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slides.md
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slides.md
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@ -10,7 +10,7 @@ themeConfig:
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# Recovering [deleted]{style="color: #fd5765"} Wagtail pages and/or Django models
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### Jake Howard{style="color: #e85537;" .mt-10 }
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### Jake Howard{.mt-10}
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<ul class="list-none! text-sm [&>li]:m-0! mt-1 uppercase">
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<li>Senior Systems Engineer @ Torchbox</li>
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@ -31,6 +31,22 @@ background: /intranet.png
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# Setting the scene
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<!--
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- People usually use Wagtail as a website or blog
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- But it works really well as an intranet too
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- At Torchbox, we use it for internal documentation ("intranet")
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- Processes
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- Company information
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- Links to other places etc
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- Been around for a while
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- In 2022, we restructured the content
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- Make it easier to find things
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- Remove duplication
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- This didn't quite go to plan
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- One afternoon, I was looking to reference a process, and couldn't find it
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- Turns out, the entire "Sysadmin" section had completely vanished
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-->
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---
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layout: cover
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background: /site-history.png
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@ -38,24 +54,56 @@ background: /site-history.png
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# Site history report
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<!--
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- First step: Understanding what happened
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- The site history report!
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- Fortunately, Wagtail showed _almost_ exactly what had happened, and what I expected
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- One staff member deleted the "Sysadmin" section a few days before
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- Which deleted every page under it, all 105 of them
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- "Radical reorganisation"
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-->
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---
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layout: image
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image: /chat.png
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backgroundSize: contain
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---
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<!--
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- I messaged the person, to better understand what happened
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- Assuming they didn't mean to delete all that content
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- Hanlon's Razor
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- They'd made a new "Sysadmin" section a while ago, before switching strategy to move pages in the existing tree
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- They then deleted the wrong one
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- Sure, Wagtail shows a confirmation when you're deleting pages, but when you're deleting a lot of pages, and expecting to delete pages, you might not read the message perfectly
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- With the content gone, I had to restore from backups.
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-->
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---
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layout: section
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---
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# Restoring from backups
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<!--
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- Our intranet is a living document, it gets updated fairly often
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- Rolling back the entire system almost 2 days would have meant potentially losing critical changes
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- Not to mention people's time they spent making the changes
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- It'd be annoying, but we _could_ do it, but I'd rather another solution
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-->
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---
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layout: section
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---
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# _Partially_ restoring from backups
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<!--
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- Ideally, what I needed was to restore only the sysadmin pages, leaving all others completely untouched.
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- Using a few tricks of Django and Wagtail internals, it's absolutely possible, and we did it
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- With 0 downtime, too!
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-->
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---
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layout: section
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---
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@ -63,6 +111,11 @@ layout: section
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## 1.
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# Spin up a database backup
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<!--
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- We backup our intranet nightly, so I downloaded a backup from before the incident
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- Start the codebase locally so I can interrogate it
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-->
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---
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layout: section
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---
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@ -82,6 +135,12 @@ child_pages = sysadmin_page.get_descendants()
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</div>
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<!--
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- Behind the scenes, Wagtail pages are a tree, implemented using `django-treebeard`.
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- When a page is deleted, treebeard is the one who finds all the child pages and deletes them too
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- And then Django and postgres deal with cascading the delete
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-->
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---
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layout: section
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---
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@ -100,6 +159,20 @@ collector.collect(list(child_pages) + [sysadmin_page])
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</div>
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<!--
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This is where the magic happens
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- Deleting a page deletes more than just a page
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- The specific model
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- Revisions
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- Related models
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- Through tables
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- `get_descendants` won't get all those
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- Calling `.delete` gives you the number of objects, and it's quite a lot
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- If you've ever used the Django admin, you know it's capable of finding every model instance before a delete
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- That's implemented with an undocumented but simple to use API
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- Yes, that's really it. It doesn't delete the models, it just tells us what _would_ be if we triggered a delete.
