technical/multi-pack-index.html

   1 <!DOCTYPE html>
   2 <html xmlns="http://www.w3.org/1999/xhtml" lang="en">
   3 <head>
   4 <meta charset="UTF-8"/>
   5 <meta http-equiv="X-UA-Compatible" content="IE=edge"/>
   6 <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
   7 <meta name="generator" content="Asciidoctor 2.0.20"/>
   8 <title>Multi-Pack-Index (MIDX) Design Notes</title>
   9 <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700"/>
  10 <style>
  11 /*! Asciidoctor default stylesheet | MIT License | https://asciidoctor.org */
  12 /* Uncomment the following line when using as a custom stylesheet */
  13 /* @import "https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700"; */
  14 html{font-family:sans-serif;-webkit-text-size-adjust:100%}
  15 a{background:none}
  16 a:focus{outline:thin dotted}
  17 a:active,a:hover{outline:0}
  18 h1{font-size:2em;margin:.67em 0}
  19 b,strong{font-weight:bold}
  20 abbr{font-size:.9em}
  21 abbr[title]{cursor:help;border-bottom:1px dotted #dddddf;text-decoration:none}
  22 dfn{font-style:italic}
  23 hr{height:0}
  24 mark{background:#ff0;color:#000}
  25 code,kbd,pre,samp{font-family:monospace;font-size:1em}
  26 pre{white-space:pre-wrap}
  27 q{quotes:"\201C" "\201D" "\2018" "\2019"}
  28 small{font-size:80%}
  29 sub,sup{font-size:75%;line-height:0;position:relative;vertical-align:baseline}
  30 sup{top:-.5em}
  31 sub{bottom:-.25em}
  32 img{border:0}
  33 svg:not(:root){overflow:hidden}
  34 figure{margin:0}
  35 audio,video{display:inline-block}
  36 audio:not([controls]){display:none;height:0}
  37 fieldset{border:1px solid silver;margin:0 2px;padding:.35em .625em .75em}
  38 legend{border:0;padding:0}
  39 button,input,select,textarea{font-family:inherit;font-size:100%;margin:0}
  40 button,input{line-height:normal}
  41 button,select{text-transform:none}
  42 button,html input[type=button],input[type=reset],input[type=submit]{-webkit-appearance:button;cursor:pointer}
  43 button[disabled],html input[disabled]{cursor:default}
  44 input[type=checkbox],input[type=radio]{padding:0}
  45 button::-moz-focus-inner,input::-moz-focus-inner{border:0;padding:0}
  46 textarea{overflow:auto;vertical-align:top}
  47 table{border-collapse:collapse;border-spacing:0}
  48 *,::before,::after{box-sizing:border-box}
  49 html,body{font-size:100%}
  50 body{background:#fff;color:rgba(0,0,0,.8);padding:0;margin:0;font-family:"Noto Serif","DejaVu Serif",serif;line-height:1;position:relative;cursor:auto;-moz-tab-size:4;-o-tab-size:4;tab-size:4;word-wrap:anywhere;-moz-osx-font-smoothing:grayscale;-webkit-font-smoothing:antialiased}
  51 a:hover{cursor:pointer}
  52 img,object,embed{max-width:100%;height:auto}
  53 object,embed{height:100%}
  54 img{-ms-interpolation-mode:bicubic}
  55 .left{float:left!important}
  56 .right{float:right!important}
  57 .text-left{text-align:left!important}
  58 .text-right{text-align:right!important}
  59 .text-center{text-align:center!important}
  60 .text-justify{text-align:justify!important}
  61 .hide{display:none}
  62 img,object,svg{display:inline-block;vertical-align:middle}
  63 textarea{height:auto;min-height:50px}
  64 select{width:100%}
  65 .subheader,.admonitionblock td.content>.title,.audioblock>.title,.exampleblock>.title,.imageblock>.title,.listingblock>.title,.literalblock>.title,.stemblock>.title,.openblock>.title,.paragraph>.title,.quoteblock>.title,table.tableblock>.title,.verseblock>.title,.videoblock>.title,.dlist>.title,.olist>.title,.ulist>.title,.qlist>.title,.hdlist>.title{line-height:1.45;color:#7a2518;font-weight:400;margin-top:0;margin-bottom:.25em}
  66 div,dl,dt,dd,ul,ol,li,h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6,pre,form,p,blockquote,th,td{margin:0;padding:0}
  67 a{color:#2156a5;text-decoration:underline;line-height:inherit}
  68 a:hover,a:focus{color:#1d4b8f}
  69 a img{border:0}
  70 p{line-height:1.6;margin-bottom:1.25em;text-rendering:optimizeLegibility}
  71 p aside{font-size:.875em;line-height:1.35;font-style:italic}
  72 h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{font-family:"Open Sans","DejaVu Sans",sans-serif;font-weight:300;font-style:normal;color:#ba3925;text-rendering:optimizeLegibility;margin-top:1em;margin-bottom:.5em;line-height:1.0125em}
  73 h1 small,h2 small,h3 small,#toctitle small,.sidebarblock>.content>.title small,h4 small,h5 small,h6 small{font-size:60%;color:#e99b8f;line-height:0}
  74 h1{font-size:2.125em}
  75 h2{font-size:1.6875em}
  76 h3,#toctitle,.sidebarblock>.content>.title{font-size:1.375em}
  77 h4,h5{font-size:1.125em}
  78 h6{font-size:1em}
