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<div class="post-date">25 Jun 2024</div><h1 class="post-title"><a href="https://chenyo.me/2024-06-25-go-patterns.html">Go patterns</a></h1>
<nav id="table-of-contents" role="doc-toc">
<h2>Table of Contents</h2>
<div id="text-table-of-contents" role="doc-toc">
<ul>
<li><a href="#org3da6ca5">1. Concurrency vs Parallelism</a></li>
<li><a href="#orgec44e31">2. Use goroutines for states</a>
<ul>
<li><a href="#org6365396">2.1. Matching a regex</a></li>
<li><a href="#org134fbdf">2.2. When the state variable cannot be avoided</a></li>
</ul>
</li>
<li><a href="#org7a43aab">3. Pattern 1: publish/subscribe server</a>
<ul>
<li><a href="#org6a0be8e">3.1. Options for slow goroutines</a></li>
</ul>
</li>
<li><a href="#orga53536b">4. Pattern 2: work scheduler</a></li>
<li><a href="#org6868eed">5. Pattern 3: replicated service client</a></li>
<li><a href="#org29de31a">6. Pattern 4: Protocol multiplexer</a></li>
</ul>
</div>
</nav>
<p>
This a personal note for the <a href="https://www.youtube.com/watch?v=IdCbMO0Ey9I">Russ Cox guest lecture</a>.
</p>
<div id="outline-container-org3da6ca5" class="outline-2">
<h2 id="org3da6ca5"><span class="section-number-2">1.</span> Concurrency vs Parallelism</h2>
<div class="outline-text-2" id="text-1">
<ul class="org-ul">
<li>Concurrency: write a program to handle lot of things at once
<ul class="org-ul">
<li>not necessarily faster</li>
</ul></li>
<li>Parallelism: the program itself can do a lot of computations at once</li>
</ul>
</div>
</div>
<div id="outline-container-orgec44e31" class="outline-2">
<h2 id="orgec44e31"><span class="section-number-2">2.</span> Use goroutines for states</h2>
<div class="outline-text-2" id="text-2">
</div>
<div id="outline-container-org6365396" class="outline-3">
<h3 id="org6365396"><span class="section-number-3">2.1.</span> Matching a regex</h3>
<div class="outline-text-3" id="text-2-1">
<ul class="org-ul">
<li>return if a given string matches a regex: start with <code>"</code>, contains arbitrary escape sequence and ends with <code>"</code></li>
<li><p>
unclear logic: store states in the data
</p>
<div class="org-src-container">
<pre class="src src-go"><span class="linenr"> 1: </span><span style="color: #dcaeea;">state</span> := <span style="color: #da8548; font-weight: bold;">0</span>
<span class="linenr"> 2: </span><span style="color: #51afef;">for</span> {
<span class="linenr"> 3: </span> <span style="color: #dcaeea;">c</span> := <span style="color: #c678dd;">read</span>()
<span class="linenr"> 4: </span> <span style="color: #51afef;">switch</span> state {
<span class="linenr"> 5: </span> <span style="color: #51afef;">case</span> <span style="color: #da8548; font-weight: bold;">0</span>:
<span class="linenr"> 6: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">first char must be "</span>
<span class="linenr"> 7: </span> <span style="color: #51afef;">if</span> c != <span style="color: #98be65;">'"'</span> {
<span class="linenr"> 8: </span> <span style="color: #51afef;">return</span> <span style="color: #a9a1e1;">false</span>
<span class="linenr"> 9: </span> }
<span class="linenr">10: </span> state = <span style="color: #da8548; font-weight: bold;">1</span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">match the next char</span>
<span class="linenr">11: </span> <span style="color: #51afef;">case</span> <span style="color: #da8548; font-weight: bold;">1</span>:
<span class="linenr">12: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">ending with " matches</span>
<span class="linenr">13: </span> <span style="color: #51afef;">if</span> c == <span style="color: #98be65;">'"'</span> {
<span class="linenr">14: </span> <span style="color: #51afef;">return</span> <span style="color: #a9a1e1;">true</span>
<span class="linenr">15: </span> }
<span class="linenr">16: </span> <span style="color: #51afef;">if</span> c == <span style="color: #98be65;">'\\'</span> {
<span class="linenr">17: </span> state = <span style="color: #da8548; font-weight: bold;">2</span>
<span class="linenr">18: </span> } <span style="color: #51afef;">else</span> {
<span class="linenr">19: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">transition to state 1 to match next char</span>
<span class="linenr">20: </span> state = <span style="color: #da8548; font-weight: bold;">1</span>
<span class="linenr">21: </span> }
<span class="linenr">22: </span> <span style="color: #51afef;">case</span> <span style="color: #da8548; font-weight: bold;">2</span>:
<span class="linenr">23: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">read the char, discard it and</span>
<span class="linenr">24: </span> state = <span style="color: #da8548; font-weight: bold;">1</span>
<span class="linenr">25: </span> }
<span class="linenr">26: </span>}
</pre>
</div></li>
<li><p>
clear logic: store states in the code
</p>
<div class="org-src-container">
<pre class="src src-go"><span class="linenr"> 1: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">no variable to store state</span>
<span class="linenr"> 2: </span><span style="color: #51afef;">if</span> <span style="color: #c678dd;">read</span>() != <span style="color: #98be65;">'"'</span> {
<span class="linenr"> 3: </span> <span style="color: #51afef;">return</span> <span style="color: #a9a1e1;">false</span>
<span class="linenr"> 4: </span>}
<span class="linenr"> 5: </span><span style="color: #51afef;">var</span> <span style="color: #dcaeea;">c</span> <span style="color: #ECBE7B;">rune</span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">c is a Unicode, alias to int32</span>
<span class="linenr"> 6: </span><span style="color: #51afef;">for</span> c != <span style="color: #98be65;">'"'</span> {
<span class="linenr"> 7: </span> c = <span style="color: #c678dd;">read</span>()
<span class="linenr"> 8: </span> <span style="color: #51afef;">if</span> c == <span style="color: #98be65;">'\\'</span> {
<span class="linenr"> 9: </span> <span style="color: #c678dd;">read</span>() <span style="color: #5B6268;">// </span><span style="color: #5B6268;">skip the next char</span>
<span class="linenr">10: </span> }
<span class="linenr">11: </span>}
<span class="linenr">12: </span><span style="color: #51afef;">return</span> <span style="color: #a9a1e1;">true</span>
</pre>
</div></li>
</ul>
</div>
</div>
<div id="outline-container-org134fbdf" class="outline-3">
<h3 id="org134fbdf"><span class="section-number-3">2.2.</span> When the state variable cannot be avoided</h3>
<div class="outline-text-3" id="text-2-2">
<ul class="org-ul">
<li><p>
the function needs to return the state
</p>
<div class="org-src-container">
<pre class="src src-go"><span class="linenr"> 1: </span><span style="color: #51afef;">type</span> <span style="color: #ECBE7B;">quoter</span> <span style="color: #51afef;">struct</span> {
<span class="linenr"> 2: </span> state <span style="color: #ECBE7B;">int</span>
<span class="linenr"> 3: </span>}
<span class="linenr"> 4: </span>
<span class="linenr"> 5: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">q</span> *<span style="color: #ECBE7B;">quoter</span>) <span style="color: #c678dd;">Init</span>() {
<span class="linenr"> 6: </span> r.state = <span style="color: #da8548; font-weight: bold;">0</span>
<span class="linenr"> 7: </span>}
<span class="linenr"> 8: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">proess each char based on current state</span>
