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387 lines
16 KiB
Markdown
387 lines
16 KiB
Markdown
# Matrix Protocol
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### Introduction
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*The authoritative place for learning about matrix is at [matrix.org](https://matrix.org) but
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it may be worthwhile to spend a moment and consider this introduction which explains things
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by distilling the formal core of the protocol before introducing all of the networking and
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communicative accoutrements...*
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#### Identity
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The Matrix-ID or `mxid` is a universally unique plain-text string allowing
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an entity to be addressed internet-wide which is fundamental to the matrix
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federation in contrast to the traditional IRC server/network. An example of an
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mxid: "@user:host" where `host` is a public DNS name, `user` is a party to
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`host`, and the '@' character is replaced to convey type information. The
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character, called a `sigil`, is defined to be '!' for `room_id` identifiers,
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'$' for `event_id` identifiers, '#' for room aliases, and '@' for users.
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#### Event
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The fundamental primitive of this protocol is the `event` object. This object
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contains some set of key/value pairs and the protocol defines a list of such keys
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which are meaningful to the protocol. Other keys which are not meaningful to the
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protocol can be included directly in the `event` object but there are no guarantees
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for if and how a party will pass these keys. To dive right in, here's the list
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of recognized keys for an `event`:
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```
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auth_events
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content
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depth
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event_id
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hashes
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membership
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origin
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origin_server_ts
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prev_events
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prev_state
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room_id
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sender
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signatures
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state_key
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type
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```
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In the event structure, the value for `sender` and `room_id` and `event_id` are
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all an `mxid` of the appropriate type.
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The `event` object is also the *only* fundamental primitive of the protocol; in other
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words: everything is an `event`. All information is conveyed in events, and governed
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by rules for proper values behind these keys. The rest of the protocol specification
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describes an *abstract state machine* which has its state updated by an event, in
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addition to providing a standard means for communication of events between parties
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over the internet. That's it.
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#### Timeline
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The data tape of the matrix machine consists of a singly-linked list of `event`
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objects with each referencing the `event_id` of its preceding parent somewhere
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in the `prev_` keys; this is called the `timeline`. Each event is signed by its
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creator and affirms all events in the chain preceding it. This is a very similar
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structure to that used by software like Git, and Bitcoin. It allows looking back
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into the past from any point, but doesn't force a party to accept a future and
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leaves dispute resolution open-ended (which will be explained later).
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#### State
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The `state` consists of a subset of events which are accumulated according to a
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few rules when playing the tape through the machine. Events which are selected
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as `state` will overwrite a matching previously selected `state event` and thus
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reduce the number of events in this set to far less than the entire `timeline`.
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The `state` is then used to satisfy queries for deciding valid transitions for
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the machine. This is like the "work tree" in Git when positioned at some commit.
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* Events with a `state_key` are considered state.
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* The identity of a `state event` is the concatenation of the `room_id`
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value with the `type` value with the `state_key` value. Thus an event
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with the same `room_id, type, state_key` replaces an older event in `state`.
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* Some `state_key` values are empty strings `""`. This is a convention for
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singleton `state` events, like an `m.room.create` event. The `state_key`
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is used to represent a set, like with `m.room.member` events, where the
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value of the `state_key` is a user `mxid`.
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#### Rooms
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The `room` structure encapsulates an instance of the matrix machine. A room
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is a container of `event` objects in the form of a timeline. The query
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complexity for information in a room timeline is as follows:
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- Message (non-state) events in the timeline have a linear lookup time:
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the timeline must be iterated in sequence to find a satisfying message.
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- State events in the timeline have a logarithmic lookup: the implementation
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is expected to maintain a map of the `type`,`state_key` values for events
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present in the timeline.
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The matrix protocol specifies certain `event` types which are recognized to
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affect the behavior of the `room`; here is a list of some types:
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```
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m.room.name
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m.room.create
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m.room.topic
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m.room.avatar
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m.room.aliases
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m.room.canonical_alias
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m.room.join_rules
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m.room.power_levels
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m.room.member
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m.room.message
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...
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```
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Some of these events are `state` events and some are ephemeral. These will be
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detailed later. All `m.room.*` namespaced events govern the functionality of the
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room. Rooms may contain events of any type, but we don't invent new `m.room.*`
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type events ourselves. This project tends to create events in the namespace
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`ircd.*` These events should not alter the room's functionality for a client
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with knowledge of only the published `m.room.*` events wouldn't understand.
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#### Coherence
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Matrix is specified as a directed acyclic graph of messages. The conversation of
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messages moves in one direction: past to future. Messages only reference other
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messages which have a lower degree of separation (depth) from the first message
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in the graph (m.room.create). Specifically, each message makes a reference to all
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known messages at the last depth.