  79 hr{border:solid #dddddf;border-width:1px 0 0;clear:both;margin:1.25em 0 1.1875em}
  80 em,i{font-style:italic;line-height:inherit}
  81 strong,b{font-weight:bold;line-height:inherit}
  82 small{font-size:60%;line-height:inherit}
  83 code{font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;font-weight:400;color:rgba(0,0,0,.9)}
  84 ul,ol,dl{line-height:1.6;margin-bottom:1.25em;list-style-position:outside;font-family:inherit}
  85 ul,ol{margin-left:1.5em}
  86 ul li ul,ul li ol{margin-left:1.25em;margin-bottom:0}
  87 ul.circle{list-style-type:circle}
  88 ul.disc{list-style-type:disc}
  89 ul.square{list-style-type:square}
  90 ul.circle ul:not([class]),ul.disc ul:not([class]),ul.square ul:not([class]){list-style:inherit}
  91 ol li ul,ol li ol{margin-left:1.25em;margin-bottom:0}
  92 dl dt{margin-bottom:.3125em;font-weight:bold}
  93 dl dd{margin-bottom:1.25em}
  94 blockquote{margin:0 0 1.25em;padding:.5625em 1.25em 0 1.1875em;border-left:1px solid #ddd}
  95 blockquote,blockquote p{line-height:1.6;color:rgba(0,0,0,.85)}
  96 @media screen and (min-width:768px){h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{line-height:1.2}
  97 h1{font-size:2.75em}
  98 h2{font-size:2.3125em}
  99 h3,#toctitle,.sidebarblock>.content>.title{font-size:1.6875em}
 100 h4{font-size:1.4375em}}
 101 table{background:#fff;margin-bottom:1.25em;border:1px solid #dedede;word-wrap:normal}
 102 table thead,table tfoot{background:#f7f8f7}
 103 table thead tr th,table thead tr td,table tfoot tr th,table tfoot tr td{padding:.5em .625em .625em;font-size:inherit;color:rgba(0,0,0,.8);text-align:left}
 104 table tr th,table tr td{padding:.5625em .625em;font-size:inherit;color:rgba(0,0,0,.8)}
 105 table tr.even,table tr.alt{background:#f8f8f7}
 106 table thead tr th,table tfoot tr th,table tbody tr td,table tr td,table tfoot tr td{line-height:1.6}
 107 h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{line-height:1.2;word-spacing:-.05em}
 108 h1 strong,h2 strong,h3 strong,#toctitle strong,.sidebarblock>.content>.title strong,h4 strong,h5 strong,h6 strong{font-weight:400}
 109 .center{margin-left:auto;margin-right:auto}
 110 .stretch{width:100%}
 111 .clearfix::before,.clearfix::after,.float-group::before,.float-group::after{content:" ";display:table}
 112 .clearfix::after,.float-group::after{clear:both}
 113 :not(pre).nobreak{word-wrap:normal}
 114 :not(pre).nowrap{white-space:nowrap}
 115 :not(pre).pre-wrap{white-space:pre-wrap}
 116 :not(pre):not([class^=L])>code{font-size:.9375em;font-style:normal!important;letter-spacing:0;padding:.1em .5ex;word-spacing:-.15em;background:#f7f7f8;border-radius:4px;line-height:1.45;text-rendering:optimizeSpeed}
 117 pre{color:rgba(0,0,0,.9);font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;line-height:1.45;text-rendering:optimizeSpeed}
 118 pre code,pre pre{color:inherit;font-size:inherit;line-height:inherit}
 119 pre>code{display:block}
 120 pre.nowrap,pre.nowrap pre{white-space:pre;word-wrap:normal}
 121 em em{font-style:normal}
 122 strong strong{font-weight:400}
 123 .keyseq{color:rgba(51,51,51,.8)}
 124 kbd{font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;display:inline-block;color:rgba(0,0,0,.8);font-size:.65em;line-height:1.45;background:#f7f7f7;border:1px solid #ccc;border-radius:3px;box-shadow:0 1px 0 rgba(0,0,0,.2),inset 0 0 0 .1em #fff;margin:0 .15em;padding:.2em .5em;vertical-align:middle;position:relative;top:-.1em;white-space:nowrap}
 125 .keyseq kbd:first-child{margin-left:0}
 126 .keyseq kbd:last-child{margin-right:0}
 127 .menuseq,.menuref{color:#000}
 128 .menuseq b:not(.caret),.menuref{font-weight:inherit}
 129 .menuseq{word-spacing:-.02em}
 130 .menuseq b.caret{font-size:1.25em;line-height:.8}
 131 .menuseq i.caret{font-weight:bold;text-align:center;width:.45em}
 132 b.button::before,b.button::after{position:relative;top:-1px;font-weight:400}
 133 b.button::before{content:"[";padding:0 3px 0 2px}
 134 b.button::after{content:"]";padding:0 2px 0 3px}
 135 p a>code:hover{color:rgba(0,0,0,.9)}
 136 #header,#content,#footnotes,#footer{width:100%;margin:0 auto;max-width:62.5em;*zoom:1;position:relative;padding-left:.9375em;padding-right:.9375em}
 137 #header::before,#header::after,#content::before,#content::after,#footnotes::before,#footnotes::after,#footer::before,#footer::after{content:" ";display:table}
 138 #header::after,#content::after,#footnotes::after,#footer::after{clear:both}
 139 #content{margin-top:1.25em}
 140 #content::before{content:none}
 141 #header>h1:first-child{color:rgba(0,0,0,.85);margin-top:2.25rem;margin-bottom:0}
 142 #header>h1:first-child+#toc{margin-top:8px;border-top:1px solid #dddddf}