<span class="linenr"> 9: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">q</span> *<span style="color: #ECBE7B;">quoter</span>) <span style="color: #c678dd;">Write</span>(<span style="color: #dcaeea;">c</span> <span style="color: #ECBE7B;">rune</span>) <span style="color: #ECBE7B;">Status</span> {
<span class="linenr">10: </span> <span style="color: #51afef;">switch</span> q.state {
<span class="linenr">11: </span> <span style="color: #51afef;">case</span> <span style="color: #da8548; font-weight: bold;">0</span>:
<span class="linenr">12: </span> <span style="color: #51afef;">if</span> c != <span style="color: #98be65;">'"'</span> {
<span class="linenr">13: </span> <span style="color: #51afef;">return</span> BadInput
<span class="linenr">14: </span> }
<span class="linenr">15: </span> q.state = <span style="color: #da8548; font-weight: bold;">1</span>
<span class="linenr">16: </span> <span style="color: #51afef;">case</span> <span style="color: #da8548; font-weight: bold;">1</span>:
<span class="linenr">17: </span> <span style="color: #51afef;">if</span> c == <span style="color: #98be65;">'"'</span> {
<span class="linenr">18: </span> <span style="color: #51afef;">return</span> Success
<span class="linenr">19: </span> }
<span class="linenr">20: </span> <span style="color: #51afef;">if</span> c == <span style="color: #98be65;">'\\'</span> {
<span class="linenr">21: </span> q.state = <span style="color: #da8548; font-weight: bold;">2</span>
<span class="linenr">22: </span> } <span style="color: #51afef;">else</span> {
<span class="linenr">23: </span> q.state = <span style="color: #da8548; font-weight: bold;">1</span>
<span class="linenr">24: </span> }
<span class="linenr">25: </span> <span style="color: #51afef;">case</span> <span style="color: #da8548; font-weight: bold;">2</span>:
<span class="linenr">26: </span> q.state = <span style="color: #da8548; font-weight: bold;">1</span>
<span class="linenr">27: </span> }
<span class="linenr">28: </span> <span style="color: #51afef;">return</span> NeedMoreInput
<span class="linenr">29: </span>}
</pre>
</div></li>
<li><p>
use additional goroutines to hold states
</p>
<div class="org-src-container">
<pre class="src src-go"><span class="linenr"> 1: </span><span style="color: #51afef;">type</span> <span style="color: #ECBE7B;">quoter</span> <span style="color: #51afef;">struct</span> {
<span class="linenr"> 2: </span> char <span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">rune</span>
<span class="linenr"> 3: </span> status <span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">Status</span>
<span class="linenr"> 4: </span>}
<span class="linenr"> 5: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">q</span> *<span style="color: #ECBE7B;">quoter</span>) <span style="color: #c678dd;">Init</span>() {
<span class="linenr"> 6: </span> q.char = <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">rune</span>)
<span class="linenr"> 7: </span> q.status = <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">Status</span>)
<span class="linenr"> 8: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">need to make sure why and when the goroutine will exit</span>
<span class="linenr"> 9: </span> <span style="color: #51afef;">go</span> q.<span style="color: #c678dd;">parse</span>()
<span class="linenr">10: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">blocks until it receives an initial status from parse()</span>
<span class="linenr">11: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">to ensure that parse() is ready, i.e., q.status = NeedMoreInput</span>
<span class="linenr">12: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">before Write() is called</span>
<span class="linenr">13: </span> <-q.status
<span class="linenr">14: </span>}
<span class="linenr">15: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">Write sends the next char to q.char, which will be receivecd by parse()</span>
<span class="linenr">16: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">the status is a public state accessible by the user</span>
<span class="linenr">17: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">q</span> *<span style="color: #ECBE7B;">quoter</span>) <span style="color: #c678dd;">Write</span>(<span style="color: #dcaeea;">r</span> <span style="color: #ECBE7B;">rune</span>) <span style="color: #ECBE7B;">Status</span> {
<span class="linenr">18: </span> q.char <- c
<span class="linenr">19: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">wait for the result</span>
<span class="linenr">20: </span> <span style="color: #51afef;">return</span> <-q.status
<span class="linenr">21: </span>}
<span class="linenr">22: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">q</span> *<span style="color: #ECBE7B;">quoteReader</span>) <span style="color: #c678dd;">parse</span>() {
<span class="linenr">23: </span> <span style="color: #51afef;">if</span> q.<span style="color: #c678dd;">read</span>() != <span style="color: #98be65;">'"'</span> {
<span class="linenr">24: </span> q.status <- SyntaxError
<span class="linenr">25: </span> <span style="color: #51afef;">return</span>
<span class="linenr">26: </span> }
<span class="linenr">27: </span> <span style="color: #51afef;">var</span> <span style="color: #dcaeea;">c</span> <span style="color: #ECBE7B;">rune</span>
<span class="linenr">28: </span> <span style="color: #51afef;">for</span> c!= <span style="color: #98be65;">'"'</span> {
<span class="linenr">29: </span> c = q.<span style="color: #c678dd;">read</span>()
<span class="linenr">30: </span> <span style="color: #51afef;">if</span> c == <span style="color: #98be65;">'\\'</span> {
<span class="linenr">31: </span> q.<span style="color: #c678dd;">read</span>()
<span class="linenr">32: </span> }
<span class="linenr">33: </span> }
<span class="linenr">34: </span> q.status <- Done
<span class="linenr">35: </span>}
<span class="linenr">36: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">a helper function used in parse() to return the next char in q.char</span>
<span class="linenr">37: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">q</span> *<span style="color: #ECBE7B;">quoter</span>) <span style="color: #c678dd;">read</span>() <span style="color: #ECBE7B;">int</span> {
<span class="linenr">38: </span> q.status <- NeedMoreInput
<span class="linenr">39: </span> <span style="color: #51afef;">return</span> <- q.char
<span class="linenr">40: </span>}
<span class="linenr">41: </span><span style="color: #51afef;">func</span> <span style="color: #c678dd;">main</span>() {
<span class="linenr">42: </span> <span style="color: #dcaeea;">q</span> := &<span style="color: #ECBE7B;">quoter</span>{}
<span class="linenr">43: </span> q.<span style="color: #c678dd;">Init</span>()
<span class="linenr">44: </span>
<span class="linenr">45: </span> <span style="color: #dcaeea;">input</span> := <span style="color: #98be65;">`"Hello, \"World\""`</span>
<span class="linenr">46: </span> <span style="color: #51afef;">for</span> <span style="color: #dcaeea;">_</span>, <span style="color: #dcaeea;">c</span> := <span style="color: #51afef;">range</span> input {
<span class="linenr">47: </span> <span style="color: #dcaeea;">status</span> := q.<span style="color: #c678dd;">Write</span>(c)
<span class="linenr">48: </span> }
<span class="linenr">49: </span>}
</pre>
</div></li>
<li>check goroutine blockage
<ul class="org-ul">
<li><code>Ctrl-\</code> sends <code>SIGQUIT</code></li>
