SvelteKit demo app 'todos' failing on delete

[JBusillo]

Describe the bug
The SvelteKit demo app seems to have a bug when deleting 'todos'. It occurs when multiple todos are added and subsequently deleted.

Logs

--- Browser log
Failed to load resource: the server responded with a status of 500 (Internal Server Error)
{"status":500,"message":"Error deleting todo"}

Server log has no error messages

To Reproduce

• Create a new SvelteKit demo app (npm init svelte@next app-dir), accepting all the defaults
• cd app-dir
• npm i
• npm run dev -- --open
• Open devtools

Perform the following steps (You may experience the error prior to completing all steps):

• Navigate to TODOS, enter 3 TODOS.
• Delete first TODO
• Delete last TODO
• Delete remaining TODO
• Enter 3 TODOs again
• Delete first TODO
• Delete last TODO

Expected behavior
TODO's are deleted when requested

Diagnostics

System:
    OS: Linux 5.8 Ubuntu 20.04.2 LTS (Focal Fossa)
    CPU: (6) x64 Intel(R) Core(TM) i5-8400 CPU @ 2.80GHz
    Memory: 25.99 GB / 31.04 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Node: 14.17.0 - /usr/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 7.12.0 - /usr/local/bin/npm
  Browsers:
    Brave Browser: 90.1.24.85
    Chrome: 90.0.4430.212
    Firefox: 88.0.1
  npmPackages:
    @sveltejs/kit: next => 1.0.0-next.109 
    svelte: ^3.34.0 => 3.38.2 

using Chrome (also fails using Firefox)

no adapter specified

Severity
Not severe. It may affect user perception of stability when evaluating SvelteKit.

Additional context
Failure is also present when running on Windows.

[jesslawnz]

Had some difficulty reproducing this bug until I double clicked the delete icon. Able to reproduce it with a single item.

• Create todo Item
• double click delete icon
• error occurs.

Found another similar bug in the TODO page:

• Focus on input '+tap to add a todo'
• Type 1 {enter} 2 {enter} 3 {enter} 4 {enter} 5 {enter} etc. as fast as possible
• Expect: sequential TODO entries 1..5
• Actual count and content of TODO entries is unpredictable.

[twobitfool]

I think the problem lies with the API server: https://api.svelte.dev/

We can see the issue by using curl to create 3 todo items, and then refetch them.

USER_ID=`uuidgen | tr '[:upper:]' '[:lower:]'`

echo "Adding 3 Tasks..."
curl -sH "Content-Type: application/json" -d '{"text":"First Todo"}'           "https://api.svelte.dev/todos/$USER_ID" | jq "."
curl -sH "Content-Type: application/json" -d '{"text":"Second Todo"}'          "https://api.svelte.dev/todos/$USER_ID" | jq "."
curl -sH "Content-Type: application/json" -d '{"text":"Third and Final Todo"}' "https://api.svelte.dev/todos/$USER_ID" | jq "."

sleep 1
echo "\nLoading Tasks..."
curl -sH "Content-Type: application/json" "https://api.svelte.dev/todos/$USER_ID" | jq ".[] | .text"

sleep 1
echo "\nLoading Tasks (Again)..."
curl -sH "Content-Type: application/json" "https://api.svelte.dev/todos/$USER_ID" | jq ".[] | .text"

sleep 1
echo "\nLoading Tasks (One More Time).."
curl -sH "Content-Type: application/json" "https://api.svelte.dev/todos/$USER_ID" | jq ".[] | .text"

When refetching the data, the API results are unpredictable.

Adding 3 Tasks...
{
  "uid": "748878bd1fd49332bfc648b63b2a2f713314",
  "created_at": 1624943002911,
  "text": "First Todo",
  "done": false
}
{
  "uid": "ec9cc1bb1329503b4abc2704ecc087008a25",
  "created_at": 1624943003178,
  "text": "Second Todo",
  "done": false
}
{
  "uid": "48221085a19461deb371ba466aea6301a0c5",
  "created_at": 1624943003569,
  "text": "Third and Final Todo",
  "done": false
}

Loading Tasks...
"First Todo"
"Second Todo"

Loading Tasks (Again)...
"First Todo"
"Second Todo"
"Third and Final Todo"                # <- The 3rd todo shows up here, but not in the previous or next request

Loading Tasks (One More Time)..
"First Todo"
"Second Todo"

Fetch All: [Todo 1, Todo 2]           # ❌  Wrong
Fetch All: [Todo 1, Todo 2, Todo 3]   # ✅  Correct
Fetch All: [Todo 1, Todo 2]           # ❌  Wrong

Looks like an "eventual consistency" issue with the API storage back-end.

---

Like a lot of people, I initially assumed that something was broken with SvelteKit. Might be worth changing the sample app to use in-memory (or local file) storage 🤷

[lunacd]

I also feel like a local store (file or sqlite) might be desirable. It can also demonstrate the ability for SvelteKit to function as a HTTP server (I think it already does in the current form but it's still ultimately relying on some external HTTP service).

If people think that is a desirable change, I can help to implement that.

[ric2b]

Opened the PR above, which simply disables the delete button while the delete request is in flight. Should get rid of this issue.

[twobitfool]

Please reopen this issue...

PR 2172 is great, because it avoids a double-submit issue, but it does not fix the root problem. The root problem is with the (eventually) consistency of the API (api.svelte.dev).

The bug can occur when clicking the delete button just once. (Note: Very tricky, because it doesn't fail consistently.)

Method 1 (for triggering bug):

1. Open web inspector
2. Create a few todos
3. Delete a todo
4. Goto step 2, and repeat, until you see the bug

---

Method 2 (for triggering bug):

1. Create several todos
2. Delete a few todos
3. Reload the page a few times
4. Deleted todos will reappear & disappear on a subsequent reloads

---

Method 3 (for triggering bug):

1. Run the curl script from my comment above.

This demonstrates the issue, by creating a few todos and reloading them three times.

---

To be clear: This is not a bug in the demo app, and it's not a bug in SvelteKit. This is a bug with the API server that is storing the todos. But it certainly needs to be addressed, because it makes SvelteKit look broken.

FWIW: Method 2 and 3 were the most reliable ways to reproduce the bug -- for me at least 🤷

[ignatiusmb]

Thanks for the investigation @twobitfool! Perhaps it's better if we move this to https://github.com/sveltejs/api.svelte.dev then

[twobitfool]

@ignatiusmb -- Yeah. That's an option.

...or option 2... We tweak the demo app to use local (or in-memory) storage, which would:

• resolve this issue
• remove the dependency on the API server
• allow the demo to run offline
• make the app lighting quick

[benmccann]

Some notes from when we created the demo:

We have deployed versions of the demo apps (example) so wanted a real backend

Vercel was doing some kind of caching that was originally causing issues on the Vercel deployed version. I think that was possibly turned off and wouldn't explain any issues occurring locally

KV has a limit of 1 write per second per key. This app has all user todos living under todolist__{userid} key. The way to get around this is to do todolist__(userid)__(todoid) entries. And then the GET /todos is a KV.list for the todolist__(userid) prefix

If consistency is an issue, Durable Objects or Fauna would be strongly consistent (perhaps higher latency as a result)

[ric2b]

@twobitfool Yeah, looks like the API itself has some weird consistency issues.

This seems to be about just that: sveltejs/api.svelte.dev#8

@benmccann Can you elaborate on what you mean by this?

so wanted a real backend

What is the benefit vs using local storage? Is it because it's an example that new users might want to look at to learn how to interact with API's or something else?