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Unverified Commit 223d8a3d by Yorick Peterse
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Prepare for zero downtime migrations

Starting with GitLab 9.1.0 we will no longer allow downtime migrations unless absolutely necessary. This commit updates the various developer guides and adds code that is necessary to make zero downtime migrations less painful.
parent a179c5ca
Showing with 502 additions and 21 deletions
doc/development/migration_style_guide.md
......@@ -4,28 +4,53 @@ When writing migrations for GitLab, you have to take into account that
these will be ran by hundreds of thousands of organizations of all sizes, some with
many years of data in their database.
In addition, having to take a server offline for a an upgrade small or big is
a big burden for most organizations. For this reason it is important that your
migrations are written carefully, can be applied online and adhere to the style guide below.
In addition, having to take a server offline for a a upgrade small or big is a
big burden for most organizations. For this reason it is important that your
migrations are written carefully, can be applied online and adhere to the style
guide below.
Migrations should not require GitLab installations to be taken offline unless
_absolutely_ necessary - see the ["What Requires Downtime?"](what_requires_downtime.md)
page. If a migration requires downtime, this should be clearly mentioned during
the review process, as well as being documented in the monthly release post. For
more information, see the "Downtime Tagging" section below.
Migrations are **not** allowed to require GitLab installations to be taken
offline unless _absolutely necessary_. Downtime assumptions should be based on
the behaviour of a migration when performed using PostgreSQL, as various
operations in MySQL may require downtime without there being alternatives.
When downtime is necessary the migration has to be approved by:
1. The VP of Engineering
1. A Backend Lead
1. A Database Specialist
An up-to-date list of people holding these titles can be found at
<https://about.gitlab.com/team/>.
The document ["What Requires Downtime?"](what_requires_downtime.md) specifies
various database operations, whether they require downtime and how to
work around that whenever possible.
When writing your migrations, also consider that databases might have stale data
or inconsistencies and guard for that. Try to make as little assumptions as possible
about the state of the database.
or inconsistencies and guard for that. Try to make as few assumptions as
possible about the state of the database.
Please don't depend on GitLab-specific code since it can change in future
versions. If needed copy-paste GitLab code into the migration to make it forward
compatible.
## Commit Guidelines
Please don't depend on GitLab specific code since it can change in future versions.
If needed copy-paste GitLab code into the migration to make it forward compatible.
Each migration **must** be added in its own commit with a descriptive commit
message. If a commit adds a migration it _should only_ include the migration and
any corresponding changes to `db/schema.rb`. This makes it easy to revert a
database migration without accidentally reverting other changes.
## Downtime Tagging
Every migration must specify if it requires downtime or not, and if it should
require downtime it must also specify a reason for this. To do so, add the
following two constants to the migration class' body:
require downtime it must also specify a reason for this. This is required even
if 99% of the migrations won't require downtime as this makes it easier to find
the migrations that _do_ require downtime.
To tag a migration, add the following two constants to the migration class'
body:
* `DOWNTIME`: a boolean that when set to `true` indicates the migration requires
downtime.
......@@ -50,12 +75,53 @@ from a migration class.
## Reversibility
Your migration should be reversible. This is very important, as it should
Your migration **must be** reversible. This is very important, as it should
be possible to downgrade in case of a vulnerability or bugs.
In your migration, add a comment describing how the reversibility of the
migration was tested.
## Multi Threading
Sometimes a migration might need to use multiple Ruby threads to speed up a
migration. For this to work your migration needs to include the module
`Gitlab::Database::MultiThreadedMigration`:
```ruby
class MyMigration < ActiveRecord::Migration
include Gitlab::Database::MigrationHelpers
include Gitlab::Database::MultiThreadedMigration
end
```
You can then use the method `with_multiple_threads` to perform work in separate
threads. For example:
```ruby
class MyMigration < ActiveRecord::Migration
include Gitlab::Database::MigrationHelpers
include Gitlab::Database::MultiThreadedMigration
def up
with_multiple_threads(4) do
disable_statement_timeout
# ...
end
end
end
```
Here the call to `disable_statement_timeout` will use the connection local to
the `with_multiple_threads` block, instead of re-using the global connection
pool. This ensures each thread has its own connection object, and won't time
out when trying to obtain one.
**NOTE:** PostgreSQL has a maximum amount of connections that it allows. This
limit can vary from installation to installation. As a result it's recommended
you do not use more than 32 threads in a single migration. Usually 4-8 threads
should be more than enough.
## Removing indices
When removing an index make sure to use the method `remove_concurrent_index` instead
......@@ -78,7 +144,10 @@ end
## Adding indices
If you need to add an unique index please keep in mind there is possibility of existing duplicates. If it is possible write a separate migration for handling this situation. It can be just removing or removing with overwriting all references to these duplicates depend on situation.
