Handshakes

Each object- or db-level log sync starts with a handshake. In that handshake, parties must introduce themselves (if necessary), describe the entity being subscribed to, their existing states, signal access modes (read/write), and so on. The full subscription life cycle has four stages:

1. handshake,
2. exchange of patches,
3. continuous update exchange,
4. closing handshake.

Handshake is a two-phase request-response exchange of .on pseudo-ops. The initiator of the handshake is also denoted as a downstream, the receiver as an upstream, message propagation directions are named accordingly (downstream is from the upstream to the downstream). Handshake's two .on messages are named "initial" and "return" .on. A typical per-object initial .on spec looks like:

> /Object#1CQQ3+XaUth1_K!1CQQ3+XaUth1_K.on+Xagn0071xQ

It contains

• the type /Object and the id #1CQQ3+XaUth1_K of the object being subscribed to;
• pseudo-op .on scoped to the subscribing replica Xagn0071xQ (a scoped op is not relayed globally);
• version stamp !1CQQ3+XaUth1_K of the recentmost op/state of that object received from the same peer before (!0 if none).

Note that Swarm op log is partially ordered. Hence, in the general case, a position in the log can only be described by a version vector. Swarm avoids VV use by relying on local de-facto arrival orders, which are linear. A position in a particular local log can be described by a single logical timestamp.

An active subscription is terminated by an .off operation having essentially the same attributes.

The value of the .on op is unspecified; it may contain e.g. access credentials. In the case of .off, the value is understood as an error message.

Three distinct types of handshakes are:

• peer handshake to subscribe to the entire database log,
• client handshake to make a partial-dataset client subscription, and
• object handshake to subscribe a client to a particular object.

Database-level and object-level handshakes have the same syntax. Database subscriptions are made by subscribing to the metadata object of the database. That minimizes the overhead and the number of primitives.