 143 #header>h1:only-child,body.toc2 #header>h1:nth-last-child(2){border-bottom:1px solid #dddddf;padding-bottom:8px}
 144 #header .details{border-bottom:1px solid #dddddf;line-height:1.45;padding-top:.25em;padding-bottom:.25em;padding-left:.25em;color:rgba(0,0,0,.6);display:flex;flex-flow:row wrap}
 145 #header .details span:first-child{margin-left:-.125em}
 146 #header .details span.email a{color:rgba(0,0,0,.85)}
 147 #header .details br{display:none}
 148 #header .details br+span::before{content:"\00a0\2013\00a0"}
 149 #header .details br+span.author::before{content:"\00a0\22c5\00a0";color:rgba(0,0,0,.85)}
 150 #header .details br+span#revremark::before{content:"\00a0|\00a0"}
 151 #header #revnumber{text-transform:capitalize}
 152 #header #revnumber::after{content:"\00a0"}
 153 #content>h1:first-child:not([class]){color:rgba(0,0,0,.85);border-bottom:1px solid #dddddf;padding-bottom:8px;margin-top:0;padding-top:1rem;margin-bottom:1.25rem}
 154 #toc{border-bottom:1px solid #e7e7e9;padding-bottom:.5em}
 155 #toc>ul{margin-left:.125em}
 156 #toc ul.sectlevel0>li>a{font-style:italic}
 157 #toc ul.sectlevel0 ul.sectlevel1{margin:.5em 0}
 158 #toc ul{font-family:"Open Sans","DejaVu Sans",sans-serif;list-style-type:none}
 159 #toc li{line-height:1.3334;margin-top:.3334em}
 160 #toc a{text-decoration:none}
 161 #toc a:active{text-decoration:underline}
 162 #toctitle{color:#7a2518;font-size:1.2em}
 163 @media screen and (min-width:768px){#toctitle{font-size:1.375em}
 164 body.toc2{padding-left:15em;padding-right:0}
 165 #toc.toc2{margin-top:0!important;background:#f8f8f7;position:fixed;width:15em;left:0;top:0;border-right:1px solid #e7e7e9;border-top-width:0!important;border-bottom-width:0!important;z-index:1000;padding:1.25em 1em;height:100%;overflow:auto}
 166 #toc.toc2 #toctitle{margin-top:0;margin-bottom:.8rem;font-size:1.2em}
 167 #toc.toc2>ul{font-size:.9em;margin-bottom:0}
 168 #toc.toc2 ul ul{margin-left:0;padding-left:1em}
 169 #toc.toc2 ul.sectlevel0 ul.sectlevel1{padding-left:0;margin-top:.5em;margin-bottom:.5em}
 170 body.toc2.toc-right{padding-left:0;padding-right:15em}
 171 body.toc2.toc-right #toc.toc2{border-right-width:0;border-left:1px solid #e7e7e9;left:auto;right:0}}
 172 @media screen and (min-width:1280px){body.toc2{padding-left:20em;padding-right:0}
 173 #toc.toc2{width:20em}
 174 #toc.toc2 #toctitle{font-size:1.375em}
 175 #toc.toc2>ul{font-size:.95em}
 176 #toc.toc2 ul ul{padding-left:1.25em}
 177 body.toc2.toc-right{padding-left:0;padding-right:20em}}
 178 #content #toc{border:1px solid #e0e0dc;margin-bottom:1.25em;padding:1.25em;background:#f8f8f7;border-radius:4px}
 179 #content #toc>:first-child{margin-top:0}
 180 #content #toc>:last-child{margin-bottom:0}
 181 #footer{max-width:none;background:rgba(0,0,0,.8);padding:1.25em}
 182 #footer-text{color:hsla(0,0%,100%,.8);line-height:1.44}
 183 #content{margin-bottom:.625em}
 184 .sect1{padding-bottom:.625em}
 185 @media screen and (min-width:768px){#content{margin-bottom:1.25em}
 186 .sect1{padding-bottom:1.25em}}
 187 .sect1:last-child{padding-bottom:0}
 188 .sect1+.sect1{border-top:1px solid #e7e7e9}
 189 #content h1>a.anchor,h2>a.anchor,h3>a.anchor,#toctitle>a.anchor,.sidebarblock>.content>.title>a.anchor,h4>a.anchor,h5>a.anchor,h6>a.anchor{position:absolute;z-index:1001;width:1.5ex;margin-left:-1.5ex;display:block;text-decoration:none!important;visibility:hidden;text-align:center;font-weight:400}
 190 #content h1>a.anchor::before,h2>a.anchor::before,h3>a.anchor::before,#toctitle>a.anchor::before,.sidebarblock>.content>.title>a.anchor::before,h4>a.anchor::before,h5>a.anchor::before,h6>a.anchor::before{content:"\00A7";font-size:.85em;display:block;padding-top:.1em}
 191 #content h1:hover>a.anchor,#content h1>a.anchor:hover,h2:hover>a.anchor,h2>a.anchor:hover,h3:hover>a.anchor,#toctitle:hover>a.anchor,.sidebarblock>.content>.title:hover>a.anchor,h3>a.anchor:hover,#toctitle>a.anchor:hover,.sidebarblock>.content>.title>a.anchor:hover,h4:hover>a.anchor,h4>a.anchor:hover,h5:hover>a.anchor,h5>a.anchor:hover,h6:hover>a.anchor,h6>a.anchor:hover{visibility:visible}
 192 #content h1>a.link,h2>a.link,h3>a.link,#toctitle>a.link,.sidebarblock>.content>.title>a.link,h4>a.link,h5>a.link,h6>a.link{color:#ba3925;text-decoration:none}
 193 #content h1>a.link:hover,h2>a.link:hover,h3>a.link:hover,#toctitle>a.link:hover,.sidebarblock>.content>.title>a.link:hover,h4>a.link:hover,h5>a.link:hover,h6>a.link:hover{color:#a53221}
 194 details,.audioblock,.imageblock,.literalblock,.listingblock,.stemblock,.videoblock{margin-bottom:1.25em}
 195 details{margin-left:1.25rem}
