Federation reads

The federation package serves local resources through the object dispatchers in packages/federation/src/object-dispatchers.ts and the outbox dispatcher in packages/federation/src/collection.ts. Both read through packages/federation/src/serving/, and share the SQL predicates in packages/federation/src/visibility.ts, so that a resource is served the same way wherever it is read.

Routes

Route	Serves
/users/{actor}/{id}	Objects attributed to the actor, including a Question
/ap/creates/{id}	Create activities
/ap/activities/{id}	Every other activity, including a Question saved as one
/users/{actor}/outbox	The actor's published activities

The common write operations allocate an IRI from the route of the resource's type, so a route answers 404 for a resource allocated to another one. No IRI changes: legacy Create activities stay at /ap/creates/{id}.

Sources

A Note or Article created by the createObject mutation, and its Create, have legacy typed rows and are serialized from them by toObject() and toCreate(), as before, also after an edit keeps the rows in step.

Every other resource, saved by the common write operations, is served from the document of its current revision:

• The document is served as saved, with its context, extension properties, IRI references and embedded values. An activity keeps the version it embedded when it was saved, so editing an object does not change an earlier activity's response. Nothing is dereferenced.
• bto and bcc are removed from the root, from every embedded document, and from every JSON-LD node object in extension values, which a JSON-LD processor reads with the context it inherits. They are found by meaning through describeDocument(), so aliases of them are removed as well; in extension values, whatever the alias's container. JSON literals stay as saved, however their keywords are spelled.
• A document saved without @context states the ActivityStreams context, which the validator read it with and which Activity Streams 2.0 asks documents to include.
• The document must name its own resource in id. Fedify does not parse it: the dispatchers hand Fedify a vocabulary object whose JSON-LD is the document, so a document with values Fedify 2.4.3 cannot read, such as a Link in audience or object, is served as saved too.
• A document the write path does not save, one the validator cannot read or one naming another resource, can only come from a change made outside it. It is logged under ["drfed", "federation", "serving"] and left out: its resource answers 404, and outbox pages and the counter skip it. Each revision is logged once per process, however often it is read, while it is among the last 4,096 logged.

Conditions

Every read needs the request's authority to be the instance of a live local actor, and Public to or cc addressing in the resource's current addressing index. Moreover:

• An activity must be in the outbox of such an actor. A saved activity that was not published is not served, nor is a received one.
• A legacy Create is not served once its object is deleted. A saved document is not served when the object of its root or of an embedded activity refers to a deleted local object, unless that activity is a Delete: the deletion hides the Like and other activities about the object, and an Undo of such a Like, and leaves its Delete retrievable, also inside an Announce that embeds it. A local object is deleted when its typed row is, or when its current revision is a local whiteout.
• A saved document that embeds a deleted local object, at any depth, is not served, so the Create and Update activities that carried its content, and an Undo of a Like embedding it, are hidden.
• A saved document that embeds, at any depth, a document addressed to recipients but not to the public in its to or cc is not served, since its content is theirs. Its recipients are read as the addressing index records them: references as resolved, embedded documents by id, and Link values by href. An embedded document without recipients of its own goes with the document that embeds it, as the Like in an Undo usually does.
• These rules read the node objects in extension values as embedded documents, with their values through @set and @list objects and their recipients by href when a type is Link, and by id otherwise. A node object whose object refers to a deleted local object, by a reference or a href, hides the document unless its type is Delete. A saved document is not served when a node object in an extension value is one describeDocument() leaves uninterpreted: one with a @context of its own, which the validator does not read, or with a value object, a language map, a string its key does not coerce to an IRI, or a relative reference where a reference belongs, more than one id or href, properties nested by @nest or reversed by @reverse, a value or a list object with keys other than keywords, or with keys a context of its own may alias, or a value that is an object but no JSON literal, since its recipients and references cannot then be told. A blank node identifier is read as an id that names no resource, but not as a reference, since it may name a node object defined apart. A node object whose id is used elsewhere in the document, as another node object's id or as a reference outside its own properties, is one node with what uses it, and is not read apart either; a Link whose href is its own id is read. Language maps are read as strings, and an object in one, which is no valid JSON-LD, is not read; a list under a key with a @language container is no language map, and its node objects are read.
• Embedded documents that share an id are one document, which none of them tells, so a saved document with two is not served either. An object reference names the href of a Link the document embeds elsewhere with that id as well. Such documents are logged once per revision as a warning.
• An object must be attributed to the actor of its route by its last snapshot. A deleted object is answered with 410 Gone and a Tombstone of its id and deleted time only.

What a saved document embeds and refers to is read from the document, through describeDocument(); every other condition is SQL.

Outbox

Outbox pages hold legacy and saved activities in one order: ascending position of the outbox's collection_items, where the newest item has the smallest position. An item without a position is not listed, and its activity is not served either. The cursor is the position of the last item of the previous page.

The saved activities their documents do not allow are left out after rows are selected, so a page reads further batches until it is full, each twice as large as the one before, up to 1,000 items. It promises a next page only when a served activity follows, so only the last page is short and no page is empty while another follows. A page after many hidden activities therefore reads all of them.

The counter counts what the pages hold. It reads the document of every saved activity in the outbox on each request, 100 at a time, so its cost grows with the outbox. Deleted objects are looked up 1,000 IRIs at a time, below the number of parameters PostgreSQL binds in one statement.

Outgoing payloads

@drfed/federation/serving exports the same serialization for outgoing payloads:

• publicDocument(document) returns a copy without blind recipients and with a stated context, or null for a document the validator cannot read offline or whose blind recipients cannot be found, as in a node object with a @context of its own in an extension value.
• parseVocab(ctx, document) parses a document into a vocabulary object that keeps it as its JSON-LD. It throws for a document Fedify cannot read, such as one with a Link in audience or object.
• loadActivitySnapshot(db, id) loads a saved activity's current document, blind recipients included, and its revision ID, applying no condition above.

An object derived from a parsed one, as by clone(), loses that JSON-LD and is serialized from the properties Fedify knows, without extensions. Delivery is not part of these reads.