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-->
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---
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layout: section
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---
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@ -109,7 +182,7 @@ layout: section
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<div class="pt-5 text-left">
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```python {all|3-5|all}
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```python {all|3-5}
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from django.core import serializers
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class NoM2MSerializer(Serializer):
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@ -136,6 +209,20 @@ with open("deleted-models.json", "w") as f:
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}
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</style>
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<!--
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- `collector.data` now contains all the model instances which were deleted, in memory on my laptop
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- My laptop isn't what's running production
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- Need to serialize the models into an intermdiary format which can be then be loaded onto production
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- If you're thinking of fixtures, you're right
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- Django's fixtures create a JSON representation of a model, so they can be saved in 1 location and loaded into another
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- Mostly useful for complex test fixtures (hence the name), but generally useful for cases like this
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- [click]`NoM2MSerializer` is a bit special
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- When Django serializes a model with a m2m which doesn't use a custom table, it inlines the definition, because it's easier to work with
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- However, `NestedObjects` still finds these through tables, and tries to load them separately
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- Resulting in duplicate objects and referential integrity issues
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- Instead, we exclude them
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-->
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---
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layout: section
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---
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@ -145,6 +232,10 @@ layout: section
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### `manage.py loaddata`
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<!--
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- We have a JSON file, the inverse is just `manage.py loaddata`
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-->
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---
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layout: center
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---
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@ -186,6 +277,10 @@ with open("deleted-models.json", "w") as f:
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}
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</style>
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<!--
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- If we combine it all together, this is the big script we end up with
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-->
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---
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layout: fact
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---
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@ -193,6 +288,16 @@ layout: fact
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### 5.
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# **Test!**
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<!--
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- For what I hope are obvious reasons, this needed to be tested!
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- I deleted the page through the wagtail admin locally, and then restored them to confirm they're all the same
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- I'm glad I did, because there was an issue: Search indexes
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- The search index objects (we use postgres) were picked up by `NestedObjects`
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- They didn't like being restored
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- So I skipped them and moved on, knowing I'd just rebuild the index later.
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- `manage.py fixtree` also reports any tree issues, which there weren't
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-->
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---
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layout: image-right
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image: /red-button.png
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@ -202,18 +307,29 @@ class: flex justify-center flex-col items-center
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### 6.
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# Showtime!
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---
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layout: image-right
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image: https://images.unsplash.com/photo-1622021134395-d26aab83c221?q=80&w=1470&auto=format&fit=crop&ixlib=rb-4.0.3&ixid=M3wxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8fA%3D%3D
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class: flex justify-center flex-col text-xl
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---
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<v-clicks>
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1. Backup!
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2. Transfer `deleted-models.json` to server
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3. `loaddata`
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4. `checktree`
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5. `update_index`
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6. `rebuild_reference_index`
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1. Backup! ✅
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2. Send `deleted-models.json` to server ✅
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3. `loaddata` ✅
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4. `checktree` ✅
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5. `update_index` ✅
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6. `rebuild_reference_index` ✅
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</v-clicks>
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<!--
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- The tense bit
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- Once I was happy, I ran the same steps on production
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- Our intranet runs on Heroku, so I had to do a few dances to get the JSON file up there.
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- [click]Before I began, I did a backup, because I'm a good sysadmin
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- [click]With the data file in place, [click]I crossed everything and ran `loaddata`
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- Pages popped up in the admin as if they never left
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- [click]`checktree` worked.
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- [click]`update_index` worked.
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- [click] As did `rebuild_reference_index`
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- The new pages were now findable
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-->
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---
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layout: cover
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@ -222,8 +338,20 @@ background: /sysadmin.png
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# Conclusion
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<!--
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- With a few hours work, the pages were back
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- There was no downtime
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- No content freeze
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- No data loss
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- Most people didn't even know there was an issue
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- I've used this trick a a few times in my career, for both Wagtail and plain Django sites
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- Ironically, just a few weeks after the blog post was published
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- Works identically for Django sites, so long as you know how to reconstruct the delete query.
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- Hopefully this helps you out as much as it has me!
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-->
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---
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layout: end
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---
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END
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https://wagtail.org/blog/recovering-deleted-pages-and-models/
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