 196 details>summary{cursor:pointer;display:block;position:relative;line-height:1.6;margin-bottom:.625rem;outline:none;-webkit-tap-highlight-color:transparent}
 197 details>summary::-webkit-details-marker{display:none}
 198 details>summary::before{content:"";border:solid transparent;border-left:solid;border-width:.3em 0 .3em .5em;position:absolute;top:.5em;left:-1.25rem;transform:translateX(15%)}
 199 details[open]>summary::before{border:solid transparent;border-top:solid;border-width:.5em .3em 0;transform:translateY(15%)}
 200 details>summary::after{content:"";width:1.25rem;height:1em;position:absolute;top:.3em;left:-1.25rem}
 201 .admonitionblock td.content>.title,.audioblock>.title,.exampleblock>.title,.imageblock>.title,.listingblock>.title,.literalblock>.title,.stemblock>.title,.openblock>.title,.paragraph>.title,.quoteblock>.title,table.tableblock>.title,.verseblock>.title,.videoblock>.title,.dlist>.title,.olist>.title,.ulist>.title,.qlist>.title,.hdlist>.title{text-rendering:optimizeLegibility;text-align:left;font-family:"Noto Serif","DejaVu Serif",serif;font-size:1rem;font-style:italic}
 202 table.tableblock.fit-content>caption.title{white-space:nowrap;width:0}
 203 .paragraph.lead>p,#preamble>.sectionbody>[class=paragraph]:first-of-type p{font-size:1.21875em;line-height:1.6;color:rgba(0,0,0,.85)}
 204 .admonitionblock>table{border-collapse:separate;border:0;background:none;width:100%}
 205 .admonitionblock>table td.icon{text-align:center;width:80px}
 206 .admonitionblock>table td.icon img{max-width:none}
 207 .admonitionblock>table td.icon .title{font-weight:bold;font-family:"Open Sans","DejaVu Sans",sans-serif;text-transform:uppercase}
 208 .admonitionblock>table td.content{padding-left:1.125em;padding-right:1.25em;border-left:1px solid #dddddf;color:rgba(0,0,0,.6);word-wrap:anywhere}
 209 .admonitionblock>table td.content>:last-child>:last-child{margin-bottom:0}
 210 .exampleblock>.content{border:1px solid #e6e6e6;margin-bottom:1.25em;padding:1.25em;background:#fff;border-radius:4px}
 211 .sidebarblock{border:1px solid #dbdbd6;margin-bottom:1.25em;padding:1.25em;background:#f3f3f2;border-radius:4px}
 212 .sidebarblock>.content>.title{color:#7a2518;margin-top:0;text-align:center}
 213 .exampleblock>.content>:first-child,.sidebarblock>.content>:first-child{margin-top:0}
 214 .exampleblock>.content>:last-child,.exampleblock>.content>:last-child>:last-child,.exampleblock>.content .olist>ol>li:last-child>:last-child,.exampleblock>.content .ulist>ul>li:last-child>:last-child,.exampleblock>.content .qlist>ol>li:last-child>:last-child,.sidebarblock>.content>:last-child,.sidebarblock>.content>:last-child>:last-child,.sidebarblock>.content .olist>ol>li:last-child>:last-child,.sidebarblock>.content .ulist>ul>li:last-child>:last-child,.sidebarblock>.content .qlist>ol>li:last-child>:last-child{margin-bottom:0}
 215 .literalblock pre,.listingblock>.content>pre{border-radius:4px;overflow-x:auto;padding:1em;font-size:.8125em}
 216 @media screen and (min-width:768px){.literalblock pre,.listingblock>.content>pre{font-size:.90625em}}
 217 @media screen and (min-width:1280px){.literalblock pre,.listingblock>.content>pre{font-size:1em}}
 218 .literalblock pre,.listingblock>.content>pre:not(.highlight),.listingblock>.content>pre[class=highlight],.listingblock>.content>pre[class^="highlight "]{background:#f7f7f8}
 219 .literalblock.output pre{color:#f7f7f8;background:rgba(0,0,0,.9)}
 220 .listingblock>.content{position:relative}
 221 .listingblock code[data-lang]::before{display:none;content:attr(data-lang);position:absolute;font-size:.75em;top:.425rem;right:.5rem;line-height:1;text-transform:uppercase;color:inherit;opacity:.5}
 222 .listingblock:hover code[data-lang]::before{display:block}
 223 .listingblock.terminal pre .command::before{content:attr(data-prompt);padding-right:.5em;color:inherit;opacity:.5}
 224 .listingblock.terminal pre .command:not([data-prompt])::before{content:"$"}
 225 .listingblock pre.highlightjs{padding:0}
 226 .listingblock pre.highlightjs>code{padding:1em;border-radius:4px}
 227 .listingblock pre.prettyprint{border-width:0}
 228 .prettyprint{background:#f7f7f8}
 229 pre.prettyprint .linenums{line-height:1.45;margin-left:2em}
 230 pre.prettyprint li{background:none;list-style-type:inherit;padding-left:0}
 231 pre.prettyprint li code[data-lang]::before{opacity:1}
 232 pre.prettyprint li:not(:first-child) code[data-lang]::before{display:none}
 233 table.linenotable{border-collapse:separate;border:0;margin-bottom:0;background:none}
 234 table.linenotable td[class]{color:inherit;vertical-align:top;padding:0;line-height:inherit;white-space:normal}
