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slides.md
126
slides.md
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@ -68,9 +68,10 @@ layout: fact
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<!--
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- Disclaimer: This isn't a general performance talk
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- Optimising Python, Django and Wagtail are their own topics
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- Caching can be a part of optimisation
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- Big topics!
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- Caching can be a part of optimisation
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- But it's not the only part
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- It's not a replacement for proper benchmarking and investigations
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- It's not a replacement for proper benchmarking and investigations
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-->
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---
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@ -83,11 +84,11 @@ layout: section
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- What is caching?
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- Caching lets your code remember things it's already done
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- Reuse a value you've already done the work to calculate
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- It's always slower to do something than nothing
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- Because it's always slower to do something than nothing
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- Caching comes in many different forms
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- From storing a variable outside a loop
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- To caching functions
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- To caching entire pages with a CDN
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- To caching entire pages
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- Mostly focusing on the higher end today
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- The higher up the list you are, the more smaller processes you're caching
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- And the more performance you can get
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@ -123,11 +124,12 @@ h1 {
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- Caching may not seem needed at some scales
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- Small sites will probably be _fine_ without it
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- As your site grows, it becomes more important
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- Crash under your own weight and success
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- Otherwise you'll crash under your own weight and success
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- But, you may not notice you need it until it's already an issue
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- What runs fine locally may not in production
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- A problem with so many things at scale
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- If a page gets popular, it'll soon meet with the hug of death
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- Where it gets overload by its on popularity
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- Or even if it's not that popular, just shared around a bit
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- Like the recently-discovered Mastodon issue
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-->
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@ -140,6 +142,7 @@ layout: section
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<!--
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- Now, back to Wagtail
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- This is Wagtail Space, after all
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- Let's talk about 2 techniques to help you out with caching
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- Both are included in all maintained versions of Wagtail
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- So you can use them right now!
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@ -158,8 +161,8 @@ layout: section
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<!--
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- Template fragment caching
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- New ish in 5.2
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- Lots of complex words, but quite a simple concept
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- Introduced ish in 5.2
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-->
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---
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@ -203,14 +206,17 @@ class: p-0 m-0 bg-black
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<!--
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- Most of what you build in Wagtail will come out as HTML
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- Pages to be shown by the browser
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- But may take lots of intensive actions to create it
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- But probably takes lots of intensive actions to create it
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- Complex lists / filters or suggestions
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- Finding lots of renditions
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- Rendering streamfield blocks etc
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- Rendering streamfield blocks
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- etc etc
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- These can all be optimised in many different ways
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- The deeper down you go, the more complex and often less impactful those optimisations usually are
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- The deeper down you go, the more complex and often less impactful those optimisations are
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- Why not cache an entire chunk of template?
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- With all of that complexity contained within it
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- Rather than cache the name of a page for the navbar
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- Cache the entire navbar
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- That's the thing people see (sort of)
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-->
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@ -227,6 +233,7 @@ background: /website-homepage.png
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- An example: My website
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- Subtle self-promotion, I know
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- It's a relatively simple blog
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- Currently built with Wagtail
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-->
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---
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@ -235,7 +242,7 @@ image: /website-posts.png
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---
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<!--
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Here's a list of posts
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- Here's a list of posts
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- Each item has a lot to it
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- Title
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- Date
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@ -327,11 +334,15 @@ layout: none
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- Makes them easier to work with
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- And makes fragment caching much easier too
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- This has no caching at all
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- [click]Let's add some
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- Every time I used the template, the template is evaluated
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- All the template tags are run
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- [click]Let's cache it
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- Nice and simple
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- It's just 2 lines of change[click]
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- The tag might look a bit weird
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- The tag might look a bit strange
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- It's not an inclusion or a filter
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- It's a fully custom template tag
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- Let's look at its structure
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- A name: To uniquely identify this template fragment from others
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- A TTL: To determine how long the cache is valid for
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- Whatever variables the tag needs
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@ -342,6 +353,7 @@ layout: none
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- They'll be cached too
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- [click]The template is in charge of caching itself
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- Keep the complexities hidden, and in 1 place
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- Pages using it don't need to worry about it too much
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-->
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---
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@ -353,11 +365,14 @@ class: text-right
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# What about<br>search?
