Glossary

Action

In the Syndicated Actor Model, an action may be performed by an actor during a turn. Actions are quasi-transactional, taking effect only if their containing turn is committed.

Four core varieties of action, each with a matching variety of event, are offered across all realisations of the SAM:

An assertion action publishes an assertion at a target entity. A unique handle names the assertion action so that it may later be retracted. For more detail, see below on Assertions.
A retraction action withdraws a previously-published assertion from the target entity.
A message action sends a message to a target entity.
A synchronization action carries a local entity reference to a target entity. When it eventually reaches the target, the target will (by default) immediately reply with a simple acknowledgement to the entity reference carried in the request. For more detail, see below on Synchronization.

Beside the core four actions, many individual implementations offer action variants such as the following:

A spawn action will, when the containing turn commits, create a new actor running alongside the acting party. In many implementations, spawned actors may optionally be linked to the spawning actor.
Replacement of a previously-established assertion, "altering" the target entity reference and/or payload. This proceeds, conventionally, by establishment of the new assertion followed immediately by retraction of the old.

Finally, implementations may offer pseudo-actions whose effects are local to the acting party:

Creation of a new facet.
Creation of a new entity reference associated with the active facet denoting a freshly-created local entity.
Shutdown (stopping) of the active facet or any other facet within the acting party.
Stopping of the current actor, either gracefully or with a simulated crash.
Creation of a new field/cell/dataflow variable.
Creation of a new dataflow block.
Creation of a new linked task associated with the active facet.
Scheduling of a new one-off or periodic alarm.

The facet associated with the event currently being processed in an active turn.

In the Syndicated Actor Model, an actor is an isolated thread of execution. An actor repeatedly takes events from its mailbox, handling each in a turn. In many implementations of the SAM, each actor is internally structured as a tree of facets.

Alarm

See timeout.

Assertion

verb. To assert (or to publish) a value is to choose a target entity and perform an action conveying an assertion to that entity.
noun. An assertion is a value carried as the payload of an assertion action, denoting a relevant portion of a public aspect of the conversational state of the sending party that it has chosen to convey to the recipient entity.

The value carried in an assertion may, in some implementations, depend on one or more dataflow variables; in those implementations, when the contents of such a variable changes, the assertion is automatically withdrawn, recomputed, and re-published (with a fresh handle).

Attenuation

To attenuate a capability (yielding an attenuated capability), a sequence of filters is prepended to the possibly-empty list of filters attached to an existing capability. Each filter either discards, rewrites, or accepts unchanged any payload directed at the underlying capability. A special pattern language exists in the Syndicate network protocol for describing filters; many implementations also allow in-memory capabilities to be filtered by the same language.

Capability

(a.k.a. Cap) Used roughly interchangeably with "reference", connoting a security-, access-control-, or privacy-relevant aspect.

Cell

See dataflow variable.

Compositional

To quote the Stanford Encyclopedia of Philosophy, the "principle of compositionality" can be understood to be that

The meaning of a complex expression is determined by its structure and the meanings of its constituents.

People often implicitly intend "... and nothing else." For example, when I claim that the object-capability model is a compositional approach to system security, I mean that the access conveyed by an assemblage of capabilties can be understood in terms of the access conveyed by each individual capability taken in isolation, and nothing else.

Configuration Scripting Language

Main article: The Configuration Scripting Language

The syndicate-server program includes a scripting language, used for configuration of the server and its clients, population of initial dataspaces for the system that the syndicate-server instance is part of, and scripting of simple behaviours in reaction to appearance of assertions or transmission of messages.

The scripting language is documented here.

Conversational State

The collection of facts and knowledge held by a component participating in an ongoing conversation about some task that the component is undertaking:

The conversational state that accumulates as part of a collaboration among components can be thought of as a collection of facts. First, there are those facts that define the frame of a conversation. These are exactly the facts that identify the task at hand; we label them “framing knowledge”, and taken together, they are the “conversational frame” for the conversation whose purpose is completion of a particular shared task. Just as tasks can be broken down into more finely-focused subtasks, so can conversations be broken down into sub-conversations. In these cases, part of the conversational state of an overarching interaction will describe a frame for each sub-conversation, within which corresponding sub-conversational state exists. The knowledge framing a conversation acts as a bridge between it and its wider context, defining its “purpose” in the sense of the [Gricean Cooperative Principle]. [The following figure] schematically depicts these relationships.

