SQLite in Production: Optimizing WAL Mode, Concurrency, and VFS Layers

That's the biggest pain of dealign with SQLite.

You can define constraints on a column that ensure it is text that's valid JSON for example:

  CREATE TABLE documents (
    id INTEGER PRIMARY KEY,
    data TEXT NOT NULL
      CHECK (
        json_valid(data)
        json_type(data) = 'object'
      )
  );

  CREATE TABLE documents (
    id INTEGER PRIMARY KEY,
    data TEXT NOT NULL CHECK (
      json_valid(data)
      AND json_type(data) = 'object'
      AND json_type(data, '$.name') = 'text'
      AND json_type(data, '$.age') = 'integer'
    )
  );

  CREATE TABLE events (
    id INTEGER PRIMARY KEY,
    occurred_on TEXT NOT NULL CHECK (
      length(occurred_on) = 10
      AND occurred_on GLOB
        '[0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]'
      AND date(occurred_on, '+0 days') = occurred_on
    )
  );

For alter table it offers a "transform" command which implements the pattern of creating a new table with your desired scheme, copying data to it from the old table, then renaming the tables (all in a transaction): https://sqlite-utils.datasette.io/en/stable/cli.html#transfo...

  sqlite-utils transform fixtures.db roadside_attractions \
    --rename pk id \
    --default name Untitled \
    --column-order id \
    --column-order longitude \
    --column-order latitude \
    --drop address

  sqlite-utils migrate creatures.db path/to/migrations.py

  from sqlite_utils import Migrations
  
  migrations = Migrations("creatures")
  
  @migrations()
  def create_table(db):
      db["creatures"].create(
          {"id": int, "name": str, "species": str},
          pk="id",
      )
  
  @migrations()
  def add_weight(db):
      db["creatures"].add_column("weight", float)

I see the migration argument come up a lot. But, in practice with sqlite you'll be using projections where you have a source of truth database (event log) and project off it into disposable/expendable sqlite databases. So schema changes are often just delete and rebuild the projection.

I don’t think that’s really a positive or negative.

And the point about migrations ideally being separate is really just your own opinion. I prefer having the database definition in the same source tree as the application, ideally just a .sql file in the project.

Why is that the ideal? With SQLite your database is 1:1 connected to your application (meaning there is no other application using that database), it doesn't make sense to move the app to a new version but not the database or vice versa. Running migrations on startup of the app is ideal.

Migrations are a bit more difficult to write for SQLite than they need to be (DROP column only being added recently...), though. I usually iterate a few times to get the column definitions just right so that I don't have to change them later.

As you say column types are limited (and enforcement lax) but in practice it's a non-issue because you convert the data to application-specific types when reading from db (and enforce by writing only right data types) anyway.

https://www.sqlite.org/lang_altertable.html

On Go you can embed your migrations in your binary:

https://oscarforner.com/blog/2023-10-10-go-embed-for-migrati...

Or at least that's how I view it. Whenever I think about using SQLite, I make sure I read these documents to see if I am fine with the limitations.

https://sqlite.org/whentouse.html

https://sqlite.org/quirks.html

No you don’t [0]. It is less convenient than being able to directly alter columns, but you do not need to mess around with writable schemas or risk corruption.

[0]: https://www.sqlite.org/lang_altertable.html#otheralter

If you're saying "do this, get that", you should be able explain how to measure and reproduce that result.

The answer to why someone might choose SQLite in production could be latency but if it is then prove it's worth the trade-offs.

Another downside is that it will take time to make it look nice.

That being said it can be done and it's pretty normal for mobile apps, desktop apps etc. You just have to make sure the migrations are run when the tenant connects/unlocks/runs the app - and make sure that you minimise the risk of it going wrong!

I don't think it scales when you have unbounded amounts of tenants.