If you need to add a unique index please keep in mind there is the possibility
of existing duplicates being present in the database. This means that should
always _first_ add a migration that removes any duplicates, before adding the
unique index.
When adding an index make sure to use the method `add_concurrent_index` instead
of the regular `add_index` method. The `add_concurrent_index` method
......@@ -90,17 +159,22 @@ so:
```ruby
class MyMigration < ActiveRecord::Migration
include Gitlab::Database::MigrationHelpers
disable_ddl_transaction!
def change
def up
add_concurrent_index :table, :column
end
def down
remove_index :table, :column if index_exists?(:table, :column)
end
end
```
## Adding Columns With Default Values
When adding columns with default values you should use the method
When adding columns with default values you must use the method
`add_column_with_default`. This method ensures the table is updated without
requiring downtime. This method is not reversible so you must manually define
the `up` and `down` methods in your migration class.
......@@ -123,6 +197,9 @@ class MyMigration < ActiveRecord::Migration
end
```
Keep in mind that this operation can easily take 10-15 minutes to complete on
larger installations (e.g. GitLab.com). As a result you should only add default
values if absolutely necessary.
## Integer column type
......@@ -147,13 +224,15 @@ add_column(:projects, :foo, :integer, default: 10, limit: 8)
## Testing
Make sure that your migration works with MySQL and PostgreSQL with data. An empty database does not guarantee that your migration is correct.
Make sure that your migration works with MySQL and PostgreSQL with data. An
empty database does not guarantee that your migration is correct.
Make sure your migration can be reversed.
## Data migration
Please prefer Arel and plain SQL over usual ActiveRecord syntax. In case of using plain SQL you need to quote all input manually with `quote_string` helper.
Please prefer Arel and plain SQL over usual ActiveRecord syntax. In case of
using plain SQL you need to quote all input manually with `quote_string` helper.
Example with Arel:
......@@ -177,3 +256,17 @@ select_all("SELECT name, COUNT(id) as cnt FROM tags GROUP BY name HAVING COUNT(i
execute("DELETE FROM tags WHERE id IN(#{duplicate_ids.join(",")})")
end
```
If you need more complex logic you can define and use models local to a
migration. For example:
```ruby
class MyMigration < ActiveRecord::Migration
class Project < ActiveRecord::Base
self.table_name = 'projects'
end
end
```
When doing so be sure to explicitly set the model's table name so it's not
derived from the class name or namespace.
doc/update/README.md
......@@ -48,6 +48,23 @@ GitLab provides official Docker images for both Community and Enterprise
editions. They are based on the Omnibus package and instructions on how to
update them are in [a separate document][omnidocker].
## Upgrading without downtime
Starting with GitLab 9.1.0 it's possible to upgrade to a newer version of GitLab
without having to take your GitLab instance offline. However, for this to work
there are the following requirements:
1. You can only upgrade 1 release at a time. For example, if 9.1.15 is the last
release of 9.1 then you can safely upgrade from that version to 9.2.0.
However, if you are running 9.1.14 you first need to upgrade to 9.1.15.
2. You have to use [post-deployment
migrations](../development/post_deployment_migrations.md).
3. You are using PostgreSQL. If you are using MySQL you will still need downtime
when upgrading.
This applies to major, minor, and patch releases unless stated otherwise in a
release post.
## Upgrading between editions
GitLab comes in two flavors: [Community Edition][ce] which is MIT licensed,
......
lib/gitlab/database/migration_helpers.rb
......@@ -89,7 +89,8 @@ module Gitlab
ADD CONSTRAINT #{key_name}
FOREIGN KEY (#{column})
REFERENCES #{target} (id)
ON DELETE #{on_delete} NOT VALID;
#{on_delete ? "ON DELETE #{on_delete}" : ''}
NOT VALID;
EOF
# Validate the existing constraint. This can potentially take a very
......@@ -250,6 +251,245 @@ module Gitlab
raise error
end
end
# Renames a column without requiring downtime.
#
# Concurrent renames work by using database triggers to ensure both the
# old and new column are in sync. However, this method will _not_ remove
# the triggers or the old column automatically; this needs to be done
# manually in a post-deployment migration. This can be done using the
# method `cleanup_concurrent_column_rename`.
#
# table - The name of the database table containing the column.
# old - The old column name.
# new - The new column name.
# type - The type of the new column. If no type is given the old column's
# type is used.
def rename_column_concurrently(table, old, new, type: nil)
if transaction_open?
