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+} + +</style> +<h1 id="contexts"><a class="header" href="#contexts">Contexts</a></h1> +<p>Contexts <a name="a001"></a> in Scalability Protocols provide for isolation of protocol-specific +state machines and associated data, allowing multiple <a name="a002"></a>concurrent <a name="a003"></a>state machines (or transactions) +to coexist on a single <a href="/TODO.html">socket</a>.</p> +<p>For example, a <a href="/proto/rep.html">REP</a> server may wish to allow many requests to be serviced concurrently, +even though some jobs may take significant time to process. Contexts provide for this ability.</p> +<p>Not all protocols have contexts, because many protocols simply have no state to manage. +The following protocols support contexts:</p> +<ul> +<li><a href="/proto/rep.html">REP</a></li> +<li><a href="/proto/req.html">REQ</a></li> +<li><a href="/proto/respondent.html">RESPONDENT</a></li> +<li><a href="/proto/surveyor.html">SURVEYOR</a></li> +<li><a href="/proto/sub.html">SUB</a></li> +</ul> +<p>For these protocols, the socket will also have a single, default, context that is used when +performing send or receive operations on the socket directly.</p> +<p>Other protocols are stateless, at least with respect to message processing, and have no use for +contexts. For the same reason, <a href="/api/sock.html#raw-mode-sockets">raw</a> mode sockets do not support contexts.</p> +<div class="mdbook-alerts mdbook-alerts-tip"> +<p class="mdbook-alerts-title"> + <span class="mdbook-alerts-icon"></span> + tip +</p> +<p>Developers with experience with [libnanomsg] may be used to using <a href="/api/sock.html#raw-mode-sockets">raw</a> sockets for concurrency. +Contexts provide a superior solution, as they are much easier to use, less error prone, and allow +for easy control of the amount of concurrency used on any given socket.</p> +</div> +<p>One drawback of contexts is that they cannot be used with file descriptor polling using +<a href="/api/sock.html#polling-socket-events"><code>nng_socket_get_recv_poll_fd</code></a> or <a href="/api/sock.html#polling-socket-events"><code>nng_socket_get_send_poll_fd</code></a>.</p> +<h2 id="context-structure"><a class="header" href="#context-structure">Context Structure</a></h2> +<pre><code class="language-c">#define NNG_CTX_INITIALIZER // opaque value + +typedef struct nng_ctx_s nng_ctx; +</code></pre> +<p>The <a name="a004"></a><code>nng_ctx</code> structure represents context. This is a handle, and +the members of it are opaque. However, unlike a pointer, it is passed by value.</p> +<p>A context may be initialized statically with the <a name="a005"></a><code>NNG_CTX_INITIALIZER</code> macro, +to ensure that it cannot be confused with a valid open context.</p> +<h2 id="creating-a-context"><a class="header" href="#creating-a-context">Creating a Context</a></h2> +<pre><code class="language-c">int nng_ctx_open(nng_ctx *ctxp, nng_socket s); +</code></pre> +<p>The <a name="a006"></a><code>nng_ctx_open</code> function creates a separate context to be used with the <a href="/TODO.html">socket</a> <em>s</em>, +and returns it at the location pointed by <em>ctxp</em>.</p> +<h2 id="context-identity"><a class="header" href="#context-identity">Context Identity</a></h2> +<pre><code class="language-c">int nng_ctx_id(nng_ctx c); +</code></pre> +<p>The <a name="a007"></a><code>nng_ctx_id</code> function returns a positive identifier for the context <em>c</em> if it is valid. +Otherwise it returns <code>-1</code>.</p> +<p>A context is considered valid if it was ever opened with <a href="/api/ctx.html#creating-a-context"><code>nng_ctx_open</code></a> function. +Contexts that are allocated on the stack or statically should be initialized with the macro <a href="/api/ctx.html#context-structure"><code>NNG_CTX_INITIALIZER</code></a> +to ensure that they cannot be confused with a valid context before they are opened.