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* The strong ordering of this system contributes to an intuitive "light cone"
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read coherence. Knowledge of any piece of information (like an event) offers
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coherent knowledge of all known information which preceded it at that point.
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* Write consistency is relaxed. Multiple messages may be issued at the same
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depth from independent actors and multiple reference chains may form
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independent of others. This provides the scalar for performance in a large
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distributed internet system.
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* Write incoherence must then be resolved with entry consistency because of the
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relaxed release sequence. While parties broadcast all of their new messages,
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they make no guarantees for their arrival integration with destinations at
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the point of release. This wouldn't be as practical. This means a write which
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wishes to be coherent can only use the best available state they have been
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made aware of and commit a new message to it.
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The system has no other method of resolving incoherence. As a future thought,
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some form of release commitment will have to be integrated among at least a
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subset of actors for a few important updates to the graph. For example, a
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two-phase commit of an important state event *or the re-introduction of
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the classic IRC mode change indicating a commitment to change state.*
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References to previous events:
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```
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[A0] <-- [A1] <-- [A2] | A has seen B1 and includes a reference in A2
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^ |
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| <---<----<
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^------ [B1] <-- [B2] | B hasn't yet seen A1 or A2
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[T0] A release A0 :
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[T1] A release A1 : B acquire A0
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[T2] : B release B1
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[T3] A acquire B1 : B release B2
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[T4] A release A2 :
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```
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Both actors will have their clock (depth) now set to 2 and will issue the
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next new message at clock cycle 3 referencing all messages from cycle 2 to
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merge the split in the illustration above which is happening.
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```
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[A0] <-- [A1] <-- [A2] [A4] | A now sees B3, B2, and B1
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^ | | |
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| <---<----< ^--<--< <--<
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^------- [B1] <-- [B2] <-- [B3] | B now sees A2, A1, and A0
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```
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### Implementation
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#### Model
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This system embraces the fact that "everything is an event." It then follows
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that everything is a room. We use rooms for both communication and storage of
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everything.
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There is only one† backend database and it stores events. For example: there
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is no "user accounts database" holding all of the user data for the server-
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instead there is an `!accounts` *room*. To use these rooms as efficient
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databases we categorize a piece of data with an event `type` and key it with
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the event `state_key` and the value is the event `content`. Iteration of these
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events is also possible. This is now a sufficient key-value store as good as
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any other approach; better though, since such a databasing room retains all
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features and distributed capabilities of any other room. We then focus our
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efforts to optimize the behavior of a room, to the benefit of all rooms, and
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all things.
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† Under special circumstances other databases may exist but they are purely
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slave to the events database: i.e one could `rm -rf` a slave database and it
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would be rebuilt from the events database. These databases only exist if an
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event is *truly* inappropriate and doesn't fit the model even by a stretch.
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An example of this is the search-terms database which specializes in indexing
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individual words to the events where they are found so content searches can be
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efficient.
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#### Flow
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This is a single-writer/multiple-reader approach. The "core" is the only writer.
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The write itself is just the saving of an event. This serves as a transaction
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advancing the state of the machine with effects visible to all future
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transactions and external actors.
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The core takes the pattern of
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`evaluate + exclude -> write commitment -> release sequence`. The single
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writer approach means that we resolve all incoherence using exclusion or
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reordering or rejection on entry and before any writing and release of the
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event. Many ircd::ctx's can orbit the inner core resolving their evaluation
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with the tightest exclusion occurring around the write at the inner core.
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This also gives us the benefit of a total serialization at this point.
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:::::::
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||||||| <-- evaluation + rejection
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\|/ <-- evaluation + exclusion / reordering
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!
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* <-- actor serialized core write commitment
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//|||\\
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//|// \\|\\
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::::::::::::: <-- release sequence propagation cone
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The evaluation phase ensures the event commitment will work: that the event
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is valid, and that the event is a valid transition of the machine according
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to the rules. This process may take some time and many yields and IO, even
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network IO -- if the server lacks a warm cache. During the evaluation phase
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locks and exclusions may be acquired to maintain the validity of the
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evaluation state through writing at the expense of other contexts contending
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for that resource.
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> Many ircd::ctx are concurrently working their way through the core. The
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> "velocity" is low when an ircd::ctx on this path may yield a lot for various
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> IO and allow other events to be processed. The velocity increases when
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> concurrent evaluation and reordering is no longer viable to maintain
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> coherence. Any yielding of an ircd::ctx at a higher velocity risks stalling
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> the whole core.
