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 >>>> >>>
