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https://github.com/matrix-construct/construct
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416 lines
18 KiB
Markdown
416 lines
18 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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redacts
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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 referenced events 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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- Ephemeral (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 indicated by the `depth` from
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the first message in the graph (where `type` was `m.room.create`). Specifically,
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each message makes a reference to all known messages at the last `depth`, or all
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previously unknown messages at some lower `depth`. Each new message is broadcast
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to all participants in a room.
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* The monotonic increase in `depth` 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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strongly ordered knowledge of all known information which preceded it at
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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 trees 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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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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Keen observers may have realized by now this system is not fully coherent.
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To be coherent, a system must leverage *entry consistency* and/or *release
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consistency*. Translated to this system:
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* *Entry* is the point where an event is created containing references to
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all previous events. *Entry consistency* would mean that the knowledge
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of all those references is revealed from all parties to the issuer such that
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the issuer would not be issuing a conflicting event.
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* *Release* is the act of broadcasting that event to other servers. *Release
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consistency* would mean that the integration of the newly issued event does not
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conflict at the point of acceptance by each and every party.
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This system appears to strive for *eventual consistency*. To be pedantic, that
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is not a third lemma supplementing the above: it's a higher order composite (like
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mutual exclusion, or other algorithms). What this system wants to achieve is a
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byzantine tolerance which can be continuously corrected as more information is
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learned. This is a *tolerance*, not a *prevention*, because the relaxed write
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consistency is of extreme practical importance.
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For *eventual consistency* to be coherent, the "seeds" of a correction have to
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be planted early on before any fault. When the fault occurs, all deviations
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can be corrected toward some single coherent state as each party learns more
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information. Once all parties learn all information from the system, there is
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no possibility for incoherence. The caveat is that some parties may need to
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roll back certain decisions they made without complete information.
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Consider the following: `Alice` is a room founder and has one other member
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`Bob` who is an op. `Alice` outranks `Bob`. Consider the following scenario:
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> 1. `Charlie` joins the room. Now the room has three members. Everyone is
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> still in full agreement.
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>
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> 2. `GNAA` ddos's `Alice` so she can't reach the internet but she can still
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> use her server on her LAN.
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>
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> 3. `Alice` likes `Charlie` so she gives him `+e` or some ban immunity.
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>
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> 4. `Bob` doesn't like `Charlie` so he bans him.
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Now there is a classic byzantine fault. The internet sees a room with two
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members `Alice` and `Bob` again while `Alice` sees a room with three: `Alice`, `Bob`
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and `Charlie`.
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> 5. `GNAA` stops the ddos.
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This fault now has to be resolved. This is called "state conflict resolution"
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and the matrix specification does not know how to do this. What is currently
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specified is that `Alice` and `Bob` can only perform actions that are valid
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with the knowledge they had when they performed them. In fact, that was true
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in this scenario.
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Intuitively, `Alice` needs to dominate the resolution because `Alice` outranks
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`Bob`. `Charlie` must not be banned and the room must continue with three
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members. Exactly how to roll back the ban and reinstate `Charlie` may seem
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obvious but there are practicalities to consider: Perhaps `Alice` is ddosed for
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something like a year straight and `Charlie` has entirely given up on socializing
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over the internet. A seemingly random and irrelevant correction will be in store
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for the room and the effects might be far more complicated.
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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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#### Technique
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The Matrix room, as described earlier, is a state machine underwritten by
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timelines of events in a directed acyclic graph with eventual consistency.
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To operate effectively, the state machine must respond to queries about
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the state of the room at the point of any event in the past. This is similar
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to a `git reset` to some specific commit where one can browse the work tree
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as it appeared at that commit.
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> Was X a member of room Y when event Z happened?
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The naive approach is to trace the graph from the event backward collecting
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all of the state events to then satisfy the query. Sequences of specific state
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event types can be held by implementations to hasten such a trace.
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Alternatively, a complete list of all state events can be created for each
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modification to the state to avoid the reference chasing of a trace at the
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cost of space.
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Our approach is more efficient. We create a b-tree to represent the complete
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state to satisfy any query in logarithmic time. When the state is updated,
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only one path in the tree is modified and the root is stored with that event.
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This structure is actually immutable: the previous versions of the affected
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nodes are not discarded allowing past configurations of the tree to be
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represented. We further benefit from the fact that each node is referenced by
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the hash of its content for efficient reuse, as well as our database being
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well compressed.
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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 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. Vertical
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tracks are contexts on their journey through each evaluation and exclusion step
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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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\ | | | | / post-commit prefetching
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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.
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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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#### 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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