Some facts define conversational frames, but every shared fact is contextualized within some conversational frame. Within a frame, then, some facts will pertain directly to the task at hand. These, we label “domain knowledge”. Generally, such facts describe global aspects of the common problem that remain valid as we shift our perspective from participant to participant. Other facts describe the knowledge or beliefs of particular components. These, we label “epistemic knowledge”.

— Excerpt from Chapter 2 of (Garnock-Jones 2017). The quoted section continues here.

In the Syndicated Actor Model, there is often a one-to-one correspondence between a facet and a conversational frame, with fate-sharing employed to connect the lifetime of the one with the lifetime of the other.

Dataflow

A programming model in which changes in stored state automatically cause re-evaluation of computations depending on that state. The results of such re-evaluations are themselves often used to update a store, potentially triggering further re-computation.

In the Syndicated Actor Model, dataflow appears in two guises: first, at a coarse granularity, among actors and entities in the form of changes in published assertions; and second, at fine granularity, many implementations include dataflow variables and dataflow blocks for intra-actor dataflow-based management of conversational state and related computation.

Dataflow Block

Implementations of the Syndicated Actor Model often include some language feature or library operation for marking a portion of code as participating in dataflow, where changes in observed dataflow variables cause re-evaluation of the code block.

For example, in a Smalltalk implementation of the SAM,

a := Turn active cell: 1.
b := Turn active cell: 2.
sum := Turn active cell: 0.
Turn active dataflow: [sum value: a value + b value].

Later, as a and b have their values updated, sum will automatically be updated by re-evaluation of the block given to the dataflow: method.

Analogous code can be written in TypeScript:

field a: number = 1;
field b: number = 2;
field sum: number = 0;
dataflow {
    sum.value = a.value + b.value;
}

in Racket:

(define-field a 1)
(define-field b 2)
(define/dataflow sum (+ (a) (b)))

in Python:

a = turn.field(1)
b = turn.field(2)
sum = turn.field(0)
@turn.dataflow
def maintain_sum():
    sum.value = a.value + b.value

and in Rust:

turn.dataflow(|turn| {
    let a_value = turn.get(&a);
    let b_value = turn.get(&b);
    turn.set(&sum, a_value + b_value);
})

Dataflow Variable

(a.k.a. Field, Cell) A dataflow variable is a store for a single value, used with dataflow blocks in dataflow programming.

When the value of a dataflow variable is read, the active dataflow block is marked as depending on the variable; and when the value of the variable is updated, the variable is marked as damaged, leading eventually to re-evaluation of dataflow blocks depending on that variable.

Dataspace

In the Syndicated Actor Model, a dataspace is a particular class of entity with prescribed behaviour. Its role is to route and replicate published assertions according to the declared interests of its peers.

See here for a full explanation of dataspaces.

Dataspace Pattern

In the Syndicated Actor Model, a dataspace pattern is a structured value describing a pattern over other values. The pattern language used in current Dataspace implementations and in the Syndicate protocol is documented here.

E

The E programming language is an object-capability model Actor language that has strongly influenced the Syndicated Actor Model.

Many good sources exist describing the language and its associated philosophy, including:

The ERights.org website, the home of E
E (programming language) on Wikipedia
Miller, Mark S. “Robust Composition: Towards a Unified Approach to Access Control and Concurrency Control.” PhD, Johns Hopkins University, 2006. [PDF]
Miller, Mark S., E. Dean Tribble, and Jonathan Shapiro. “Concurrency Among Strangers.” In Proc. Int. Symp. on Trustworthy Global Computing, 195–229. Edinburgh, Scotland, 2005. [DOI] [PDF]

Embedded References

In the Syndicated Actor Model, the values carried by assertions and messages may include references to entities. Because the SAM uses Preserves as its data language, the Preserves concept of an embedded value is used in the SAM to reliably mark portions of a datum referring to SAM entities.

