github.com/anovateam/migrate@v3.5.4+incompatible/MIGRATIONS.md (about)

     1  # Migrations
     2  
     3  ## Migration Filename Format
     4  
     5  A single logical migration is represented as two separate migration files, one
     6  to migrate "up" to the specified version from the previous version, and a second
     7  to migrate back "down" to the previous version.  These migrations can be provided
     8  by any one of the supported [migration sources](./README.md#migration-sources).
     9  
    10  The ordering and direction of the migration files is determined by the filenames
    11  used for them.  `migrate` expects the filenames of migrations to have the format:
    12  
    13      {version}_{title}.up.{extension}
    14      {version}_{title}.down.{extension}
    15  
    16  The `title` of each migration is unused, and is only for readability.  Similarly,
    17  the `extension` of the migration files is not checked by the library, and should
    18  be an appropriate format for the database in use (`.sql` for SQL variants, for
    19  instance).
    20  
    21  Versions of migrations may be represented as any 64 bit unsigned integer.
    22  All migrations are applied upward in order of increasing version number, and
    23  downward by decreasing version number.
    24  
    25  Common versioning schemes include incrementing integers:
    26  
    27      1_initialize_schema.down.sql
    28      1_initialize_schema.up.sql
    29      2_add_table.down.sql
    30      2_add_table.up.sql
    31      ...
    32  
    33  Or timestamps at an appropriate resolution:
    34  
    35      1500360784_initialize_schema.down.sql
    36      1500360784_initialize_schema.up.sql
    37      1500445949_add_table.down.sql
    38      1500445949_add_table.up.sql
    39      ...
    40  
    41  But any scheme resulting in distinct, incrementing integers as versions is valid.
    42  
    43  It is suggested that the version number of corresponding `up` and `down` migration
    44  files be equivalent for clarity, but they are allowed to differ so long as the
    45  relative ordering of the migrations is preserved.
    46  
    47  The migration files are permitted to be empty, so in the event that a migration
    48  is a no-op or is irreversible, it is recommended to still include both migration
    49  files, and either leaving them empty or adding a comment as appropriate.
    50  
    51  ## Migration Content Format
    52  
    53  The format of the migration files themselves varies between database systems.
    54  Different databases have different semantics around schema changes and when and
    55  how they are allowed to occur
    56  (for instance, [if schema changes can occur within a transaction](https://wiki.postgresql.org/wiki/Transactional_DDL_in_PostgreSQL:_A_Competitive_Analysis)).
    57  
    58  As such, the `migrate` library has little to no checking around the format of
    59  migration sources.  The migration files are generally processed directly by the
    60  drivers as raw operations.
    61  
    62  ## Reversibility of Migrations
    63  
    64  Best practice for writing schema migration is that all migrations should be
    65  reversible.  It should in theory be possible for run migrations down and back up
    66  through any and all versions with the state being fully cleaned and recreated
    67  by doing so.
    68  
    69  By adhering to this recommended practice, development and deployment of new code
    70  is cleaner and easier (cleaning database state for a new feature should be as
    71  easy as migrating down to a prior version, and back up to the latest).
    72  
    73  As opposed to some other migration libraries, `migrate` represents up and down
    74  migrations as separate files.  This prevents any non-standard file syntax from
    75  being introduced which may result in unintended behavior or errors, depending
    76  on what database is processing the file.
    77  
    78  While it is technically possible for an up or down migration to exist on its own
    79  without an equivalently versioned counterpart, it is strongly recommended to
    80  always include a down migration which cleans up the state of the corresponding
    81  up migration.