Hi all! Unless there are objections, I propose the following:
1. Introduce an opt-in UTF-8 validating SerDe for std::string (validation OFF by default). 2. For AoS string items, enable UTF-8 validation at update() by default, with an explicit opt-out. If this direction looks reasonable, I will proceed accordingly in the AoS PR and follow up with a separate PR for the SerDe option. Thanks, Hyeonho On Fri, Feb 20, 2026 at 11:59 PM Hyeonho Kim <[email protected]> wrote: > Thanks all for the feedback. > > > We can preserve backward compatibility for existing C++ users while also > providing a clear path for cross-language portability. > > How do you think about the following approach? > > - SerDe with string: Add an option to validate whether the string contains > valid UTF-8 sequences. The default would be validation OFF to preserve > existing compatibility. > > - AoS tuple sketch: Validate UTF-8 at the update method (fail-fast). > Enabling validation by default, with an explicit opt-out for users who want. > > > For DS-Go, we can follow the same policy as C++. > > > Feedback is welcome. > > On Wed, Feb 18, 2026 at 3:24 AM Jon Malkin <[email protected]> wrote: > >> Gonna agree with Alexander here. I think we should provide a serde option >> for c++, but that we should not reject non-UTF-8 strings. >> >> That wouldn’t just be an API-breaking change. It would break >> compatibility of c++ with itself for anyone who doesn’t need language >> portability. >> >> A separate utf8_serde option gets my vote. >> >> jon >> >> On Tue, Feb 17, 2026 at 10:12 AM Alexander Saydakov via dev < >> [email protected]> wrote: >> >>> Regarding C++, I would think that the easiest approach is to instruct >>> the user to use a UTF8-validating string substitute instead of std::string. >>> I am not sure whether we should provide such a thing or let the user to >>> come up with their own implementation. >>> Consider having a uft8_string that would validate the input in the >>> constrtuctor but otherwise identical to std::string >>> So the user can instantiate, for example, >>> frequent_items_sketch<utf8_string> instead of >>> frequent_items_sketch<std::string> if validation is necessary. >>> >>> >>> On Sun, Feb 15, 2026 at 8:38 PM Hyeonho Kim <[email protected]> wrote: >>> >>>> Thanks for the feedback. I agree that for container sketches that >>>> retain and serialize strings, we should validate that string payloads are >>>> valid UTF-8 sequences to preserve cross-language portability. >>>> >>>> On *where* to validate in DS-CPP: validating at update() (ingest time) >>>> is attractive because it is fail-fast, but it also adds additional cost on >>>> the hot path. If the community is comfortable with that overhead for >>>> string-based container sketches, I’m happy to pursue the update()-time >>>> validation approach. >>>> >>>> If performance sensitivity is a concern, an alternative would be to >>>> always validate at (de)serialization boundaries (to guarantee artifact >>>> correctness), and optionally provide a “fail-fast” mode that enables >>>> validation at update() as well. >>>> >>>> For DS-Go, we can follow the same policy. Go’s situation is a bit >>>> simpler in implementation because it provides UTF-8 validation in the >>>> standard library (unicode/utf8), so we wouldn’t need an external >>>> dependency for the validator. >>>> >>>> On Mon, Feb 16, 2026 at 6:29 AM Lee Rhodes <[email protected]> wrote: >>>> >>>>> This issue, raised by Hyeonho Kim, relates to sketches that allow a >>>>> user to update the sketch with a string and the sketch also retains within >>>>> the sketch a sample of the input strings seen. When serialized, there is >>>>> an >>>>> implicit assumption that another user, possibly in a different language, >>>>> can successfully deserialize those sketch images. These sketches include >>>>> KLL, >>>>> REQ, Classic Quantiles, Sampling, FrequentItems, and Tuple. We >>>>> informally call these "container" sketches, because they contain actual >>>>> samples from the input stream. HLL, Theta, CPC, BloomFilter, etc., are >>>>> not >>>>> container sketches. >>>>> >>>>> In the DS-Java library, all container sketches that allow strings >>>>> always use UTF_8. So the sketch images produced will contain proper UTF_8 >>>>> sequences. >>>>> >>>>> In the DS-CPP library, all the various data types are abstracted via >>>>> templates. The serialization operation is declared similar to >>>>> >>>>> >>>>> *sketch<T>::serialize(std::ostream& os, const SerDe& sd)where T *is >>>>> the item type*, os is the output stream and sd* *is the SerDe that >>>>> performs the conversion to bytes. * >>>>> >>>>> >>>>> If the user wants to use an item of type string, *T* would typically >>>>> be of type *std::string*, which is just a blob of bytes and no >>>>> requirement that it is UTF_8. >>>>> >>>>> >>>>> So far, we have trusted users of the library to know that if they >>>>> update one of these container classes with a type *T,* that the >>>>> downstream user can successfully decode it. But this could be >>>>> catastrophic: A downstream user of a sketch image could be separated from >>>>> the creation of the sketch image by years and be using a different >>>>> language. >>>>> >>>>> One of the big advantages