A blog and website by Peter Bengtsson

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Concurrent download with hashin without --update-all

18 December 2018 0 comments   Python, Web development

Last week, I landed concurrent downloads in hashin. The example was that you do something like...

$ time hashin -r some/requirements.txt --update-all

...and the whole thing takes ~2 seconds even though it that some/requirements.txt file might contain 50 different packages, and thus 50 different lookups.

Just wanted to point out, this is not unique to use with --update-all. It's for any list of packages. And I want to put some better numbers on that so here goes...

Suppose you want to create a requirements file for every package in the current virtualenv you might do it like this:

# the -e filtering removes locally installed packages from git URLs
$ pip freeze | grep -v '-e ' | xargs hashin -r /tmp/reqs.txt

Before running that I injected a little timer on each download. It looked like this:

def get_package_data(package, verbose=False):
    url = "" % package
    if verbose:
+   t0 = time.time()
    content = json.loads(_download(url))
    if "releases" not in content:
        raise PackageError("package JSON is not sane")
+   t1 = time.time()
+   print(t1 - t0)

I also put a print around the call to pre_download_packages(lookup_memory, specs, verbose=verbose) to see what the "total time" was.

The output looked like this:

▶ pip freeze | grep -v '-e ' | xargs python -r /tmp/reqs.txt
SUM TOTAL TOOK 0.8481268882751465

If you sum up all the individual times it would have become 17.3 seconds. It's 43 individual packages and 8 CPUs multiplied by 5 means it had to wait with some before downloading the rest.

Clearly, this works nicely.

How I performance test PostgreSQL locally on macOS

10 December 2018 2 comments   PostgreSQL, MacOSX, Web development

It's weird to do performance analysis of a database you run on your laptop. When testing some app, your local instance probably has 1/1000 the amount of realistic data compared to a production server. Or, you're running a bunch of end-to-end integration tests whose PostgreSQL performance doesn't make sense to measure.

Anyway, if you are doing some performance testing of an app that uses PostgreSQL one great tool to use is pghero. I use it for my side-projects and it gives me such nice insights into slow queries that I'm willing to live with the cost that it is to run it on a production database.

This is more of a brain dump of how I run it locally:

First, you need to edit your postgresql.conf. Even if you used Homebrew to install it, it's not clear where the right config file is. Start psql (on any database) and type this to find out which file is the one:

$ psql kintobench
kintobench=# show config_file;
(1 row)

Now, open /usr/local/var/postgres/postgresql.conf and add the following lines:

# Peterbe: From Pghero's configuration help.
shared_preload_libraries = 'pg_stat_statements'
pg_stat_statements.track = all

Now, to restart the server use:

▶ brew services restart postgresql
Stopping `postgresql`... (might take a while)
==> Successfully stopped `postgresql` (label: homebrew.mxcl.postgresql)
==> Successfully started `postgresql` (label: homebrew.mxcl.postgresql)

The next thing you need is pghero itself and it's easy to run in docker. So to start, you need Docker for mac installed. You also need to know the database URL. Here's how I ran it:

docker run -ti -e DATABASE_URL=postgres://peterbe:@host.docker.internal:5432/kintobench -p 8080:8080 ankane/pghero

Duplicate indexes

Note the trick of peterbe:@host.docker.internal because I don't use a password but inside the Docker container it doesn't know my terminal username. And the host.docker.internal is so the Docker container can reach the PostgreSQL installed on the host.

Once that starts up you can go to http://localhost:8080 in a browser and see a listing of all the cumulatively slowest queries. There are other cool features in pghero too that you can immediately benefit from such as hints about unused/redundent database indices.

Hope it helps!

React 16.6 with Suspense and lazy loading components with react-router-dom

26 October 2018 0 comments   ReactJS, Javascript, Web development

If you're reading this, you might have thought one of two thoughts about this blog post title (or both); "Cool buzzwords!" or "Yuck! So much hyped buzzwords!"

