7586c5ccbd
This pull request introduces a new caching mechanism for client options in the Alternator and transport layers, refactors how client metadata is stored and accessed, and extends the `system.clients` virtual table to surface richer client information. The changes improve efficiency by deduplicating commonly used strings (like driver names/versions and client options), and ensure that client data is handled in a way that's safe for cross-shard access. Additionally, the test suite and virtual table schema are updated to reflect the new client options data. **Caching and client metadata refactoring:** * The largest and most repeatable items in the connection state before this PR were a `driver_name` and a `driver_version` which were stored as an `sstring` object which means that the corresponding memory consumption was 16 bytes per each such value at least (the smallest size of the `seastar`'s `sstring` object) **per-connection**. In reality the driver name is usually longer than 15 characters, e.g. "ScyllaDB Python Driver" is 23 characters and this is not the longest driver name there is. In such cases the actual memory usage of a corresponding `sstring` object jumps to 8 + 4 + 1 + (string length, 23 in our example) + 1. So, for "ScyllaDB Python Driver" it would be 37 bytes (in reality it would be a bit more due to natural alignment of other allocations since the `contents` size is not well aligned (13 bytes), but let's ignore this for now). * These bytes add up quickly as there are more connections and, sometimes we are talking about millions of connections per-shard. * Using a smart pointer (`lw_shared_ptr`) referencing a corresponding cached value will effectively reduce the per-connection memory usage to be 8 bytes (a size of a pointer on 64-bit CPU platform) for each such value. While storing a corresponding `sstring` value only once. * This will would reduce the "variable" (per-connection) memory usage by **at least 50%**. And in case of "ScyllaDB Python Driver" driver version - by 78%! * And all this for a price of a single `loading_shared_values` object **per-shard** (implements a hash table) and a minor overhead for each value **stored** in it. * Introduced a new cache type (`client_options_cache_type`) for deduplicating and sharing client option strings, and refactored `client_data`, `client_state`, and related classes to use `foreign_ptr<std::unique_ptr<client_data>>` and cached entry types for fields like driver name, driver version, and client options. (`client_data.hh`, `service/client_state.hh`, `alternator/server.hh`, `alternator/controller.hh`, `transport/controller.hh`, `transport/protocol_server.hh`) [[1]](diffhunk://#diff-664a3b19e905481bdf8eb3843fc4d34691067bb97ab11cfd6e652e74aac51d9fR33-R36) [[2]](diffhunk://#diff-664a3b19e905481bdf8eb3843fc4d34691067bb97ab11cfd6e652e74aac51d9fL40-R56) [[3]](diffhunk://#diff-daadce1a2de3667511e59558f3a8f077b5ee30a14bcc6a99d588db90d0fcd2bdL105-R107) [[4]](diffhunk://#diff-daadce1a2de3667511e59558f3a8f077b5ee30a14bcc6a99d588db90d0fcd2bdL154-R182) [[5]](diffhunk://#diff-5fce246edf5abffb2351bd02e2eb1e9850880f7a00607ccaa90c3eee7ef57c6bL91-R92) [[6]](diffhunk://#diff-5fce246edf5abffb2351bd02e2eb1e9850880f7a00607ccaa90c3eee7ef57c6bL110-R111) [[7]](diffhunk://#diff-31730ba8e7374f784a88dc27c1512291cf73b7f24e08768f7466a3c8cfcc7a1aL96-R96) [[8]](diffhunk://#diff-19a97c0247cc08155ee49b277e43859ca32d6ef8cbff0ed7368ec5fa19e0a11eL172-R172) [[9]](diffhunk://#diff-eea7e2db5d799a25e717a72ac8ce5842bd4adb72b694d38d8f47166d9cd926faL356-R356) [[10]](diffhunk://#diff-d0b4ec3a144bbc5dc993866cf0b940850a457ff6156064f7e2b4b10ad0a95fefL80-R80) [[11]](diffhunk://#diff-4293b94c444d9bd5ecd17ce7eda8c00685d35ecf6e07f844efc91a91bbe85be1L46-R48) * Updated