added to the participant’s buffer of unacknowledged outgoing messages.

Receive Message​

Upon receiving a message,

  • the participant MUST review the ACK status of messages in its unacknowledged outgoing buffer using the received message's causal history and bloom filter.
  • the participant MUST include the received message ID in its local bloom filter.
  • the participant MUST verify that all causal dependencies are met for the received message. Dependencies are met if the message IDs in the causal_history of the received message appear in the local history of the receiving participant.

If all dependencies are met, the participant MUST deliver the message. If dependencies are unmet, the participant MUST add the message to the incoming buffer of messages with unmet causal dependencies.

Deliver Message​

Triggered by the Receive Message procedure.

If the received message’s Lamport timestamp is greater than the participant's local Lamport timestamp, the participant MUST update its local Lamport timestamp to match the received message. The participant MUST insert the message ID into its local log, based on Lamport timestamp. If one or more message IDs with the same Lamport timestamp already exists, the participant MUST follow the Resolve Conflicts procedure.

Resolve Conflicts​

Triggered by the Deliver Message procedure.

The participant MUST order messages with the same Lamport timestamp in ascending order of message ID. If the message ID is implemented as a hash of the message, this means the message with the lowest hash would precede other messages with the same Lamport timestamp in the local log.

Review ACK Status​

Triggered by the Receive Message procedure.

For each message in the unacknowledged outgoing buffer, based on the received bloom_filter and causal_history:

  • the participant MUST mark all messages in the received causal_history as acknowledged.
  • the participant MUST mark all messages included in the bloom_filter as possibly acknowledged. If a message appears as possibly acknowledged in multiple received bloom filters, the participant MAY mark it as acknowledged based on probabilistic grounds, taking into account the bloom filter size and hash number.

Periodic Incoming Buffer Sweep​

The participant MUST periodically check causal dependencies for each message in the incoming buffer. For each message in the incoming buffer:

  • the participant MAY attempt to retrieve missing dependencies from the Store node (high-availability cache) or other peers.
  • if all dependencies of a message are met, the participant MUST proceed to deliver the message.

If a message's causal dependencies have failed to be met after a predetermined amount of time, the participant MAY mark them as irretrievably lost.

Periodic Outgoing Buffer Sweep​

The participant MUST rebroadcast unacknowledged outgoing messages after a set period. The participant SHOULD use distinct resend periods for unacknowledged and possibly acknowledged messages, prioritizing unacknowledged messages.

Periodic Sync Message​

For each channel of communication, participants SHOULD periodically send an empty-content message to maintain sync state, without incrementing the Lamport timestamp. To avoid network activity bursts in large groups, a participant MAY choose to only send periodic sync messages if no other messages have been broadcast in the channel after a random backoff period.

Participants MUST process these sync messages following the same steps as regular messages.

Ephemeral Messages​

Participants MAY choose to send short-lived messages for which no synchronization or reliability is required. These messages are termed ephemeral.

Ephemeral messages SHOULD be sent with lamport_timestamp, causal_history, and bloom_filter unset. Ephemeral messages SHOULD NOT be added to the unacknowledged outgoing buffer after broadcast. Upon reception, ephemeral messages SHOULD be delivered immediately without buffering for causal dependencies or including in the local log.

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