Idempotence in the query builder¶

SELECT statements¶

SELECT statements don’t modify the contents of the database. They’re always considered idempotent, regardless of the other rules below.

Unsafe terms¶

If you use the result of a user-defined function in an INSERT or UPDATE statement, there is no way of knowing if that function is idempotent:

SimpleStatement statement = insertInto("foo").value("k", function("generate_id")).build();
// INSERT INTO foo (k) VALUES (generate_id())
assert !statement.isIdempotent();

This extends to arithmetic operations using such terms:

SimpleStatement statement =
    insertInto("foo").value("k", add(function("generate_id"), literal(1))).build();
// INSERT INTO foo (k) VALUES (generate_id()+1)
assert !statement.isIdempotent();

Raw terms could be anything, so they are also considered unsafe by default:

SimpleStatement statement =
    insertInto("foo").value("k", raw("generate_id()+1")).build();
// INSERT INTO foo (k) VALUES (generate_id()+1)
assert !statement.isIdempotent();

Unsafe WHERE clauses¶

If a WHERE clause in an UPDATE or DELETE statement uses a comparison with an unsafe term, it could potentially apply to different rows for each execution:

SimpleStatement statement =
    update("foo")
        .setColumn("v", bindMarker())
        .whereColumn("k").isEqualTo(function("non_idempotent_func"))
        .build();
// UPDATE foo SET v=? WHERE k=non_idempotent_func()
assert !statement.isIdempotent();

Unsafe updates¶

Counter updates are never idempotent:

SimpleStatement statement =
    update("foo")
        .increment("c")
        .whereColumn("k").isEqualTo(bindMarker())
        .build();
// UPDATE foo SET c+=1 WHERE k=?
assert !statement.isIdempotent();

Nor is appending or prepending an element to a list:

SimpleStatement statement =
    update("foo")
        .appendListElement("l", literal(1))
        .whereColumn("k").isEqualTo(bindMarker())
        .build();
// UPDATE foo SET l=l+[1] WHERE k=?
assert !statement.isIdempotent();

The generic append and prepend methods apply to any kind of collection, so we have to consider them unsafe by default too:

SimpleStatement statement =
    update("foo")
        .prepend("l", literal(Arrays.asList(1, 2, 3)))
        .whereColumn("k").isEqualTo(bindMarker())
        .build();
// UPDATE foo SET l=[1,2,3]+l WHERE k=?
assert !statement.isIdempotent();

The generic remove method is however safe since collection removals are idempotent:

SimpleStatement statement =
    update("foo")
        .remove("l", literal(Arrays.asList(1, 2, 3)))
        .whereColumn("k").isEqualTo(bindMarker())
        .build();
// UPDATE foo SET l=l-[1,2,3] WHERE k=?
assert statement.isIdempotent();

When appending, prepending or removing a single element to/from a collection, it is possible to use the dedicated methods listed below; their idempotence depends on the collection type (list, set or map), the operation (append, prepend or removal) and the idempotence of the element being added/removed:

1. appendListElement : not idempotent

2. prependListElement : not idempotent

3. removeListElement : idempotent if element is idempotent

4. appendSetElement : idempotent if element is idempotent

5. prependSetElement : idempotent if element is idempotent

6. removeSetElement : idempotent if element is idempotent

7. appendMapElement : idempotent if both key and value are idempotent

8. prependMapElement : idempotent if both key and value are idempotent

9. removeMapElement : idempotent if both key and value are idempotent

In practice, most invocations of the above methods will be idempotent because most collection elements are. For example, the following statement is idempotent since literal(1) is also idempotent:

SimpleStatement statement =
    update("foo")
        .removeListElement("l", literal(1))
        .whereColumn("k").isEqualTo(bindMarker())
        .build();
// UPDATE foo SET l=l-[1] WHERE k=?
assert statement.isIdempotent();

However, in rare cases the resulting statement won’t be marked idempotent, e.g. if you use a function to select a collection element:

SimpleStatement statement =
    update("foo")
        .removeListElement("l", function("myfunc"))
        .whereColumn("k").isEqualTo(bindMarker())
        .build();
// UPDATE foo SET l=l-[myfunc()] WHERE k=?
assert !statement.isIdempotent();

Unsafe deletions¶

Deleting from a list is not idempotent:

SimpleStatement statement =
    deleteFrom("foo")
        .element("l", literal(0))
        .whereColumn("k").isEqualTo(bindMarker())
        .build();
// DELETE l[0] FROM foo WHERE k=?
assert !statement.isIdempotent();

Conditional statements¶

All conditional statements are considered non-idempotent:

• INSERT with IF NOT EXISTS;

• UPDATE and DELETE with IF EXISTS or IF conditions on columns.

This might seem counter-intuitive, as these queries can sometimes be safe to execute multiple times. For example, consider the following query:

update("foo")
    .setColumn("v", literal(4))
    .whereColumn("k").isEqualTo(literal(1))
    .ifColumn("v").isEqualTo(literal(1));
// UPDATE foo SET v=4 WHERE k=1 IF v=1

If we execute it twice, the IF condition will fail the second time, so the second execution will do nothing and v will still have the value 4.

However, the problem appears when we consider multiple clients executing the query with retries:

1. v has the value 1;

2. client 1 executes the query above, performing a a CAS (compare and set) from 1 to 4;

3. client 1’s connection drops, but the query completes successfully. v now has the value 4;

4. client 2 executes a CAS from 4 to 2;

5. client 2’s transaction succeeds. v now has the value 2;

6. since client 1 lost its connection, it considers the query as failed, and transparently retries the CAS from 1 to 4. But since the column now has value 2, it receives a “not applied” response.

One important aspect of lightweight transactions is linearizability: given a set of concurrent operations on a column from different clients, there must be a way to reorder them to yield a sequential history that is correct. From our clients’ point of view, there were two operations:

• client 1 executed a CAS from 1 to 4, that was not applied;

• client 2 executed a CAS from 4 to 2, that was applied.

But overall the column changed from 1 to 2. There is no ordering of the two operations that can explain that change. We broke linearizability by doing a transparent retry at step 6.