</p> +<h2 id="closing-a-context"><a class="header" href="#closing-a-context">Closing a Context</a></h2> +<pre><code class="language-c">int nng_ctx_close(nng_ctx ctx); +</code></pre> +<p>The <a name="a008"></a><code>nng_ctx_close</code> function closes the context <em>ctx</em>. +Messages that have been submitted for sending may be flushed or delivered, +depending upon the transport.</p> +<p>Further attempts to use the context after this call returns will result in <code>NNG_ECLOSED</code>. +Threads waiting for operations on the context when this +call is executed may also return with an <code>NNG_ECLOSED</code> result.</p> +<div class="mdbook-alerts mdbook-alerts-note"> +<p class="mdbook-alerts-title"> + <span class="mdbook-alerts-icon"></span> + note +</p> +<p>Closing the socket associated with <em>ctx</em> using <code>nng_socket_close</code>] also closes this context.</p> +</div> +<h2 id="sending-messages"><a class="header" href="#sending-messages">Sending Messages</a></h2> +<pre><code class="language-c">int nng_ctx_sendmsg(nng_ctx ctx, nng_msg *msg, int flags); +void nng_ctx_send(nng_ctx ctx, nng_aio *aio); +</code></pre> +<p>These functions (<a name="a009"></a><code>nng_ctx_sendmsg</code> and <a name="a010"></a><code>nng_ctx_send</code>) send +messages over the socket <em>s</em>. The differences in their behaviors are as follows.</p> +<div class="mdbook-alerts mdbook-alerts-note"> +<p class="mdbook-alerts-title"> + <span class="mdbook-alerts-icon"></span> + note +</p> +<p>The semantics of what sending a message means varies from protocol to +protocol, so examination of the protocol documentation is encouraged. +Additionally, some protocols may not support sending at all or may require other pre-conditions first. +(For example, <a href="/proto/rep.html">REP</a> sockets cannot normally send data until they have first received a request, +while <a href="/proto/sub.html">SUB</a> sockets can only receive data and never send it.)</p> +</div> +<h3 id="nng_ctx_sendmsg"><a class="header" href="#nng_ctx_sendmsg">nng_ctx_sendmsg</a></h3> +<p>The <code>nng_ctx_sendmsg</code> function sends the <em>msg</em> over the context <em>ctx</em>.</p> +<p>If this function returns zero, then the socket will dispose of <em>msg</em> when the transmission is complete. +If the function returns a non-zero status, then the call retains the responsibility for disposing of <em>msg</em>.</p> +<p>The <em>flags</em> can contain the value <a href="/TODO.html"><code>NNG_FLAG_NONBLOCK</code></a>, indicating that the function should not wait if the socket +cannot accept more data for sending. In such a case, it will return <a href="/api/errors.html#NNG_EAGAIN"><code>NNG_EAGAIN</code></a>.</p> +<h3 id="nng_ctx_send"><a class="header" href="#nng_ctx_send">nng_ctx_send</a></h3> +<p>The <code>nng_ctx_send</code> function sends a message asynchronously, using the <a href="/api/aio.html#asynchronous-io-handle"><code>nng_aio</code></a> <em>aio</em>, over the context <em>ctx</em>. +The message to send must have been set on <em>aio</em> using the <a href="/api/aio.html#messages"><code>nng_aio_set_msg</code></a> function.</p> +<p>If the operation completes successfully, then the context will have disposed of the message. +However, if it fails, then callback of <em>aio</em> should arrange for a final disposition of the message. +(The message can be retrieved from <em>aio</em> with <a href="/api/aio.html#messages"><code>nng_aio_get_msg</code></a>.)</p> +<p>Note that callback associated with <em>aio</em> may be called <em>before</em> the message is finally delivered to the recipient. +For example, the message may be sitting in queue, or located in TCP buffers, or even in flight.