<li>use the HTTP server’s <code>/debug/pprof/goroutine</code> if importing <code>net/http</code></li>
</ul></li>
</ul>
</div>
</div>
</div>
<div id="outline-container-org7a43aab" class="outline-2">
<h2 id="org7a43aab"><span class="section-number-2">3.</span> Pattern 1: publish/subscribe server</h2>
<div class="outline-text-2" id="text-3">
<ul class="org-ul">
<li>the information goes one way: server -> client</li>
<li>close a channel to signal no new values will be sent</li>
<li><p>
prefer <code>defer</code> when unlocking the mutex
</p>
<div class="org-src-container">
<pre class="src src-go"><span class="linenr"> 1: </span><span style="color: #51afef;">type</span> <span style="color: #ECBE7B;">Server</span> <span style="color: #51afef;">struct</span> {
<span class="linenr"> 2: </span> mu <span style="color: #ECBE7B;">sync.Mutex</span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">protect sub</span>
<span class="linenr"> 3: </span> sub <span style="color: #51afef;">map</span>[<span style="color: #51afef;">chan</span><- <span style="color: #ECBE7B;">Event</span>]<span style="color: #ECBE7B;">bool</span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">whether a channel should be closed</span>
<span class="linenr"> 4: </span>}
<span class="linenr"> 5: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">s</span> *<span style="color: #ECBE7B;">Server</span>) <span style="color: #c678dd;">Init</span>() {
<span class="linenr"> 6: </span> s.sub = <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">map</span>[<span style="color: #51afef;">chan</span><- <span style="color: #ECBE7B;">Event</span>]<span style="color: #ECBE7B;">bool</span>)
<span class="linenr"> 7: </span>}
<span class="linenr"> 8: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">publish an event to all subscribed channel</span>
<span class="linenr"> 9: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">s</span> *<span style="color: #ECBE7B;">Server</span>) <span style="color: #c678dd;">Publish</span>(<span style="color: #dcaeea;">e</span> <span style="color: #ECBE7B;">Event</span>) {
<span class="linenr">10: </span> s.mu.<span style="color: #c678dd;">Lock</span>() <span style="color: #5B6268;">// </span><span style="color: #5B6268;">each method could be called by many clients</span>
<span class="linenr">11: </span> <span style="color: #51afef;">defer</span> s.mu.<span style="color: #c678dd;">Unlock</span>()
<span class="linenr">12: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">need mutex here since it needs to read s.sub state</span>
<span class="linenr">13: </span> <span style="color: #51afef;">for</span> <span style="color: #dcaeea;">c</span> := <span style="color: #51afef;">range</span> s.sub {
<span class="linenr">14: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">if a goroutine consumes the channel events too slow</span>
<span class="linenr">15: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">then a new event publish has to wait</span>
<span class="linenr">16: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">before it can send to the channel</span>
<span class="linenr">17: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">can add channel buffer to mitigate this</span>
<span class="linenr">18: </span> c <- e
<span class="linenr">19: </span> }
<span class="linenr">20: </span>}
<span class="linenr">21: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">a channel starts to subscribe</span>
<span class="linenr">22: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">s</span> *<span style="color: #ECBE7B;">Server</span>) <span style="color: #c678dd;">Subscribe</span>(<span style="color: #dcaeea;">c</span> <span style="color: #51afef;">chan</span><- <span style="color: #ECBE7B;">Event</span>) {
<span class="linenr">23: </span> s.mu.<span style="color: #c678dd;">Lock</span>()
<span class="linenr">24: </span> <span style="color: #51afef;">defer</span> s.mu.<span style="color: #c678dd;">Unlock</span>()
<span class="linenr">25: </span> <span style="color: #51afef;">if</span> s.sub[c] {
<span class="linenr">26: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">the mutex wil also be unlocked with defer</span>
<span class="linenr">27: </span> <span style="color: #c678dd;">panic</span>(<span style="color: #98be65;">"pubsub: already subscribed"</span>) }
<span class="linenr">28: </span> s.sub[c] = <span style="color: #a9a1e1;">true</span>
<span class="linenr">29: </span>}
<span class="linenr">30: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">a channel cancels the subscription</span>
<span class="linenr">31: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">s</span> *<span style="color: #ECBE7B;">Server</span>) <span style="color: #c678dd;">Cancel</span>(<span style="color: #dcaeea;">c</span> <span style="color: #51afef;">chan</span><- <span style="color: #ECBE7B;">Event</span>) {
<span class="linenr">32: </span> s.mu.<span style="color: #c678dd;">Lock</span>()
<span class="linenr">33: </span> <span style="color: #51afef;">defer</span> s.mu.<span style="color: #c678dd;">Unlock</span>()
<span class="linenr">34: </span> <span style="color: #51afef;">if</span> <span style="color: #51afef; font-weight: bold;">!</span>s.sub[c] {
<span class="linenr">35: </span> <span style="color: #c678dd;">panic</span>(<span style="color: #98be65;">"pubsub: not subscribed"</span>)
<span class="linenr">36: </span> }
<span class="linenr">37: </span> <span style="color: #c678dd;">close</span>(c)
<span class="linenr">38: </span> <span style="color: #c678dd;">delete</span>(s.sub, c)
<span class="linenr">39: </span>}
</pre>
</div></li>
</ul>
</div>
<div id="outline-container-org6a0be8e" class="outline-3">
<h3 id="org6a0be8e"><span class="section-number-3">3.1.</span> Options for slow goroutines</h3>
<div class="outline-text-3" id="text-3-1">
<ul class="org-ul">
<li>slow down event generation</li>
<li>drop events if it cannot be sent, e.g., <code>os/signal</code>, <code>runtime/pprof</code></li>
<li><p>
queue events, e.g., add a <code>helper</code> between the server and each client, which also separates the concerns
</p>
<div class="org-src-container">
<pre class="src src-go"><span class="linenr"> 1: </span><span style="color: #51afef;">func</span> <span style="color: #c678dd;">helper</span>(<span style="color: #dcaeea;">in</span> <-<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">Event</span>, <span style="color: #dcaeea;">out</span> <span style="color: #51afef;">chan</span><- <span style="color: #ECBE7B;">Event</span>) {
<span class="linenr"> 2: </span> <span style="color: #51afef;">var</span> <span style="color: #dcaeea;">q</span> []<span style="color: #ECBE7B;">Event</span>
<span class="linenr"> 3: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">if the in is closed, flash out the pending events in q</span>
<span class="linenr"> 4: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">and close out</span>
<span class="linenr"> 5: </span> <span style="color: #51afef;">for</span> in != <span style="color: #a9a1e1;">nil</span> || <span style="color: #c678dd;">len</span>(q) > <span style="color: #da8548; font-weight: bold;">0</span> {
<span class="linenr"> 6: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">decide whether and what to send</span>
<span class="linenr"> 7: </span> <span style="color: #51afef;">var</span> <span style="color: #dcaeea;">sendOut</span> <span style="color: #51afef;">chan</span><- <span style="color: #dcaeea;">Event</span>
