How to add headers to ContainerClient requests?

[kate-goldenring]

I am looking to implement compare-and-swap functionality with the new v0.22.0 CosmosDB SDK. With the previous v0.21.0 SDK (example), i was able to use the if_match_condition(azure_core::request_options::IfMatchCondition::Match(etag)) method on the DocumentClient. That client has been replaced by the ContainerClient in the v0.22.0 SDK, which is missing many methods for adding headers to requests. What is the best way to add a header (such as the "if-match" one) to an individual request issued by the ContainerClient? I created a policy for the cases I wanted to add headers for all requests; however, I am not sure how to add headers on individual requests like i used to be able to with the predefined methods.

[analogrelay]

A number of things from the previous unofficial versions of the client haven't quite made it over yet, and this is one of them. We'll be adding specific options as we continue, but for now, using a Policy is your best bet. As you noticed, the policy runs on every request, but you can "configure" it using the Context object.

Every method takes an "Options" value as it's final parameter, and every ...Options type has a method_options of type ClientMethodOptions. That type allows you to specify a Context value, which will flow down through all the policies. The Context value you provide in the options for the individual method is the one that ends up being passed to your Policy's send method. The Context is a generic key-value store that is keyed off of the type of the value you pass in, so you could do something like this:

pub struct IfMatch(String);

let context = Context::new().with_value(IfMatch(etag));
container_client.upsert_item(..., ItemOptions {
    method_options: ClientMethodOptions {
        context,
        ..Default::default()
    },
    ..Default::default()
}).await;

And then in your policy:

if let Some(IfMatch(if_match)) = ctx.value::<IfMatch>() {
    // Add your header here
}

This is definitely a bit clunky and not the long-term plan, but if you'd like to move to 0.22.0 and were using this feature from the previous versions, it's a way to get unblocked!

[kate-goldenring]

Thank you @analogrelay! This unblocks me. I appreciate the support.

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