- 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.
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
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
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.
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
column. If we were to run the migration this would result in the GitLab instance
producing errors whenever it tries to use the
As a result of the above downtime is required when removing a column, even when using PostgreSQL.
Renaming columns requires downtime as running GitLab instances will continue using the old column name until a new version is deployed. This can result in the instance producing errors, which in turn can impact the user experience.
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.
This operation requires downtime.
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
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 an index does not require downtime on both PostgreSQL and MySQL.
This operation is safe as there's no code using the table just yet.
This operation requires downtime as application code may still be using the table.
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.
This operation should not require downtime on both PostgreSQL and MySQL.
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.