Creating a change log with JDBC using the commit timestamp feature of Cloud Spanner

This document describes how to use the commit timestamp feature to track the date and time when changes are made to your database records. When you have a large database with many transactions that change records and you want to capture changes made to the datasets, the commit timestamp feature simplifies this task.

Through a series of code samples, this document shows you how to rewrite existing data manipulation language (DML) to create a change log. The document extends Using Cloud Spanner commit timestamps to create a change log with Go by demonstrating the methods using Java and JDBC. The logic is represented in SQL rather than as Spanner client objects.

How the tables work

Assume you have a facts table called msgs and a history table called msg_history. Each time the facts table gets modified, you want to concurrently save a record in the msg_history table. You can use the contents of the history table later for other uses—for example, for auditing or as an event source.

The msgs table holds the facts, represented by a transaction ID (id) and the message (msg). The data definition language (DDL) statement that you use to create the table looks like the following:

CREATE TABLE msgs (
    id INT64,
    msg STRING(MAX),
) PRIMARY KEY (id)

The msg_history table holds the transaction history. The following DDL creates the history table. The column ts stores the commit timestamp.

CREATE TABLE msgs_history (
    id INT64,
    ts TIMESTAMP OPTIONS (allow_commit_timestamp=true),
    previous_msg STRING(MAX)
) PRIMARY KEY (ts, id)

The examples

Instead of writing only to the facts table, you must do the operation to the history table in the same transaction. Spanner's JDBC driver supports declaration of the start and end of a transaction, which is a standard JDBC operation.

Step 1: Rewrite the insert operations

The first step is to rewrite the insert operations from the following form:

insert into msgs (id, msg) values (1, 'a real msg')
  • Change the inserts to look like the following:

    insert into msgs_history (id, previous_msg, ts) values (1, Null, PENDING_COMMIT_TIMESTAMP());
    insert into msgs (id, msg) values (1, 'a real msg');
    

    Here's the Java code:

    // insert into history
    stmt.executeUpdate(
        "insert into msgs_history (id, previous_msg, ts)"
            + " values (1, Null, PENDING_COMMIT_TIMESTAMP())");
    // insert into real table
    stmt.executeUpdate("insert into msgs (id, msg)" + " values (1, 'a real msg')");
    conn.commit();
    

Step 2: Rewrite the update operations

Next, you rewrite the update operations from the following form:

update msgs set msg = 'new message' where id = 1
  • Change the updates to look like the following:

    insert msgs_history (id, previous_msg, ts) values ((select id from msgs where id =1 ), (select msg from msgs where id = 1), PENDING_COMMIT_TIMESTAMP());
    update msgs set msg = 'new message' where id = 1;
    

    Here's the Java code:

    // insert into historystmt.executeUpdate(
        "insert msgs_history (id, previous_msg, ts)"
            + " values  ((select id from msgs where id =1),"
            + " (select msg from msgs where id = 1), PENDING_COMMIT_TIMESTAMP())");
    // update into fact table
    stmt.executeUpdate("update msgs set msg = 'new message' where id = 1");
    conn.commit();
    

Step 3: Rewrite the delete operations

Finally, you rewrite the delete operations from the following form:

delete from msgs where id = 1
  • Change the deletes to look like the following:

    insert msgs_history (id, previous_msg, ts) values ((select id from msgs where id =1 ), (select msg from msgs where id = 1), PENDING_COMMIT_TIMESTAMP());
    delete from msgs where id = 1;
    

    Here's the Java code:

    // insert into history
    stmt.executeUpdate(
        "insert msgs_history (id, previous_msg, ts)"
            + " values  ((select id from msgs where id =1),"
            + " (select msg from msgs where id = 1), PENDING_COMMIT_TIMESTAMP()) ");
    // delete from real table
    stmt.executeUpdate("delete from msgs where id = 1");
    conn.commit();
    

Using the history table

  • Access the change history as follows:

    select id,msg,current_timestamp() as ts
    from msgs where id = 1
    union all
    select id,previous_msg, ts as msg
    from msgs_history where id = 1 order by ts desc
    

    The results look similar to the following:

    id    msg             ts
    1     new message     2020-02-07T07:44:10.24833726Z
    1     a real msg      2020-01-14T10:07:20.137935Z
    1                     2020-01-14T10:07:20.070374Z
    

What's next