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 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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  <title>P1269R0: Three Years with the Networking TS</title>
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
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 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">P1269R0<br>Three Years with the Networking TS</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Published Proposal, <time class="dt-updated" datetime="2018-10-07">2018-10-07</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="wg21.link/P1269">wg21.link/P1269</a>
     <dt>Author:
     <dd>
      <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jcarey@argv.me">Jason Carey</a> (<span class="p-org org">MongoDB</span>)
     <dt>Audience:
     <dd>EWG, LEWG
     <dt>Project:
     <dd>ISO/IEC JTC1/SC22/WG21 14882: Programming Language — C++
     <dt>Latest Version:
     <dd><a href="https://hanumantmk.github.io/wg21/p1269.html">Click here</a>
     <dt>Source:
     <dd><a href="https://github.com/hanumantmk/wg21/blob/master/p1269/p1269.bs">No real reason to click here</a>
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>Before the Networking TS reaches inclusion in the standard, we feel its important to share our experience using it.  This paper intends to highlight limitations we’ve found in ergonomics and performance, along with suggestions on how they might be remedied.  In particular, through tighter integration with coroutines, futures and executors.</p>
  </div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#introduction"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li><a href="#a-summary-of-how-we-use-asio-and-modifications-weve-made"><span class="secno">2</span> <span class="content">A summary of how we use ASIO and modifications we’ve made</span></a>
    <li>
     <a href="#areas-of-concern"><span class="secno">3</span> <span class="content">Areas of Concern</span></a>
     <ol class="toc">
      <li><a href="#synchronous-io"><span class="secno">3.1</span> <span class="content">Synchronous IO</span></a>
      <li><a href="#lifetime"><span class="secno">3.2</span> <span class="content">Lifetime</span></a>
      <li><a href="#timers"><span class="secno">3.3</span> <span class="content">Timers</span></a>
      <li>
       <a href="#execution"><span class="secno">3.4</span> <span class="content">Execution</span></a>
       <ol class="toc">
        <li><a href="#relationship-with-the-executors-ts"><span class="secno">3.4.1</span> <span class="content">Relationship with the Executors TS</span></a>
        <li><a href="#over-generalization"><span class="secno">3.4.2</span> <span class="content">Over Generalization</span></a>
       </ol>
     </ol>
    <li>
     <a href="#key-takeaways"><span class="secno">4</span> <span class="content">Key Takeaways</span></a>
     <ol class="toc">
      <li><a href="#core-of-the-critique"><span class="secno">4.1</span> <span class="content">Core of the critique</span></a>
      <li><a href="#suggestions"><span class="secno">4.2</span> <span class="content">Suggestions</span></a>
     </ol>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="introduction"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#introduction"></a></h2>
   <p>Over the past three years we’ve re-hosted the vast majority of the networking components in our
system on top of a close proxy for the Networking TS (via its upstream in <a href="https://github.com/chriskohlhoff/asio/tree/master/asio">ASIO</a>).  It’s provided us with
opportunities to use it in a wide variety of capacities and to come to terms with its strengths, and
its limitations.  We believe those limitations to be compelling enough to warrant significant
changes before inclusion in the standard.</p>
   <h2 class="heading settled" data-level="2" id="a-summary-of-how-we-use-asio-and-modifications-weve-made"><span class="secno">2. </span><span class="content">A summary of how we use ASIO and modifications we’ve made</span><a class="self-link" href="#a-summary-of-how-we-use-asio-and-modifications-weve-made"></a></h2>
   <p>At MongoDB we have an architecture that is a mixture of sync and async.  For certain stages of
ingress processing and for scatter/gather IO, we’re in a good position to be fully async.  For other
portions of our operation lifecycle we are forced into a sync model, either for practical (accessing
disk) or performance (the overhead of repeated context switches when needing to perform variably
expensive computation) reasons.</p>
   <p>We do all of our networking on top of ASIO, but have added to it to support:</p>
   <ul>
    <li data-md>
     <p>Handling sockets outside their io_context to do limited amounts of async networking synchronously</p>
    <li data-md>
     <p>TLS, including native support for SChannel and SecureTransport (not available in ASIO)</p>
    <li data-md>
     <p>A MongoDB native implementation of a Promise/Future type, via ASIO’s future’s extension API</p>
   </ul>
   <h2 class="heading settled" data-level="3" id="areas-of-concern"><span class="secno">3. </span><span class="content">Areas of Concern</span><a class="self-link" href="#areas-of-concern"></a></h2>
   <p>This section enumerates areas of weakness we’ve found in the current API.</p>
   <h3 class="heading settled" data-level="3.1" id="synchronous-io"><span class="secno">3.1. </span><span class="content">Synchronous IO</span><a class="self-link" href="#synchronous-io"></a></h3>
   <p>The Networking TS offers limited support for synchronous networking.  In particular, it offers
timeouts exclusively via its async api.  A sync api caller many not early return from
send/recv/poll, and worse, risks near-perpetual hangs in the case of networking black holes.