of our DataSketches project is that our >>>>> serialization images should be language and platform independent, allowing >>>>> cross-language and cross platform interchange of sketches. >>>>> >>>>> Hyeonho Kim's recommendation makes sense: For serialized sketch images >>>>> that contain strings, those strings must be UTF_8. >>>>> >>>>> So how do we implement that? My thoughts are as follows: >>>>> >>>>> 1. We should document now in the website and in appropriate places >>>>> in the library the potential danger of not using UTF_8 strings. (At >>>>> least >>>>> until we have a more robust solution) >>>>> 2. I think implementing validation checks on UTF_8 strings at the >>>>> SerDe boundaries may be too late. A user could have processed a large >>>>> stream of data only to discover a failure at serialization time, which >>>>> could be much later in time. The other possibility would be to >>>>> validate >>>>> the strings at the input into the sketch, typically in the *update() >>>>> *method. >>>>> 3. For C++, there are 3rd party libraries that specialize in UTF_8 >>>>> validation, including ICU >>>>> >>>>> <https://urldefense.com/v3/__https://github.com/unicode-org/icu__;!!Op6eflyXZCqGR5I!Hr1GVWHWpCX58DUhmQXYJ9srUYP2YzNW09vCpXOXZ8v4t3inaSAg9EewqhWEuJKCGoolYxZAnpPC5K7q2A$> >>>>> , UTF8-CPP >>>>> >>>>> <https://urldefense.com/v3/__https://github.com/nemtrif/utfcpp__;!!Op6eflyXZCqGR5I!Hr1GVWHWpCX58DUhmQXYJ9srUYP2YzNW09vCpXOXZ8v4t3inaSAg9EewqhWEuJKCGoolYxZAnpNk0hS7xg$> >>>>> and simjson >>>>> >>>>> <https://urldefense.com/v3/__https://lemire.me/blog/2020/10/20/ridiculously-fast-unicode-utf-8-validation/__;!!Op6eflyXZCqGR5I!Hr1GVWHWpCX58DUhmQXYJ9srUYP2YzNW09vCpXOXZ8v4t3inaSAg9EewqhWEuJKCGoolYxZAnpMVUko1NQ$>. >>>>> (These have standard licensing). From what I've read, UTF-8 >>>>> validation, if >>>>> done correctly, can be done very fast, with only a small section of >>>>> code. >>>>> 4. I am not sure what the solutions are for Rust or Go. >>>>> >>>>> I welcome your feedback. >>>>> >>>>> >>>>> On Sat, Feb 14, 2026 at 1:47 AM tison <[email protected]> wrote: >>>>> >>>>>> This PR [1] of datasketches-rust demonstrates how the Rust impl >>>>>> deserializes String values. >>>>>> >>>>>> [1] https://github.com/apache/datasketches-rust/pull/82 >>>>>> <https://urldefense.com/v3/__https://github.com/apache/datasketches-rust/pull/82__;!!Op6eflyXZCqGR5I!Hr1GVWHWpCX58DUhmQXYJ9srUYP2YzNW09vCpXOXZ8v4t3inaSAg9EewqhWEuJKCGoolYxZAnpN3yo3d3w$> >>>>>> >>>>>> If it's std::string::String, then it must be of UTF-8 encoding. And >>>>>> we check the encoding on deserialization. >>>>>> >>>>>> However, the Rust ecosystem also supports "strings" that do not use >>>>>> UTF-8, such as BStr. >>>>>> >>>>>> So, my opinions are: >>>>>> >>>>>> 1. It's good to assume serialized string data to be valid UTF-8. >>>>>> 2. Even if it isn't, for datasketches-rust, users should be able to >>>>>> choose a proper type to deserialize the bytes into a type that doesn't >>>>>> require UTF-8 encoding. >>>>>> >>>>>> Best, >>>>>> tison. >>>>>> >>>>>> >>>>>> Hyeonho Kim <[email protected]> 于2026年2月14日周六 17:24写道： >>>>>> >>>>>>> Hi all, >>>>>>> >>>>>>> While working on UTF-8 validation for the AoS tuple sketch in C++ >>>>>>> (ref: https://github.com/apache/datasketches-cpp/pull/476 >>>>>>> <https://urldefense.com/v3/__https://github.com/apache/datasketches-cpp/pull/476__;!!Op6eflyXZCqGR5I!Hr1GVWHWpCX58DUhmQXYJ9srUYP2YzNW09vCpXOXZ8v4t3inaSAg9EewqhWEuJKCGoolYxZAnpPslrtDnQ$>), >>>>>>> a broader design question came up that may affect multiple sketches. >>>>>>> >>>>>>> Based on my current understanding: >>>>>>> >>>>>>> - In datasketches-java, string serialization already produces valid >>>>>>> UTF-8 bytes via getBytes(StandardCharsets.UTF_8). So Java-generated >>>>>>> artifacts already assume valid UTF-8 string encoding. >>>>>>> - Rust and Python string types represent Unicode text and can be >>>>>>> encoded to UTF-8. Please correct me if I am mistaken. (I don't know Rust >>>>>>> and Python well) >>>>>>> - In Go, string is a byte sequence and may contain invalid UTF-8 >>>>>>> unless explicitly validated. So during serialization, it may produce >>>>>>> invalid UTF-8 sequences. >>>>>>> - In C++, std::string is also a byte container and does not enforce >>>>>>> UTF-8 validity. So during serialization, it may produce invalid UTF-8 >>>>>>> sequences. >>>>>>> >>>>>>> If I am mistaken on any of these points, I would appreciate >>>>>>> corrections. >>>>>>> >>>>>>> If we want to maintain cross-language portability for serialized >>>>>>> artifacts, one possible approach would be to ensure that any serialized >>>>>>> string data is valid UTF-8. This could potentially apply to any sketches >>>>>>> that serialize or deserialize string data. >>>>>>> >>>>>>> There seem to be several possible approaches: >>>>>>> - Validate UTF-8 at serialization boundaries >>>>>>> - Document that input strings must be valid UTF-8 and rely on caller >>>>>>> discipline >>>>>>> >>>>>>> At this point I am not proposing a specific solution. I would like >>>>>>> to hear opinions from the community on: We want to require serialized >>>>>>> string data to be valid UTF-8 for cross-language portability >>>>>>> >>>>>>> Thanks, >>>>>>> >>>>>>> Hyeonho >>>>>>> >>>>>>