</p> +<div class="mdbook-alerts mdbook-alerts-tip"> +<p class="mdbook-alerts-title"> + <span class="mdbook-alerts-icon"></span> + tip +</p> +<p>This is the preferred function to use for sending data on a context. While it does require a few extra +steps on the part of the application, the lowest latencies and highest performance will be achieved by using +this function instead of <a href="/api/ctx.html#nng_ctx_sendmsg"><code>nng_ctx_sendmsg</code></a>.</p> +</div> +<h2 id="receiving-messages"><a class="header" href="#receiving-messages">Receiving Messages</a></h2> +<pre><code class="language-c">int nng_ctx_recvmsg(nng_ctx ctx, nng_msg **msgp, int flags); +void nng_ctx_recv(nng_ctx ctx, nng_aio *aio); +</code></pre> +<p>These functions (, <a name="a011"></a><code>nng_ctx_recvmsg</code> and <a name="a012"></a><code>nng_ctx_recv</code>) receive +messages over the context <em>ctx</em>. The differences in their behaviors are as follows.</p> +<div class="mdbook-alerts mdbook-alerts-note"> +<p class="mdbook-alerts-title"> + <span class="mdbook-alerts-icon"></span> + note +</p> +<p>The semantics of what receving a message means varies from protocol to +protocol, so examination of the protocol documentation is encouraged. +Additionally, some protocols may not support receiving at all or may require other pre-conditions first. +(For example, <a href="/proto/req.html">REQ</a> sockets cannot normally receive data until they have first sent a request.)</p> +</div> +<h3 id="nng_recvmsg"><a class="header" href="#nng_recvmsg">nng_recvmsg</a></h3> +<p>The <code>nng_ctx_recvmsg</code> function receives a message and stores a pointer to the <a href="/api/msg.html#message-structure"><code>nng_msg</code></a> for that message in <em>msgp</em>.</p> +<p>The <em>flags</em> can contain the value <a href="/TODO.html"><code>NNG_FLAG_NONBLOCK</code></a>, indicating that the function should not wait if the socket +has no messages available to receive. In such a case, it will return <a href="/api/errors.html#NNG_EAGAIN"><code>NNG_EAGAIN</code></a>.</p> +<h3 id="nng_recv_aio"><a class="header" href="#nng_recv_aio">nng_recv_aio</a></h3> +<p>The <code>nng_ctx_send</code> function receives a message asynchronously, using the <a href="/api/aio.html#asynchronous-io-handle"><code>nng_aio</code></a> <em>aio</em>, over the context <em>ctx</em>. +On success, the received message can be retrieved from the <em>aio</em> using the <a href="/api/aio.html#messages"><code>nng_aio_get_msg</code></a> function.</p> +<div class="mdbook-alerts mdbook-alerts-note"> +<p class="mdbook-alerts-title"> + <span class="mdbook-alerts-icon"></span> + note +</p> +<p>It is important that the application retrieves the message, and disposes of it accordingly. +Failure to do so will leak the memory.</p> +</div> +<div class="mdbook-alerts mdbook-alerts-tip"> +<p class="mdbook-alerts-title"> + <span class="mdbook-alerts-icon"></span> + tip +</p> +<p>This is the preferred function to use for receiving data on a context. While it does require a few extra +steps on the part of the application, the lowest latencies and highest performance will be achieved by using +this function instead of <a href="/api/ctx.html#nng_ctx_recvmsg"><code>nng_ctx_recvmsg</code></a>.</p> +</div> +<h2 id="context-options"><a class="header" href="#context-options">Context Options</a></h2> +<pre><code class="language-c">int nng_ctx_get_bool(nng_ctx ctx, const char *opt, bool *valp); +int nng_ctx_get_int(nng_ctx ctx, const char *opt, int *valp); +int nng_ctx_get_ms(nng_ctx ctx, const char *opt, nng_duration *valp); +int nng_ctx_get_size(nng_ctx ctx, const char *opt, size_t *valp); +int nng_ctx_get_uint64(nng_ctx ctx, const char *opt, uint64_t *valp); + +int nng_ctx_set_bool(nng_ctx ctx, const char *opt, int val); +int nng_ctx_set_int(nng_ctx ctx, const char *opt, int val); +int nng_ctx_set_ms(nng_ctx ctx, const char *opt, nng_duration val); +int nng_ctx_set_size(nng_ctx ctx, const char *opt, size_t val); +int nng_ctx_set_uint64(nng_ctx ctx, const char *opt, uint64_t val); +</code></pre> +<p>These functions are used to retrieve or change the value of an option named <em>opt</em> from the context <em>ctx</em>. +The <code>nng_ctx_get_</code> functions retrieve the value, and store it in the location <em>valp</em> references. +The <code>nng_ctx_set_</code> functions change the value for the <em>ctx</em>, taking it from <em>val</em>.