Dataspace Programming with Syndicate

1 Overview

Syndicate is an actor language where all communication occurs through a tightly controlled shared memory, dubbed the dataspace. The values in the dataspace are called assertions, representing the information that the actors in the system are currently sharing with each other. Assertions are read-only and owned by the actor that entered them into the dataspace. Only the originating actor has permission to withdraw an assertion. Assertions are linked to the lifetime of their actor, and are withdrawn from the dataspace when that actor exits, either normally or via exception.

To respond to assertions in the dataspace, an actor expresses an interest in the shape of assertions it wishes to receive. An interest is an assertion constructed with observe and wildcards where the actor wishes to receive any matching assertion. When an actor makes an assertion of interest, the dataspace dispatches the set of all matching assertions to that actor. Moreover, the dataspace keeps the actor up-to-date, informing it when a new assertion appears matching its interest, as well as when a matching assertion disappears from the dataspace. Thus, dataspaces implement a form of publish/subscribe communication.

In addition to assertions, dataspaces support instantaneous message broadcast. At the time a message is sent, all actors with a matching interest receive notification.

Thus, each individual Syndicate actor has three fudamental concerns:

• Defining local state and updating it in response to events;

• Publishing aspects of local state/knowledge as assertions; and

• Reacting to relevant assertions and messages.

Each concern is addressed by a separate language construct, which are collectively dubbed endpoints:

• The fields of an actor hold its state;

• An actor publishes information using assert; and

• An event-handler endpoint uses on to define reactions to particular messages and assertions.

Endpoints are tied together via dataflow. Thus, the assertions of an actor automatically reflect the current value of its fields.

Implementing an actor’s role in a particular conversation typically involves some combination of these behaviors; a facet is a collection of related endpoints constituting the actor’s participation in a particular conversation.

Each actor starts with a single facet, and may add new facets or terminate current ones in response to events. The facets of an actor form a tree, where the parent of a particular facet is the facet in which it was created. The tree structure affects facet shutdown; terminating a facet also terminates all of its descendants.

To recap: an actor is a tree of facets, each of which comprises of a collection of endpoints.

2 Programming Syndicate Actors with Facets

The actions permitted by the two contexts are mutually exclusive, and trying to perform an action in the wrong context will give rise to a run-time error.

Within the following descriptions, we use EI as a shorthand for expressions that execute in an endpoint-installation context and S for expressions in a script context.

2.1 Script Actions: Starting and Stopping Actors and Facets

An optionally provided name-expr is associated with the created actor. The name is only used for error and log messages, thus is mainly useful for debugging.

The actor may optionally be given some initial assertions, which come into being at the same time as the actor. (Otherwise, the actor spawns, then boots its initial facet(s), then establishes any ensuing assertions.) When assertion-expr is provided, the actors initial assertions are the result of interpreting the expression as a trie pattern, with ? giving rise to infinte sets. On the other hand, assertions-expr may be used to specify an entire set of initial assertions as an arbitrary trie.

The optional linkage-exprs are executed during facet startup; your simple documentation author is not sure why they are useful, as opposed to just putting them in the body of the spawn.

The optional script actions S ... function like a continuation. They run after the facet and all of its children finish shutting down, i.e. after all stop handlers have executed. Moreover, S ... runs in the context of the parent of fid. Thus, any facet created by the script survives termination and will have fid’s parent as its own parent.

Note that fid must be an ancestor of the current facet.

Allowed within a script.

Allowed within a script.

2.2 Installing Endpoints

Fields may optionally have a contract; the in contract is applied when writing to a field, while the (optional) out contract applies when reading a value from a field.

Allowed within an endpoint installation context.

If the expression exp refers to any fields, then the assertion created by the endpoint is automatically kept up-to-date each time any of those fields is updated. More specifically, the will issue a patch retracting the assertion of the previous value, replacing it with the results of reevaluating exp with the current values of each field.

Allowed within an endpoint installation context.

The actor will make an assertion of interest in events that could match event-description. Like with assert, the interest will be refreshed any time a field referenced within the event-description pattern changes.

The event handler can optionally be made conditional on a boolean expression by supplying a #:when predicate, in which case the endpoint only reacts to events, and only expresses the corresponding assertion of interest, when pred evaluates to a truthy value.

Allowed within an endpoint installation context.

(on (asserted pattern)

(react

EI ...

(on (retracted inst-pattern)

(stop-current-facet))))

where inst-pattern is the pattern with variables instantiated to their matching values.

Allowed within an endpoint installation context.

