71 lines
3.8 KiB
Markdown
71 lines
3.8 KiB
Markdown
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# FAQ
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#### How is the code base structured?
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```
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/ package migrate (the heart of everything)
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/cli the CLI wrapper
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/database database driver and sub directories have the actual driver implementations
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/source source driver and sub directories have the actual driver implementations
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```
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#### Why is there no `source/driver.go:Last()`?
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It's not needed. And unless the source has a "native" way to read a directory in reversed order,
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it might be expensive to do a full directory scan in order to get the last element.
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#### What is a NilMigration? NilVersion?
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NilMigration defines a migration without a body. NilVersion is defined as const -1.
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#### What is the difference between uint(version) and int(targetVersion)?
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version refers to an existing migration version coming from a source and therefor can never be negative.
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targetVersion can either be a version OR represent a NilVersion, which equals -1.
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#### What's the difference between Next/Previous and Up/Down?
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```
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1_first_migration.up.extension next -> 2_second_migration.up.extension ...
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1_first_migration.down.extension <- previous 2_second_migration.down.extension ...
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```
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#### Why two separate files (up and down) for a migration?
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It makes all of our lives easier. No new markup/syntax to learn for users
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and existing database utility tools continue to work as expected.
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#### How many migrations can migrate handle?
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Whatever the maximum positive signed integer value is for your platform.
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For 32bit it would be 2,147,483,647 migrations. Migrate only keeps references to
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the currently run and pre-fetched migrations in memory. Please note that some
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source drivers need to do build a full "directory" tree first, which puts some
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heat on the memory consumption.
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#### Are the table tests in migrate_test.go bloated?
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Yes and no. There are duplicate test cases for sure but they don't hurt here. In fact
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the tests are very visual now and might help new users understand expected behaviors quickly.
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Migrate from version x to y and y is the last migration? Just check out the test for
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that particular case and know what's going on instantly.
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#### What is Docker being used for?
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Only for testing. See [testing/docker.go](testing/docker.go)
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#### Why not just use docker-compose?
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It doesn't give us enough runtime control for testing. We want to be able to bring up containers fast
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and whenever we want, not just once at the beginning of all tests.
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#### Can I maintain my driver in my own repository?
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Yes, technically thats possible. We want to encourage you to contribute your driver to this respository though.
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The driver's functionality is dictated by migrate's interfaces. That means there should really
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just be one driver for a database/ source. We want to prevent a future where several drivers doing the exact same thing,
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just implemented a bit differently, co-exist somewhere on Github. If users have to do research first to find the
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"best" available driver for a database in order to get started, we would have failed as an open source community.
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#### Can I mix multiple sources during a batch of migrations?
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No.
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#### What does "dirty" database mean?
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Before a migration runs, each database sets a dirty flag. Execution stops if a migration fails and the dirty state persists,
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which prevents attempts to run more migrations on top of a failed migration. You need to manually fix the error
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and then "force" the expected version.
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#### What happens if two programs try and update the database at the same time?
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Database-specific locking features are used by *some* database drivers to prevent multiple instances of migrate from running migrations at the same time
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the same database at the same time. For example, the MySQL driver uses the `GET_LOCK` function, while the Postgres driver uses
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the `pg_advisory_lock` function.
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