gitlab-org--gitlab-foss/app/models/awareness_session.rb

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# frozen_string_literal: true
# A Redis backed session store for real-time collaboration. A session is defined
# by its documents and the users that join this session. An online user can have
# two states within the session: "active" and "away".
#
# By design, session must eventually be cleaned up. If this doesn't happen
# explicitly, all keys used within the session model must have an expiry
# timestamp set.
class AwarenessSession # rubocop:disable Gitlab/NamespacedClass
# An awareness session expires automatically after 1 hour of no activity
SESSION_LIFETIME = 1.hour
private_constant :SESSION_LIFETIME
# Expire user awareness keys after some time of inactivity
USER_LIFETIME = 1.hour
private_constant :USER_LIFETIME
PRESENCE_LIFETIME = 10.minutes
private_constant :PRESENCE_LIFETIME
KEY_NAMESPACE = "gitlab:awareness"
private_constant :KEY_NAMESPACE
class << self
def for(value = nil)
# Creates a unique value for situations where we have no unique value to
# create a session with. This could be when creating a new issue, a new
# merge request, etc.
value = SecureRandom.uuid unless value.present?
# We use SHA-256 based session identifiers (similar to abbreviated git
# hashes). There is always a chance for Hash collisions (birthday
# problem), we therefore have to pick a good tradeoff between the amount
# of data stored and the probability of a collision.
#
# The approximate probability for a collision can be calculated:
#
# p ~= n^2 / 2m
# ~= (2^18)^2 / (2 * 16^15)
# ~= 2^36 / 2^61
#
# n is the number of awareness sessions and m the number of possibilities
# for each item. For a hex number, this is 16^c, where c is the number of
# characters. With 260k (~2^18) sessions, the probability for a collision
# is ~2^-25.
#
# The number of 15 is selected carefully. The integer representation fits
# nicely into a signed 64 bit integer and eventually allows Redis to
# optimize its memory usage. 16 chars would exceed the space for
# this datatype.
id = Digest::SHA256.hexdigest(value.to_s)[0, 15]
AwarenessSession.new(id)
end
end
def initialize(id)
@id = id
end
def join(user)
user_key = user_sessions_key(user.id)
with_redis do |redis|
redis.pipelined do |pipeline|
pipeline.sadd(user_key, id_i)
pipeline.expire(user_key, USER_LIFETIME.to_i)
pipeline.zadd(users_key, timestamp.to_f, user.id)
# We also mark for expiry when a session key is created (first user joins),
# because some users might never actively leave a session and the key could
# therefore become stale, w/o us noticing.
reset_session_expiry(pipeline)
end
end
nil
end
def leave(user)
user_key = user_sessions_key(user.id)
with_redis do |redis|
redis.pipelined do |pipeline|
pipeline.srem(user_key, id_i)
pipeline.zrem(users_key, user.id)
end
# cleanup orphan sessions and users
#
# this needs to be a second pipeline due to the delete operations being
# dependent on the result of the cardinality checks
user_sessions_count, session_users_count = redis.pipelined do |pipeline|
pipeline.scard(user_key)
pipeline.zcard(users_key)
end
redis.pipelined do |pipeline|
pipeline.del(user_key) unless user_sessions_count > 0
unless session_users_count > 0
pipeline.del(users_key)
@id = nil
end
end
end
nil
end
def present?(user, threshold: PRESENCE_LIFETIME)
with_redis do |redis|
user_timestamp = redis.zscore(users_key, user.id)
break false unless user_timestamp.present?
timestamp - user_timestamp < threshold
end
end
def away?(user, threshold: PRESENCE_LIFETIME)
!present?(user, threshold: threshold)
end
# Updates the last_activity timestamp for a user in this session
def touch!(user)
with_redis do |redis|
redis.pipelined do |pipeline|
pipeline.zadd(users_key, timestamp.to_f, user.id)
# extend the session lifetime due to user activity
reset_session_expiry(pipeline)
end
end
nil
end
def size
with_redis do |redis|
redis.zcard(users_key)
end
end
def to_param
id&.to_s
end
def to_s
"awareness_session=#{id}"
end
def online_users_with_last_activity(threshold: PRESENCE_LIFETIME)
users_with_last_activity.filter do |_user, last_activity|
user_online?(last_activity, threshold: threshold)
end
end
def users
User.where(id: user_ids)
end
def users_with_last_activity
# where in (x, y, [...z]) is a set and does not maintain any order, we need
# to make sure to establish a stable order for both, the pairs returned from
# redis and the ActiveRecord query. Using IDs in ascending order.
user_ids, last_activities = user_ids_with_last_activity
.sort_by(&:first)
.transpose
return [] if user_ids.blank?
users = User.where(id: user_ids).order(id: :asc)
users.zip(last_activities)
end
private
attr_reader :id
def user_online?(last_activity, threshold:)
last_activity.to_i + threshold.to_i > Time.zone.now.to_i
end
# converts session id from hex to integer representation
def id_i
Integer(id, 16) if id.present?
end
def users_key
"#{KEY_NAMESPACE}:session:#{id}:users"
end
def user_sessions_key(user_id)
"#{KEY_NAMESPACE}:user:#{user_id}:sessions"
end
def with_redis
Gitlab::Redis::SharedState.with do |redis|
yield redis if block_given?
end
end
def timestamp
Time.now.to_i
end
def user_ids
with_redis do |redis|
redis.zrange(users_key, 0, -1)
end
end
# Returns an array of tuples, where the first element in the tuple represents
# the user ID and the second part the last_activity timestamp.
def user_ids_with_last_activity
pairs = with_redis do |redis|
redis.zrange(users_key, 0, -1, with_scores: true)
end
# map data type of score (float) to Time
pairs.map do |user_id, score|
[user_id, Time.zone.at(score.to_i)]
end
end
# We want sessions to cleanup automatically after a certain period of
# inactivity. This sets the expiry timestamp for this session to
# [SESSION_LIFETIME].
def reset_session_expiry(redis)
redis.expire(users_key, SESSION_LIFETIME)
nil
end
end