Concretely, in Preserves text syntax, embedded values appear prepended with #:. In messages transferred across links using the Syndicate network protocol, references might appear as #:[0 123], #:[1 555], etc. etc.

Entity

In the Syndicated Actor Model, an entity is a stateful programming-language construct, located within an actor, that is the target of events. Each entity has its own behaviour, specifying in code how it responds to incoming events.

An entity is the SAM analogue of "object" in E-style languages: an addressable construct logically contained within and fate-sharing with an actor. The concept of "entity" differs from "object" in that entities are able to respond to assertions, not just messages.

In many implementations of the SAM, entities fate-share with individual facets within their containing actor rather than with the actor as a whole: when the facet associated with an entity is stopped, the entity becomes unresponsive.

Erlang

Erlang is a process-style Actor language that has strongly influenced the Syndicated Actor Model. In particular, Erlang's approach to failure-handling, involving supervisors arranged in supervision trees and processes (actors) connected via links and monitors, has been influential on the SAM. In the SAM, links and monitors become special cases of assertions, and Erlang's approach to process supervision is used directly and is an important aspect of SAM system organisation.

Event

In the Syndicated Actor Model, an event is processed by an entity during a turn, and describes the outcome of an action taken by some other actor.

Events come in four varieties corresponding to the four core actions in the SAM:

An assertion event notifies the recipient entity of an assertion published by some peer. A unique handle names the event so that later retraction of the assertion can be correlated with the assertion event.
A retraction event notifies the recipient entity of withdrawal of a previously-published assertion.
A message event notifies the recipient entity of a message sent by some peer.
A synchronization event, usually not handled explicitly by an entity, carries an entity reference. The recipient should arrange for an acknowledgement to be delivered to the referenced entity once previously-received events that might modify the recipient's state (or the state of a remote entity that it is proxy for) have been completely processed. For more detail, see below on Synchronization.

In many implementations of the Syndicated Actor Model, a facet is a programming-language construct representing a conversation and corresponding to a conversational frame. Facets are similar to the "nested threads" of Martin Sústrik's idea of Structured Concurrency (see also Wikipedia).

Every actor is structured as a tree of facets. (Compare and contrast with the diagram in the entry for Conversational State.)

Every facet is either "running" or "stopped". Each facet is the logical owner of zero or more entities as well as of zero or more published assertions. A facet's entities and published assertions share its fate. While a facet is running, its associated entities are responsive to incoming events; when it stops, its entities become permanently unresponsive. A stopped facet never starts running again. When a facet is stopped, all its assertions are retracted and all its subfacets are also stopped.

Facets may have stop handlers associated with them: when a facet is stopped, its stop handlers are executed, one at a time. The stop handlers of each facet are executed before the stop handlers of its parent and before its assertions are withdrawn.

Facets may be explicitly stopped by a stop action, or implicitly stopped when an actor crashes. When an actor crashes, its stop handlers are not run: stop handlers are for orderly processing of conversation termination. Instead, many implementations allow actors to have associated crash handlers which run only in case of an actor crash. In the limit, of course, even crash handlers cannot be guaranteed to run, because the underlying hardware or operating system may suffer some kind of catastrophic failure.

A design principle from large-scale network engineering, due to David Clark:

The fate-sharing model suggests that it is acceptable to lose the state information associated with an entity if, at the same time, the entity itself is lost.

— David D. Clark, “The Design Philosophy of the DARPA Internet Protocols.” ACM SIGCOMM Computer Communication Review 18, no. 4 (August 1988): 106–14. [DOI]

In the Syndicated Actor Model, fate-sharing is used in connecting the lifetime of conversational state with the programming language representation of a conversational frame, a facet.

Field

See dataflow variable.

Handle

In the Syndicated Actor Model, every assertion action (and the corresponding event) includes a scope-lifetime-unique handle that denotes the specific action/event concerned, for purposes of later correlation with a retraction action.