 235 table.linenotable td.code{padding-left:.75em}
 236 table.linenotable td.linenos,pre.pygments .linenos{border-right:1px solid;opacity:.35;padding-right:.5em;-webkit-user-select:none;-moz-user-select:none;-ms-user-select:none;user-select:none}
 237 pre.pygments span.linenos{display:inline-block;margin-right:.75em}
 238 .quoteblock{margin:0 1em 1.25em 1.5em;display:table}
 239 .quoteblock:not(.excerpt)>.title{margin-left:-1.5em;margin-bottom:.75em}
 240 .quoteblock blockquote,.quoteblock p{color:rgba(0,0,0,.85);font-size:1.15rem;line-height:1.75;word-spacing:.1em;letter-spacing:0;font-style:italic;text-align:justify}
 241 .quoteblock blockquote{margin:0;padding:0;border:0}
 242 .quoteblock blockquote::before{content:"\201c";float:left;font-size:2.75em;font-weight:bold;line-height:.6em;margin-left:-.6em;color:#7a2518;text-shadow:0 1px 2px rgba(0,0,0,.1)}
 243 .quoteblock blockquote>.paragraph:last-child p{margin-bottom:0}
 244 .quoteblock .attribution{margin-top:.75em;margin-right:.5ex;text-align:right}
 245 .verseblock{margin:0 1em 1.25em}
 246 .verseblock pre{font-family:"Open Sans","DejaVu Sans",sans-serif;font-size:1.15rem;color:rgba(0,0,0,.85);font-weight:300;text-rendering:optimizeLegibility}
 247 .verseblock pre strong{font-weight:400}
 248 .verseblock .attribution{margin-top:1.25rem;margin-left:.5ex}
 249 .quoteblock .attribution,.verseblock .attribution{font-size:.9375em;line-height:1.45;font-style:italic}
 250 .quoteblock .attribution br,.verseblock .attribution br{display:none}
 251 .quoteblock .attribution cite,.verseblock .attribution cite{display:block;letter-spacing:-.025em;color:rgba(0,0,0,.6)}
 252 .quoteblock.abstract blockquote::before,.quoteblock.excerpt blockquote::before,.quoteblock .quoteblock blockquote::before{display:none}
 253 .quoteblock.abstract blockquote,.quoteblock.abstract p,.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{line-height:1.6;word-spacing:0}
 254 .quoteblock.abstract{margin:0 1em 1.25em;display:block}
 255 .quoteblock.abstract>.title{margin:0 0 .375em;font-size:1.15em;text-align:center}
 256 .quoteblock.excerpt>blockquote,.quoteblock .quoteblock{padding:0 0 .25em 1em;border-left:.25em solid #dddddf}
 257 .quoteblock.excerpt,.quoteblock .quoteblock{margin-left:0}
 258 .quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{color:inherit;font-size:1.0625rem}
 259 .quoteblock.excerpt .attribution,.quoteblock .quoteblock .attribution{color:inherit;font-size:.85rem;text-align:left;margin-right:0}
 260 p.tableblock:last-child{margin-bottom:0}
 261 td.tableblock>.content{margin-bottom:1.25em;word-wrap:anywhere}
 262 td.tableblock>.content>:last-child{margin-bottom:-1.25em}
 263 table.tableblock,th.tableblock,td.tableblock{border:0 solid #dedede}
 264 table.grid-all>*>tr>*{border-width:1px}
 265 table.grid-cols>*>tr>*{border-width:0 1px}
 266 table.grid-rows>*>tr>*{border-width:1px 0}
 267 table.frame-all{border-width:1px}
 268 table.frame-ends{border-width:1px 0}
 269 table.frame-sides{border-width:0 1px}
 270 table.frame-none>colgroup+*>:first-child>*,table.frame-sides>colgroup+*>:first-child>*{border-top-width:0}
 271 table.frame-none>:last-child>:last-child>*,table.frame-sides>:last-child>:last-child>*{border-bottom-width:0}
 272 table.frame-none>*>tr>:first-child,table.frame-ends>*>tr>:first-child{border-left-width:0}
 273 table.frame-none>*>tr>:last-child,table.frame-ends>*>tr>:last-child{border-right-width:0}
 274 table.stripes-all>*>tr,table.stripes-odd>*>tr:nth-of-type(odd),table.stripes-even>*>tr:nth-of-type(even),table.stripes-hover>*>tr:hover{background:#f8f8f7}
 275 th.halign-left,td.halign-left{text-align:left}
 276 th.halign-right,td.halign-right{text-align:right}
 277 th.halign-center,td.halign-center{text-align:center}
 278 th.valign-top,td.valign-top{vertical-align:top}
 279 th.valign-bottom,td.valign-bottom{vertical-align:bottom}
 280 th.valign-middle,td.valign-middle{vertical-align:middle}
 281 table thead th,table tfoot th{font-weight:bold}
 282 tbody tr th{background:#f7f8f7}
 283 tbody tr th,tbody tr th p,tfoot tr th,tfoot tr th p{color:rgba(0,0,0,.8);font-weight:bold}
 284 p.tableblock>code:only-child{background:none;padding:0}
 285 p.tableblock{font-size:1em}
 286 ol{margin-left:1.75em}
 287 ul li ol{margin-left:1.5em}
 288 dl dd{margin-left:1.125em}
 289 dl dd:last-child,dl dd:last-child>:last-child{margin-bottom:0}
 290 li p,ul dd,ol dd,.olist .olist,.ulist .ulist,.ulist .olist,.olist .ulist{margin-bottom:.625em}
 291 ul.checklist,ul.none,ol.none,ul.no-bullet,ol.no-bullet,ol.unnumbered,ul.unstyled,ol.unstyled{list-style-type:none}
 292 ul.no-bullet,ol.no-bullet,ol.unnumbered{margin-left:.625em}