<span class="linenr"> 8: </span> <span style="color: #51afef;">var</span> <span style="color: #dcaeea;">next</span> <span style="color: #ECBE7B;">Event</span>
<span class="linenr"> 9: </span> <span style="color: #51afef;">if</span> <span style="color: #c678dd;">len</span>(q) > <span style="color: #da8548; font-weight: bold;">0</span> {
<span class="linenr">10: </span> sendOut = out
<span class="linenr">11: </span> next = q[<span style="color: #da8548; font-weight: bold;">0</span>]
<span class="linenr">12: </span> }
<span class="linenr">13: </span> <span style="color: #51afef;">select</span> {
<span class="linenr">14: </span> <span style="color: #51afef;">case</span> <span style="color: #dcaeea;">e</span>, <span style="color: #dcaeea;">ok</span> := <-in: <span style="color: #5B6268;">// </span><span style="color: #5B6268;">never reaches here after in = nil</span>
<span class="linenr">15: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">ok tells whether in is closed</span>
<span class="linenr">16: </span> <span style="color: #51afef;">if</span> <span style="color: #51afef; font-weight: bold;">!</span>ok {
<span class="linenr">17: </span> in = <span style="color: #a9a1e1;">nil</span>
<span class="linenr">18: </span> <span style="color: #51afef;">break</span>
<span class="linenr">19: </span> }
<span class="linenr">20: </span> q = <span style="color: #c678dd;">append</span>(q, e)
<span class="linenr">21: </span> <span style="color: #51afef;">case</span> sendOut <- next: <span style="color: #5B6268;">// </span><span style="color: #5B6268;">if len(q) == 0, sendOut = nil</span>
<span class="linenr">22: </span> q = q[<span style="color: #da8548; font-weight: bold;">1</span>:]
<span class="linenr">23: </span> }
<span class="linenr">24: </span> }
<span class="linenr">25: </span> <span style="color: #c678dd;">close</span>(out)
<span class="linenr">26: </span>}
</pre>
</div></li>
<li><p>
convert mutexes into goroutines, not suitable for Raft where state transition is complex
</p>
<div class="org-src-container">
<pre class="src src-go"><span class="linenr"> 1: </span><span style="color: #51afef;">type</span> <span style="color: #ECBE7B;">Server</span> <span style="color: #51afef;">struct</span> {
<span class="linenr"> 2: </span> publish <span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">Event</span>
<span class="linenr"> 3: </span> subscribe <span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">subReq</span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">a channel to queue unhandled subscription</span>
<span class="linenr"> 4: </span> cancel <span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">subReq</span>
<span class="linenr"> 5: </span>}
<span class="linenr"> 6: </span><span style="color: #51afef;">type</span> <span style="color: #ECBE7B;">subReq</span> <span style="color: #51afef;">struct</span> {
<span class="linenr"> 7: </span> c <span style="color: #51afef;">chan</span><- <span style="color: #ECBE7B;">Event</span>
<span class="linenr"> 8: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">a signal of whether an operation succeeds</span>
<span class="linenr"> 9: </span> ok <span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">bool</span>
<span class="linenr">10: </span>}
<span class="linenr">11: </span>
<span class="linenr">12: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">s</span> *<span style="color: #ECBE7B;">Server</span>) <span style="color: #c678dd;">Init</span>() {
<span class="linenr">13: </span> s.publish = <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">Event</span>)
<span class="linenr">14: </span> s.subscribe = <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">subReq</span>)
<span class="linenr">15: </span> s.cancel = <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">subReq</span>)
<span class="linenr">16: </span> <span style="color: #51afef;">go</span> s.<span style="color: #c678dd;">loop</span>()
<span class="linenr">17: </span>}
<span class="linenr">18: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">s</span> *<span style="color: #ECBE7B;">Server</span>) <span style="color: #c678dd;">Publish</span>(<span style="color: #dcaeea;">e</span> <span style="color: #ECBE7B;">Event</span>) {
<span class="linenr">19: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">no mutex is required here</span>
<span class="linenr">20: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">as it does not read state</span>
<span class="linenr">21: </span> s.publish <- e
<span class="linenr">22: </span>}
<span class="linenr">23: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">s</span> *<span style="color: #ECBE7B;">Server</span>) <span style="color: #c678dd;">Subscribe</span>(<span style="color: #dcaeea;">c</span> <span style="color: #51afef;">chan</span><- <span style="color: #ECBE7B;">Event</span>) {
<span class="linenr">24: </span> <span style="color: #dcaeea;">r</span> := <span style="color: #ECBE7B;">subReq</span>{<span style="color: #a9a1e1;">c</span>: c, <span style="color: #a9a1e1;">ok</span>: <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">bool</span>)}
<span class="linenr">25: </span> s.subscribe <- r
<span class="linenr">26: </span> <span style="color: #51afef;">if</span> <span style="color: #51afef; font-weight: bold;">!</span><-r.ok { <span style="color: #5B6268;">// </span><span style="color: #5B6268;">wait for loop() handle result</span>
<span class="linenr">27: </span> <span style="color: #c678dd;">panic</span>(<span style="color: #98be65;">"pubsub: already subscribed"</span>)
<span class="linenr">28: </span> }
<span class="linenr">29: </span>}
<span class="linenr">30: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">s</span> *<span style="color: #ECBE7B;">Server</span>) <span style="color: #c678dd;">Cancel</span>(<span style="color: #dcaeea;">c</span> <span style="color: #51afef;">chan</span><- <span style="color: #ECBE7B;">Event</span>) {
<span class="linenr">31: </span> <span style="color: #dcaeea;">r</span> := <span style="color: #ECBE7B;">subReq</span>{<span style="color: #a9a1e1;">c</span>: c, <span style="color: #a9a1e1;">ok</span>: <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">bool</span>)}
<span class="linenr">32: </span> s.cancel <- r
<span class="linenr">33: </span> <span style="color: #51afef;">if</span> <span style="color: #51afef; font-weight: bold;">!</span><-r.ok {
<span class="linenr">34: </span> <span style="color: #c678dd;">panic</span>(<span style="color: #98be65;">"pubusb: not subscribed"</span>)
<span class="linenr">35: </span> }
<span class="linenr">36: </span>}
<span class="linenr">37: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">s</span> *<span style="color: #ECBE7B;">Server</span>) <span style="color: #c678dd;">loop</span>() {
<span class="linenr">38: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">now sub is a local variable, no lock is needed</span>
<span class="linenr">39: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">sub maps from a subscribed channel to a helper channel</span>
<span class="linenr">40: </span> <span style="color: #dcaeea;">sub</span> := <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">map</span>[<span style="color: #51afef;">chan</span><- <span style="color: #ECBE7B;">Event</span>]<span style="color: #51afef;">chan</span><- <span style="color: #ECBE7B;">Event</span>)