Handles are, in many cases, implemented as unsigned integers, allocated using a simple scope-wide counter.

Initial OID

In the Syndicate network protocol, the initial OID is a special OID value understood by prior arrangement to denote an entity (specified by the "initial ref") owned by some remote peer across some network medium. The initial OID of a session is used to bootstrap activity within that session.

Initial Ref

In the Syndicate network protocol, the initial ref is a special entity reference associated by prior arrangement with an initial OID in a session in order to bootstrap session activity.

Linked Actor

Many implementations of the Syndicated Actor Model offer a feature whereby an actor can be spawned so that its root facet is linked to the spawning facet in the spawning actor, so that when one terminates, so does the other (by default).

Links are implemented as a pair of "presence" assertions, atomically established at the time of the spawn action, each indicating to a special entity with "stop on retraction" behaviour the presence of its peer. When one of these assertions is withdrawn, the targetted entity stops its associated facet, automatically terminating any subfacets and executing any stop handlers.

This allows a "parent" actor to react to termination of its child, perhaps releasing associated resources, and the corresponding "child" actor to be automatically terminated when the facet in its parent that spawned the actor terminates.

This idea is inspired by Erlang, whose "links" are symmetric, bidirectional, failure-propagating connections among Erlang processes (actors) and whose "monitors" are unidirectional connections similar to the individual "presence" assertions described above.

Linked Task

Many implementations of the Syndicated Actor Model offer the ability to associate a facet with zero or more native threads, coroutines, objects, or other language-specific representations of asynchronous activities. When such a facet stops (either by explicit stop action or by crash-termination of the facet's actor), its linked tasks are also terminated. By default, the converse is also the case: a terminating linked task will trigger termination of its associated facet. This allows for resource management patterns similar to those enabled by the related idea of linked actors.

Macaroon

A macaroon is an access token for authorization of actions in distributed systems. Macaroons were introduced in the paper:

“Macaroons: Cookies with Contextual Caveats for Decentralized Authorization in the Cloud.”, by Arnar Birgisson, Joe Gibbs Politz, Úlfar Erlingsson, Ankur Taly, Michael Vrable, and Mark Lentczner. In Proc. Network and Distributed System Security Symposium (NDSS), 2014. [PDF]

In the Syndicated Actor Model, a variation of the macaroon concept is used to represent "sturdyrefs". A sturdyref is a long-lived token authorizing interaction with some entity, which can be upgraded to a live entity reference by presenting it to a gatekeeper entity across a session of the Syndicate network protocol. (The term "sturdyref" is lifted directly from the E language and associated ecosystem.)

Mailbox

Every actor notionally has a mailbox which receives events resulting from its peers' actions. Each actor spends its existence waiting for an incoming event to appear in its mailbox, removing the event, taking a turn to process it, and repeating the cycle.

Membrane

A membrane is a structure used in implementations of the Syndicate network protocol to keep track of wire symbols.

Message

In the Syndicated Actor Model, a message is a value carried as the payload or body of a message action (and associated event), conveying transient information from some sending actor to a recipient entity.

Network

A network is a group of peers (actors), plus a medium of communication (a transport), an addressing model (references), and an associated scope.

Object-Capability Model

The Object-capability model is a compositional means of expressing access control in a distributed system. It has its roots in operating systems research stretching back decades, but was pioneered in a programming language setting by the E language and the Scheme dialect W7.

In the Syndicated Actor Model, object-capabilities manifest as potentially-attenuated entity references.

Observe

In the Syndicated Actor Model, assertion of an Observe record at a dataspace declares an interest in receiving notifications about matching assertions and messages as they are asserted, retracted and sent through the dataspace.

Each Observe record contains a dataspace pattern describing a structural predicate over assertion and message payloads, plus an entity reference to the entity which should be informed as matching events appear at the dataspace.

OID

An OID is an "object identifier", a small, session-unique integer acting as an entity reference across a transport link in an instance of the Syndicate network protocol.

Publishing

To publish something is to assert it; see assertion.

Preserves

Main article: Preserves

Many implementations of the SAM use Preserves, a programming-language-independent language for data, as the language defining the possible values that may be exchanged among entities in assertions and values.