raise 'rename_column_concurrently can not be run inside a transaction'
end
trigger_name = rename_trigger_name(table, old, new)
quoted_table = quote_table_name(table)
quoted_old = quote_column_name(old)
quoted_new = quote_column_name(new)
if Database.postgresql?
install_rename_triggers_for_postgresql(trigger_name, quoted_table,
quoted_old, quoted_new)
else
install_rename_triggers_for_mysql(trigger_name, quoted_table,
quoted_old, quoted_new)
end
old_col = column_for(table, old)
new_type = type || old_col.type
add_column(table, new, new_type,
limit: old_col.limit,
default: old_col.default,
null: old_col.null,
precision: old_col.precision,
scale: old_col.scale)
update_column_in_batches(table, new, Arel::Table.new(table)[old])
copy_indexes(table, old, new)
copy_foreign_keys(table, old, new)
end
# Changes the type of a column concurrently.
#
# table - The table containing the column.
# column - The name of the column to change.
# new_type - The new column type.
def change_column_type_concurrently(table, column, new_type)
temp_column = "#{column}_for_type_change"
rename_column_concurrently(table, column, temp_column, type: new_type)
end
# Performs cleanup of a concurrent type change.
#
# table - The table containing the column.
# column - The name of the column to change.
# new_type - The new column type.
def cleanup_concurrent_column_type_change(table, column)
temp_column = "#{column}_for_type_change"
transaction do
# This has to be performed in a transaction as otherwise we might have
# inconsistent data.
cleanup_concurrent_column_rename(table, column, temp_column)
rename_column(table, temp_column, column)
end
end
# Cleans up a concurrent column name.
#
# This method takes care of removing previously installed triggers as well
# as removing the old column.
#
# table - The name of the database table.
# old - The name of the old column.
# new - The name of the new column.
def cleanup_concurrent_column_rename(table, old, new)
trigger_name = rename_trigger_name(table, old, new)
if Database.postgresql?
remove_rename_triggers_for_postgresql(table, trigger_name)
else
remove_rename_triggers_for_mysql(trigger_name)
end
remove_column(table, old)
end
# Performs a concurrent column rename when using PostgreSQL.
def install_rename_triggers_for_postgresql(trigger, table, old, new)
execute <<-EOF.strip_heredoc
CREATE OR REPLACE FUNCTION #{trigger}()
RETURNS trigger AS
$BODY$
BEGIN
NEW.#{new} := NEW.#{old};
RETURN NEW;
END;
$BODY$
LANGUAGE 'plpgsql'
VOLATILE
EOF
execute <<-EOF.strip_heredoc
CREATE TRIGGER #{trigger}
BEFORE INSERT OR UPDATE
ON #{table}
FOR EACH ROW
EXECUTE PROCEDURE #{trigger}()
EOF
end
# Installs the triggers necessary to perform a concurrent column rename on
# MySQL.
def install_rename_triggers_for_mysql(trigger, table, old, new)
execute <<-EOF.strip_heredoc
CREATE TRIGGER #{trigger}_insert
BEFORE INSERT
ON #{table}
FOR EACH ROW
SET NEW.#{new} = NEW.#{old}
EOF
execute <<-EOF.strip_heredoc
CREATE TRIGGER #{trigger}_update
BEFORE UPDATE
ON #{table}
FOR EACH ROW
SET NEW.#{new} = NEW.#{old}
EOF
end
# Removes the triggers used for renaming a PostgreSQL column concurrently.
def remove_rename_triggers_for_postgresql(table, trigger)
execute("DROP TRIGGER #{trigger} ON #{table}")
execute("DROP FUNCTION #{trigger}()")
end
# Removes the triggers used for renaming a MySQL column concurrently.
def remove_rename_triggers_for_mysql(trigger)
execute("DROP TRIGGER #{trigger}_insert")
execute("DROP TRIGGER #{trigger}_update")
end
# Returns the (base) name to use for triggers when renaming columns.
def rename_trigger_name(table, old, new)
'trigger_' + Digest::SHA256.hexdigest("#{table}_#{old}_#{new}").first(12)
end
# Returns an Array containing the indexes for the given column
def indexes_for(table, column)
column = column.to_s
indexes(table).select { |index| index.columns.include?(column) }
end
# Returns an Array containing the foreign keys for the given column.
def foreign_keys_for(table, column)
column = column.to_s
foreign_keys(table).select { |fk| fk.column == column }
end
# Copies all indexes for the old column to a new column.
#
# table - The table containing the columns and indexes.
# old - The old column.
# new - The new column.
def copy_indexes(table, old, new)
old = old.to_s
new = new.to_s
indexes_for(table, old).each do |index|
new_columns = index.columns.map do |column|
column == old ? new : column
end
# This is necessary as we can't properly rename indexes such as
# "ci_taggings_idx".
unless index.name.include?(old)
raise "The index #{index.name} can not be copied as it does not "\
"mention the old column. You have to rename this index manually first."
