Move model to client-server for now.

1 files changed, 0 insertions, 249 deletions

diff --git a/docs/model/presence.rst b/docs/model/presence.rst
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-========
-Presence
-========
-
-A description of presence information and visibility between users.
-
-Overview
-========
-
-Each user has the concept of Presence information. This encodes a sense of the
-"availability" of that user, suitable for display on other user's clients.
-
-
-Presence Information
-====================
-
-The basic piece of presence information is an enumeration of a small set of
-state; such as "free to chat", "online", "busy", or "offline". The default state
-unless the user changes it is "online". Lower states suggest some amount of
-decreased availability from normal, which might have some client-side effect
-like muting notification sounds and suggests to other users not to bother them
-unless it is urgent. Equally, the "free to chat" state exists to let the user
-announce their general willingness to receive messages moreso than default.
-
-Home servers should also allow a user to set their state as "hidden" - a state
-which behaves as offline, but allows the user to see the client state anyway and
-generally interact with client features such as reading message history or
-accessing contacts in the address book.
-
-This basic state field applies to the user as a whole, regardless of how many
-client devices they have connected. The home server should synchronise this
-status choice among multiple devices to ensure the user gets a consistent
-experience.
-
-Idle Time
----------
-
-As well as the basic state field, the presence information can also show a sense
-of an "idle timer". This should be maintained individually by the user's
-clients, and the homeserver can take the highest reported time as that to
-report. Likely this should be presented in fairly coarse granularity; possibly
-being limited to letting the home server automatically switch from a "free to
-chat" or "online" mode into "idle".
-
-When a user is offline, the Home Server can still report when the user was last
-seen online, again perhaps in a somewhat coarse manner.
-
-Device Type
------------
-
-Client devices that may limit the user experience somewhat (such as "mobile"
-devices with limited ability to type on a real keyboard or read large amounts of
-text) should report this to the home server, as this is also useful information
-to report as "presence" if the user cannot be expected to provide a good typed
-response to messages.
-
-
-Presence List
-=============
-
-Each user's home server stores a "presence list" for that user. This stores a
-list of other user IDs the user has chosen to add to it (remembering any ACL
-Pointer if appropriate).
-
-To be added to a contact list, the user being added must grant permission. Once
-granted, both user's HS(es) store this information, as it allows the user who
-has added the contact some more abilities; see below. Since such subscriptions
-are likely to be bidirectional, HSes may wish to automatically accept requests
-when a reverse subscription already exists.
-
-As a convenience, presence lists should support the ability to collect users
-into groups, which could allow things like inviting the entire group to a new
-("ad-hoc") chat room, or easy interaction with the profile information ACL
-implementation of the HS.
-
-
-Presence and Permissions
-========================
-
-For a viewing user to be allowed to see the presence information of a target
-user, either
-
- * The target user has allowed the viewing user to add them to their presence
-   list, or
-
- * The two users share at least one room in common
-
-In the latter case, this allows for clients to display some minimal sense of
-presence information in a user list for a room.
-
-Home servers can also use the user's choice of presence state as a signal for
-how to handle new private one-to-one chat message requests. For example, it
-might decide:
-
-  "free to chat": accept anything
-  "online": accept from anyone in my addres book list
-  "busy": accept from anyone in this "important people" group in my address
-    book list
-
-
-API Efficiency
-==============
-
-A simple implementation of presence messaging has the ability to cause a large
-amount of Internet traffic relating to presence updates. In order to minimise
-the impact of such a feature, the following observations can be made:
-
- * There is no point in a Home Server polling status for peers in a user's
-   presence list if the user has no clients connected that care about it.
-
- * It is highly likely that most presence subscriptions will be symmetric - a
-   given user watching another is likely to in turn be watched by that user.
-
- * It is likely that most subscription pairings will be between users who share
-   at least one Room in common, and so their Home Servers are actively
-   exchanging message PDUs or transactions relating to that Room.
-
- * Presence update messages do not need realtime guarantees. It is acceptable to
-   delay delivery of updates for some small amount of time (10 seconds to a
-   minute).
-
-The general model of presence information is that of a HS registering its
-interest in receiving presence status updates from other HSes, which then
-promise to send them when required. Rather than actively polling for the
-currentt state all the time, HSes can rely on their relative stability to only
-push updates when required.
