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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.
  • Participant ID: Each participant has a globally unique, immutable ID visible to other participants in the communication.
  • Sender ID: The Participant ID of the original sender of a message, often coupled with a 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 HistoryEntry {
  string message_id = 1; // Unique identifier of the SDS message, as defined in `Message`
  optional bytes retrieval_hint = 2; // Optional information to help remote parties retrieve this SDS message; For example, A Waku deterministic message hash or routing payload hash

  optional string sender_id = 3; // Participant ID of original message sender. Only populated if using optional SDS Repair extension
}

message Message {
  string sender_id = 1;           // Participant ID of the message sender
  string message_id = 2;          // Unique identifier of the message
  string channel_id = 3;          // Identifier of the channel to which the message belongs
  optional uint64 lamport_timestamp = 10;    // Logical timestamp for causal ordering in channel
  repeated HistoryEntry 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

  repeated HistoryEntry repair_request = 13; // Capped list of history entries missing from sender's causal history. Only populated if using the optional SDS Repair extension.

  optional bytes content = 20;             // Actual content of the message
}

The sending participant MUST include its own globally unique identifier in the sender_id field. In addition, it MUST include a globally unique identifier for the message 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

> Note: Close readers may notice that, outside of filtering messages originating from the sender itself, the sender_id field is not used for much. Its importance is expected to increase once a p2p retrieval mechanism is added to SDS, as is planned for the protocol.

Participant state​

Each participant MUST maintain:

  • A Lamport timestamp for each channel of communication, initialized to current epoch time in millisecond resolution. The Lamport timestamp is increased as described in the protocol steps to maintain a logical ordering of events while staying close to the current epoch time. This allows the messages from new joiners to be correctly ordered with other recent messages, without these new participants first having to synchronize past messages to discover the current Lamport timestamp.
  • 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 set its local Lamport timestamp to the maximum between the current value + 1 and the current epoch time in milliseconds. In other words the local Lamport timestamp is set to max(timeNowInMs, current_lamport_timestamp + 1).
  • the participant MUST include the increased Lamport timestamp in the message's 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 MAY include a retrieval_hint in the HistoryEntry for each message ID in the causal_history field. This is an application-specific field to facilitate retrieval of messages, e.g. from high-availability caches.
  • 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 SHOULD ignore the message if it has a sender_id matching its own.
  • the participant MAY deduplicate the message by comparing its message_id to previously received message IDs.
  • the participant MUST review the ACK status of messages in its unacknowledged outgoing buffer using the received message's causal history and bloom filter.
  • if the message has a populated content field, 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 and the message has a populated content field, 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. It MAY use the application-specific retrieval_hint in the HistoryEntry to facilitate retrieval.
  • 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 sync messages to maintain state. These sync messages:

  • MUST be sent with empty content
  • MUST include a Lamport timestamp increased to max(timeNowInMs, current_lamport_timestamp + 1), where timeNowInMs is the current epoch time in milliseconds.
  • MUST include causal history and bloom filter according to regular message rules
  • MUST NOT be added to the unacknowledged outgoing buffer
  • MUST NOT be included in causal histories of subsequent messages
  • MUST NOT be included in bloom filters
  • MUST NOT be added to the local log

Since sync messages are not persisted, they MAY have non-unique message IDs without impacting the protocol. 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 the causal history and bloom filter of these sync messages following the same steps as regular messages, but MUST NOT persist the sync messages themselves.

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.

SDS Repair (SDS-R)​

SDS Repair (SDS-R) is an optional extension module for SDS, allowing participants in a communication to collectively repair any gaps in causal history (missing messages) preferably over a limited time window. Since SDS-R acts as coordinated rebroadcasting of missing messages, which involves all participants of the communication, it is most appropriate in a limited use case for repairing relatively recent missed dependencies. It is not meant to replace mechanisms for long-term consistency, such as peer-to-peer syncing or the use of a high-availability centralised cache (Store node).

SDS-R message fields​

SDS-R adds the following fields to SDS messages:

  • sender_id in HistoryEntry: the original message sender's participant ID. This is used to determine the group of participants who will respond to a repair request.
  • repair_request in Message: a capped list of history entries missing for the message sender and for which it's requesting a repair.

SDS-R participant state​

SDS-R adds the following to each participant state:

  • Outgoing repair request buffer: a list of locally missing HistoryEntrys each mapped to a future request timestamp, T_req, after which this participant will request a repair if at that point the missing dependency has not been repaired yet. T_req is computed as a pseudorandom backoff from the timestamp when the dependency was detected missing. Determining T_req is described below. We RECOMMEND that the outgoing repair request buffer be chronologically ordered in ascending order of T_req.
  • Incoming repair request buffer: a list of locally available HistoryEntrys that were requested for repair by a remote participant AND for which this participant might be an eligible responder, each mapped to a future response timestamp, T_resp, after which this participant will rebroadcast the corresponding requested Message if at that point no other participant had rebroadcast the Message. T_resp is computed as a pseudorandom backoff from the timestamp when the repair was first requested. Determining T_resp is described below. We describe below how a participant can determine if they're an eligible responder for a specific repair request.
  • Augmented local history log: for each message ID kept in the local log for which the participant could be a repair responder, the full SDS Message must be cached rather than just the message ID, in case this participant is called upon to rebroadcast the message. We describe below how a participant can determine if they're an eligible responder for a specific message.