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<!--
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This is a component, it's used in a few different places
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- The 'listing item' is a component, it's used in a few different places
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- There's a posts list, and a search
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- And a few others
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- But the UI for each item is the same
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- It takes the same context: A page, and creates the same HTML
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- It takes the same context: A page
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- And creates the same HTML
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- If it looks the same, just on different pages, can I reuse it?
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- Including its cache?
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- Yes!
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-->
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@ -393,6 +408,7 @@ image: /website-search.png
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<!--
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- Caching HTML blocks has a few nice benefits over caching the whole page
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- For example, a search page
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- Searches are still performed live, but the display of the results are cached
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- Published pages appear immediately, without needing to invalidate a cache
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-->
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@ -436,13 +452,16 @@ code {
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</style>
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<!--
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If I change a page, I want the content to update
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- Cache invalidation is one of the 2 hardest problems in computer science
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- Alongside naming things and off-by-one errors
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- If I change a page, I want the content to update
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- How does it know when to regenerate the HTML?
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- [click]`page.cache_key`
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- Custom attribute on all your pages
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- [click]You can use it for other things if you need it
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- If a change is published, or the page is moved, the cache key changes, so the existing cache is ignored
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- And the old cached item will eventually expire
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- If that's not enough, and you want to manually clear the cache, you can do that too
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-->
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---
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@ -461,31 +480,37 @@ layout: center
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```jinja
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{% load wagtail_cache %}
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{% wagtailcache 500 sidebar request.user.username %}
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{% wagtailpagecache 500 sidebar request.user.username %}
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.. sidebar for logged in user ..
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{% endwagtailpagecache %}
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```
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```jinja
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{% load wagtail_cache %}
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{% wagtailcache 500 sidebar request.user.username page.cache_key site.id %}
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.. sidebar for logged in user ..
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{% endwagtailcache %}
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```
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````
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<v-click>
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- Current page
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- Current site
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- Skip in preview
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</v-click>
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<!--
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- This might sound familiar
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- Django's `{% cache %}`?
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- This might sound familiar to some
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- Django's `{% cache %}` tag
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- This isn't new, Django has had a cache tag for a very long time
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- In fact, 2007
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- But it's dangerous to use it with Wagtail unmodified
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- Preview content can get cached
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- Per-site settings cache the wrong value
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- [click]It's only a few small changes to use Wagtail's
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- [click]Which wraps this tag, but is aware of the current page and site automatically
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- A host of other weird bugs
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- [click]It's only a few small changes to use Wagtail's `wagtailpagecache`
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- Which wraps this tag, but is aware of the current page and site automatically
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- If you modify the page, the cache is automatically invalidated
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- If you use the on different sites, they're cached separately
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- If that's not enough, and you want something more manual
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- [click]There's `wagtailcache`
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- This doesn't automatically use the page and site
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- But _does_ ignore preview requests
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- Making it a closer analogue for Django's `cache` tag
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-->
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---
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@ -496,6 +521,7 @@ layout: section
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# Frontend Caching
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<!--
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- Part 2
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- Frontend caching
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- You can spend ages optimising a request
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- Caching common chunks of HTML
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@ -520,7 +546,7 @@ flowchart LR
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- What if we put something in front of Wagtail to serve the requests instead
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- A CDN (eg Cloudflare, CloudFront)
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- They cache the page, and serve it themselves
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- A CDN is generally helping you in 2 main ways:
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- A CDN generally helps you in 2 main ways:
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-->
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---
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@ -562,15 +588,16 @@ flowchart TD
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```
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<!--
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Serve a static copy of your website, so your servers aren't processing every request
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- Handle many more requests concurrently
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- Firstly: Serve a static copy of your website, so your servers aren't processing every request
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- Allows you to handle many more requests concurrently
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- Cloudflare handles 50 million requests per second globally (on average)
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- Your servers can't
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- Handle more traffic with fewer servers
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- Imagine it like converting your site to be static
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- 0 Database requests
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- 0 Database queries
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- 0 templates used
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- 0 cache hits
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- 0 lines of code
