Exit Zen
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string ID 54
type: table cell
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[link beast.ref.boost__beast__tcp_stream `tcp_stream`]
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string ID 54
type: table cell
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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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string ID 54
type: table cell
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[link beast.ref.boost__beast__basic_stream `basic_stream`]
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string ID 54
type: table cell
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[itemized_list [Timeouts for logical operations] [Configurable __Protocol__ type] [Configurable __Executor__ type] [Configurable __RatePolicy__ type] ]
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string ID 76
type: heading
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Construction
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string ID 78
type: paragraph
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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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string ID 88
type: paragraph
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Alternatively, we can construct the stream from an executor:
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14
string ID 92
type: paragraph
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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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type: heading
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Connecting
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string ID 108
type: paragraph
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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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string ID 118
type: paragraph
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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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string ID 127
type: heading
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Reading and Writing
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string ID 129
type: paragraph
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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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type: paragraph
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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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string ID 144
type: paragraph
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When a timeout is set, it cancels any previous read or write timeout for which no outstanding operation is in progress. Algorithms which loop over logical operations simply need to set the timeout once before the logical operation, it is not necessary to call `expires_never` in this case. Here we implement an algorithm which continuously echoes lines back, with a timeout. This example is implemented as a complete function.
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string ID 157
type: paragraph
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It is important to note that all of the examples thus far which perform reads and writes with a timeout, make use of the existing networking stream algorithms. As these algorithms are written generically to work with any object meeting the stream requirements, they transparently support timeouts when used with `tcp_stream`. This can be used to enable timeouts for stream wrappers that do not currently support timeouts.
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string ID 164
type: paragraph
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The following code establishes an encrypted connection, writes an HTTP request, reads the HTTP response, and closes the connection gracefully. If these operations take longer than 30 seconds total, a timeout occurs. This code is intended to show how `tcp_stream` can be used to enable timeouts across unmodified stream algorithms which were not originally written to support timing out, and how a blocking algorithm may be written from asynchronous intermediate operations.
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string ID 178
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Rate Limiting __example__
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string ID 180
type: paragraph
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The [link beast.ref.boost__beast__basic_stream `basic_stream`] class template supports an additional `RatePolicy` template parameter. Objects of this type must meet the requirements of __RatePolicy__. They are used to implement rate limiting or bandwidth management. The default policy for `basic_stream` and `tcp_stream` is [link beast.ref.boost__beast__unlimited_rate_policy `unlimited_rate_policy`], which places no limits on reading and writing. The library comes with the [link beast.ref.boost__beast__simple_rate_policy `simple_rate_policy`], allowing for independent control of read and write limits expressed in terms of bytes per second. The follow code creates an instance of the basic stream with a simple rate policy, and sets the read and write limits:
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string ID 195
type: paragraph
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More sophisticated rate policies can be implemented as user-defined types which meet the requirements of __RatePolicy__. Here, we develop a rate policy that measures the instantaneous throughput of reads and writes. First we write a small utility class that applies an exponential smoothing function to a series of discrete rate samples, to calculate instantaneous throughput.
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