 293 ul.unstyled,ol.unstyled{margin-left:0}
 294 li>p:empty:only-child::before{content:"";display:inline-block}
 295 ul.checklist>li>p:first-child{margin-left:-1em}
 296 ul.checklist>li>p:first-child>.fa-square-o:first-child,ul.checklist>li>p:first-child>.fa-check-square-o:first-child{width:1.25em;font-size:.8em;position:relative;bottom:.125em}
 297 ul.checklist>li>p:first-child>input[type=checkbox]:first-child{margin-right:.25em}
 298 ul.inline{display:flex;flex-flow:row wrap;list-style:none;margin:0 0 .625em -1.25em}
 299 ul.inline>li{margin-left:1.25em}
 300 .unstyled dl dt{font-weight:400;font-style:normal}
 301 ol.arabic{list-style-type:decimal}
 302 ol.decimal{list-style-type:decimal-leading-zero}
 303 ol.loweralpha{list-style-type:lower-alpha}
 304 ol.upperalpha{list-style-type:upper-alpha}
 305 ol.lowerroman{list-style-type:lower-roman}
 306 ol.upperroman{list-style-type:upper-roman}
 307 ol.lowergreek{list-style-type:lower-greek}
 308 .hdlist>table,.colist>table{border:0;background:none}
 309 .hdlist>table>tbody>tr,.colist>table>tbody>tr{background:none}
 310 td.hdlist1,td.hdlist2{vertical-align:top;padding:0 .625em}
 311 td.hdlist1{font-weight:bold;padding-bottom:1.25em}
 312 td.hdlist2{word-wrap:anywhere}
 313 .literalblock+.colist,.listingblock+.colist{margin-top:-.5em}
 314 .colist td:not([class]):first-child{padding:.4em .75em 0;line-height:1;vertical-align:top}
 315 .colist td:not([class]):first-child img{max-width:none}
 316 .colist td:not([class]):last-child{padding:.25em 0}
 317 .thumb,.th{line-height:0;display:inline-block;border:4px solid #fff;box-shadow:0 0 0 1px #ddd}
 318 .imageblock.left{margin:.25em .625em 1.25em 0}
 319 .imageblock.right{margin:.25em 0 1.25em .625em}
 320 .imageblock>.title{margin-bottom:0}
 321 .imageblock.thumb,.imageblock.th{border-width:6px}
 322 .imageblock.thumb>.title,.imageblock.th>.title{padding:0 .125em}
 323 .image.left,.image.right{margin-top:.25em;margin-bottom:.25em;display:inline-block;line-height:0}
 324 .image.left{margin-right:.625em}
 325 .image.right{margin-left:.625em}
 326 a.image{text-decoration:none;display:inline-block}
 327 a.image object{pointer-events:none}
 328 sup.footnote,sup.footnoteref{font-size:.875em;position:static;vertical-align:super}
 329 sup.footnote a,sup.footnoteref a{text-decoration:none}
 330 sup.footnote a:active,sup.footnoteref a:active{text-decoration:underline}
 331 #footnotes{padding-top:.75em;padding-bottom:.75em;margin-bottom:.625em}
 332 #footnotes hr{width:20%;min-width:6.25em;margin:-.25em 0 .75em;border-width:1px 0 0}
 333 #footnotes .footnote{padding:0 .375em 0 .225em;line-height:1.3334;font-size:.875em;margin-left:1.2em;margin-bottom:.2em}
 334 #footnotes .footnote a:first-of-type{font-weight:bold;text-decoration:none;margin-left:-1.05em}
 335 #footnotes .footnote:last-of-type{margin-bottom:0}
 336 #content #footnotes{margin-top:-.625em;margin-bottom:0;padding:.75em 0}
 337 div.unbreakable{page-break-inside:avoid}
 338 .big{font-size:larger}
 339 .small{font-size:smaller}
 340 .underline{text-decoration:underline}
 341 .overline{text-decoration:overline}
 342 .line-through{text-decoration:line-through}
 343 .aqua{color:#00bfbf}
 344 .aqua-background{background:#00fafa}
 345 .black{color:#000}
 346 .black-background{background:#000}
 347 .blue{color:#0000bf}
 348 .blue-background{background:#0000fa}
 349 .fuchsia{color:#bf00bf}
 350 .fuchsia-background{background:#fa00fa}
 351 .gray{color:#606060}
 352 .gray-background{background:#7d7d7d}
 353 .green{color:#006000}
 354 .green-background{background:#007d00}
 355 .lime{color:#00bf00}
 356 .lime-background{background:#00fa00}
 357 .maroon{color:#600000}
 358 .maroon-background{background:#7d0000}
 359 .navy{color:#000060}
 360 .navy-background{background:#00007d}
 361 .olive{color:#606000}
 362 .olive-background{background:#7d7d00}
 363 .purple{color:#600060}
 364 .purple-background{background:#7d007d}
 365 .red{color:#bf0000}
 366 .red-background{background:#fa0000}
 367 .silver{color:#909090}
 368 .silver-background{background:#bcbcbc}
 369 .teal{color:#006060}
 370 .teal-background{background:#007d7d}
 371 .white{color:#bfbfbf}
 372 .white-background{background:#fafafa}
 373 .yellow{color:#bfbf00}
 374 .yellow-background{background:#fafa00}
 375 span.icon>.fa{cursor:default}
 376 a span.icon>.fa{cursor:inherit}
 377 .admonitionblock td.icon [class^="fa icon-"]{font-size:2.5em;text-shadow:1px 1px 2px rgba(0,0,0,.5);cursor:default}
 378 .admonitionblock td.icon .icon-note::before{content:"\f05a";color:#19407c}
 379 .admonitionblock td.icon .icon-tip::before{content:"\f0eb";text-shadow:1px 1px 2px rgba(155,155,0,.8);color:#111}