See the chapter on Preserves in this manual for more information.

Record

The Preserves data language defines the notion of a record, a tuple containing a label and zero or more numbered fields. The dataspace pattern language used by dataspaces allows for patterns over records as well as over other compound data structures.

Reference

(a.k.a. Ref, Entity Reference, Capability) A reference is a pointer or handle denoting a live, stateful entity running within an actor. The entity accepts Preserves-format messages and/or assertions. The capability may be attenuated to restrict the messages and assertions that may be delivered to the denoted entity by way of this particular reference.

Retraction

In the Syndicated Actor Model, a retraction is an action (and corresponding event) which withdraws a previous assertion. Retractions can be explicitly performed within a turn, or implicitly performed during facet shutdown or actor termination (both normal termination and crash stop).

The SAM guarantees that an actor's assertions will be retracted when it terminates, no matter whether an orderly shutdown or an exceptional or crashing situation was the cause.

Relay

A relay connects scopes, allowing references to denote entities resident in remote networks, making use of the Syndicate network protocol to do so.

See the Syndicate network protocol for more on relays.

Relay Entity

A relay entity is a local proxy for an entity at the other side of a relay link. It forwards events delivered to it across its transport to its counterpart at the other end.

See the Syndicate network protocol for more on relay entities.

S6

S6, "Skarnet's Small Supervision Suite", is

a small suite of programs for UNIX, designed to allow process supervision (a.k.a service supervision), in the line of daemontools and runit, as well as various operations on processes and daemons.

— The S6 website

Synit uses s6-log to capture standard error output from the root system bus.

Schema

A schema defines a mapping between values and host-language types in various programming languages. The mapping describes how to parse values into host-language data, as well as how to unparse host-language data, generating equivalent values. Another way of thinking about a schema is as a specification of the allowable shapes for data to be used in a particular context.

Synit, and many programs making use of the Syndicated Actor Model, uses Preserves' schema language to define schemas for many different applications.

For more, see the section on schemas in the chapter on Preserves.

Scope

A scope maps refs to the entities they denote. Scopes exist in one-to-one relationship to networks. Because message bodies and asserted values contain embedded references, each message and assertion transmitted via some network is also inseparable from its scope.

Most actors will participate in a single scope. However, relay actors participate in two or more scopes, translating refs back and forth as messages and assertions traverse the relay.

Examples.

A process is a scope for in-memory values: in-memory refs contain direct pointers to entities, which cannot be interpreted outside the context of the process's address space. The "network" associated with the process's scope is the intra-process graph of object references.
A TCP/IP socket (or serial link, or WebSocket, or Unix socket, etc.) is a scope for values travelling between two connected processes: refs on the wire denote entities owned by one or the other of the two participants. The "network" for a socket's scope is exactly the two connected peers (NB. and is not the underlying TCP/IP network, HTTP network, or Unix kernel that supports the point-to-point link).
An ethernet segment is a scope for values broadcast among stations: the embedded refs are (MAC address, OID) pairs. The network is the set of participating peers.
A running web page is a scope for the JavaScript objects it contains: both local and remote entities are represented by JavaScript objects. The "network" is the JavaScript heap.

Subscription

See observation.

Supervision tree

A supervision tree is a concept borrowed from Erlang, where a root supervisor supervises other supervisors, which in turn supervise worker actors engaged in some task. As workers fail, their supervisors restart them; if the failures are too severe or too frequent, their direct supervisors fail in turn, and the supervisors' supervisors take action to recover from the failures.

Supervisor

A supervisor is an actor or facet whose role is to monitor the state of some service, taking action to ensure its availability to other portions of a complete system. When the service fails, the supervisor is able to restart it. If the failures are too severe or too frequent, the supervisor can take an alternative action, perhaps pausing for some time before retrying the service, or perhaps even terminating itself to give its own supervisor in a supervision tree a chance to get things back on track.

Synit uses supervisors extensively to monitor system daemons and other system services.

Sync Peer Entity

The sync peer entity is the entity reference carried in a synchronization action or event.

