debian-mirror-gitlab/doc/development/what_requires_downtime.md

What requires downtime?

When working with a database certain operations can be performed without taking GitLab offline, others do require a downtime period. This guide describes various operations and their impact.

Adding Columns

On PostgreSQL you can safely add a new column to an existing table as long as it does not have a default value. For example, this query would not require downtime:

ALTER TABLE projects ADD COLUMN random_value int;

Add a column with a default however does require downtime. For example, consider this query:

ALTER TABLE projects ADD COLUMN random_value int DEFAULT 42;

This requires updating every single row in the projects table so that random_value is set to 42 by default. This requires updating all rows and indexes in a table. This in turn acquires enough locks on the table for it to effectively block any other queries.

As of MySQL 5.6 adding a column to a table is still quite an expensive operation, even when using ALGORITHM=INPLACE and LOCK=NONE. This means downtime may be required when modifying large tables as otherwise the operation could potentially take hours to complete.

Adding a column with a default value can be done without requiring downtime when using the migration helper method Gitlab::Database::MigrationHelpers#add_column_with_default. This method works similar to add_column except it updates existing rows in batches without blocking access to the table being modified. See "Adding Columns With Default Values" for more information on how to use this method.

Dropping Columns

On PostgreSQL you can safely remove an existing column without the need for downtime. When you drop a column in PostgreSQL it's not immediately removed, instead it is simply disabled. The data is removed on the next vacuum run.

On MySQL this operation requires downtime.

While database wise dropping a column may be fine on PostgreSQL this operation still requires downtime because the application code may still be using the column that was removed. For example, consider the following migration:

class MyMigration < ActiveRecord::Migration
  def change
    remove_column :projects, :dummy
  end
end

Now imagine that the GitLab instance is running and actively uses the dummy column. If we were to run the migration this would result in the GitLab instance producing errors whenever it tries to use the dummy column.

As a result of the above downtime is required when removing a column, even when using PostgreSQL.

Changing Column Constraints

Generally changing column constraints requires checking all rows in the table to see if they meet the new constraint, unless a constraint is removed. For example, changing a column that previously allowed NULL values to not allow NULL values requires the database to verify all existing rows.

The specific behaviour varies a bit between databases but in general the safest approach is to assume changing constraints requires downtime.

Changing Column Types

This operation requires downtime.

Adding Indexes

Adding indexes is an expensive process that blocks INSERT and UPDATE queries for the duration. When using PostgreSQL one can work arounds this by using the CONCURRENTLY option:

CREATE INDEX CONCURRENTLY index_name ON projects (column_name);

Migrations can take advantage of this by using the method add_concurrent_index. For example:

class MyMigration < ActiveRecord::Migration
  def change
    add_concurrent_index :projects, :column_name
  end
end

When running this on PostgreSQL the CONCURRENTLY option mentioned above is used. On MySQL this method produces a regular CREATE INDEX query.

MySQL doesn't really have a workaround for this. Supposedly it can create indexes without the need for downtime but only for variable width columns. The details on this are a bit sketchy. Since it's better to be safe than sorry one should assume that adding indexes requires downtime on MySQL.

Dropping Indexes

Dropping an index does not require downtime on both PostgreSQL and MySQL.

Adding Tables

This operation is safe as there's no code using the table just yet.

Dropping Tables

This operation requires downtime as application code may still be using the table.

Adding Foreign Keys

Adding foreign keys acquires an exclusive lock on both the source and target tables in PostgreSQL. This requires downtime as otherwise the entire application grinds to a halt for the duration of the operation.

On MySQL this operation also requires downtime unless foreign key checks are disabled. Because this means checks aren't enforced this is not ideal, as such one should assume MySQL also requires downtime.

Removing Foreign Keys

This operation should not require downtime on both PostgreSQL and MySQL.

Updating Data

Updating data should generally be safe. The exception to this is data that's being migrated from one version to another while the application still produces data in the old version.

For example, imagine the application writes the string 'dog' to a column but it really is meant to write 'cat' instead. One might think that the following migration is all that is needed to solve this problem:

class MyMigration < ActiveRecord::Migration
  def up
    execute("UPDATE some_table SET column = 'cat' WHERE column = 'dog';")
  end
end

Unfortunately this is not enough. Because the application is still running and using the old value this may result in the table still containing rows where column is set to dog, even after the migration finished.

In these cases downtime is required, even for rarely updated tables.