pghops 1.0.0

A highly opionated Postgresql migration tool

pghops is a command line PostgreSQL schema migration utility written in Python.

Features

• Simple version file syntax: pghops version files are yaml files with keys representing directories and values of one or more sql file names.
• Executes scripts with psql: pghops uses psql to execute all sql, leveraging the extensive functionality of the PostgreSQL client. Use any psql command.
• All or nothing migrations: Wrap your entire migration in a single transaction or each migration script in a transaction if you prefer.
• All sql commands saved to version table pghops saves all sql executed during migrations to it version table.

Usage Overview

When you install PostgreSQL you initialize a storage area on disk called a database cluster, which is a collection of databases managed by a single instance of PostgreSQL. pghops expects you to place all files associated to building and defining your cluster in a single directory, referred to henceforth as the cluster_directory. Each sub-directory in cluster_directory should be the name of a database within your cluster (if not, you can add a file named databases to list the database directories).

For example, say your cluster_directory is /tmp/pghops/main and you have two databases - dba and dbb. Your directory structure would look like:

└── main
    ├── dba
    └── dbb

pghops requires each database directory to have a directory named versions which contain, you guessed it, all of you database migration files. Each migration file must follow the following versioning convention:

<major>.<minor>.<patch>.<label>.yml

This allows you to follow Semantic Versioning if you choose. pghops parses these file names and saves them to the pghops.version table.

If pghops detects the database does not exist on the cluster, pghops will create it if the database directory has a file named create_database.sql containing the database creation commands. pghops records all migrations in a table named version in the schema pghops. If this table does not exists pghops will run the included 0000.0000.0000.pghops-init.yaml script first to create it.

Each version file must be in yaml format and have a yaml or yml suffix. The file can only contain comments and key / value pairs, with keys representing directories and values of either a single file or a list of files to execute. Directories can be absolute or relative to either the database directory or a directory named schemas within the database directory. We recommend laying out your directory structure the same as pgAdmin's. For example, if your cluster_directory looks like:

├── cluster_a
│   ├── databases
│   ├── db_a1
│   │   ├── create_database.sql
│   │   ├── schemas
│   │   │   └── public
│   │   │       ├── functions
│   │   │       ├── tables
│   │   │       │   └── visits.sql
│   │   │       └── views
│   │   │           ├── vistor_view.sql
│   │   │           └── location_view.sql
│   │   └── versions
│   │       ├── 0000.0011.0001.change-a.yml
│   │       ├── 0000.0021.0002.change-b.yml
│   │       └── 0000.0032.0000.change-c.yml
│   ├── db_a2
│   │   ├── create_database.sql
│   │   ├── data
│   │   │   └── init-data.sql
│   │   ├── schemas
│   │   │   └── public
│   │   │       ├── functions
│   │   │       ├── tables
│   │   │       │   └── user.sql
│   │   │       └── views
│   │   │           ├── user_view.sql
│   │   │           └── accounts_view.sql
│   │   └── versions
│   │       ├── 0000.0001.0001.feature-a.yml
│   │       ├── 0000.0001.0002.feature-b.yml
│   │       └── 0000.0002.0000.feature-c.yml

and you want to use pghops to create new views defined in visitor_view and location_view, create a new migration script in db_a1/versions such as 0000.0033.0000.new-views.yml and add lines such as:

schemas/public/views:
  - visitor_view.sql
  - location_view.sql

You can optionally omit the sql suffix. Again, schemas is optional as well.

Run pghops by cd'ing into cluster_directory and running

pghops

See below for command line parameters. You can also pass the path of cluster_directory as the first argument.

When you run pghops, it will concatenate the contents of visitor_view.sql and location_view.sql into a single file and then execute it via psql in a single transaction. If successful, a new record is added to pghops.version and your migration is complete! For more examples see the test clusters.

Installation

pghops requires python 3.7 and the psql client. Install with pip:

pip3 install pghops

This should add the executeables pghops and create_indexes to your path.

Best Practices

Directory layout for your sql code

We recommend following the same layout as pgAdmin. For example, if you have a database named dba, one possibility is:

├── dba
│   ├── data
│   ├── schemas
│   │   ├── myschema
│   │   │   ├── functions
│   │   │   ├── tables
│   │   │   └── views
│   │   └── public
│   │       ├── functions
│   │       ├── tables
│   │       └── views
│   └── versions

The data directory can contain scripts to load data during your migrations.

Versioning

pghops is liberal when determining which migration files to execute. It ignores the major, minor, and patch fields in the pghops.version table and only looks at file_names.

As such, you can use whichever versioning scheme you like. Semantic Versioning is definitely a solid option. Another scheme, which requires slightly more effort for tracking but works well when dealing with multiple people working with many branches is to an auto-incrementing number for major that increases on every merge into your master/production branch. For minor, use something that refers to either a feature branch or something that links back to a ticketing system. For patch, use an auto-incrementing number for each migration file you create for the feature. Use label to differentiate between two people creating migration scripts for the same feature at the same time. This also helps to prevent merge conflicts.

Idempotency

Essentially this means if you execute the same sql twice all changes will only take affect once. So use "if [not] exists" when write DDL statements and check for the presence of your records first when executing update statements (or use the on conflict do nothing clause).

Keep old migration files up to date

The pghops.version table and Git (or another VCS) should be all you need for auditing and history purposes. If you make changes that would break older migration scripts if run on a new database, best to go back and update the older scripts. Then you can use pghops to create a new database from scratch for failover, setting up new environments, or testing purposes.

Passwords and psql

Normally you want to avoid having to enter your password for every psql call. A couple options:

1. Setup the user that runs pghops with password-less authentication, such as trust or peer. Best then to run pghops on the same box as PostgreSQL.
2. Use a password file.