 380 .admonitionblock td.icon .icon-warning::before{content:"\f071";color:#bf6900}
 381 .admonitionblock td.icon .icon-caution::before{content:"\f06d";color:#bf3400}
 382 .admonitionblock td.icon .icon-important::before{content:"\f06a";color:#bf0000}
 383 .conum[data-value]{display:inline-block;color:#fff!important;background:rgba(0,0,0,.8);border-radius:50%;text-align:center;font-size:.75em;width:1.67em;height:1.67em;line-height:1.67em;font-family:"Open Sans","DejaVu Sans",sans-serif;font-style:normal;font-weight:bold}
 384 .conum[data-value] *{color:#fff!important}
 385 .conum[data-value]+b{display:none}
 386 .conum[data-value]::after{content:attr(data-value)}
 387 pre .conum[data-value]{position:relative;top:-.125em}
 388 b.conum *{color:inherit!important}
 389 .conum:not([data-value]):empty{display:none}
 390 dt,th.tableblock,td.content,div.footnote{text-rendering:optimizeLegibility}
 391 h1,h2,p,td.content,span.alt,summary{letter-spacing:-.01em}
 392 p strong,td.content strong,div.footnote strong{letter-spacing:-.005em}
 393 p,blockquote,dt,td.content,td.hdlist1,span.alt,summary{font-size:1.0625rem}
 394 p{margin-bottom:1.25rem}
 395 .sidebarblock p,.sidebarblock dt,.sidebarblock td.content,p.tableblock{font-size:1em}
 396 .exampleblock>.content{background:#fffef7;border-color:#e0e0dc;box-shadow:0 1px 4px #e0e0dc}
 397 .print-only{display:none!important}
 398 @page{margin:1.25cm .75cm}
 399 @media print{*{box-shadow:none!important;text-shadow:none!important}
 400 html{font-size:80%}
 401 a{color:inherit!important;text-decoration:underline!important}
 402 a.bare,a[href^="#"],a[href^="mailto:"]{text-decoration:none!important}
 403 a[href^="http:"]:not(.bare)::after,a[href^="https:"]:not(.bare)::after{content:"(" attr(href) ")";display:inline-block;font-size:.875em;padding-left:.25em}
 404 abbr[title]{border-bottom:1px dotted}
 405 abbr[title]::after{content:" (" attr(title) ")"}
 406 pre,blockquote,tr,img,object,svg{page-break-inside:avoid}
 407 thead{display:table-header-group}
 408 svg{max-width:100%}
 409 p,blockquote,dt,td.content{font-size:1em;orphans:3;widows:3}
 410 h2,h3,#toctitle,.sidebarblock>.content>.title{page-break-after:avoid}
 411 #header,#content,#footnotes,#footer{max-width:none}
 412 #toc,.sidebarblock,.exampleblock>.content{background:none!important}
 413 #toc{border-bottom:1px solid #dddddf!important;padding-bottom:0!important}
 414 body.book #header{text-align:center}
 415 body.book #header>h1:first-child{border:0!important;margin:2.5em 0 1em}
 416 body.book #header .details{border:0!important;display:block;padding:0!important}
 417 body.book #header .details span:first-child{margin-left:0!important}
 418 body.book #header .details br{display:block}
 419 body.book #header .details br+span::before{content:none!important}
 420 body.book #toc{border:0!important;text-align:left!important;padding:0!important;margin:0!important}
 421 body.book #toc,body.book #preamble,body.book h1.sect0,body.book .sect1>h2{page-break-before:always}
 422 .listingblock code[data-lang]::before{display:block}
 423 #footer{padding:0 .9375em}
 424 .hide-on-print{display:none!important}
 425 .print-only{display:block!important}
 426 .hide-for-print{display:none!important}
 427 .show-for-print{display:inherit!important}}
 428 @media amzn-kf8,print{#header>h1:first-child{margin-top:1.25rem}
 429 .sect1{padding:0!important}
 430 .sect1+.sect1{border:0}
 431 #footer{background:none}
 432 #footer-text{color:rgba(0,0,0,.6);font-size:.9em}}
 433 @media amzn-kf8{#header,#content,#footnotes,#footer{padding:0}}
 434 </style>
 435 </head>
 436 <body class="article">
 437 <div id="header">
 438 <h1>Multi-Pack-Index (MIDX) Design Notes</h1>
 439 <div class="details">
 440 <span id="revdate">2024-08-07</span>
 441 </div>
 442 </div>
 443 <div id="content">
 444 <div id="preamble">
 445 <div class="sectionbody">
 446 <div class="paragraph">
 447 <p>The Git object directory contains a <em>pack</em> directory containing
 448 packfiles (with suffix ".pack") and pack-indexes (with suffix
 449 ".idx"). The pack-indexes provide a way to lookup objects and
 450 navigate to their offset within the pack, but these must come
 451 in pairs with the packfiles. This pairing depends on the file
 452 names, as the pack-index differs only in suffix with its pack-
 453 file. While the pack-indexes provide fast lookup per packfile,
 454 this performance degrades as the number of packfiles increases,
 455 because abbreviations need to inspect every packfile and we are
 456 more likely to have a miss on our most-recently-used packfile.
