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

layout	name	title	date	author	published	permalink	categories	summary	image
post	P2P Data Sync with a Remote Log	P2P Data Sync with a Remote Log	2019-10-01 12:00:00 +0800	oskarth	false	/remote-log	research	A research log. Asynchronous P2P messaging? Remote logs to the rescue!	/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 battery-draining, 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/proof-of-concept can be found here. The rest of this post follows the current spec closely.

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.

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 in 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

vac.mvds.Message payloads are the only payloads that MUST be uploaded. Other messages types MAY be uploaded, depending on the implementation.

Future work

The spec is still in an early draft stage, so it is expected to change. Same with the proof of concept. More work is needed on getting a fully featured proof of concept with specific CAS and NAS instances. E.g. Swarm and Swarm Feeds, or IPFS and IPNS, or something else.

For data sync in general:

• Make consistency guarantees more explicit for app developers with support for sequence numbers and DAGs, as well as the ability to send non-synced messages. E.g. ephemeral typing notifications, linear/sequential history and casual consistency/DAG history
• Better semantics and scalability for multi-user sync contexts, e.g. CRDTs and joining multiple logs together
• Better usability in terms of application layer usage (data sync clients) and supporting more transports

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PS. 1 Thanks everyone who submitted great logo proposals for Vac! PPS. 2 Next week on October 10th decanus and I will be presenting Vac at Devcon, come say hi :)