Options

pghops has many congratulation options, which you can set via the command line, environment variables or various property files. Options are loaded in the following order, from highest to lowest priority:

1. Command line arguments
2. Properties in the file specified by the --options-file command line argument.
3. Environment variables.
4. Properties in //pghops.properties
5. Properties in /pghops.properties
6. Properties in pghops/conf/default.properties

Property files should be in yaml format and contain key/value pairs.

pghops treats options in property files that only differ in case or usage of underscore versus hyphen the same. For example:

wrap-all-in-transaction wrap_all_in_transaction Wrap_All_In_Transaction

all refer to the same option. Environment variables should use underscores instead of hyphens, be in all caps, and have a prefix of PGHOPS_. For example, the environment varaible for the wrap-all-in-transaction property above is PGHOPS_WRAP_ALL_IN_TRANSACTION.

psql's environment variables are also in effect.

pghops options are as follows:

cluster_directory - The first argument to pghops. Defaults to the current working directory. The base directory containing your database sql.

dbname - By default pghops will migrate all dbs in the cluster directory. Use this option to only update the specified db.

cluster_map - Path to a yaml file containing a map of cluster names to directories. The cluster name can then be supplied as the cluster_directory argument instead of a directory.

dry_run - Do not execute the migration, only print the files that would have executed.

verbosity - Verbosity level. One of default, verbose, or terse. "terse" only prints errors. "verbose" echos all executed sql.

psql_base_args - "Base" arguments to psql. Defaults to "--set ON_ERROR_STOP=1". Use this in combination with psql_arguments.

psql_arguments - A list of arguments to provide to psql, such as --host, --port, etc.

db_conninfo - Alternative way to specify the connection parameters to psql.

wrap_all_in_transaction - When true, the default, pghops will wrap the entire migration in a single transaction.

wrap_each_version_in_transaction - When true, each version script will run in its own transaction, not the entire migration.

fail_if_unable_to_connect - When true, the default, pghops will raise an error if it cannot connect to the database server.

fail_if_standby - When true, the default, pghops will raise an error if it can connect to the database server but the database server is in standby mode.

save_sql_to_version_table - When true, the default, pghops will save all executed sql to the pghops.version table. Consider setting to false for large migrations or migrations that contain sensitive info.

save_indexes - When true, the default, pghops scans you sql code for create index statements and saves them to the pghops.index table. See below for more details.

migration_file - Use this option to only execute the supplied file instead of all files in the versions directory.

script_suffixes - A case-insensitive comma separated list of suffixes that migration file names must match in order to be executed. Defaults to yml and yaml.

Managing Indexes

As your schema evolves, you may find the need to create new indexes on large, existing tables. If creating indexes during the migration is unacceptable, you can have pghops manage indexes for you so you can create them asynchronously at a later time.

By setting the option save-indexes to true (the default), pghops will scan your sql code for create index statements and save any to pghops.index. For pghops to track an index, ensure the following:

1. The index statement resides in a file with a '.sql' suffix.
2. The entire index statement resides on a single line.
3. The index statement begins on the first column of the line. pghops ignores any indexes statements preceded by white space. Useful if, for example, you have a function that creates a temp table and defines indexes on said temp table, you do not want pghops to manage this index.
4. Use fully qualified table names in you index definitions (schema.table_name). The create indexes script first checks for the existence of the table before executing the index statement, and when pghops saves an index it does not analyze any preceding set path statements. If you do not use a fully qualified table name pghops will not save the index.
5. The statement uses if not exists so it can be run multiple times without causing an error.
6. The scanning for indexes is not perfect. If you use unconventional names for your index or table which requires quoting the name, pghops cannot parse the statement correctly.

By scanning your code for indexes, you can define indexes in the same files as their table and pghops will add them to pghops.index automatically during the next migration.

For every record in pghops.index, create_indexes will first check to see if the table_name is a valid table. It then checks the enabled flag and, if set, executes the sql in definition. The script runs in parallel based on the number of cpu cores, although this advantage is mitigated in more recent PostgreSQLs that can create a single index in parallel automatically.

FAQ

What does pghops stand for?

Either PostGresql Highly OPinionated migrationS. Or maybe you can use pghops to "hop" to your next database version. Take your pick.

Why make pghops PostgresSQL specific? Why not make it database agnosistic by using drivers for the Python Database API?

By using psql you can leverage all of its power - your sql can contain any psql meta command which is not possible to do with adapters such as Psycopg.

Is there support for rolling back migrations?

No built in support. In a perfect world each database migration script would be accompanied by a rollback script. But if something goes wrong on production and you need to roll back, do you really feel comfortable executing the rollback script? Have you tested all possible state that the rollback script can encounter?

In my experience the need to rolling back is infrequent and when it is necessary, careful examination of the database must happen before any changes can take place. However, if you insist on having rollback scripts, you can initially create rollback files in the same versions directory and name them with a non-yaml suffix, such as .rollback. Then when you need to rollback, run pghops with the --migration-file option to run the rollback script. If you wish to erase the records from the pghops.version table, you will have to do that manually.

I have dependencies between my databases and I need pghops to execute migrations in a particular order.

In your cluster_directory, create a file named databases and list the databases in the required order.

What happens if I need to execute sql that cannot be in a transaction?

Probably best to include a commit statement immediately preceding the sql that cannot run inside a transaction, followed by a begin statement to start a new transaction. You could also omit transactions for this pghops run by setting the options wrap-all-in-transaction and wrap-each-version-in-transaction to false.

Miscellaneous

pghops was developed and tested on a GNU/Linux. Feel free to report bugs and contribute patches.

License

GPLv3. See COPYING.