-
-A Home Server should not rely on the longterm validity of this presence
-information, however, as this would not cover such cases as a user's server
-crashing and thus failing to inform their peers that users it used to host are
-no longer available online. Therefore, each promise of future updates should
-carry with a timeout value (whether explicit in the message, or implicit as some
-defined default in the protocol), after which the receiving HS should consider
-the information potentially stale and request it again.
-
-However, because of the likelyhood that two home servers are exchanging messages
-relating to chat traffic in a room common to both of them, the ongoing receipt
-of these messages can be taken by each server as an implicit notification that
-the sending server is still up and running, and therefore that no status changes
-have happened; because if they had the server would have sent them. A second,
-larger timeout should be applied to this implicit inference however, to protect
-against implementation bugs or other reasons that the presence state cache may
-become invalid; eventually the HS should re-enquire the current state of users
-and update them with its own.
-
-The following workflows can therefore be used to handle presence updates:
-
- 1 When a user first appears online their HS sends a message to each other HS
-   containing at least one user to be watched; each message carrying both a
-   notification of the sender's new online status, and a request to obtain and
-   watch the target users' presence information. This message implicitly
-   promises the sending HS will now push updates to the target HSes.
-
- 2 The target HSes then respond a single message each, containing the current
-   status of the requested user(s). These messages too implicitly promise the
-   target HSes will themselves push updates to the sending HS.
-
-   As these messages arrive at the sending user's HS they can be pushed to the
-   user's client(s), possibly batched again to ensure not too many small
-   messages which add extra protocol overheads.
-
-At this point, all the user's clients now have the current presence status
-information for this moment in time, and have promised to send each other
-updates in future.
-
- 3 The HS maintains two watchdog timers per peer HS it is exchanging presence
-   information with. The first timer should have a relatively small expiry
-   (perhaps 1 minute), and the second timer should have a much longer time
-   (perhaps 1 hour).
-
- 4 Any time any kind of message is received from a peer HS, the short-term
-   presence timer associated with it is reset.
-
- 5 Whenever either of these timers expires, an HS should push a status reminder
-   to the target HS whose timer has now expired, and request again from that
-   server the status of the subscribed users.
-
- 6 On receipt of one of these presence status reminders, an HS can reset both
-   of its presence watchdog timers.
-
-To avoid bursts of traffic, implementations should attempt to stagger the expiry
-of the longer-term watchdog timers for different peer HSes.
-
-When individual users actively change their status (either by explicit requests
-from clients, or inferred changes due to idle timers or client timeouts), the HS
-should batch up any status changes for some reasonable amount of time (10
-seconds to a minute). This allows for reduced protocol overheads in the case of
-multiple messages needing to be sent to the same peer HS; as is the likely
-scenario in many cases, such as a given human user having multiple user
-accounts.
-
-
-API Requirements
-================
-
-The data model presented here puts the following requirements on the APIs:
-
-Client-Server
--------------
-
-Requests that a client can make to its Home Server
-
- * get/set current presence state
-   Basic enumeration + ability to set a custom piece of text
-
- * report per-device idle time
-   After some (configurable?) idle time the device should send a single message
-   to set the idle duration. The HS can then infer a "start of idle" instant and
-   use that to keep the device idleness up to date. At some later point the
-   device can cancel this idleness.
-
- * report per-device type
-   Inform the server that this device is a "mobile" device, or perhaps some
-   other to-be-defined category of reduced capability that could be presented to
-   other users.
-
- * start/stop presence polling for my presence list
-   It is likely that these messages could be implicitly inferred by other
-   messages, though having explicit control is always useful.
-
- * get my presence list
-   [implicit poll start?]
-   It is possible that the HS doesn't yet have current presence information when
-   the client requests this. There should be a "don't know" type too.
-
- * add/remove a user to my presence list
-
-Server-Server
--------------
-
-Requests that Home Servers make to others
-
- * request permission to add a user to presence list
-
- * allow/deny a request to add to a presence list
-
- * perform a combined presence state push and subscription request
-   For each sending user ID, the message contains their new status.
-   For each receiving user ID, the message should contain an indication on
-   whether the sending server is also interested in receiving status from that
-   user; either as an immediate update response now, or as a promise to send
-   future updates.
-
-Server to Client
-----------------
-
-[[TODO(paul): There also needs to be some way for a user's HS to push status
-updates of the presence list to clients, but the general server-client event
-model currently lacks a space to do that.]]