<span class="linenr">41: </span> <span style="color: #51afef;">for</span> {
<span class="linenr">42: </span> <span style="color: #51afef;">select</span> {
<span class="linenr">43: </span> <span style="color: #51afef;">case</span> <span style="color: #dcaeea;">e</span> := <-s.publish:
<span class="linenr">44: </span> <span style="color: #51afef;">for</span> <span style="color: #dcaeea;">_</span>, <span style="color: #dcaeea;">h</span> := <span style="color: #51afef;">range</span> sub {
<span class="linenr">45: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">the event is published to a helper channel</span>
<span class="linenr">46: </span> h <- e
<span class="linenr">47: </span> }
<span class="linenr">48: </span> <span style="color: #51afef;">case</span> <span style="color: #dcaeea;">r</span> := <-s.subscribe:
<span class="linenr">49: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">the helper channel exists</span>
<span class="linenr">50: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">meaning the subscriber has been handled before</span>
<span class="linenr">51: </span> <span style="color: #51afef;">if</span> sub[r.c] != <span style="color: #a9a1e1;">nil</span> {
<span class="linenr">52: </span> r.ok <- <span style="color: #a9a1e1;">false</span>
<span class="linenr">53: </span> <span style="color: #51afef;">break</span>
<span class="linenr">54: </span> }
<span class="linenr">55: </span> h = <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">Event</span>)
<span class="linenr">56: </span> <span style="color: #51afef;">go</span> <span style="color: #c678dd;">helper</span>(h, r.c)
<span class="linenr">57: </span> sub[r.c] = h
<span class="linenr">58: </span> r.ok <- <span style="color: #a9a1e1;">true</span>
<span class="linenr">59: </span> <span style="color: #51afef;">case</span> <span style="color: #dcaeea;">c</span> := <-s.cancel:
<span class="linenr">60: </span> <span style="color: #51afef;">if</span> <span style="color: #51afef; font-weight: bold;">!</span>sub[r.c] == <span style="color: #a9a1e1;">nil</span>{
<span class="linenr">61: </span> r.ok <- <span style="color: #a9a1e1;">false</span>
<span class="linenr">62: </span> <span style="color: #51afef;">break</span>
<span class="linenr">63: </span> }
<span class="linenr">64: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">close the helper channel</span>
<span class="linenr">65: </span> <span style="color: #c678dd;">close</span>(sub[r.c])
<span class="linenr">66: </span> <span style="color: #c678dd;">delete</span>(sub, r.c)
<span class="linenr">67: </span> r.ok <- <span style="color: #a9a1e1;">true</span>
<span class="linenr">68: </span> }
<span class="linenr">69: </span> }
<span class="linenr">70: </span>}
</pre>
</div></li>
</ul>
</div>
</div>
</div>
<div id="outline-container-orga53536b" class="outline-2">
<h2 id="orga53536b"><span class="section-number-2">4.</span> Pattern 2: work scheduler</h2>
<div class="outline-text-2" id="text-4">
<ul class="org-ul">
<li><p>
\(M\) tasks assigned to \(N\) servers/workers, \( M >> N\).
</p>
<div class="org-src-container">
<pre class="src src-go"><span class="linenr"> 1: </span><span style="color: #51afef;">func</span> <span style="color: #c678dd;">Schedule</span>(<span style="color: #dcaeea;">servers</span> []<span style="color: #ECBE7B;">string</span>, <span style="color: #dcaeea;">numTask</span> <span style="color: #ECBE7B;">int</span>,
<span class="linenr"> 2: </span> <span style="color: #dcaeea;">call</span> <span style="color: #51afef;">func</span>(<span style="color: #dcaeea;">srv</span> <span style="color: #ECBE7B;">string</span>, <span style="color: #dcaeea;">task</span> <span style="color: #ECBE7B;">int</span>)) {
<span class="linenr"> 3: </span>
<span class="linenr"> 4: </span> <span style="color: #dcaeea;">idle</span> := <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">string</span>, <span style="color: #c678dd;">len</span>(servers))
<span class="linenr"> 5: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">initialize a channel of idle servers</span>
<span class="linenr"> 6: </span> <span style="color: #51afef;">for</span> <span style="color: #dcaeea;">_</span>, <span style="color: #dcaeea;">srv</span> := <span style="color: #51afef;">range</span> servers {
<span class="linenr"> 7: </span> idle <- srv
<span class="linenr"> 8: </span> }
<span class="linenr"> 9: </span>
<span class="linenr">10: </span> <span style="color: #51afef;">for</span> <span style="color: #dcaeea;">task</span> := <span style="color: #da8548; font-weight: bold;">0</span>, task < numTask; task++ {
<span class="linenr">11: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">if using task in the for loop rather than a local task,</span>
<span class="linenr">12: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">there is a race: the loop goes on before the goroutinue starts,</span>
<span class="linenr">13: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">so that some tasks are skipped.</span>
<span class="linenr">14: </span> <span style="color: #dcaeea;">task</span> := task
<span class="linenr">15: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">if moving srv := <- idle inside goroutine</span>
<span class="linenr">16: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">a lot of goroutines are created simoutaneously and hung</span>
<span class="linenr">17: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">due to non-idle server</span>
<span class="linenr">18: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">leaving it outside so that a goroutine is only created when</span>
<span class="linenr">19: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">there is an idle server (but it slows down the main loop)</span>
<span class="linenr">20: </span> <span style="color: #dcaeea;">srv</span> := <-idle
<span class="linenr">21: </span> <span style="color: #51afef;">go</span> <span style="color: #51afef;">func</span>() {
<span class="linenr">22: </span> <span style="color: #c678dd;">call</span>(srv, task) <span style="color: #5B6268;">// </span><span style="color: #5B6268;">server does the task</span>
<span class="linenr">23: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">serve finishes the task and becomes idle again</span>
<span class="linenr">24: </span> idle <- srv
<span class="linenr">25: </span> }()
<span class="linenr">26: </span> }
<span class="linenr">27: </span>
<span class="linenr">28: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">determine when all tasks are done / all servers are idle</span>
<span class="linenr">29: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">this is used to prevent early exit when all tasks have been assigned</span>
<span class="linenr">30: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">but the last servers have not finished</span>
<span class="linenr">31: </span> <span style="color: #51afef;">for</span> <span style="color: #dcaeea;">i</span> :=<span style="color: #da8548; font-weight: bold;">0</span>; i < <span style="color: #c678dd;">len</span>(servers); i++ {
<span class="linenr">32: </span> <-idle
<span class="linenr">33: </span> }
<span class="linenr">34: </span>}
</pre>
</div></li>
<li><p>
Optimization for the above code: while the task loop creates goroutines \(M\) times, actually there are only at most \(N\) active goroutines at any time.
