id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,pull_request,body,repo,type,active_lock_reason,performed_via_github_app,reactions,draft,state_reason
1870672704,PR_kwDOBm6k_c5Y-7Em,2162,"Add new `--internal internal.db` option, deprecate legacy `_internal` database",15178711,closed,0,,,4,2023-08-29T00:05:07Z,2023-08-29T03:24:23Z,2023-08-29T03:24:23Z,CONTRIBUTOR,simonw/datasette/pulls/2162,"refs #2157 

This PR adds a new `--internal` option to datasette serve. If provided, it is the path to a persistent internal database that Datasette core and Datasette plugins can use to store data, as discussed in the proposal issue. 

This PR also removes and deprecates the previous in-memory `_internal` database. Those tables now appear in the `internal` database, with `core_` prefixes (ex `tables` in `_internal` is now `core_tables` in `internal`).


## A note on the new `core_` tables
However, one important notes about those new `core_` tables: If a `--internal` DB is passed in, that means those `core_` tables will persist across multiple Datasette instances. This wasn't the case before, since `_internal` was always an in-memory database created from scratch.

I tried to put those `core_` tables as `TEMP` tables - after all, there's always one 1 `internal` DB connection at a time, so I figured it would work. But, since we use the `Database()` wrapper for the internal DB, it has two separate connections: a default read-only connection and a write connection that is created when a write operation occurs. Which meant the `TEMP` tables would be created by the write connection, but not available in the read-only connection. 

So I had a brillant idea: Attach an in-memory named database with `cache=shared`, and create those tables there! 

```sql
ATTACH DATABASE 'file:datasette_internal_core?mode=memory&cache=shared' AS core;
```

We'd run this on both the read-only connection and the write-only connection. That way, those tables would stay in memory, they'd communicate with the `cache=shared` feature, and we'd be good to go.


However, I couldn't find an easy way to run a `ATTACH DATABASE` command on the read-only query. 

Using `Database()` as a wrapper for the internal DB is pretty limiting - it's meant for Datasette ""data"" databases, where we want multiple readers and possibly 1 write connection at a time. But the internal database doesn't really require that kind of support - I think we could get away with a single read/write connection, but it seemed like too big of a rabbithole to go through now. 


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:books: Documentation preview :books:: https://datasette--2162.org.readthedocs.build/en/2162/

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