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simonw/sqlite-chronicle: Use triggers to track when rows in a SQLite table were updated or deleted

Use triggers to track when rows in a SQLite table were updated or deleted

pip install sqlite-chronicle

You can enable chronicle for specific tables in a SQLite database using the command-line interface, passing in one or more table names:

python -m sqlite_chronicle database.db table_1 table_2

This package exposes two Python functions for configuring and using chronicle tables:

This module provides a function: sqlite_chronicle.enable_chronicle(conn, table_name), which does the following:

1. Checks if a _chronicle_{table_name} table exists already. If so, it does nothing. Otherwise...
2. Creates that table, with the same primary key columns as the original table plus integer columns __added_ms, __updated_ms, __version and __deleted
3. Creates a new row in the chronicle table corresponding to every row in the original table, setting __added_ms and __updated_ms to the current timestamp in milliseconds, and __version column that starts at 1 and increments for each subsequent row
4. Sets up three triggers on the table:

• An AFTER INSERT trigger, which creates a new row in the chronicle table, sets __added_ms and __updated_ms to the current time and sets the __version to one higher than the current maximum version for that table
• An AFTER UPDATE trigger, which updates the __updated_ms timestamp and increments the __version - but only if at least one column in the row has changed
• An AFTER DELETE trigger, which updates the __updated_ms, increments the __version and places a 1 in the deleted column

The function will raise a sqlite_chronicle.ChronicleError exception if the table does not exist or if it does not have a single or compound primary key,

Note that the __version for a table is a globally incrementing number, so every time it is set it will be set to the current max(__version) + 1 for that entire table.

The end result is a chronicle table that looks something like this:

This function detects if the specified table has previously had an older version of the chronicle table and triggers created for it, and if so it will upgrade that table to the latest implementation, preserving existing timestamp and version data.

The sqlite_chronicle.updates_since() function returns a generator over a list of Change objects.

These objects represent changes that have occurred to rows in the table since the since version number, or since the beginning of time if since is not provided.

• conn is a SQLite connection object
• table_name is a string containing the name of the table to get changes for
• since is an optional integer version number - if not provided, all changes will be returned
• batch_size is an internal detail, controlling the number of rows that are returned from the database at a time. You should not need to change this as the function implements its own internal pagination.

Each Change returned from the generator looks something like this:

Change(
    pks=(5,),
    added_ms=1701836971223,
    updated_ms=1701836971223,
    version=5,
    row={'id': 5, 'name': 'Simon'},
    deleted=False
)

A Change is a dataclass with the following properties:

• pks is a tuple of the primary key values for the row - this will be a tuple with a single item for normal primary keys, or multiple items for compound primary keys
• added_ms is the timestamp in milliseconds when the row was added
• updated_ms is the timestamp in milliseconds when the row was last updated
• version is the version number for the row - you can use this as a since value to get changes since that point
• row is a dictionary containing the current values for the row - these will be None if the row has been deleted (except for the primary keys)
• deleted is 0 if the row has not been deleted, or 1 if it has been deleted

Any time you call this you should track the last version number that you see, so you can pass it as the since value in future calls to get changes that occurred since that point.

Note that if a row had multiple updates in between calls to this function you will still only see one Change object for that row - the updated_ms and version will reflect the most recent update.

• If you run INSERT OR REPLACE INTO ... and update an existing record in a way that does not change any of the fields, this system will still treat that record as if it has been updated. Use INSERT ... ON CONFLICT SET upserts instead to avoid this problem.
• Updates to columns that are part of a primary key for the record is not currently supported.

Chronicle tables can be used to efficiently answer the question "what rows have been inserted, updated or deleted since I last checked" - by looking at the version column which has an index to make it fast to answer that question.

This has numerous potential applications, including:

• Synchronization and replication: other databases can "subscribe" to tables, keeping track of when they last refreshed their copy and requesting just rows that changed since the last time - and deleting rows that have been marked as deleted.
• Indexing: if you need to update an Elasticsearch index or a vector database embeddings index or similar you can run against just the records that changed since your last run - see also The denormalized query engine design pattern
• Enrichments: datasette-enrichments needs to to persist something that says "every address column should be geocoded" - then have an enrichment that runs every X seconds and looks for newly inserted or updated rows and enriches just those.
• Showing people what has changed since their last visit - "52 rows have been updated and 16 deleted since yesterday" kind of thing.

Here's an example SQL schema showing the _chronicle_dogs table that would be created for a dogs table, along with its triggers:

CREATE TABLE dogs (id integer primary key, name text, age integer);

CREATE TABLE "_chronicle_dogs" (
  "id" INTEGER,
  __added_ms INTEGER,
  __updated_ms INTEGER,
  __version INTEGER,
  __deleted INTEGER DEFAULT 0,
  PRIMARY KEY("id")
);

CREATE INDEX "_chronicle_dogs__version_idx"
  ON "_chronicle_dogs"(__version);

CREATE TRIGGER "chronicle_dogs_ai"
AFTER INSERT ON "dogs"
FOR EACH ROW
BEGIN
  INSERT OR IGNORE INTO "_chronicle_dogs" (
    "id", __added_ms, __updated_ms, __version, __deleted
  ) VALUES (
    NEW."id",
    CAST((julianday('now') - 2440587.5)*86400*1000 AS INTEGER),
    CAST((julianday('now') - 2440587.5)*86400*1000 AS INTEGER),
    COALESCE((SELECT MAX(__version) FROM "_chronicle_dogs"),0) + 1,
    0
  );
END;

CREATE TRIGGER "chronicle_dogs_au"
AFTER UPDATE ON "dogs"
FOR EACH ROW
WHEN OLD."name" IS NOT NEW."name" OR OLD."age" IS NOT NEW."age"
BEGIN
  UPDATE "_chronicle_dogs"
  SET __updated_ms = CAST((julianday('now') - 2440587.5)*86400*1000 AS INTEGER),
    __version = COALESCE((SELECT MAX(__version) FROM "_chronicle_dogs"),0) + 1
  WHERE "id"=NEW."id";
END;

CREATE TRIGGER "chronicle_dogs_ad"
AFTER DELETE ON "dogs"
FOR EACH ROW
BEGIN
  UPDATE "_chronicle_dogs"
    SET __updated_ms = CAST((julianday('now') - 2440587.5)*86400*1000 AS INTEGER),
      __version = COALESCE((SELECT MAX(__version) FROM "_chronicle_dogs"),0) + 1,
      __deleted = 1
  WHERE "id"=OLD."id";
END

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