Either way, React v16.6 came out a couple of days ago and it brings with it React.lazy: Code-Splitting with Suspense.

React.lazy is React's built-in way of lazy loading components. With Suspense you can make that lazy loading be smart and know to render a fallback component (or JSX element) whilst waiting for that slowly loading chunk for the lazy component.

The sample code in the announcement was deliciously simple but I was curious; how does that work with react-router-dom??

Without furher ado, here's a complete demo/example. The gist is an app that has two sub-components loaded with react-router-dom:

  <div className="App">
      <Route path="/" exact component={Home} />
      <Route path="/:id" component={Post} />

The idea is that the Home component will list all the blog posts and the Post component will display the full details of that blog post. In my demo, the Post component never bothers to actually do the fetching of the full details to display. It just displays the passed in ID from the react-router-dom match prop. You get the idea.

That's standard React with react-router-dom stuff. Next up, lazy loading. Basically, instead of importing the Post component, you make it lazy:

-import Post from "./post";
+const Post = React.lazy(() => import("./post"));

And here comes the magic sauce. Instead of referencing component={Post} in the <Route/> you use this badboy:

function WaitingComponent(Component) {
  return props => (
    <Suspense fallback={<div>Loading...</div>}>
      <Component {...props} />

Complete prototype

The final thing looks like this:

import React, { lazy, Suspense } from "react";
import ReactDOM from "react-dom";
import { MemoryRouter as Router, Route, Switch } from "react-router-dom";

import Home from "./home";
const Post = lazy(() => import("./post"));

function App() {
  return (
      <div className="App">
          <Route path="/" exact component={Home} />
          <Route path="/:id" component={WaitingComponent(Post)} />

function WaitingComponent(Component) {
  return props => (
    <Suspense fallback={<div>Loading...</div>}>
      <Component {...props} />

const rootElement = document.getElementById("root");
ReactDOM.render(<App />, rootElement);

(sorry about the weird syntax highlighting with the red boxes.)

And it totally works! It's hard to show this with the demo but if you don't believe me, you can download the whole codesandbox as a .zip, run yarn && yarn run build && serve -s build and then you can see it doing its magic as if this was the complete foundation of a fully working client-side app.

1. Loading the "Home" page, then click one of the links

Loading the "Home" page

2. Lazy loading the Post component

Lazy loading the Post component

3. Post component lazily loaded once and for all

Post component lazily loaded once and for all


One thing that can happen is that you might load the app when the Wifi is honky dory but when you eventually make a click that causes a lazy loading to actually need to go out on the Internet and download that .js file it might fail. For example, because the file has been removed from the server or your network just fails for some reason. To deal with that, simply wrap the whole <Suspense> component in an error boundary component.

See this demo which is a fork of the main demo but with error boundaries added.

In conclusion

No surprise that it works. React is pretty awesome. I just wasn't sure how it would look like with react-router-dom.

A word of warning, from the v16.6 announcement: "This feature is not yet available for server-side rendering. Suspense support will be added in a later release."

I think lazy loading isn't actually that big of a deal. It's nice that it works but how likely is it really that you have a sub-tree of components that is so big and slow that you can't just pay for it up front as part of one big fat build. If you really care about a really great web performance for those people who reach your app rarely and sporadically, the true ticket to success is server-side rendering and shipping a gzipped HTML document with all the React client-side code non-blocking rendering so that the user can download the HTML, start reading/consuming it immediately and then whilst the user is doing that you download the rest of the .js that is going to be needed once the user clicks around. Start there.

How much HTML is too much for optimal web performance

17 October 2018 4 comments   Web Performance, Web development

Right off the bat; I don't know. All I know is that it's complicated.

I have this page which is just a blog post page. It's entirely rendered on the server, comments and all. At the time of writing, the total size of the HTML document is 119KB (30KB gzipped). If you remove all the comments, which makes up the bulk of the HTML it reduces down to 31KB (7KB gzipped). Fair enough. That's 23KB less to download. But, does it matter (much)?


