Re: rgw: thoughts on the http client

On Thu, Oct 28, 2021 at 10:41 AM Yuval Lifshitz &lt;ylifshit(a)redhat.com&gt; wrote: historically, we have seen very bad behavior from tcmalloc at high thread counts in rgw, and we've been making general efforts both to reduce allocations and the number of threads required. i don't think anyone has tried to measure the impact of RGWHTTPArgs itself, but i do see it's use of map<string, string> as low hanging fruit. and because this piece is on rgw's http server side, replacing this map wouldn't require any of the client stuff described above i agree! i wanted to start by documenting where the gaps are, to help us understand the scope of a project here even without dropping libcurl, i think there's a lot of potential cleanup in the several layers (rgw_http_client, rgw_rest_client, rgw_rest_conn, rgw_cr_rest) between libcurl and multisite. for multisite in general, i would really like to see it adopt similar async primitives to the rest of the rgw codebase so that we can share more code beast has generally positioned itself as a low-level http protocol library, to serve as building blocks for higher-level client and server libraries/applications. the http ecosystem is vast, so it makes sense to limit the scope of any individual library. libcurl is enormous, yet still only covers the client side though with the addition of the tcp_stream in boost 1.70 (https://www.boost.org/doc/libs/1_70_0/libs/beast/doc/html/beast/release_not…), beast did take a step toward this higher level of abstraction. it's definitely worth discussing whether additional features like client connection pooling would be in scope for the project. it's also worth researching what other asio-compatible http client libraries are out there > Yuval > > > On Tue, Oct 26, 2021 at 9:34 PM Casey Bodley &lt;cbodley(a)redhat.com&gt; wrote: >> >> dear Adam and list, >> >> aside from rgw's frontend, which is the server side of http, we also >> have plenty of http client code that sends http requests to other >> servers. the biggest user of the client is multisite sync, which uses >> http to read replication logs and fetch objects from other zones. all >> of this http client code is based on libcurl, and uses its 'multi api' >> to provide an async interface with a background thread that polls for >> completions >> >> it's hard to beat libcurl for stability and features, but there has >> also been interest in using asio+beast for the client ever since we >> added it to the frontend. benefits there would include a nicer c++ >> interface, better integration with the asio async model (we do >> currently have wrappers for libcurl, but they're specific to >> coroutines), and the potential to use custom allocators to avoid most >> of the per-request allocations >> >> >> to help with a comparison against beast, these are the features of >> libcurl that we rely on: >> >> - asynchronous using the 'multi api' and a background thread >> (https://everything.curl.dev/libcurl/drive/multi) >> - connection pooling (see https://everything.curl.dev/libcurl/connectionreuse) >> - ssl context and optional certificate verification >> - connect/request timeouts >> - rate limits >> >> see RGWHTTPClient::init_request() in rgw_http_client.cc for all of the >> specific CURLOPT_ features we're using now >> >> also noteworthy is curl's support for http/1.1, http/2, and http/3 >> (https://everything.curl.dev/libcurl-http/versions) >> >> >> asio does not have connection pooling or connect timeouts (though it >> has the components necessary to build them), and beast only supports >> http/1.1. i think everything else in the list is covered: >> >> ssl support comes from boost::asio::ssl and ssl_stream >> >> there's a tcp_stream class >> (https://www.boost.org/doc/libs/1_70_0/libs/beast/doc/html/beast/ref/boost__…) >> that wraps a tcp socket and adds rate limiting and timeouts. we use >> that in the frontend, though we're tracking a performance regression >> related to its timeouts in https://tracker.ceph.com/issues/52333 >> >> there's a very nice http::fields class >> (https://www.boost.org/doc/libs/1_70_0/libs/beast/doc/html/beast/ref/boost__…) >> for headers that has custom allocator support. there's an >> 'http_server_fast' example at >> https://www.boost.org/doc/libs/1_70_0/libs/beast/example/http/server/fast/h… >> that uses the custom allocator in >> https://www.boost.org/doc/libs/1_70_0/libs/beast/example/http/server/fast/f…. >> i'd love to see something like that replace our use of map<string, >> string> for headers in RGWHTTPArgs during request processing >> >> >> for connection pooling with asio, i did explore this for a while with >> Abhishek in https://github.com/cbodley/nexus/tree/wip-connection-pool/include/nexus/htt…. >> it had connect timeouts and some test coverage in >> https://github.com/cbodley/nexus/blob/wip-connection-pool/test/http/test_co…, >> but needs more work. for example, each connection_pool is constructed >> with one hostname/port. there also needs to be a map of these pools, >> keyed either on hostname/port or resolved address, so we can cache >> connections for any url the client requests >> >> i was also imagining higher-level interfaces like http::async_get() >> (and head/put/post/etc) that would hide the use of connection pooling >> entirely, and use beast's request/response concepts to write the >> request and read its response. this is also a good place to implement >> retries. i explored this idea in a separate repo here >> https://github.com/cbodley/requests/tree/master/include/requests >> >> with asio, we can attach a connection pooling service as an >> io_context::service that gets created automatically on first use, and >> saved over the lifetime of the io_context. the application would have >> the option to configure it, but doesn't have to know anything about it >> otherwise >> >> overloading those high-level interfaces could also provide a good >> abstraction to support http 2 and 3, where their connection pools >> would just have one connection per address, and each request would >> open its own stream >> >> _______________________________________________ >> Dev mailing list -- dev(a)ceph.io >> To unsubscribe send an email to dev-leave(a)ceph.io >>