A query is idempotent if it can be applied multiple times without changing the result of the initial application. For example:
Idempotence matters for
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and speculative query executions . The driver will bypass those features if the Statement#isIdempotent() flag is set to
, to ensure that the statement does not get executed more than once.
In most cases, you must set that flag manually. The driver does not parse query strings, so it can’t infer it automatically (except for statements coming from the query builder, see below).
Statements start out as non-idempotent by default. You can override the flag on each statement:
The default is also configurable: if you want all statements to start out as idempotent, do this:
Any statement on which you didn’t call
gets this default value.
Bound statements inherit the flag from the prepared statement they were created from:
DSL tries to infer the
flag on the statements it generates. The following statements will be marked
prepend, append or deletion operations on lists:
queries that insert the result of a function call or a “raw” string in a column (or as an element in a collection column):
This is a conservative approach, since the driver can’t guess whether a function is idempotent, or what a raw string contains. It might yield false negatives, that you’ll have to fix manually.
lightweight transactions (see the next section for a detailed explanation):
If these rules produce a false negative, you can manually override the flag on the built statement:
As explained in the previous section, the query builder considers lightweight transactions as non-idempotent. This might sound counter-intuitive, as these queries can sometimes be safe to execute multiple times. For example, consider the following query:
If we execute it twice, the
condition will fail the second time, so the second execution will do nothing and
will still have the value 4.
However, the problem appears when we consider multiple clients executing the query with retries:
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:
returns a byte sequence containing the representation of
argument has the same meaning as for
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but return the result as a string instead of a byte sequence.
marshals the value
, storing its byte representation in the sequence
, starting at index
, and writing at most
bytes. It returns the number of bytes actually written to the sequence. If the byte representation of
does not fit in
characters, the exception
reads from channel
the byte representation of a structured value, as produced by one of the
functions, and reconstructs and returns the corresponding value.
if the function has already reached the end of file when starting to read from the channel, and raises
if it reaches the end of file later during the unmarshalling.
unmarshals a structured value like
does, except that the byte representation is not read from a channel, but taken from the byte sequence
, starting at position
. The byte sequence is not mutated.
but take a string as argument instead of a byte sequence.
The bytes representing a marshaled value are composed of a fixed-size header and a variable-sized data part, whose size can be determined from the header.
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is the size, in bytes, of the data part, assuming a valid header is stored in
starting at position
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is the total size, in bytes, of the marshaled value. Both
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does not contain a valid header.