PutIf 与 PutIfExists 和 PutIfNotExists 有何不同

Question

I am using ScalarDB which adds ACID support in Cassandra . Referring to the documentation, how is PutIf different from PutIfExists and PutIfNotExists ?

I suppose PutIfExists is like an update PutIfNotExists is like a new addition

What is PutIf? When to use it?

Answer 1

PutIf can add conditions to update a record. You can use PutIfExists when you want to just update without any other condition. On the other hand, You have to use PutIfNotExists when you want to insert a new record without unexpected overwriting.

For example, a table has a record as below. We want to update the pass of a record if the score is greater than or equal to 60. (We assume that records have been inserted beforehand, and we have calculated the threshold (mean, median, etc.), then we decide the ID can pass the threshold or not.)

|pID|cID|score| pass|
|  0|  0|   80|false|
|  1|  0|   45|false|

The following put will update the first record to set true in pass .

PartitionKey pk = new Key(new IntValue("pID", 0));
ClusteringKey ck = new Key(new IntValue("cID", 0));

PutIf condition = new PutIf(new ConditionalExpression("score", new IntValue(60), Operator.GTE));

Put put = new Put(pk, ck).withValue(new BooleanValue("pass", true)).withCondition(condition);

storage.put(put);

If we try to update the second record with the same condition, it won't be updated because the score is less than 60.