Synchronization

An actor may synchronize with an entity by scheduling a synchronization action targeted at that entity. The action will carry a local entity reference acting as a continuation. When the target entity eventually responds, it will transmit an acknowledgement to the continuation entity reference carried in the request.

An entity receiving a synchronization event should arrange for an acknowledgement to be delivered to the referenced continuation entity once previously-received events that might modify the recipient's state (or the state of a remote entity that it is proxy for) have been completely processed.

Most entities do not explicitly include code for responding to synchronization requests. The default code, which simply replies to the continuation immediately, usually suffices. However, sometimes the default is not appropriate. For example, when relay entity is proxying for some remote entity via a relay across a transport, it should react to synchronization events by forwarding them to the remote entity. When the remote entity receives the forwarded request, it will reply to its local proxy for the continuation entity, which will in turn forward the reply back across the transport.

Syndicate Protocol

Main article: The Syndicate Protocol

The Syndicate Protocol (a.k.a the Syndicate Network Protocol) allows relays to proxy entities from remote scopes into the local scope.

For more, see the protocol specification document.

Syndicated Actor Model

Main article: The Syndicated Actor Model

The Syndicated Actor Model (often abbreviated SAM) is the model of concurrency and communication underpinning Synit. The SAM offers a “conversational” metaphor: programs meet and converse in virtual locations, building common understanding of the state of their shared tasks.

In the SAM, source entities running within an actor publish assertions and send messages to target entities, possibly in other actors. The essential idea of the SAM is that state replication is more useful than message-passing; message-passing protocols often end up simulating state replication.

A thorough introduction to the Syndicated Actor Model is available.

System Layer

The system layer is an essential part of an operating system, mediating between user-facing programs and the kernel. It provides the technical foundation for many qualities relevant to system security, resilience, connectivity, maintainability and usability.

The concept of a system layer has only been recently recognised—the term itself was coined by Benno Rice in a 2019 conference presentation—although many of the ideas it entails have a long history.

The hypothesis that the Synit system explores is that the Syndicated Actor model provides a suitable theoretical and practical foundation for a system layer. The system layer demands, and the SAM supplies, well-integrated expression of features such as service naming, presence, discovery and activation; security mechanism and policy; subsystem isolation; and robust handling of partial failure.

System Dataspace

The system dataspace in Synit is the primary dataspace entity, owned by an actor running within the root system bus, and (selectively) made available to daemons, system services, and user programs.

Timeout

Many implementations of the Syndicated Actor Model offer actions for establishing timeouts, i.e. one-off or repeating alarms. Timeouts are frequently implemented as linked tasks.

Transport

A transport is the underlying medium connecting one relay to its counterpart(s) in an instance of the Syndicate network protocol. For example, a TLS-on-TCP/IP socket may connect a pair of relays to one another, or a UDP multicast socket may connect an entire group of relays across an ethernet.

Turn

Each time an event arrives at an actor's mailbox, the actor takes a turn. A turn is the process of handling the triggering event, from the moment of its withdrawal from the mailbox to the moment of the completion of its interpretation.

Relatedly, the programming-language representation of a turn is a convenient place to attach the APIs necessary for working with the Syndicated Actor Model. In many implementations, some class named Turn or similar exposes methods corresponding to the actions available in the SAM.

In the SAM, a turn comprises

the event that triggered the turn,
the entity addressed by the event,
the facet owning the targeted entity, and
the collection of pending actions produced during execution.

If a turn proceeds to completion without an exception or other crash, its pending actions are committed (finalised and/or delivered to their target entities). If, on the other hand, the turn is aborted for some reason, its pending actions are rolled back (discarded), the actor is terminated, its assertions retracted, and all its resources released.

Value

A Preserves Value with embedded data. The embedded data are often embedded references but, in some implementations, may be other kinds of datum. Every message body and every assertion payload is a value.

Wire Symbol

A wire symbol is a structure used in implementations of the Syndicate network protocol to maintain a connection between an in-memory entity reference and the equivalent name for the entity as used in packets sent across the network.

The Synit Manual