</p>
<ul class="org-ul">
<li>Better to spin off a goroutine for each server.</li>
<li>The number of servers can be dynamic.</li>
</ul>
<div class="org-src-container">
<pre class="src src-go"><span class="linenr"> 1: </span><span style="color: #51afef;">func</span> <span style="color: #c678dd;">Schedule</span>(<span style="color: #dcaeea;">servers</span> <span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">string</span>, <span style="color: #dcaeea;">numTask</span> <span style="color: #ECBE7B;">int</span>,
<span class="linenr"> 2: </span> <span style="color: #dcaeea;">call</span> <span style="color: #51afef;">func</span>(<span style="color: #dcaeea;">srv</span> <span style="color: #ECBE7B;">string</span>, <span style="color: #dcaeea;">task</span> <span style="color: #ECBE7B;">int</span>)) {
<span class="linenr"> 3: </span>
<span class="linenr"> 4: </span> <span style="color: #dcaeea;">work</span> := <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">int</span>) <span style="color: #5B6268;">// </span><span style="color: #5B6268;">a queue of all works yet to be done</span>
<span class="linenr"> 5: </span> <span style="color: #dcaeea;">done</span> := <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">bool</span>) <span style="color: #5B6268;">// </span><span style="color: #5B6268;">a queue of all done tasks</span>
<span class="linenr"> 6: </span> <span style="color: #dcaeea;">exit</span> := <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">bool</span>) <span style="color: #5B6268;">// </span><span style="color: #5B6268;">signal when should not pull new servers</span>
<span class="linenr"> 7: </span>
<span class="linenr"> 8: </span> <span style="color: #dcaeea;">runTasks</span> := <span style="color: #51afef;">func</span>(<span style="color: #dcaeea;">srv</span> <span style="color: #ECBE7B;">string</span>) {
<span class="linenr"> 9: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">keep polling until work is closed</span>
<span class="linenr">10: </span> <span style="color: #51afef;">for</span> <span style="color: #dcaeea;">task</span> := <span style="color: #51afef;">range</span> work {
<span class="linenr">11: </span> <span style="color: #51afef;">if</span> <span style="color: #c678dd;">call</span>(srv, task) {
<span class="linenr">12: </span> done <- <span style="color: #a9a1e1;">true</span>
<span class="linenr">13: </span> } <span style="color: #51afef;">else</span> {
<span class="linenr">14: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">repush the task if it failed</span>
<span class="linenr">15: </span> work <- task
<span class="linenr">16: </span> }
<span class="linenr">17: </span> }
<span class="linenr">18: </span> }
<span class="linenr">19: </span>
<span class="linenr">20: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">use a goroutine to avoid hanging when</span>
<span class="linenr">21: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">no server is available</span>
<span class="linenr">22: </span> <span style="color: #51afef;">go</span> <span style="color: #51afef;">func</span>() {
<span class="linenr">23: </span> <span style="color: #51afef;">for</span> <span style="color: #dcaeea;">_</span>, <span style="color: #dcaeea;">srv</span> := <span style="color: #51afef;">range</span> servers {
<span class="linenr">24: </span> <span style="color: #51afef;">for</span> {
<span class="linenr">25: </span> <span style="color: #51afef;">select</span> {
<span class="linenr">26: </span> <span style="color: #51afef;">case</span> <span style="color: #dcaeea;">src</span> := <-servers:
<span class="linenr">27: </span> <span style="color: #51afef;">go</span> <span style="color: #c678dd;">runTasks</span>(srv)
<span class="linenr">28: </span> <span style="color: #51afef;">case</span> <-exit:
<span class="linenr">29: </span> <span style="color: #51afef;">return</span>
<span class="linenr">30: </span> }
<span class="linenr">31: </span> }
<span class="linenr">32: </span> }
<span class="linenr">33: </span> }()
<span class="linenr">34: </span>
<span class="linenr">35: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">The following code has a deadlock!</span>
<span class="linenr">36: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">In the runTasks, the server pushes to done channel when a task is done.</span>
<span class="linenr">37: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">However, the done channel is only pulled when the main routine has</span>
<span class="linenr">38: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">pushed all tasks and close the work channel.</span>
<span class="linenr">39: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">Therefore any server hangs when trying push the second done work.</span>
<span class="linenr">40: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">for taks := 0; task < numTask; task++ {</span>
<span class="linenr">41: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">work <- task</span>
<span class="linenr">42: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">}</span>
<span class="linenr">43: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">// signal no more task so that servers know</span>
<span class="linenr">44: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">// when to termiante</span>
<span class="linenr">45: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">close(work)</span>
<span class="linenr">46: </span>
<span class="linenr">47: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">// wait until all tasks are done</span>
<span class="linenr">48: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">for i := 0; i < numTask; i++ {</span>
<span class="linenr">49: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;"><-done</span>
<span class="linenr">50: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">}</span>
<span class="linenr">51: </span>
<span class="linenr">52: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">fix 1: one can switch between work and donw channel</span>
<span class="linenr">53: </span> <span style="color: #dcaeea;">i</span> := <span style="color: #da8548; font-weight: bold;">0</span>
<span class="linenr">54: </span><span style="color: #a9a1e1;">WorkLoop</span>:
<span class="linenr">55: </span> <span style="color: #51afef;">for</span> <span style="color: #dcaeea;">task</span> := <span style="color: #da8548; font-weight: bold;">0</span>; task < numTask; task++ {
<span class="linenr">56: </span> <span style="color: #51afef;">for</span> {
<span class="linenr">57: </span> <span style="color: #51afef;">select</span> {
<span class="linenr">58: </span> <span style="color: #51afef;">case</span> work <- task:
<span class="linenr">59: </span> <span style="color: #51afef;">continue</span> <span style="color: #a9a1e1;">WorkLoop</span>
<span class="linenr">60: </span> <span style="color: #51afef;">case</span> <-done:
<span class="linenr">61: </span> i++
<span class="linenr">62: </span> }
<span class="linenr">63: </span> }
<span class="linenr">64: </span> }
<span class="linenr">65: </span>
<span class="linenr">66: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">wait for the last assigned tasks to be done</span>
<span class="linenr">67: </span> <span style="color: #51afef;">for</span> ; i < numTask; i++ {
<span class="linenr">68: </span> <-done
<span class="linenr">69: </span> }
<span class="linenr">70: </span>
<span class="linenr">71: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">only close work channel in the end,</span>
<span class="linenr">72: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">in case some tasks failed and need to be redo</span>
<span class="linenr">73: </span> <span style="color: #c678dd;">close</span>(work)
<span class="linenr">74: </span> exit <- <span style="color: #a9a1e1;">true</span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">stop pulling new servers</span>
<span class="linenr">75: </span>
<span class="linenr">76: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">fix 2: move the work assignment to a separate go routine</span>
<span class="linenr">77: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">go func() {</span>
<span class="linenr">78: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">for task := ; task < numTask; task++ {</span>
<span class="linenr">79: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">work <- task</span>
<span class="linenr">80: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">}</span>
<span class="linenr">81: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">close(work)</span>
<span class="linenr">82: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">}()</span>
<span class="linenr">83: </span>
<span class="linenr">84: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">fix 3: increase buffer for the work channel</span>
<span class="linenr">85: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">work := make(chan int, numTask)</span>
<span class="linenr">86: </span>}
</pre>
</div></li>
</ul>
</div>
</div>
<div id="outline-container-org6868eed" class="outline-2">
<h2 id="org6868eed"><span class="section-number-2">5.</span> Pattern 3: replicated service client</h2>
<div class="outline-text-2" id="text-5">
<ul class="org-ul">
<li><p>
A client replicates its requests to multiple servers, waits for the first reply and changes its preferred server.
