91b752dce6
This changes the BackgroundMigration worker so it checks for the health of the DB before performing a background migration. This in turn allows us to reduce the minimum interval, without having to worry about blowing things up if we schedule too many migrations. In this setup, the BackgroundMigration worker will reschedule jobs as long as the database is considered to be in an unhealthy state. Once the database has recovered, the migration can be performed. To determine if the database is in a healthy state, we look at the replication lag of any replication slots defined on the primary. If the lag is deemed to great (100 MB by default) for too many slots, the migration is rescheduled for a later point in time. The health checking code is hidden behind a feature flag, allowing us to disable it if necessary.
32 lines
1.2 KiB
Ruby
32 lines
1.2 KiB
Ruby
# frozen_string_literal: true
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module Postgresql
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class ReplicationSlot < ActiveRecord::Base
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self.table_name = 'pg_replication_slots'
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# Returns true if the lag observed across all replication slots exceeds a
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# given threshold.
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#
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# max - The maximum replication lag size, in bytes. Based on GitLab.com
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# statistics it takes between 1 and 5 seconds to replicate around
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# 100 MB of data.
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def self.lag_too_great?(max = 100.megabytes)
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lag_function = "#{Gitlab::Database.pg_wal_lsn_diff}" \
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"(#{Gitlab::Database.pg_current_wal_insert_lsn}(), restart_lsn)::bigint"
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# We force the use of a transaction here so the query always goes to the
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# primary, even when using the EE DB load balancer.
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sizes = transaction { pluck(lag_function) }
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too_great = sizes.count { |size| size >= max }
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# If too many replicas are falling behind too much, the availability of a
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# GitLab instance might suffer. To prevent this from happening we require
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# at least 1 replica to have data recent enough.
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if sizes.any? && too_great.positive?
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(sizes.length - too_great) <= 1
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else
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false
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end
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end
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end
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end
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