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class FixReblogsInFeeds < ActiveRecord::Migration[5.1]
def up
redis = Redis.current
fm = FeedManager.instance
# Old scheme:
# Each user's feed zset had a series of score:value entries,
# where "regular" statuses had the same score and value (their
# ID). Reblogs had a score of the reblogging status' ID, and a
# value of the reblogged status' ID.
# New scheme:
# The feed contains only entries with the same score and value.
# Reblogs result in the reblogging status being added to the
# feed, with an entry in a reblog tracking zset (where the score
# is once again set to the reblogging status' ID, and the value
# is set to the reblogged status' ID). This is safe for Redis'
# float coersion because in this reblog tracking zset, we only
# need the rebloggging status' ID to be able to stop tracking
# entries after they have gotten too far down the feed, which
# does not require an exact value.
# This process reads all feeds and writes 3 times for each reblogs.
# So we use Lua script to avoid overhead between Ruby and Redis.
script = <<-LUA
local timeline_key = KEYS[1]
local reblog_key = KEYS[2]
-- So, first, we iterate over the user's feed to find any reblogs.
local items = redis.call('zrange', timeline_key, 0, -1, 'withscores')
for i = 1, #items, 2 do
local reblogged_id = items[i]
local reblogging_id = items[i + 1]
if (reblogged_id ~= reblogging_id) then
-- The score and value don't match, so this is a reblog.
-- (note that we're transitioning from IDs < 53 bits so we
-- don't have to worry about the loss of precision)
-- Remove the old entry
redis.call('zrem', timeline_key, reblogged_id)
-- Add a new one for the reblogging status
redis.call('zadd', timeline_key, reblogging_id, reblogging_id)
-- Track the fact that this was a reblog
redis.call('zadd', reblog_key, reblogging_id, reblogged_id)
end
end
LUA
script_hash = redis.script(:load, script)
# find_each is batched on the database side.
User.includes(:account).find_each do |user|
account = user.account
timeline_key = fm.key(:home, account.id)
reblog_key = fm.key(:home, account.id, 'reblogs')
redis.evalsha(script_hash, [timeline_key, reblog_key])
end
end
def down
# We *deliberately* do nothing here. This means that reverting
# this and the associated changes to the FeedManager code could
# allow one superfluous reblog of any given status, but in the case
# where things have gone wrong and a revert is necessary, this
# appears preferable to requiring a database hit for every status
# in every users' feed simply to revert.
# Note that this is operating under the assumption that entries
# with >53-bit IDs have already been entered. Otherwise, we could
# just use the data in Redis to reverse this transition.
end
end
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