First of all, I noticed this:

WebPagetest with iPhone 6, 4G on the same US coast as the datacenter

That's a WebPagetest using iPhone 6 on 4G and, lemme emphasize this, it took 126ms to download the HTML document . If you subtract "DNS Lookup" (283ms), "Initial Connection" (1013ms), and "SSL Negotiation" (733ms) it took 684ms serve the file, download it, and parse it. Remember, this is all on 4G. Pretty fast. In conclusion, it's probably not too much HTML in that page to download. This downloadingness is fraction of the total "web performance cost". Let's dig deeper.

Note! With WebPagetest all those numbers like DNS Lookup, Initial Connection and SSL Negotiation are wildly unpredictable between tests. Chances are, the numbers are very different the next time you run a test using the exact same input. Who knows. Deep internet plumbings beyond the control of WebPagetest.

Note! I ran it one more time with the exact same parameters and this time it was 535ms (instead of 684ms) to serve, download, and parse.

Parsing & layout

Parsing is hard to measure but here's what I found when using the Google Chrome dev tools:

Google Chrome Performance devtools
Google Chrome Performance devtools

It says it took...

That's half a second just loading and rendering. Definitely sucks. But note, this test uses 4x CPU slowdown and 3G simulation. So perhaps it's not so bad.

Let's try again with a smaller HTML document

So I butchered up a hybrid version that has almost the same HTML except all but 1 of those 166'ish div.comment DOM nodes. It's now 31KB (7KB gzipped´) to download instead of 119KB (30KB gzipped).

Same WebPagetest parameters but now this this smaller HTML document:

WebPagetest with a much smaller HTML footprint
WebPagetest with a much smaller HTML footprint

Now it says it only took 39ms to download and 232ms (it was 684ms before) to serve the file, download it, and parse it. Interesting!

Note! I ran it one more time with exact same parameters and this time it was 237ms (instead of 232ms) to serve, download, and parse.

Clearly it's working. The smaller the HTML document the faster it performs. No surprise. But stick around for the conclusion.

Parsing & layout with a smaller HTML document

Check this out:

Google Chrome Performance devtools (smaller HTML document)
Google Chrome Performance devtools (smaller HTML document)

It says it took...

Mind you, all of these numbers are at the mercy of what my laptop is up to at the moment as it can affect Chrome's rendering if it has, at that moment, less (or more) access to CPU and memory caching.

Either way, it parses + layout in 126ms instead of 523ms for the larger HTML document.


The best test to see how much faster the smaller HTML document variant is, is to compare them side-by-side. It looks like this:

Visual comparison on WebPagetest (using 4G)

Two major takeaways from this:

  1. The smaller HTML version starts rendering half a second before the original one.
  2. The complete time favors the smaller HTML version by 2.5 seconds but that's possibly influenced by the ads that load more than any slow layout rendering.
  3. This is using 4G which isn't unheard of but definitely much less common than better speeds.

Here they are compared on "Desktop" which appears to give the smaller HTML version a 0.2 second advantage:

Visual comparison on WebPagetest (using "Desktop")
Visual comparison on WebPagetest (using "Desktop")

And here are the Lighthouse reports side-by-side:

Side-by-side using Lighthouse


The above concludes rather unsurprisingly that a smaller HTML footprint downloads, parses and lays out quicker.

The killer reason that page is so large, with all those comments rendered in the original HTML is simple: SEO. Google loves comments because comments indicate that the page is thriving and a place where people go, spend time, and stick around. I've experimented with this in the past and found that if I make the HTML document smaller (or loading the rest after document load) the SEO takes a big hit. Yes, Google's bot renders with JavaScript but not always and even if it does, I assume it's smart enough to appreciate that content that is loaded (async or post-DOMContentLoad) is less important and thus not what the page is about.

Regarding SEO, we know that Google loves fast sites. Especially for mobile. But content is still king my gut tells me. Left as an exercise to the reader to take a stand on this.

