vac.dev/_posts/2019-10-01-remote-log.md

layout	name	title	date	author	published	permalink	categories	summary	image
post	Remote log	P2P Data Sync with a Remote Log	2019-10-01 12:00:00 +0800	oskarth	true	/remote-log	research	A research log. Reliable and decentralized, pick two.	/assets/img/remote_log.png

A big problem when doing end-to-end data sync between mobile nodes is that most devices are offline most of the time. With a naive approach, you quickly run into issues of 'ping-pong' behavior, where messages have to be constantly retransmitted. We saw some basic calculations of what this bandwidth multiplier looks like in a previous post.

While you could do some background processing, this is really draining the battery, and on iOS these capabilities are limited. A better approach instead is to loosen the constraint that two nodes need to be online at the same time. How do we do this? There are two main approaches, one is the store and forward model, and the other is a remote log.

In the store and forward model, we use an intermediate node that forward messages on behalf of the recipient. In the remote log model, you instead replicate the data onto some decentralized storage, and have a mutable reference to the latest state, similar to DNS. While both work, the latter is somewhat more elegant and "pure", as it has less strict requirements of an individual node's uptime. Both act as a highly-available cache to smoothen over non-overlapping connection windows between endpoints.

In this post we are going to describe how such a remote log schema could work. Specifically, how it enhances p2p data sync and takes care of the following requirements:

3. MUST allow for mobile-friendly usage. By mobile-friendly we mean devices that are resource restricted, mostly-offline and often changing network.

4. MAY use helper services in order to be more mobile-friendly. Examples of helper services are decentralized file storage solutions such as IPFS and Swarm. These help with availability and latency of data for mostly-offline devices.

Remote log

A remote log is a replication of a local log. This means a node can read data from a node that is offline.

The spec is in an early draft stage and can be found here. A very basic spike can be found here.

Definitions

Term	Definition
CAS	Content-addressed storage. Stores data that can be addressed by its hash.
NS	Name system. Associates mutable data to a name.
Remote log	Replication of a local log at a different location.

Payloads

Payloads are implemented using protocol buffers v3.

CAS service:

syntax = "proto3";

package vac.cas;

service CAS {
  rpc Add(Content) returns (Address) {}
  rpc Get(Address) returns (Content) {}
}

message Address {
  bytes id = 1;
}

message Content {
  bytes data = 1;
}

NS service:

syntax = "proto3";

package vac.cas;

service NS {
  rpc Update(NameUpdate) returns (Response) {}
  rpc Fetch(Query) returns (Content) {}
}

message NameUpdate {
  string name = 1;
  bytes content = 2;
}

message Query {
  string name = 1;
}

message Content {
  bytes data = 1;
}

message Response {
  bytes data = 1;
}

Remote log:

syntax = "proto3";

package vac.cas;

message RemoteLog {
  repeated Pair pair = 1;
  bytes tail = 2;

  message Pair {
    bytes remoteHash = 1;
    bytes localHash = 2;
    bytes data = 3;
  }
}

Synchronization

Roles

There are four fundamental roles:

1. Alice
2. Bob
3. Name system (NS)
4. Content-addressed storage (CAS)

The remote log protobuf is what is stored at the Name system.

"Bob" can represent anything from 0 to N participants. Unlike Alice, Bob only needs read-only access to NS and CAS.

Flow

Figure 1: Remote log data synchronization.

Remote log

The remote log lets receiving nodes know what data they are missing. Depending on the specific requirements and capabilities of the nodes and name system, the information can be referred to differently. We distinguish between three rough modes:

1. Fully replicated log
2. Normal sized page with CAS mapping
3. "Linked list" mode - minimally sized page with CAS mapping

Data format:

| H1_3 | H2_3 |
| H1_2 | H2_2 |
| H1_1 | H2_1 |
| ------------|
| next_page   |

Here the upper section indicates a list of ordered pairs, and the lower section contains the address for the next page chunk. H1 is the native hash function, and H2 is the one used by the CAS. The numbers corresponds to the messages.

To indicate which CAS is used, a remote log SHOULD use a multiaddr.

Embedded data:

A remote log MAY also choose to embed the wire payloads that corresponds to the native hash. This bypasses the need for a dedicated CAS and additional round-trips, with a trade-off in bandwidth usage.

| H1_3 | | C_3 |
| H1_2 | | C_2 |
| H1_1 | | C_1 |
| -------------|
| next_page    |

Here C stands for the content that would be stored at the CAS.

Both patterns can be used in parallel, e,g. by storing the last k messages directly and use CAS pointers for the rest. Together with the next_page page semantics, this gives users flexibility in terms of bandwidth and latency/indirection, all the way from a simple linked list to a fully replicated log. The latter is useful for things like backups on durable storage.

Next page semantics

The pointer to the 'next page' is another remote log entry, at a previous point in time.

Interaction with MVDS

TBD.

Future work

TBD.