Add a default value for serial consistency: LocalSerial #291

psarna · 2021-09-17T07:40:27Z

This series adds a default serial consistency level for queries - LocalSerial. The decision is based on the fact that LocalSerial is the one that provides acceptable performance. Users interested in strong, cross-dc consistency - which could also cause prohibitive latencies - should override to Serial.

The series also comes with a doc entry which briefly explains LWT queries (they weren't mentioned at all so far), as well as an extra validation layer which would prevent users from setting a wrong consistency level, without introducing any backward incompatible changes.

This series introduces a breaking change - serial consistency is now defined as a separate enum. Fixing the existing aplications is trivial and consists of replacing all occurrences of Consistency::LocalSerial to SerialConsistency::LocalSerial and Consistency::Serial to SerialConsistency::Serial.

Fixes #277

Pre-review checklist

I have split my patch into logically separate commits.
All commit messages clearly explain what they change and why.
I added relevant tests for new features and bug fixes.
All commits compile, pass static checks and pass test.
PR description sums up the changes and reasons why they should be introduced.
I added appropriate Fixes: annotations to PR description.

piodul · 2021-09-17T09:05:33Z

scylla/src/frame/types.rs

+    pub fn validate_serial(cl: &Option<Self>) {
+        match cl {
+            None | Some(Consistency::Serial) | Some(Consistency::LocalSerial) => (),
+            _ => panic!("Serial consistency can only be set to Serial or LocalSerial"),


By the way, I wonder if we couldn't split the Consistency type into a regular Consistency + SerialConsistency - there would be no need for runtime validation and less confusion for the users. Does it make sense to set Serial or LocalSerial for regular consistency?

On the other hand other drivers always seem to use the same consistency level type both for regular and serial consistency, maybe there is a good reason for that...

That would indeed be best, but that's also why I mentioned maintaining backward compatibility - existing code could break for some users. That's why I went with a runtime check.

Sigh, on the other hand, we're still in 0.x, so we can make breaking changes, and it makes so much sense to split the types... they are absolutely mutually exclusive, as it's also not legal to set a LOCAL_SERIAL level to the regular consistency level. I'll rework this pull request to make this breaking change and send it for review.

There's also no point in following other drivers in that matter - if anything, our solution would just be more comprehensible to the users. It's ultimately CQL's fault that these levels were standardized in such a weird manner.

@kostja Could you comment on #291 (comment) ? The rust driver has a type which represents the CQL's [consistency], but would like to split it into two - one which has SERIAL and LOCAL_SERIAL consistency levels, and one which has the others. Is there a good reason not to do that? I don't think any other driver which I know does that, but I'm not sure if there is a good reason behind that.

I would be wonderful if there are two distinct types.

I think go driver went that path

SERIAL and LOCAL-SERIAL should be part of Consistency data type, but it's best to make SerialConsistency a distinct type to avoid programming bugs.

piodul · 2021-09-17T09:07:05Z

scylla/src/statement/batch.rs

@@ -53,6 +53,7 @@ impl Batch {
    /// Sets the serial consistency to be used when executing this batch.
    /// (Ignored unless the batch is an LWT)
    pub fn set_serial_consistency(&mut self, sc: Option<Consistency>) {


If we decide not to split the Consistency type, it would be nice if the set_serial_consistency functions would have a comment which warns that the function will panic if the consistency level is invalid.

psarna · 2021-09-17T10:46:46Z

v2:

instead of runtime checks, split the types so that the compiler can check correctness for us

This breaking change splits the Consistency enum into two: - Consistency, used for all queries - SerialConsistency, an additional flag used only for lightweight transactions The reason for the split is that these values are mutually exclusive: - it's only legal to use serial consistency for LWT, and it's not possible to use it in any other scope. Respectively, it's not possible to use any of the regular consistency values for defining serial consistency.

... to improve user experience. Serial consistency level can be reset to stronger, cross-DC consistency by calling set_serial_consistency explicitly.

psarna · 2021-11-03T07:51:40Z

@piodul rebased, review ping

psarna · 2021-11-03T07:55:50Z

There's some clash with a recent bump of docs version, I'll debug. Meanwhile, the code itself is still reviewable

The entry explains that lightweight transaction queries are almost like regular queries, except they have a serial consistency level in addition to the regular consistency level.

piodul

LGTM

psarna requested a review from piodul September 17, 2021 07:40

piodul requested changes Sep 17, 2021

View reviewed changes

psarna force-pushed the serial_consistency_default branch from ffd22ab to 787f880 Compare September 17, 2021 10:46

psarna added 2 commits November 3, 2021 08:50

statement: add default serial consistency: LocalSerial

dcd9c05

... to improve user experience. Serial consistency level can be reset to stronger, cross-DC consistency by calling set_serial_consistency explicitly.

psarna force-pushed the serial_consistency_default branch from 787f880 to c1e8293 Compare November 3, 2021 07:51

docs: add an entry for LWT queries

70a4cd1

The entry explains that lightweight transaction queries are almost like regular queries, except they have a serial consistency level in addition to the regular consistency level.

psarna force-pushed the serial_consistency_default branch from c1e8293 to 70a4cd1 Compare November 3, 2021 08:01

piodul approved these changes Nov 3, 2021

View reviewed changes

piodul merged commit c1447c3 into main Nov 3, 2021

Lorak-mmk deleted the serial_consistency_default branch October 12, 2023 13:33

wprzytula mentioned this pull request Jul 5, 2025

Change default serial consistency from ANY to LOCAL_SERIAL scylladb/cpp-rust-driver#335

Open

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Add a default value for serial consistency: LocalSerial #291

Add a default value for serial consistency: LocalSerial #291

Uh oh!

psarna commented Sep 17, 2021 •

edited by wprzytula

Loading

Uh oh!

piodul Sep 17, 2021

Uh oh!

psarna Sep 17, 2021

Uh oh!

psarna Sep 17, 2021

Uh oh!

psarna Sep 17, 2021

Uh oh!

piodul Sep 21, 2021

Uh oh!

kostja Sep 21, 2021

Uh oh!

kostja Sep 21, 2021

Uh oh!

kostja Sep 21, 2021

Uh oh!

piodul Sep 17, 2021

Uh oh!

psarna commented Sep 17, 2021

Uh oh!

psarna commented Nov 3, 2021

Uh oh!

psarna commented Nov 3, 2021

Uh oh!

piodul left a comment

Uh oh!

Uh oh!

Add a default value for serial consistency: LocalSerial #291

Add a default value for serial consistency: LocalSerial #291

Uh oh!

Conversation

psarna commented Sep 17, 2021 • edited by wprzytula Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Pre-review checklist

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

psarna commented Sep 17, 2021

Uh oh!

psarna commented Nov 3, 2021

Uh oh!

psarna commented Nov 3, 2021

Uh oh!

piodul left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

psarna commented Sep 17, 2021 •

edited by wprzytula

Loading