end
name = index.name.gsub(old, new)
options = {
unique: index.unique,
name: name,
length: index.lengths,
order: index.orders
}
# These options are not supported by MySQL, so we only add them if
# they were previously set.
options[:using] = index.using if index.using
options[:where] = index.where if index.where
unless index.opclasses.blank?
opclasses = index.opclasses.dup
# Copy the operator classes for the old column (if any) to the new
# column.
opclasses[new] = opclasses.delete(old) if opclasses[old]
options[:opclasses] = opclasses
end
add_concurrent_index(table, new_columns, options)
end
end
# Copies all foreign keys for the old column to the new column.
#
# table - The table containing the columns and indexes.
# old - The old column.
# new - The new column.
def copy_foreign_keys(table, old, new)
foreign_keys_for(table, old).each do |fk|
add_concurrent_foreign_key(fk.from_table,
fk.to_table,
column: new,
on_delete: fk.on_delete)
end
end
# Returns the column for the given table and column name.
def column_for(table, name)
name = name.to_s
columns(table).find { |column| column.name == name }
end
end
end
end
lib/gitlab/database/multi_threaded_migration.rb 0 → 100644
module Gitlab
module Database
module MultiThreadedMigration
MULTI_THREAD_AR_CONNECTION = :thread_local_ar_connection
# This overwrites the default connection method so that every thread can
# use a thread-local connection, while still supporting all of Rails'
# migration methods.
def connection
Thread.current[MULTI_THREAD_AR_CONNECTION] ||
ActiveRecord::Base.connection
end
# Starts a thread-pool for N threads, along with N threads each using a
# single connection. The provided block is yielded from inside each
# thread.
#
# Example:
#
# with_multiple_threads(4) do
# execute('SELECT ...')
# end
#
# thread_count - The number of threads to start.
#
# join - When set to true this method will join the threads, blocking the
# caller until all threads have finished running.
#
# Returns an Array containing the started threads.
def with_multiple_threads(thread_count, join: true)
pool = Gitlab::Database.create_connection_pool(thread_count)
threads = Array.new(thread_count) do
Thread.new do
pool.with_connection do |connection|
begin
Thread.current[MULTI_THREAD_AR_CONNECTION] = connection
yield
ensure
Thread.current[MULTI_THREAD_AR_CONNECTION] = nil
end
end
end
end
threads.each(&:join) if join
threads
end
end
end
end
spec/lib/gitlab/database/multi_threaded_migration_spec.rb 0 → 100644
require 'spec_helper'
describe Gitlab::Database::MultiThreadedMigration do
let(:migration) do
Class.new { include Gitlab::Database::MultiThreadedMigration }.new
end
describe '#connection' do
after do
Thread.current[described_class::MULTI_THREAD_AR_CONNECTION] = nil
end
it 'returns the thread-local connection if present' do
Thread.current[described_class::MULTI_THREAD_AR_CONNECTION] = 10
expect(migration.connection).to eq(10)
end
it 'returns the global connection if no thread-local connection was set' do
expect(migration.connection).to eq(ActiveRecord::Base.connection)
end
end
describe '#with_multiple_threads' do
it 'starts multiple threads and yields the supplied block in every thread' do
output = Queue.new
migration.with_multiple_threads(2) do
output << migration.connection.execute('SELECT 1')
end
expect(output.size).to eq(2)
end
it 'joins the threads when the join parameter is set' do
expect_any_instance_of(Thread).to receive(:join).and_call_original
migration.with_multiple_threads(1) { }
end
end
end
spec/models/concerns/ignorable_column_spec.rb 0 → 100644
require 'spec_helper'
describe IgnorableColumn do
let :base_class do
Class.new do
def self.columns
# This method does not have access to "double"
[Struct.new(:name).new('id'), Struct.new(:name).new('title')]
end
end
end
let :model do
Class.new(base_class) do
include IgnorableColumn
end
end
describe '.columns' do
it 'returns the columns, excluding the ignored ones' do
model.ignore_column(:title)
expect(model.columns.map(&:name)).to eq(%w(id))
end
end
describe '.ignored_columns' do
it 'returns a Set' do
expect(model.ignored_columns).to be_an_instance_of(Set)
end
it 'returns the names of the ignored columns' do
model.ignore_column(:title)
expect(model.ignored_columns).to eq(Set.new(%w(title)))
end
end
end
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