</p>
<div class="org-src-container">
<pre class="src src-go"><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">c</span> *<span style="color: #ECBE7B;">Client</span>) <span style="color: #c678dd;">Call</span>(<span style="color: #dcaeea;">args</span> <span style="color: #ECBE7B;">Args</span>) <span style="color: #ECBE7B;">Reply</span> {
<span style="color: #51afef;">type</span> <span style="color: #ECBE7B;">result</span> <span style="color: #51afef;">struct</span> {
serverID <span style="color: #ECBE7B;">int</span>
reply <span style="color: #ECBE7B;">Reply</span>
}
<span style="color: #51afef;">const</span> <span style="color: #a9a1e1;">timeout</span> = <span style="color: #da8548; font-weight: bold;">1</span> * time.Second
<span style="color: #dcaeea;">t</span> := time.<span style="color: #c678dd;">NewTimer</span>(timeout)
<span style="color: #51afef;">defer</span> t.<span style="color: #c678dd;">Stop</span>()
<span style="color: #5B6268;">// </span><span style="color: #5B6268;">a channel for all servers to send reply</span>
<span style="color: #5B6268;">// </span><span style="color: #5B6268;">so that even if the client has received a reply</span>
<span style="color: #5B6268;">// </span><span style="color: #5B6268;">other later replies don't hang</span>
<span style="color: #dcaeea;">done</span> := <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">result</span>, <span style="color: #c678dd;">len</span>(c.servers))
c.mu.<span style="color: #c678dd;">Lock</span>()
<span style="color: #dcaeea;">prefer</span> := c.prefer
c.mu.<span style="color: #c678dd;">Unlock</span>()
<span style="color: #51afef;">var</span> <span style="color: #dcaeea;">r</span> <span style="color: #ECBE7B;">result</span>
<span style="color: #51afef;">for</span> <span style="color: #dcaeea;">off</span> := <span style="color: #da8548; font-weight: bold;">0</span>; off < <span style="color: #c678dd;">len</span>(c.servers); off++ {
<span style="color: #5B6268;">// </span><span style="color: #5B6268;">start from the preferred server</span>
<span style="color: #dcaeea;">id</span> := (prefer + off) % <span style="color: #c678dd;">len</span>(c.servers)
<span style="color: #51afef;">go</span> <span style="color: #51afef;">func</span>() {
done <- <span style="color: #ECBE7B;">result</span>{id, c.<span style="color: #c678dd;">callOne</span>(c.servers[id], arfs)}
}()
<span style="color: #5B6268;">// </span><span style="color: #5B6268;">now wait for either a done signal or a timeout</span>
<span style="color: #5B6268;">// </span><span style="color: #5B6268;">if it is done, don't send to other servers</span>
<span style="color: #5B6268;">// </span><span style="color: #5B6268;">otherwise, reset the timer and sends to the next server</span>
<span style="color: #51afef;">select</span> {
<span style="color: #51afef;">case</span> r = <-done:
<span style="color: #51afef;">goto</span> <span style="color: #a9a1e1;">Done</span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">use a goto if it makes code clear</span>
<span style="color: #51afef;">case</span> <-t.C:
<span style="color: #5B6268;">// </span><span style="color: #5B6268;">timeout</span>
t.<span style="color: #c678dd;">Reset</span>(timeout)
}
}
r = <-done <span style="color: #5B6268;">// </span><span style="color: #5B6268;">wait for the first reply even if it is a RPC timeout</span>
<span style="color: #a9a1e1;">Done</span>:
c.mu.<span style="color: #c678dd;">Lock</span>()
c.prefer = r.serverID <span style="color: #5B6268;">// </span><span style="color: #5B6268;">update preference</span>
c.mu.<span style="color: #c678dd;">Unlock</span>()
<span style="color: #51afef;">return</span> r.reply
}
</pre>
</div></li>
</ul>
</div>
</div>
<div id="outline-container-org29de31a" class="outline-2">
<h2 id="org29de31a"><span class="section-number-2">6.</span> Pattern 4: Protocol multiplexer</h2>
<div class="outline-text-2" id="text-6">
<ul class="org-ul">
<li><p>
A multiplexer sits in front of a service and forward messages between multiple clients and the service, e.g., an RPC.
</p>
<div class="org-src-container">
<pre class="src src-go"><span class="linenr"> 1: </span><span style="color: #51afef;">type</span> <span style="color: #ECBE7B;">ProtocolMux</span> <span style="color: #51afef;">interface</span> {
<span class="linenr"> 2: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">A mux is binded to a specific service</span>
<span class="linenr"> 3: </span> <span style="color: #c678dd;">Init</span>(<span style="color: #ECBE7B;">Service</span>)
<span class="linenr"> 4: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">A client uses this method to send message to the service</span>
<span class="linenr"> 5: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">and wait for the service reply</span>
<span class="linenr"> 6: </span> <span style="color: #c678dd;">Call</span>(<span style="color: #ECBE7B;">Msg</span>) <span style="color: #ECBE7B;">Msg</span>
<span class="linenr"> 7: </span>}
<span class="linenr"> 8: </span>
<span class="linenr"> 9: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">Methods that a service exposes to let a mux use</span>
<span class="linenr">10: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">Underlining messgae processing are in the implementation</span>
<span class="linenr">11: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">of the actual service struct</span>
<span class="linenr">12: </span><span style="color: #51afef;">type</span> <span style="color: #ECBE7B;">Service</span> <span style="color: #51afef;">interface</span> {
<span class="linenr">13: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">A tag is a muxing identifier in the request or reply message,</span>
<span class="linenr">14: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">e.g., which client channel to send the reply</span>
<span class="linenr">15: </span> <span style="color: #c678dd;">ReadTag</span>(<span style="color: #ECBE7B;">Msg</span>) <span style="color: #ECBE7B;">int64</span>
<span class="linenr">16: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">Send a request message to the service</span>
<span class="linenr">17: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">multiple sends cannot be called concurrently</span>
<span class="linenr">18: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">probably due to only a single channel between</span>
<span class="linenr">19: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">mux and the service (serialization)</span>
<span class="linenr">20: </span> <span style="color: #c678dd;">Send</span>(<span style="color: #ECBE7B;">Msg</span>)
<span class="linenr">21: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">Waits and return the reply message,</span>
<span class="linenr">22: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">multiple recvs cannot be called concurrently</span>
<span class="linenr">23: </span> <span style="color: #c678dd;">Recv</span>() <span style="color: #ECBE7B;">Msg</span>
<span class="linenr">24: </span>}
</pre>
</div></li>
<li><p>
The mux maintains a channel to queue unsent requests and a channel to queue unsent replies.
