Can Alternator correctly handle HTTP requests that include the "Expect: 100-continue" HTTP request header?

[Felix-zhoux]

Installation details
Scylla version (or git commit hash): 4.0.0-0.20200601.d95aa77b6
Cluster size: 1
OS (RHEL/CentOS/Ubuntu/AWS AMI): Centos

When I use Dynamo SDK to send a request to Scylla Alternator, if I send the request directly to Alternator, I will get a delay of more than 1s. If I send it to apisix first, and then apisix forwards it to Alternator, I can get a normal ms level. delay.

After tcpdump debug, I found that the delay of 1s comes from the stage of establishing TCP connection and sending request data. The Alternator delay monitoring in the Scylla monitor also verifies this. The delay of the Alternator processing PutItem and GetItem operations is still ms level, but the dynamo SDK client gets a delay of about 1s.

After apisix, you can get a normal delay, because apisix can normally handle HTTP requests with the "Expect: 100-continue" header, and the requests forwarded to the Alternator no longer have the "Expect: 100-continue" header.
Attached below are some screenshots of my debug process.
I’m not sure if my guess is correct, I need your help to verify

HTTP request sent directly to ScyllaAlternator
The client gets a delay of about 1s

HTTP request is forwarded to ScyllaAlternator via apisix routing
The client gets a normal ms-level delay

ps: Dynamo SDK client(10.10.10.35), apisix(10.10.10.92), Scylla(10.10.10.31)

[nyh]

Nice detective work!
I haven't seen these 1 second delays in anu of my tests (what is this "Dynamo SDK" - which language is it?), but you are right - the "Expect: 100-continue" is a part of the HTTP 1.1 RFC (see https://tools.ietf.org/html/rfc7231#section-5.1.1) and Seastar's HTTP server is missing support for it :-( I will have to add it.

[Felix-zhoux]

I haven't seen these 1 second delays in anu of my tests (what is this "Dynamo SDK" - which language is it?)

aws-cpp-sdk-dynamodb （Sorry, that's my spoken language. -_-!）

[psarna]

By the way, AWS C++ SDK documentation mentions a quite easy workaround (source: https://docs.aws.amazon.com/sdk-for-cpp/v1/developer-guide/client-config.html):

disableExpectHeader

• Applicable only for CURL HTTP clients. By default, CURL adds an “Expect: 100-Continue” header in an HTTP request to avoid sending the HTTP payload in situations where the server responds with an error immediately after receiving the header. This behavior can save a round-trip and is useful in situations where the payload is small and network latency is relevant. The variable’s default setting is false. If set to true, CURL is instructed to send both the HTTP request header and body payload together.

[nyh]

This will need to be fixed in the Seastar library, where the HTTP server code that we use lives. I opened an issue there: scylladb/seastar#768

[nyh]

I haven't seen these 1 second delays in anu of my tests (what is this "Dynamo SDK" - which language is it?)

aws-cpp-sdk-dynamodb （Sorry, that's my spoken language. -_-!）

My question was about the programming language of that SDK (C++? Java? Python? Go?), not your spoken language :-)
So the answer is C++. Thanks. Surprisingly, I think that's the only language that we haven't experimented with - we test heavily with the Python SDK, and experimented less heavily with the Java, Go, and Node.JS SDKs. So it's good that you are testing it.

[avikivity]

@nyh please evaluate for backport. It doesn't look like a regression, but if it makes alternator unusable, it's a major bug.

[nyh]

Yes, it does make Alternator unusable for users of a specific driver (a C++ one) which appears to use this HTTP feature. However, I'm not sure how to backport. I guess that instead of putting a new version of Seastar in the branches, I should backport the specific Seastar patch to the branch-specific-seastar we have? So I should basically follow the instructions for that in the maintainers.md?

[avikivity]

I suppose we need an instructions.md that tells you to follow the instructions.

[nyh]

Backported the Seastar patch to the Seastar branch of Scylla 4.2, 4.1 and 4.0