Another problem with lazy loading the comments (or whatever else might be applicable to your site) is that it might cause "flicker". I put that word in quote because sometimes flicker is literally visual flicker and sometimes it's moments of browser sluggishness. The XHR request and the subsequent post-rendering will cause a bunch of work that strains the browser and might make it unpleasant when your eyes and brain is in the midst of committing to consuming it.

Basically, there are significant real benefits of not trying to squeeze every little millisecond out by making the HTML smaller upfront. Remember the fact that the "smaller HTML" version in this test is drastic. I butchered it from 119KB to 31KB which might be so drastic that it's not necessarily applicable at all. In other words, had I reduced the HTML size by just 20% it might not even register on the performance graph but could be significant in terms SEO keywords.


The majority of the time spend making a web page useful to a user is a sum of all sorts of metrics. The size of the HTML document does matter but remember that it's just one of multiple aspects to watch out for.

In conclusion, it's complicated and depends on your needs and context. I hope you can benefit a little bit from the metrics above.

Fancy linkifying of text with Bleach and domain checks (with Python)

10 October 2018 2 comments   Web development, Python

Bleach is awesome. Thank you for it @willkg! It's a Python library for sanitizing text as well as "linkifying" text for HTML use. For example, consider this:

>>> import bleach
>>> bleach.linkify("Here is some text with a")
'Here is some text with a <a href="" rel="nofollow"></a>.'

Note that sanitizing is separate thing, but if you're curious, consider this example:

>>> bleach.linkify(bleach.clean("Here is <script> some text with a"))
'Here is &lt;script&gt; some text with a <a href="" rel="nofollow"></a>.'

With that output you can confidently template interpolate that string straight into your HTML.

Getting fancy

That's a great start but I wanted a more. For one, I don't always want the rel="nofollow" attribute on all links. In particular for links that are within the site. Secondly, a lot of things look like a domain but isn't. For example This is a the start which would naively become...:

>>> bleach.linkify("This is a the start")
'This is a <a href="" rel="nofollow"></a> the start'

...because looks like a domain.

So here is how I use it here on to linkify blog comments:

def custom_nofollow_maker(attrs, new=False):
    href_key = (None, u"href")

    if href_key not in attrs:
        return attrs

    if attrs[href_key].startswith(u"mailto:"):
        return attrs

    p = urlparse(attrs[href_key])
    if p.netloc not in settings.NOFOLLOW_EXCEPTIONS:
        # Before we add the `rel="nofollow"` let's first check that this is a
        # valid domain at all.
        root_url = p.scheme + "://" + p.netloc
            response = requests.head(root_url)
            if response.status_code == 301:
                redirect_p = urlparse(response.headers["location"])
                # If the only difference is that it redirects to https instead
                # of http, then amend the href.
                if (
                    redirect_p.scheme == "https"
                    and p.scheme == "http"
                    and p.netloc == redirect_p.netloc
                    attrs[href_key] = attrs[href_key].replace("http://", "https://")

        except ConnectionError:
            return None

        rel_key = (None, u"rel")
        rel_values = [val for val in attrs.get(rel_key, "").split(" ") if val]
        if "nofollow" not in [rel_val.lower() for rel_val in rel_values]:
        attrs[rel_key] = " ".join(rel_values)

    return attrs

html = bleach.linkify(text, callbacks=[custom_nofollow_maker])

This basically taking the default nofollow callback and extending it a bit.

By the way, here is the complete code I use for sanitizing and linkifying blog comments here on this site: render_comment_text.


This is slow because it requires network IO every time a piece of text needs to be linkified (if it has domain looking things in it) but that's best alleviated by only doing it once and either caching it or persistently storing the cleaned and rendered output.

Also, the check uses try: requests.head() except requests.exceptions.ConnectionError: as the method to see if the domain works. I considered doing a whois lookup or something but that felt a little wrong because just because a domain exists doesn't mean there's a website there. Either way, it could be that the domain/URL is perfectly fine but in that very unlucky instant you checked your own server's internet or some other DNS lookup thing is busted. Perhaps wrapping it in a retry and doing try: requests.head() except requests.exceptions.RetryError: instead.