</p>
<div class="org-src-container">
<pre class="src src-go"><span class="linenr"> 1: </span><span style="color: #51afef;">type</span> <span style="color: #ECBE7B;">Mux</span> <span style="color: #51afef;">struct</span> {
<span class="linenr"> 2: </span> srv <span style="color: #ECBE7B;">Service</span>
<span class="linenr"> 3: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">stores unsent requests</span>
<span class="linenr"> 4: </span> send <span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">Msg</span>
<span class="linenr"> 5: </span> mu <span style="color: #ECBE7B;">sync.Mutex</span>
<span class="linenr"> 6: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">maps channel tag to channel</span>
<span class="linenr"> 7: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">whose replies have not been sent out</span>
<span class="linenr"> 8: </span> pending <span style="color: #51afef;">map</span>[<span style="color: #ECBE7B;">int64</span>]<span style="color: #51afef;">chan</span><- <span style="color: #ECBE7B;">Msg</span>
<span class="linenr"> 9: </span>}
<span class="linenr">10: </span>
<span class="linenr">11: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">m</span> *<span style="color: #ECBE7B;">Mux</span>) <span style="color: #c678dd;">Init</span>(<span style="color: #dcaeea;">srv</span> <span style="color: #ECBE7B;">Service</span>) {
<span class="linenr">12: </span> m.srv = srv
<span class="linenr">13: </span> m.pending = <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">map</span>[<span style="color: #ECBE7B;">int64</span>]<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">Msg</span>)
<span class="linenr">14: </span> <span style="color: #51afef;">go</span> m.<span style="color: #c678dd;">sendLoop</span>()
<span class="linenr">15: </span> <span style="color: #51afef;">go</span> m.<span style="color: #c678dd;">recvLoop</span>()
<span class="linenr">16: </span>}
<span class="linenr">17: </span>
<span class="linenr">18: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">sending out queued requests</span>
<span class="linenr">19: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">m</span> *<span style="color: #ECBE7B;">Mux</span>) <span style="color: #ECBE7B;">sendLoop</span> {
<span class="linenr">20: </span> <span style="color: #51afef;">for</span> <span style="color: #dcaeea;">args</span> := <span style="color: #51afef;">range</span> m.send {
<span class="linenr">21: </span> m.srv.<span style="color: #c678dd;">Send</span>(args)
<span class="linenr">22: </span> }
<span class="linenr">23: </span>}
<span class="linenr">24: </span>
<span class="linenr">25: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">m</span> *<span style="color: #ECBE7B;">Mux</span>) <span style="color: #c678dd;">recvLoop</span>() {
<span class="linenr">26: </span> <span style="color: #51afef;">for</span> {
<span class="linenr">27: </span> <span style="color: #dcaeea;">reply</span> := m.srv.<span style="color: #c678dd;">Recv</span>()
<span class="linenr">28: </span> <span style="color: #dcaeea;">tag</span> := m.srv.<span style="color: #c678dd;">ReadTag</span>(reply)
<span class="linenr">29: </span> m.mu.<span style="color: #c678dd;">Lock</span>()
<span class="linenr">30: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">get the reply channel</span>
<span class="linenr">31: </span> <span style="color: #dcaeea;">done</span> := m.pending[tag]
<span class="linenr">32: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">clear the channel since the message loop</span>
<span class="linenr">33: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">is complete</span>
<span class="linenr">34: </span> <span style="color: #c678dd;">delete</span>(m.pending, tag)
<span class="linenr">35: </span> m.mu.<span style="color: #c678dd;">Unlock</span>()
<span class="linenr">36: </span>
<span class="linenr">37: </span> <span style="color: #51afef;">if</span> done == <span style="color: #a9a1e1;">nil</span> {
<span class="linenr">38: </span> <span style="color: #c678dd;">panic</span>(<span style="color: #98be65;">"unexpected reply"</span>)
<span class="linenr">39: </span> }
<span class="linenr">40: </span> done <- reply
<span class="linenr">41: </span> }
<span class="linenr">42: </span>
<span class="linenr">43: </span>}
<span class="linenr">44: </span>
<span class="linenr">45: </span><span style="color: #5B6268;">// </span><span style="color: #5B6268;">Clients call this method concurrently</span>
<span class="linenr">46: </span><span style="color: #51afef;">func</span> (<span style="color: #dcaeea;">m</span> *<span style="color: #ECBE7B;">Mux</span>) <span style="color: #c678dd;">Call</span>(<span style="color: #dcaeea;">args</span> <span style="color: #ECBE7B;">Msg</span>) (<span style="color: #dcaeea;">reply</span> <span style="color: #ECBE7B;">Msg</span>) {
<span class="linenr">47: </span> <span style="color: #dcaeea;">tag</span> := m.srv.<span style="color: #c678dd;">ReadTag</span>(args)
<span class="linenr">48: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">to record which message should reply</span>
<span class="linenr">49: </span> <span style="color: #5B6268;">// </span><span style="color: #5B6268;">to which client</span>
<span class="linenr">50: </span> <span style="color: #dcaeea;">done</span> := <span style="color: #c678dd;">make</span>(<span style="color: #51afef;">chan</span> <span style="color: #ECBE7B;">Msg</span>, <span style="color: #da8548; font-weight: bold;">1</span>)
<span class="linenr">51: </span> m.mu.<span style="color: #c678dd;">Lock</span>()
<span class="linenr">52: </span> <span style="color: #51afef;">if</span> m.pending[tag] != <span style="color: #a9a1e1;">nil</span> {
<span class="linenr">53: </span> m.mu.<span style="color: #c678dd;">Unlock</span>()
<span class="linenr">54: </span> <span style="color: #c678dd;">panic</span>(<span style="color: #98be65;">"duplicate request"</span>)
<span class="linenr">55: </span> }
<span class="linenr">56: </span> m.pending[tag] = done
<span class="linenr">57: </span> m.mu.<span style="color: #c678dd;">Unlock</span>()
<span class="linenr">58: </span> m.send <- args
<span class="linenr">59: </span> <span style="color: #51afef;">return</span> <-done <span style="color: #5B6268;">// </span><span style="color: #5B6268;">hang until a reply is received</span>
<span class="linenr">60: </span>}
</pre>
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