</p> +<p>These functions access an option as a specific type. The protocol will have details about which options +are available for contexts, and which type they may be accessed using.</p> +<h2 id="examples"><a class="header" href="#examples">Examples</a></h2> +<p>These examples show building blocks for a concurrent service based on contexts.</p> +<h3 id="example-1-context-echo-server"><a class="header" href="#example-1-context-echo-server">Example 1: Context Echo Server</a></h3> +<p>The following program fragment demonstrates the use of contexts to implement +a concurrent <a href="/proto/rep.html">REP</a> service that simply echos messages back +to the sender.</p> +<pre><code class="language-c">struct echo_context { + nng_ctx ctx; + nng_aio *aio; + enum { INIT, RECV, SEND } state; +}; + +void +echo(void *arg) +{ + struct echo_context *ec = arg; + + switch (ec->state) { + case INIT: + ec->state = RECV; + nng_ctx_recv(ec->ctx, ec->aio); + return; + case RECV: + if (nng_aio_result(ec->aio) != 0) { + // ... handle error + } + // We reuse the message on the ec->aio + ec->state = SEND; + nng_ctx_send(ec->ctx, ec->aio); + return; + case SEND: + if (nng_aio_result(ec->aio) != 0) { + // ... handle error + } + ec->state = RECV; + nng_ctx_recv(ec->ctx, ec->aio); + return; + } +} +</code></pre> +<h3 id="example-2-starting-the-echo-service"><a class="header" href="#example-2-starting-the-echo-service">Example 2: Starting the Echo Service</a></h3> +<p>Given the above fragment, the following example shows setting up the service. +It assumes that the <a href="/TODO.html">socket</a> has already been +created and any transports set up as well with functions such as <a href="/TODO.html"><code>nng_dial</code></a> or <a href="/TODO.html"><code>nng_listen</code></a>.</p> +<pre><code class="language-c"> +#define CONCURRENCY 1024 + +static struct echo_context ecs[CONCURRENCY]; + +void +start_echo_service(nng_socket rep_socket) +{ + for (int i = 0; i < CONCURRENCY; i++) { + // error checks elided for clarity + nng_ctx_open(&ecs[i].ctx, rep_socket); + nng_aio_alloc(&ecs[i].aio, echo, ecs+i); + ecs[i].state = INIT; + echo(ecs+i); // start it running + } +} +</code></pre> +<!-- Symbol cross reference --> +<!-- Macros --> +<!-- Protocols --> +<!-- Transports --> +<!-- Concept index --> + + </main> + + <nav class="nav-wrapper" aria-label="Page navigation"> + <!-- Mobile navigation buttons --> + <a rel="prev" href="../api/sock.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + + <a rel="next prefetch" href="../api/memory.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></i> + </a> + + <div style="clear: both"></div> + </nav> + </div> + </div> + + <nav class="nav-wide-wrapper" aria-label="Page navigation"> + <a rel="prev" href="../api/sock.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> + <i class="fa fa-angle-left"></i> + </a> + + <a rel="next prefetch" href="../api/memory.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> + <i class="fa fa-angle-right"></i> + </a> + </nav> + + </div> + + + + + <script> + window.playground_copyable = true; + </script> + + + <script src="../elasticlunr.min.js"></script> + <script src="../mark.min.js"></script> + <script src="../searcher.js"></script> + + <script src="../clipboard.min.js"></script> + <script src="../highlight.js"></script> + <script src="../book.js"></script> + + <!-- Custom JS scripts --> + <script src="../theme/pagetoc.js"></script> + + + </div> + </body> +</html> |