 457 For some large repositories, repacking into a single packfile
 458 is not feasible due to storage space or excessive repack times.</p>
 459 </div>
 460 <div class="paragraph">
 461 <p>The multi-pack-index (MIDX for short) stores a list of objects
 462 and their offsets into multiple packfiles. It contains:</p>
 463 </div>
 464 <div class="ulist">
 465 <ul>
 466 <li>
 467 <p>A list of packfile names.</p>
 468 </li>
 469 <li>
 470 <p>A sorted list of object IDs.</p>
 471 </li>
 472 <li>
 473 <p>A list of metadata for the ith object ID including:</p>
 474 <div class="ulist">
 475 <ul>
 476 <li>
 477 <p>A value j referring to the jth packfile.</p>
 478 </li>
 479 <li>
 480 <p>An offset within the jth packfile for the object.</p>
 481 </li>
 482 </ul>
 483 </div>
 484 </li>
 485 <li>
 486 <p>If large offsets are required, we use another list of large
 487 offsets similar to version 2 pack-indexes.</p>
 488 <div class="ulist">
 489 <ul>
 490 <li>
 491 <p>An optional list of objects in pseudo-pack order (used with MIDX bitmaps).</p>
 492 </li>
 493 </ul>
 494 </div>
 495 </li>
 496 </ul>
 497 </div>
 498 <div class="paragraph">
 499 <p>Thus, we can provide O(log N) lookup time for any number
 500 of packfiles.</p>
 501 </div>
 502 </div>
 503 </div>
 504 <div class="sect1">
 505 <h2 id="_design_details">Design Details</h2>
 506 <div class="sectionbody">
 507 <div class="ulist">
 508 <ul>
 509 <li>
 510 <p>The MIDX is stored in a file named <em>multi-pack-index</em> in the
 511 .git/objects/pack directory. This could be stored in the pack
 512 directory of an alternate. It refers only to packfiles in that
 513 same directory.</p>
 514 </li>
 515 <li>
 516 <p>The core.multiPackIndex config setting must be on (which is the
 517 default) to consume MIDX files.  Setting it to <code>false</code> prevents
 518 Git from reading a MIDX file, even if one exists.</p>
 519 </li>
 520 <li>
 521 <p>The file format includes parameters for the object ID hash
 522 function, so a future change of hash algorithm does not require
 523 a change in format.</p>
 524 </li>
 525 <li>
 526 <p>The MIDX keeps only one record per object ID. If an object appears
 527 in multiple packfiles, then the MIDX selects the copy in the
 528 preferred packfile, otherwise selecting from the most-recently
 529 modified packfile.</p>
 530 </li>
 531 <li>
 532 <p>If there exist packfiles in the pack directory not registered in
 533 the MIDX, then those packfiles are loaded into the <code>packed_git</code>
 534 list and <code>packed_git_mru</code> cache.</p>
 535 </li>
 536 <li>
 537 <p>The pack-indexes (.idx files) remain in the pack directory so we
 538 can delete the MIDX file, set core.midx to false, or downgrade
 539 without any loss of information.</p>
 540 </li>
 541 <li>
 542 <p>The MIDX file format uses a chunk-based approach (similar to the
 543 commit-graph file) that allows optional data to be added.</p>
 544 </li>
 545 </ul>
 546 </div>
 547 </div>
 548 </div>
 549 <div class="sect1">
 550 <h2 id="_future_work">Future Work</h2>
 551 <div class="sectionbody">
 552 <div class="ulist">
 553 <ul>
 554 <li>
 555 <p>The multi-pack-index allows many packfiles, especially in a context
 556 where repacking is expensive (such as a very large repo), or
 557 unexpected maintenance time is unacceptable (such as a high-demand
 558 build machine). However, the multi-pack-index needs to be rewritten
 559 in full every time. We can extend the format to be incremental, so
 560 writes are fast. By storing a small "tip" multi-pack-index that
 561 points to large "base" MIDX files, we can keep writes fast while
 562 still reducing the number of binary searches required for object
 563 lookups.</p>
 564 </li>
 565 <li>
 566 <p>If the multi-pack-index is extended to store a "stable object order"
 567 (a function Order(hash) = integer that is constant for a given hash,
 568 even as the multi-pack-index is updated) then MIDX bitmaps could be
 569 updated independently of the MIDX.</p>
 570 </li>
 571 <li>
 572 <p>Packfiles can be marked as "special" using empty files that share
 573 the initial name but replace ".pack" with ".keep" or ".promisor".
 574 We can add an optional chunk of data to the multi-pack-index that
 575 records flags of information about the packfiles. This allows new
 576 states, such as <em>repacked</em> or <em>redeltified</em>, that can help with
 577 pack maintenance in a multi-pack environment. It may also be
 578 helpful to organize packfiles by object type (commit, tree, blob,
 579 etc.) and use this metadata to help that maintenance.</p>
 580 </li>
 581 </ul>
 582 </div>
 583 </div>
 584 </div>
 585 <div class="sect1">
 586 <h2 id="_related_links">Related Links</h2>
 587 <div class="sectionbody">
 588 <div class="paragraph">
 589 <p>[0] <a href="https://bugs.chromium.org/p/git/issues/detail?id=6" class="bare">https://bugs.chromium.org/p/git/issues/detail?id=6</a>
 590     Chromium work item for: Multi-Pack Index (MIDX)</p>
 591 </div>
 592 <div class="paragraph">
 593 <p>[1] <a href="https://lore.kernel.org/git/20180107181459.222909-1-dstolee@microsoft.com/" class="bare">https://lore.kernel.org/git/20180107181459.222909-1-dstolee@microsoft.com/</a>
 594     An earlier RFC for the multi-pack-index feature</p>
 595 </div>
 596 <div class="paragraph">
 597 <p>[2] <a href="https://lore.kernel.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/" class="bare">https://lore.kernel.org/git/alpine.DEB.2.20.1803091557510.23109@alexmv-linux/</a>
 598     Git Merge 2018 Contributor&#8217;s summit notes (includes discussion of MIDX)</p>
 599 </div>
 600 </div>
 601 </div>
 602 </div>
 603 <div id="footer">
 604 <div id="footer-text">
 605 Last updated 2022-02-16 17:29:08 -0800
 606 </div>
 607 </div>
 608 </body>
 609 </html>