Exit Zen
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Timeouts __example__
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Network programs must handle adverse connection conditions; the most common is that a connected peer goes offline unexpectedly. Protocols have no way of identifying this reliably: the peer is offline after all, and unable to send a message announcing the absence. A peer can go offline for various reasons:
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To determine when a peer is offline or idle, a program will implement a [@https://en.wikipedia.org/wiki/Timeout_(computing) timeout] algorithm, which closes the connection after a specified amount of time if some condition is met. For example, if no data is received for the duration. A timeout may be used to:
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Traditionally, programs use a [@boost:/doc/html/boost_asio/reference/steady_timer.html `net::steady_timer`] to determine when a timeout occurs, and then call [@boost:/doc/html/boost_asio/reference/basic_socket/close/overload2.html `close`] on the socket to release the resources. The complexity of managing a separate timer is often a source of [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1269r0.html#timers frustration] for non-experts.
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For portability reasons, networking does not provide timeouts
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To simplify the handling of timeouts, these provided types wrap a [@boost:/doc/html/boost_asio/reference/basic_stream_socket.html `net::basic_stream_socket`] to provide additional features:
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[link beast.ref.boost__beast__tcp_stream `tcp_stream`]
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[itemized_list [Timeouts for logical operations] [[@boost:/doc/html/boost_asio/reference/ip__tcp.html `net::ip::tcp`] protocol] [[@boost:/doc/html/boost_asio/reference/executor.html `net::executor`] executor] [[link beast.ref.boost__beast__unlimited_rate_policy `unlimited_rate_policy`] rate limits] ]
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[link beast.ref.boost__beast__basic_stream `basic_stream`]
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[itemized_list [Timeouts for logical operations] [Configurable __Protocol__ type] [Configurable __Executor__ type] [Configurable __RatePolicy__ type] ]
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Construction
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The `tcp_stream` is designed as a replacement for [@boost:/doc/html/boost_asio/reference/ip__tcp/socket.html `net::ip::tcp::socket`]. Any program which currently uses a socket, can switch to a `tcp_stream` and achieve the features above (although some interfaces are different, see below). Networking now allows I/O objects to construct with any instance of __ExecutionContext__ or __Executor__ objects. Here we construct a stream which uses a particular I/O context to dispatch completion handlers:
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Alternatively, we can construct the stream from an executor:
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The function [@boost:/doc/html/boost_asio/reference/make_strand.html `make_strand`] returns a strand constructed from an execution context or executor. When a [@boost:/doc/html/boost_asio/reference/strand.html `net::strand`] is chosen for the stream's executor, all completion handlers which do not already have an associated executor will use the strand. This is both a notational convenience (no need for `strand::wrap` or `bind_executor` at call sites) and a measure of safety, as it is no longer possible to forget to use the strand.
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Connecting
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Before data can be exchanged, the stream needs to be connected to a peer. The following code sets a timeout for an asynchronous connect operation. In Beast, functions to connect to a range of endpoints (such as the range returned by [@boost:/doc/html/boost_asio/reference/ip__basic_resolver/resolve/overload3.html `net::ip::tcp::resolver::resolve`]) are members of the class rather than free functions such as [@boost:/doc/html/boost_asio/reference/async_connect.html `net::async_connect`].
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A server will use an acceptor bound to a particular IP address and port to listen to and receive incoming connection requests. The acceptor returns an ordinary socket. A `tcp_stream` can be move-constructed from the underlying `basic_stream_socket` thusly:
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Reading and Writing
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Timeouts apply to the logical operation, expressed as a series of asynchronous calls, rather than just the next call. This code reads a line from the stream and writes it back. Both the read and the write must complete within 30 seconds from when the timeout was set; the timer is not reset between operations.
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Since reads and writes can take place concurrently, it is possible to have two simultaneous logical operations where each operation either only reads, or only writes. The beginning of a new read or write operation will use the most recently set timeout. This will not affect operations that are already outstanding.
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