Lastly, the business logic I chose was to rewrite all http:// to https:// only if the URL http://domain does a 301 redirect to https://domain. So if the original link was it leaves it as is. Perhaps a fancier version would be to look at the domain name ending. For example HEAD 301 redirects to so you could use the fact that "".endswith("").

UPDATE Oct 10 2018

Moments after publishing this, I discovered a bug where it would fail badly if the text contained a URL with an ampersand in it. Turns out, it was a known bug in Bleach. It only happens when you try to pass a filter to the bleach.Cleaner() class.

So I simplified my code and now things work. Apparently, using bleach.Cleaner(filters=[...]) is faster so I'm losing that. But, for now, that's OK in my context.

Also, in another later fix, I improved the function some more by avoiding non-HTTP links (with the exception of mailto: and tel:). Otherwise it would attempt to run requests.head('ssh://') which doesn't make sense.

Inline scripts in create-react-app 2.0 and CSP hashes

05 October 2018 0 comments   ReactJS, Javascript, Web development


My understanding of how to generate the CSP nonces was wrong. What I initially posted was a confusion between nonces and hashes. Sorry. The blog post has been updated to use hashing.


Shortly after publishing this I changed my mind entirely. I decided I don't want any inline scripts no matter how small. Reasons are: 1) with HTTP2 it's cheap to send another file and thus that critical precious first HTML document becomes smaller and 2) when you load it as an external you have the power to load it async if it's applicable.

Check out this new script, it's hackish but works: uninline_scripts.js

UPDATE (Oct 18, 2018)

If you use INLINE_RUNTIME_CHUNK=false yarn run build no scripts, independent of size, are inlined. See this pull request for details.


I have an app that is hosted on github-pages and because I can't control Content Security Policy HTTP headers I have to do it with a <meta http-equiv="Content-Security-Policy" content="${csp}"> tag in the HTML. That's working fine and the way I do it is that I have a script that looks like this:

#!/usr/bin/env node
const fs = require("fs");
const crypto = require("crypto");

const CSP_TEMPLATE = `
default-src 'none';
connect-src 'self';
img-src 'self' https://*;
script-src 'self'%SCRIPT_HASHES%;
style-src 'self' 'unsafe-inline';
font-src 'self' data:;
manifest-src 'self'

const htmlFile = process.argv[2];
if (!htmlFile) throw new Error("missing file argument");
let html = fs.readFileSync(htmlFile, "utf8");

let hashes = "";
let csp = CSP_TEMPLATE;
const matches = html.match(/<script>.*<\/script>/g);
if (matches) {
  matches.forEach(scriptTag => {
    const hash = crypto.createHash("sha256");
    hash.update(scriptTag.replace(/<script>/, "").replace("</script>", ""));
    const digest = hash.digest("hex");
    hashes += ` 'sha256-${digest.toString("base64")}'`;
csp = csp.replace(/%SCRIPT_HASHES%/, hashes);

const metatag = `
  <meta http-equiv="Content-Security-Policy" content="${csp}">
  .replace(/\n/g, "")
if ( > -1)
  throw new Error("already has CSP metatag in HTML");
const anchor = '<meta charset="utf-8">';
const newHtml = html.replace(anchor, `${anchor}${metatag}`);
fs.writeFileSync(htmlFile, newHtml, "utf8");

Laugh all you like at my hurried node scripting but it works. It finds any <script>ANYTHING</script> tags (which means it disregards any <script src="... tags), calculates a sha256 hash string out of it and then puts that into the CSP block.

The output becomes something like this:

<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8">
      content="default-src 'none';script-src 'self' 'sha256-bb84aa7f904e73495b9e99f08531053f3a86f3c1b2e232e3abbac252bf723f1f';">

I don't know if I've done it right but at least what didn't use to work now works; the page loads in my browsers now.