Seastar
High performance C++ framework for concurrent servers
file_demo.cc

A program demonstrating the use of seastar::with_file and seastar::with_file_close_on_failure

/*
* This file is open source software, licensed to you under the terms
* of the Apache License, Version 2.0 (the "License"). See the NOTICE file
* distributed with this work for additional information regarding copyright
* ownership. You may not use this file except in compliance with the License.
*
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing,
* software distributed under the License is distributed on an
* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
* KIND, either express or implied. See the License for the
* specific language governing permissions and limitations
* under the License.
*/
/*
* Copyright 2020 ScyllaDB
*/
// Demonstration of seastar::with_file
#include <cstring>
#include <limits>
#include <random>
#include <seastar/core/app-template.hh>
#include <seastar/core/aligned_buffer.hh>
#include <seastar/core/file.hh>
#include <seastar/core/seastar.hh>
#include <seastar/core/sstring.hh>
#include <seastar/core/temporary_buffer.hh>
#include <seastar/core/loop.hh>
#include <seastar/util/log.hh>
#include <seastar/util/tmp_file.hh>
using namespace seastar;
constexpr size_t aligned_size = 4096;
future<> verify_data_file(file& f, temporary_buffer<char>& rbuf, const temporary_buffer<char>& wbuf) {
return f.dma_read(0, rbuf.get_write(), aligned_size).then([&rbuf, &wbuf] (size_t count) {
assert(count == aligned_size);
fmt::print(" verifying {} bytes\n", count);
assert(!memcmp(rbuf.get(), wbuf.get(), aligned_size));
});
}
future<file> open_data_file(sstring meta_filename, temporary_buffer<char>& rbuf) {
fmt::print(" retrieving data filename from {}\n", meta_filename);
return with_file(open_file_dma(meta_filename, open_flags::ro), [&rbuf] (file& f) {
return f.dma_read(0, rbuf.get_write(), aligned_size).then([&rbuf] (size_t count) {
assert(count == aligned_size);
auto data_filename = sstring(rbuf.get());
fmt::print(" opening {}\n", data_filename);
return open_file_dma(data_filename, open_flags::ro);
});
});
}
future<> demo_with_file() {
fmt::print("Demonstrating with_file():\n");
return tmp_dir::do_with_thread([] (tmp_dir& t) {
auto rnd = std::mt19937(std::random_device()());
auto dist = std::uniform_int_distribution<char>(0, std::numeric_limits<char>::max());
auto wbuf = temporary_buffer<char>::aligned(aligned_size, aligned_size);
sstring meta_filename = (t.get_path() / "meta_file").native();
sstring data_filename = (t.get_path() / "data_file").native();
// `with_file` is used to create/open `filename` just around the call to `dma_write`
auto write_to_file = [] (const sstring filename, temporary_buffer<char>& wbuf) {
auto count = with_file(open_file_dma(filename, open_flags::rw | open_flags::create), [&wbuf] (file& f) {
return f.dma_write(0, wbuf.get(), aligned_size);
}).get0();
assert(count == aligned_size);
};
// print the data_filename into the write buffer
std::fill(wbuf.get_write(), wbuf.get_write() + aligned_size, 0);
std::copy(data_filename.cbegin(), data_filename.cend(), wbuf.get_write());
// and write it to `meta_filename`
fmt::print(" writing \"{}\" into {}\n", data_filename, meta_filename);
write_to_file(meta_filename, wbuf);
// now write some random data into data_filename
fmt::print(" writing random data into {}\n", data_filename);
std::generate(wbuf.get_write(), wbuf.get_write() + aligned_size, [&dist, &rnd] { return dist(rnd); });
write_to_file(data_filename, wbuf);
// verify the data via meta_filename
fmt::print(" verifying data...\n");
auto rbuf = temporary_buffer<char>::aligned(aligned_size, aligned_size);
with_file(open_data_file(meta_filename, rbuf), [&rbuf, &wbuf] (file& f) {
return verify_data_file(f, rbuf, wbuf);
}).get();
});
}
future<> demo_with_file_close_on_failure() {
fmt::print("\nDemonstrating with_file_close_on_failure():\n");
return tmp_dir::do_with_thread([] (tmp_dir& t) {
auto rnd = std::mt19937(std::random_device()());
auto dist = std::uniform_int_distribution<char>(0, std::numeric_limits<char>::max());
auto wbuf = temporary_buffer<char>::aligned(aligned_size, aligned_size);
sstring meta_filename = (t.get_path() / "meta_file").native();
sstring data_filename = (t.get_path() / "data_file").native();
// with_file_close_on_failure will close the opened file only if
// `make_file_output_stream` returns an error. Otherwise, in the error-free path,
// the opened file is moved to `file_output_stream` that in-turn closes it
// when the stream is closed.