The assertion triggering the during/spawn may disappear before the spawned actor boots, in which case it fails to see the retraction event. To avoid potential glitches, the spawning actor maintains an assertion that lets the spawned actor know whether the originial assertion still exists.

The maybe-name and maybe-assertions have the same meaning they do for spawn, applied to the newly spawned actor.

The wrapper-stx serves as an interposition point; it may be provided to change the meaning of "spawning an actor" in response to an assertion. By default, it is #'spawn.

The optional #:let clauses can be used to read the values of fields in the spawning actor so that they can be used in the spawned actor. Otherwise, the spawned actor has no access to the parent’s fields, and trying to read or write to such a field will cause a runtime error.

The on-crash-expr provides a hook for script actions that can be performed in the spawning actor if the spawned actor crashes.

Allowed within an endpoint installation context.

Allowed within an endpoint installation context.

2.3 Handling Facet Startup and Shutdown

In addition to external events, such as assertion (dis)appearance and message broadcast, facets can react to their own startup and shutdown. This provides a handy way to perform initialization, cleanup, as well as setting up and tearing down resources.

Allowed within an endpoint installation context.

The script S ... differs from that of stop-facet in that it executes in the context of the terminating facet, not its parent. Thus, any facets created in S ... will start up and then immediately shut down.

Allowed within an endpoint installation context.

Note that a single facet may have any number of on-start and on-stop handlers, which do not compete with each other. That is, each on-start handler runs during facet startup and, likewise, each on-stop during facet shutdown.

2.4 Streaming Query Fields

Syndicate actors often aggregate information about current assertions as part of their local state, that is, in a field. Since these patterns are exceedingly common, Syndicate provides a number of forms for defining fields that behave as streaming queries over the assertions in the dataspace.

The optional on-add-expr is performed inside the on asserted handler, while on-remove-expr runs in the on retracted handler.

Allowed within an endpoint installation context.

The optional maybe-on-add and maybe-on-expr behave the same way they do for define/query-set.

Allowed within an endpoint installation context.

The optional maybe-on-add and maybe-on-expr behave the same way they do for define/query-set.

Allowed within an endpoint installation context.

The optional maybe-on-add and maybe-on-expr behave the same way they do for define/query-set.

Allowed within an endpoint installation context.

2.5 Generalizing Dataflow

The dataflow mechanism that automatically refreshes assert endpoints when a referenced field changes may be used to react to local state updates in arbitrary ways using begin/dataflow.

Conceptually, begin/dataflow may be thought of as an event handler endpoint in the vein of on, where the event of interest is update of local state.

Allowed within an endpoint installation context.

The value of name is either #f or, if provided, default-expr. This initial value is observable for a short time during facet startup.

Note that when a field referenced by expr changes, there may be some time before name refreshes, during which "stale" values may be read from the field.

Allowed within an endpoint installation context.

2.6 Generalizing Actor-Internal Communication

Talk about internal assertions and messages.

2.7 Nesting Dataspaces

Nested dataspaces, inbound and outbound assertions, quit-datapace.

Allowed within a script.

3 #lang syndicate Programs

In a #lang syndicate program, the results of top-level expressions define the initial group of actors in the dataspace. That is, evaluating spawn or dataspace in the context of the module top-level adds that actor specification to the initial dataspace of the program. For example, a module such as:

0 #lang syndicate

1

2 (define (spawn-fun)

3   (spawn ...))

4

5 (spawn ...)

6

7 (spawn-fun)

launches a syndicate program with two initial actors, one the result of the spawn expression on line 5 and one the result of evaluating the spawn expresion on line 3 during the course of calling spawn-fun on line 7.

The initial dataspace is referred to as the ground dataspace, and it plays a special role in Syndicate programming; see below.

4 Interacting with the Outside World

ground dataspace, drivers, etc.

5 Actors with an Agenda

Here we talk about spawn* and react/suspend.

6 Odds and Ends

The optional #:init [I ...] provides a sequence of initialization actions. The initial actions are executed before the ongoing behaviors begin but after the interests of the state actor are established.

The optional #:collect [(id init) ...] clause introduces bindings that are visible within the body of the state actor. Each binding id is initialized to the corresponding init expression. The bindings are updated when an ongoing behavior executes an instantaneous event, such as the result of an on behavior. The new bindings are in the form of a values form, with the new values in the same order and number as in the #:collect.

The ongoing behaviors O ... are run simultaneously until the state actor exits.

Each [E I ...] specifies a termination event E of the actor. When a termination event E activates, the corresponding Is are executed. The state actor then exits, with the same result of the final I action.