the methods for setting and getting driver name, driver version, and client options in `client_state` to be asynchronous and use the new cache. (`service/client_state.hh`, `service/client_state.cc`) [[1]](diffhunk://#diff-daadce1a2de3667511e59558f3a8f077b5ee30a14bcc6a99d588db90d0fcd2bdL154-R182) [[2]](diffhunk://#diff-99634aae22e2573f38b4e2f050ed2ac4f8173ff27f0ae8b3609d1f0cc1aeb775R347-R362) **Virtual table and API enhancements:** * Extended the `system.clients` virtual table schema and implementation to include a new `client_options` column (a map of option key/value pairs), and updated the table population logic to use the new cached types and foreign pointers. (`db/virtual_tables.cc`) [[1]](diffhunk://#diff-05f7bff3edb39fb8759c90b445e860189f2f30e04717ed58bae42716082af3d1R752) [[2]](diffhunk://#diff-05f7bff3edb39fb8759c90b445e860189f2f30e04717ed58bae42716082af3d1L769-R770) [[3]](diffhunk://#diff-05f7bff3edb39fb8759c90b445e860189f2f30e04717ed58bae42716082af3d1L809-R816) [[4]](diffhunk://#diff-05f7bff3edb39fb8759c90b445e860189f2f30e04717ed58bae42716082af3d1L828-R879) **API and interface changes:** * Changed the signatures of `get_client_data` methods throughout the codebase to return vectors of `foreign_ptr<std::unique_ptr<client_data>>` instead of plain `client_data` objects, to ensure safe cross-shard access. (`alternator/controller.hh`, `alternator/controller.cc`, `alternator/server.hh`, `alternator/server.cc`, `transport/controller.hh`, `transport/protocol_server.hh`) [[1]](diffhunk://#diff-31730ba8e7374f784a88dc27c1512291cf73b7f24e08768f7466a3c8cfcc7a1aL96-R96) [[2]](diffhunk://#diff-19a97c0247cc08155ee49b277e43859ca32d6ef8cbff0ed7368ec5fa19e0a11eL172-R172) [[3]](diffhunk://#diff-5fce246edf5abffb2351bd02e2eb1e9850880f7a00607ccaa90c3eee7ef57c6bL110-R111) [[4]](diffhunk://#diff-a7e2cda866c03a75afcf3b087de1c1dcd2e7aa996214db67f9a11ed6451e596dL988-R995) [[5]](diffhunk://#diff-eea7e2db5d799a25e717a72ac8ce5842bd4adb72b694d38d8f47166d9cd926faL356-R356) [[6]](diffhunk://#diff-d0b4ec3a144bbc5dc993866cf0b940850a457ff6156064f7e2b4b10ad0a95fefL80-R80) [[7]](diffhunk://#diff-4293b94c444d9bd5ecd17ce7eda8c00685d35ecf6e07f844efc91a91bbe85be1L46-R48) **Testing and validation:** * Updated the Python test for the `system.clients` table to verify the new `client_options` column and its contents, ensuring that driver name and version are present in the options map. (`test/cqlpy/test_virtual_tables.py`) [[1]](diffhunk://#diff-6dd8bd4a6a82cd642252a29dc70726f89a46ceefb991c3e63fc67e283f323f03R79) [[2]](diffhunk://#diff-6dd8bd4a6a82cd642252a29dc70726f89a46ceefb991c3e63fc67e283f323f03R88-R90) Closes scylladb/scylladb#25746 * github.com:scylladb/scylladb: transport/server: declare a new "CLIENT_OPTIONS" option as supported service/client_state and alternator/server: use cached values for driver_name and driver_version fields system.clients: add a client_options column controller: update get_client_data to use foreign_ptr for client_data
Scylla developer documentation
This folder contains developer-oriented documentation concerning the ScyllaDB codebase. We also have a wiki, which contains additional developer-oriented documentation. There is currently no clear definition of what goes where, so when looking for something be sure to check both.
Seastar documentation can be found here.
User documentation can be found on docs.scylladb.com
For information on how to build Scylla and how to contribute visit HACKING.md and CONTRIBUTING.md.