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::::::: <-- event input (low velocity)
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||||||| <-- evaluation process (low velocity)
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\|/ <-- serialization process (higher velocity)
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The write commitment saves the event to the database. This is a relatively
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fast operation which probably won't even yield the ircd::ctx, and all
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future reads to the database will see this write.
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! <-- serial write commitment (highest velocity)
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The release sequence broadcasts the event so its effects can be consumed.
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This works by yielding the ircd::ctx so all consumers can view the event
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and apply its effects for their feature module or send the event out to
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clients. This is usually faster than it sounds, as the consumers try not to
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hold up the release sequence for more than their first execution-slice,
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and copy the event if their output rate is slower.
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* <-- event revelation (higher velocity)
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//|||\\
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//|// \\|\\
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::::::::::::: <-- release sequence propagation cone (low velocity)
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The entire core commitment process relative to an event riding through it
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on an ircd::ctx has a duration tolerable for something like a REST interface,
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so the response to the user can wait for the commitment to succeed or fail
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and properly inform them after.
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The core process is then optimized by the following facts:
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* The resource exclusion zone around most matrix events is either
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small or non-existent because of its relaxed write consistency.
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* Writes in this implementation will not delay.
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"Core dilation" is a phenomenon which occurs when large numbers of events
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which have relaxed dependence are processed concurrently because none of
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them acquire any exclusivity which impede the others.
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:::::::
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|||||||
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||||||| <-- Core dilation; flow shape optimized for volume.
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|||||||
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/|||||\
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///|||\\\
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//|/|||\|\\
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:::::::::::::
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Close up of the charybdis's write head when tight to one schwarzschild-radius of
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matrix room surface which propagates only one event through at a time.
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Vertical tracks are contexts on their journey through each evaluation and exclusion
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step to the core.
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Input Events Phase
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:::::::::::::::::::::::::::::::::::::::::::::::::::::: validation / dupcheck
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|||||||||||||||||||||||||||||||||||||||||||||||||||||| identity/key resolution
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|||||||||||||||||||||||||||||||||||||||||||||||||||||| verification
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|||| ||||||||||||||| ||||||||||||||| ||||||||||||||||| head resolution
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--|--|----|-|---|--|--|---|---|---|---------|---|---|- graph resolutions
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----------|-|---|---------|-------|-----------------|- module evaluations
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\ | | | | /
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== ==============| | == Lowest velocity locks
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\ | | /
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== | | == Mid velocity locks
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\ | | /
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== | / == High velocity locks
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\ | / /
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== =====/= == Highest velocity lock
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\ / /
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\__ / __/
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_ | _
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! Write commitment
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Above, two contexts are illustrated as contending for the highest velocity
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lock. The highest velocity lock is not held for significant time, as the
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holder has very little work left to be done within the core, and will
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release the lock to the other context quickly. The lower velocity locks
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may have to be held longer, but are also less exclusive to all contexts.
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* Singularity
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[ ]
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/-------------[---]-------------\
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/ : : \ Federation send
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/ /---------[---]---------\ \
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/ : : \ Client sync
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out / /------[---]------\ \ out
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/ / : : \ \
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/ out / | | \ out \
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/ out / \ out \
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/ \
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return
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| result to |
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| evaluator |
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-------------
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Above, a close-up of the release sequence. The new event is being "viewed" by
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each consumer context separated by the horizontal lines representing a context
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switch from the perspective of the event travelling down. Each consumer
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performs its task for how to propagate the commissioned event.
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Each consumer has a shared-lock of the event which will hold up the completion
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of the commitment until all consumers release that. The ideal consumer will only
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hold their lock for a single context-slice while they play their part in applying
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the event, like non-blocking copies to sockets etc. These consumers then go on
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to do the rest of their output without the original event data which was memory
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supplied by the evaluator (like an HTTP client). Then all locks acquired on
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the entry side of the core can be released. The evaluator then gets the result
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of the successful commitment.
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#### Scaling
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Scaling beyond the limit of a single CPU core can be done with multiple instances
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of IRCd which form a cluster of independent actors. This cluster can extend
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to other machines on the network too. The independent actors leverage the weak
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write consistency and strong ordering of the matrix protocol to scale the same
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way the federation scales.
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Interference pattern of two IRCd'en:
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```
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::::::::::::::::::::::::::::::::::::
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--------\:::::::/--\:::::::/--------
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||||||| |||||||
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\|/ \|/
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! !
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* *
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//|||\\ //|||\\
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//|// \\|\\//|// \\|\\
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/|/|/|\|\|\/|/|/|\|\|\|\
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```
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