Further, its current embodiment in ASIO makes it impossible to work around yourself, as retry loops
are hidden behind most every function.  While these loops are required for interrupt handling on
POSIX systems, its unfortunate that core functions like ::read() and ::write() aren’t instead named
::read_all() and ::write_all() as access to a lower level api would allow customization at a layer
above the os native handles.</p>
   <p>An additional quality of implementation complaint is that we’ve seen technical limitations around
transitioning sockets between asynchronous and synchronous.  For the epoll reactor, async accept
followed by sync operations, forced spurious wakeups on the original context for the lifetime of
those sockets.  While a workaround was available, in the form of accepting sockets onto an alternate
context, it’s emblematic of the complexity that ASIO is attempting to hide, and the difficulty it
has in doing so durably.  Contexts wrap things like IO completion ports and epoll fds, and it’s
important to understand precisely how ASIO is using them.</p>
   <h3 class="heading settled" data-level="3.2" id="lifetime"><span class="secno">3.2. </span><span class="content">Lifetime</span><a class="self-link" href="#lifetime"></a></h3>
   <p>This is an artifact of how difficult callbacks are to use in C++, but it’s quite awkward to
correctly manage the lifetime of objects used asynchronously inside of ASIO.  Objects generally need
to be held by shared_ptr and shared_ptr anchors need to be added to successive callback chains.
While this is easier to handle with Future’s, and more or less obviated with coroutines, we’re
currently considering a Networking TS blocked behind neither of those.  I have strong reservations
about the usability of any async framework in C++ built only on top of callbacks.</p>
   <h3 class="heading settled" data-level="3.3" id="timers"><span class="secno">3.3. </span><span class="content">Timers</span><a class="self-link" href="#timers"></a></h3>
   <p>More directly relevant for ASIO, the intermixing of lifetime for socket operations and timers to
time them out is difficult to manage.  Timers and their sockets need to live as least as long as
both sets of callbacks, and timers need to be cancelled after socket operations have proceeded past
certain points.  This interplay requires careful orchestration with external concurrency primitives
due to guarantees around when timers fire (timer callbacks, after cancellation, may fire any time
after cancellation, on destruction or not at all).  Our experience suggests that directly attaching
timeouts to async networking operations would substantially improve ease of use.</p>
   <h3 class="heading settled" data-level="3.4" id="execution"><span class="secno">3.4. </span><span class="content">Execution</span><a class="self-link" href="#execution"></a></h3>
   <h4 class="heading settled" data-level="3.4.1" id="relationship-with-the-executors-ts"><span class="secno">3.4.1. </span><span class="content">Relationship with the Executors TS</span><a class="self-link" href="#relationship-with-the-executors-ts"></a></h4>
   <p>Long before the current Executors TS, ASIO had io_service’s with executor style APIs.  Today, it has
context objects with methods like poll, poll_one, run,
run_one, run_for and run_one_for, which control the dedication of
caller resources to processing ASIO requests.  On the other side, we see methods like
post, dispatch and defer for allowing "normal", inline or lazy
execution semantics.  Beyond quibbling about the names of those methods, there’s a longer term
problem where the Executors TS is almost certain to codify behavior and API inconsistent with the
Networking Executor.</p>
   <h4 class="heading settled" data-level="3.4.2" id="over-generalization"><span class="secno">3.4.2. </span><span class="content">Over Generalization</span><a class="self-link" href="#over-generalization"></a></h4>
   <p>While there is a common subset of functionality that all operating systems make available, there are
profound differences in how they work.  If an application isn’t performance sensitive, often these
differences don’t matter.  But if an application isn’t performance sensitive, it often doesn’t make
sense to use async networking at all, due to the added complexity callbacks introduce.  And this is
an area where ASIO’s "one size fits all" executor strategy falls down.  Your choice of /dev/poll,
epoll, kqueue, select or IOCP is chosen for you based on your target OS, and that’s all you can get.
The option to use poll on a smaller number of sockets for a smaller number of calls; the ability to
use one shot mode in epoll; the ability to get meaningful diagnostics about how many events are
ready at once; all are out.</p>
   <p>ASIO appears targeted for a use case that binds one io_context to one thread and stays on that
thread.  Additional cores are applied by replicating that stack across multiple threads.  To the
extent to which your application directly maps to that idiom, the abstraction stays tight.  If you
need something else (more threads, either to mix in extra computation, or to perform disk IO), the
lack of control over underlying execution begins to show.</p>
   <h2 class="heading settled" data-level="4" id="key-takeaways"><span class="secno">4. </span><span class="content">Key Takeaways</span><a class="self-link" href="#key-takeaways"></a></h2>
   <h3 class="heading settled" data-level="4.1" id="core-of-the-critique"><span class="secno">4.1. </span><span class="content">Core of the critique</span><a class="self-link" href="#core-of-the-critique"></a></h3>
   <p>As a general purpose networking layer, ASIO fails insofar as async programming in C++ fails.
Without the addition of futures or coroutines, callbacks compose poorly and timeouts in particular
are difficult to manage.  If the sync api was more fully featured, it might be possible to ignore
these problems (by avoiding them entirely), but the lack of timeout support makes the sync api
unsuitable.  The lack of TLS support further limits its available audience.</p>
   <p>As a high performance networking layer, ASIO is too abstract.  It fails to deliver high performance
outside a specific kind of application architecture, and fails to offer direct access to OS
primitives that would allow a user to work around those problems.  In particular, the lack of
control over underlying execution and overhead in non-share-nothing multi-threading is a real
problem.</p>
   <h3 class="heading settled" data-level="4.2" id="suggestions"><span class="secno">4.2. </span><span class="content">Suggestions</span><a class="self-link" href="#suggestions"></a></h3>
   <p>We believe the Networking TS can be the future networking stack for the language, but it requires
real and substantial modification.  At a minimum, a re-work of timers to make their management
easier in async code, and possible at all in sync, is needed.  Putting a wait on the Networking TS
until after futures or coroutines have settled more fully will solve real problems in ergonomics
around async code.  And waiting until after Executors have landed will allow a re-working of the
internal executor inside of the TS, with better tools for both executing Networking tasks as well as
waiting on their completion.</p>
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