250 lines
10 KiB
ReStructuredText
250 lines
10 KiB
ReStructuredText
|
========
|
||
|
|
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.]]
|