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-->
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---
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@ -580,7 +607,7 @@ class: bg-white
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---
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<!--
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- Store said cached copy closer to your users
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- Secondly: Store said cached copy closer to your users
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- Reduced latency
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- Makes your site seem faster
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- Even helps on your Google Lighthouse scores
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@ -595,12 +622,6 @@ background: /wagtail-homepage.png
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## Example
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# https://wagtail.org
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<style>
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code {
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background-color: transparent !important;
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}
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</style>
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<!--
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- Let's look at an example: Wagtail.org
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-->
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@ -614,6 +635,7 @@ transition: fade
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<!--
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- Wagtail.org is hosted in Dublin, Ireland (AWS `eu-west-1`)
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- But I'm in Arnhem
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- Quite far away
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-->
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---
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background: /map.jpg
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transition: fade
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---
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## Arnhem → Ireland
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# ~XXms
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@ -633,6 +656,7 @@ h1 {
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<!--
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- XXms away
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- But responses are being served quickly
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- Page loads take just XXms
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- That's the CDN (Cloudflare, in our case)
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-->
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<!--
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- This was actually served from XXX
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- Just XXms to get a response
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- Just XXms from me
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- It's closer to us
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- Quicker
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- Closer to us
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-->
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---
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@ -682,9 +706,12 @@ flowchart LR
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Let's cache our content!
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- Put a caching layer between users and the site
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- Requests are sent via cache
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- Usually just adding a DNS record
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- It's always DNS
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- If they're cached, return them
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- Otherwise, get the actual content from Wagtail
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- Ensure the CDN knows how long to cache pages for
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- Somewhere between "Forever" and "1 second"
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- An hour is probably fine
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- Lots of different options
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- Cache headers
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- Configuration in the CDN itself
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- There's another talk about this
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@ -719,7 +746,7 @@ flowchart LR
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- Make sure you skip caching authenticated requests (ie the admin)
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- These pages are dynamic based on who is viewing them, so we can't cache them easily
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- Wagtail does its best to make sure the admin is fast anyway
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- May need to include the session id as part of the cache key
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- Exactly how to do this will again depend on your CDN
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-->
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---
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@ -755,6 +782,7 @@ flowchart LR
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- Server load should drop drastically
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- Might even be able to scale down
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- Wagtail.org serves ~70% of all page loads from the CDN
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- And with some work we could probably get that higher
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-->
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---
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- The whole point of it is it doesn't need to hit Wagtail for every request
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- So it can't know
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- The cache will expire _eventually_
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- After an hour or so
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- What if it needs to go out sooner
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- What if it's more serious than just a typo
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- What if it's more serious than just a typo
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-->
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---
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@ -887,7 +916,7 @@ code {
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<!--
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- Integrating is simple
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- [click]Add entry to `INSTALLED_APPS`
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- [click]dd credentials and configuration
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- [click]Add configuration and credentials
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- Wagtail supports quite a few backends!
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- [click]That's it!
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- All the hooks into all the right places are set up for you!
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@ -920,7 +949,7 @@ flowchart LR
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- When you publish a page, it'll automatically be purged
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- It can take a minute or so, depending on your CDN
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- Usually much faster than that
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- Then when a user loads the page, it's cached, and subsequent loads go back to being lightning fast
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- Then when a user loads the page from Wagtail, it's cached, and subsequent loads go back to being lightning fast
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-->
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---
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@ -934,6 +963,9 @@ background: https://d1nvwtjgmbo5v5.cloudfront.net/media/images/Screen_Shot_2015-
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- When most people think of "performance", they tend to think "database queries"
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- Which is true, that's where a lot of time goes
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- But there's more to optimising a site than sprinkling `select_related` / `prefetch_related`
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- "Performance" means lots of things to different people
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- Your application is made up of lots of different components
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- So too must your performance investigations
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- It's faster to do less than to do more
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- It's faster still to do nothing than something
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- The higher up the stack your cache is, the more impactful it can be
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