Note: The required state can likely be significantly reduced in future by simply requiring that a responding participant should reconstruct the original Message when rebroadcasting, rather than the simpler, but heavier, requirement of caching the entire received Message content in local history.

SDS-R global state​

For a specific channel (that is, within a specific SDS-controlled communication) the following SDS-R configuration state SHOULD be common for all participants in the conversation:

  • T_min: the minimum time period to wait before a missing causal entry can be repaired. We RECOMMEND a value of at least 30 seconds.
  • T_max: the maximum time period over which missing causal entries can be repaired. We RECOMMEND a value of between 120 and 600 seconds.

Furthermore, to avoid a broadcast storm with multiple participants responding to a repair request, participants in a single channel MAY be divided into discrete response groups. Participants will only respond to a repair request if they are in the response group for that request. The global num_response_groups variable configures the number of response groups for this communication. Its use is described below. A reasonable default value for num_response_groups is one response group for every 128 participants. In other words, if the (roughly) expected number of participants is expressed as num_participants, then num_response_groups = num_participants div 128 + 1. In other words, if there are fewer than 128 participants in a communication, they will all belong to the same response group.

We RECOMMEND that the global state variables T_min, T_max and num_response_groups be set statically for a specific SDS-R application, based on expected number of group participants and volume of traffic.

Note: Future versions of this protocol will recommend dynamic global SDS-R variables, based on the current number of participants.

SDS-R send message​

SDS-R adds the following steps when sending a message:

Before broadcasting a message,

  • the participant SHOULD populate the repair_request field in the message with eligible entries from the outgoing repair request buffer. An entry is eligible to be included in a repair_request if its corresponding request timestamp, T_req, has expired (in other words, T_req <= current_time). The maximum number of repair request entries to include is up to the application. We RECOMMEND that this quota be filled by the eligible entries from the outgoing repair request buffer with the lowest T_req. We RECOMMEND a maximum of 3 entries. If there are no eligible entries in the buffer, this optional field MUST be left unset.

SDS-R receive message​

On receiving a message,

  • the participant MUST remove entries matching the received message ID from its outgoing repair request buffer. This ensures that the participant does not request repairs for dependencies that have now been met.
  • the participant MUST remove entries matching the received message ID from its incoming repair request buffer. This ensures that the participant does not respond to repair requests that another participant has already responded to.
  • the participant SHOULD add any unmet causal dependencies to its outgoing repair request buffer against a unique T_req timestamp for that entry. It MUST compute the T_req for each such HistoryEntry according to the steps outlined in Determine T_req.
  • for each item in the repair_request field:
    • the participant MUST remove entries matching the repair message ID from its own outgoing repair request buffer. This limits the number of participants that will request a common missing dependency.
    • if the participant has the requested Message in its local history and is an eligible responder for the repair request, it SHOULD add the request to its incoming repair request buffer against a unique T_resp timestamp for that entry. It MUST compute the T_resp for each such repair request according to the steps outlined in Determine T_resp. It MUST determine if it's an eligible responder for a repair request according to the steps outlined in Determine response group.

Determine T_req​

A participant determines the repair request timestamp, T_req, for a missing HistoryEntry as follows:

T_req = current_time + hash(participant_id, message_id) % (T_max - T_min) + T_min

where current_time is the current timestamp, participant_id is the participant's own participant ID (not the sender_id in the missing HistoryEntry), message_id is the missing HistoryEntry's message ID, and T_min and T_max are as set out in SDS-R global state.

This allows T_req to be pseudorandomly and linearly distributed as a backoff of between T_min and T_max from current time.

> Note: placing T_req values on an exponential backoff curve will likely be more appropriate and is left for a future improvement.

Determine T_resp​

A participant determines the repair response timestamp, T_resp, for a HistoryEntry that it could repair as follows:

distance = hash(participant_id) XOR hash(sender_id)
T_resp = current_time + distance*hash(message_id) % T_max

where current_time is the current timestamp, participant_id is the participant's own (local) participant ID, sender_id is the requested HistoryEntry sender ID, message_id is the requested HistoryEntry message ID, and T_max is as set out in SDS-R global state.

We first calculate the logical distance between the local participant_id and the original sender_id. If this participant is the original sender, the distance will be 0. It should then be clear that the original participant will have a response backoff time of 0, making it the most likely responder. The T_resp values for other eligible participants will be pseudorandomly and linearly distributed as a backoff of up to T_max from current time.

> Note: placing T_resp values on an exponential backoff curve will likely be more appropriate and is left for a future improvement.

Determine response group​

Given a message with sender_id and message_id, a participant with participant_id is in the response group for that message if

hash(participant_id, message_id) % num_response_groups == hash(sender_id, message_id) % num_response_groups

where num_response_groups is as set out in SDS-R global state. This ensures that a participant will always be in the response group for its own published messages. It also allows participants to determine immediately on first reception of a message or a history entry if they are in the associated response group.

SDS-R incoming repair request buffer sweep​

An SDS-R participant MUST periodically check if there are any incoming requests in the incoming repair request buffer that is due for a response. For each item in the buffer, the participant SHOULD broadcast the corresponding Message from local history if its corresponding response timestamp, T_resp, has expired (in other words, T_resp <= current_time).

SDS-R Periodic Sync Message​

If the participant is due to send a periodic sync message, it SHOULD send the message according to SDS-R send message if there are any eligible items in the outgoing repair request buffer, regardless of whether other participants have also recently broadcast a Periodic Sync message.

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