auto make_output_stream = [] (const sstring filename) {
return with_file_close_on_failure(open_file_dma(std::move(filename), open_flags::rw | open_flags::create), [] (file f) {
return make_file_output_stream(std::move(f), aligned_size);
});
};
// writes the buffer one byte at a time, to demonstrate output stream
auto write_to_stream = [] (output_stream<char>& o, const temporary_buffer<char>& wbuf) {
return seastar::do_for_each(wbuf, [&o] (char c) {
return o.write(&c, 1);
}).finally([&o] {
return o.close();
});
};
// print the data_filename into the write buffer
std::fill(wbuf.get_write(), wbuf.get_write() + aligned_size, 0);
std::copy(data_filename.cbegin(), data_filename.cend(), wbuf.get_write());
// and write it to `meta_filename`
fmt::print(" writing \"{}\" into {}\n", data_filename, meta_filename);
// with_file_close_on_failure will close the opened file only if
// `make_file_output_stream` returns an error. Otherwise, in the error-free path,
// the opened file is moved to `file_output_stream` that in-turn closes it
// when the stream is closed.
output_stream<char> o = make_output_stream(meta_filename).get0();
write_to_stream(o, wbuf).get();
// now write some random data into data_filename
fmt::print(" writing random data into {}\n", data_filename);
std::generate(wbuf.get_write(), wbuf.get_write() + aligned_size, [&dist, &rnd] { return dist(rnd); });
o = make_output_stream(data_filename).get0();
write_to_stream(o, wbuf).get();
// verify the data via meta_filename
fmt::print(" verifying data...\n");
auto rbuf = temporary_buffer<char>::aligned(aligned_size, aligned_size);
with_file(open_data_file(meta_filename, rbuf), [&rbuf, &wbuf] (file& f) {
return verify_data_file(f, rbuf, wbuf);
}).get();
});
}
int main(int ac, char** av) {
return app.run(ac, av, [] {
return demo_with_file().then([] {
return demo_with_file_close_on_failure();
});
});
}
seastar
Seastar API namespace.
Definition: abort_on_ebadf.hh:24
seastar::do_for_each
future do_for_each(Iterator begin, Iterator end, AsyncAction action) noexcept
Call a function for each item in a range, sequentially (iterator version).
Definition: loop.hh:449
fstream.hh
seastar::app_template
Definition: app-template.hh:31
seastar::file::dma_write
future< size_t > dma_write(uint64_t pos, const CharType *buffer, size_t len, const io_priority_class &pc=default_priority_class()) noexcept
Definition: file.hh:312
seastar::output_stream::close
future close()
Definition: iostream-impl.hh:476
seastar::with_file
auto with_file(future< file > file_fut, Func func) noexcept
Helper for ensuring a file is closed after func is called.
Definition: file.hh:438
seastar::temporary_buffer< char >
seastar::api_v2::make_file_output_stream
output_stream< char > make_file_output_stream(file file, uint64_t buffer_size=8192)
seastar::file::dma_read
future< size_t > dma_read(uint64_t aligned_pos, CharType *aligned_buffer, size_t aligned_len, const io_priority_class &pc=default_priority_class()) noexcept
Definition: file.hh:240
seastar::tmp_dir
Definition: tmp_file.hh:120
seastar::with_file_close_on_failure
auto with_file_close_on_failure(future< file > file_fut, Func func) noexcept
Helper for ensuring a file is closed if func fails.
Definition: file.hh:465
seastar::future
A representation of a possibly not-yet-computed value.
Definition: future.hh:1337
seastar::temporary_buffer::get
const CharType * get() const
Gets a pointer to the beginning of the buffer.
Definition: temporary_buffer.hh:120
seastar::output_stream< char >
seastar::temporary_buffer::aligned
static temporary_buffer aligned(size_t alignment, size_t size)
Definition: temporary_buffer.hh:207
seastar::file
Definition: file.hh:158
seastar::temporary_buffer::get_write
CharType * get_write()
Definition: temporary_buffer.hh:123