This is a series of posts on migration from Apache and MySQL to Nginx+uwsgi and PostgreSQL. In this post, I’ll be detailing the steps we took to migrate the database from MySQL to PostgreSQL, with as little downtime as possible. Please leave a comment if you have suggestions!
One-time Pre-migration Steps
The default encoding on PostgreSQL is SQL_ASCII and you probably want UTF-8. If you don’t know what you want, you want UTF-8 (trust me). The easiest way was to blow away the default cluster and re-create it (Thanks jacobian!)
Make PostgreSQL listen on all interfaces
/etc/postgresql/9.1/main/postgresql.conf and ensure PostgreSQL is listening on all interfaces.
Allow access to PostgreSQL from the old server
/etc/postgresql/9.1/main/pg_hba.conf and add an entry for the old server (where 188.8.131.52 is the IP address of the old server).
Install client libraries on the old server
We use sqlalchemy for db access and I had to do
apt-get install python-psycopg2.
Creating Users and Databases
Our process is to create a user for each app and have that app’s database be owned by this user, here’s a script that automated creating the user and database.
Create user and database on the new server with the script above. Remember to set a password for this new user.
The most worrisome bit about the whole migration was exporting the data from MySQL and importing it into PostgreSQL. We used mysql2psql and it didn’t give a lot of troubles except for the bit where floats got a little messed up. My personal recommendation is to not use
real, but use
numeric(7,4) with the accuracy adjusted for what you need (this particular definition is used for our lat/long definitions.
mysql2psql on your command line, this will create the config file.
Now edit the
mysql2psql.yml file and add your appropriate entries. Here’s what ours looked like
When you run
psql2mysql again, it will export the database
mydb.sql. Before we did that, we removed this particular site from /etc/apache2/sites-enabled and restarted apache. We didn’t want the sql file to go stale as soon as it was exported. This is where the downtime starts
Copy the file over to the new server and import it into PostgreSQL with psql.
In retrospect, I should have just imported it directly with
mysql2psql. I was initially hesitant because it involved creating a user that could access that machine from outside. But I later realized I needed it anyway.
Now change the settings on the old server to use the postgres database as the backend, enable the site in Apache and you’re all set to serve this site from PostgreSQL!