General rules for applications v5

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Background

PGD uses replica identity values to identify the rows to change. Applications can cause difficulties if they insert, delete, and then later reuse the same unique identifiers. This is known as the ABA problem. PGD can't know whether the rows are the current row, the last row, or much older rows.

Similarly, since PGD uses table names to identify the table against which changes are replayed, a similar ABA problem exists with applications that create, drop, and then later reuse the same object names.

Rules for applications

These issues give rise to some simple rules for applications to follow:

  • Use unique identifiers for rows (INSERT).
  • Avoid modifying unique identifiers (UPDATE).
  • Avoid reusing deleted unique identifiers.
  • Avoid reusing dropped object names.

In the general case, breaking those rules can lead to data anomalies and divergence. Applications can break those rules as long as certain conditions are met. However, use caution: while anomalies are unlikely, they aren't impossible. For example, you can reuse a row value as long as the DELETE was replayed on all nodes, including down nodes. This might normally occur in less than a second but can take days if a severe issue occurred on one node that prevented it from restarting correctly.

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