compile error when using with_state and exracting and modifying whole request in handler

[liy9426]

Version

├── axum v0.6.0-rc.5
│ ├── axum-core v0.3.0-rc.3

Platform

Darwin -Pro.local 22.1.0 Darwin Kernel Version 22.1.0: Sun Oct 9 20:15:09 PDT 2022; root:xnu-8792.41.9~2/RELEASE_ARM64_T6000 arm64

Description

so I was building my web server based on the key-value-store example, as I needed the feature to have a globally shared state sha among route handlers as well as being able to change them.

The code looks like this:

and the handler:

but I got the compile error below:

I've tried various ways to avoid the compile error, such as mutex and even not using state, but couldn't make it work.

I‘m wondering is there anything wrong with my code and how can fix this error?

Please let me know if anything is missing.

Thanks very much!

[davidpdrsn]

Try https://docs.rs/axum/0.6.0-rc.5/axum/attr.debug_handler.html

[liy9426]

After using debug_handler I see I got the same error message as in this post #873,

the error is " future cannot be sent between threads safely", however the difference is that I wasn't using mutex but used RwLock as in the Axum-main key-value-store example.

Do I need to switch to mutex or is there anyway to make it work with Rwlock?

[davidpdrsn]

There shouldn't be any need to keep a hyper::Client inside a lock since Client::request only requires &self. It is also cheap to clone so you don't need an Arc either. I'd recommend splitting your state into things that actually need the lock and things that don't.

[liy9426]

Problem solved. Thanks a lot.
followup question: the std::sync::RwLock usage in key-value-store example seems to be confusing. I was able to resolve the error by using tokio::sync::RwLock. Is it intentional to put std::sync::RwLock there?

[davidpdrsn]

Yes. The lock isn't held across any await points and just updates in-memory things so the critical section is short. In those cases a sync mutex is often recommended. See https://docs.rs/tokio/latest/tokio/sync/struct.Mutex.html#which-kind-of-mutex-should-you-use

[Daniel-Xu]

I saw state is cloned inside with_state, so:

1. when should we need to wrap AppState with Arc?

2. when we extract the state in the handler? is it cloned into the handler?
3. for each router, I can only attach one with_state to it, is this by design?

[davidpdrsn]

when should we need to wrap AppState with Arc?

If it's expensive to clone then an arc is a good idea.

when we extract the state in the handler? is it cloned into the handler?

Yes

for each router, I can only attach one with_state to it, is this by design?

Yes but you can use FromRef to get sub states. See the docs for more details.