issue_comments: 655652679

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html_url issue_url id node_id user created_at updated_at author_association body reactions issue performed_via_github_app 655652679 MDEyOklzc3VlQ29tbWVudDY1NTY1MjY3OQ== 79913 2020-07-08T17:24:46Z 2020-07-08T17:24:46Z CONTRIBUTOR

Better transaction handling would be really great. Some of my thoughts on implementing better transaction discipline are in

My preferences:

  • Each CLI command should operate in a single transaction so that either the whole thing succeeds or the whole thing is rolled back. This avoids partially completed operations when an error occurs part way through processing. Partially completed operations are typically much harder to recovery from gracefully and may cause inconsistent data states.

  • The Python API should be transaction-agnostic and rely on the caller to coordinate transactions. Only the caller knows how individual insert, create, update, etc operations/methods should be bundled conceptually into transactions. When the caller is the CLI, for example, that bundling would be at the CLI command-level. Other callers might want to break up operations into multiple transactions. Transactions are usually most useful when controlled at the application-level (like logging configuration) instead of the library level. The library needs to provide an API that's conducive to transaction use, though.

  • The Python API should provide a context manager to provide consistent transactions handling with more useful defaults than Python's sqlite3 module. The latter issues implicit BEGIN statements by default for most DML (INSERT, UPDATE, DELETE, … but not SELECT, I believe), but not DDL (CREATE TABLE, DROP TABLE, CREATE VIEW, …). Notably, the sqlite3 module doesn't issue the implicit BEGIN until the first DML statement. It does not issue it when entering the with conn block, like other DBAPI2-compatible modules do. The with conn block for sqlite3 only arranges to commit or rollback an existing transaction when exiting. Including DDL and SELECTs in transactions is important for operation consistency, though. There are several existing tickets about this and future changes are in the works, but sqlite-utils can provide its own API sooner. sqlite-utils's Database class could itself be a context manager (built on the sqlite3 connection context manager) which additionally issues an explicit BEGIN when entering. This would then let Python API callers do something like:

db = sqlite_utils.Database(path)

with db: # ← BEGIN issued here by Database.__enter__
# ← COMMIT/ROLLBACK issue here by sqlite3.connection.__exit__
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