Some troubles with dart:io client side WebSocket implementation

[Vadim Tsushko]

One of mine primary usage case for dart is all sorts of scripting to help in my everyday job - BI development with Qlik (as qlik.com) products.

My immediate task in that area is to migrate existing infrastructure supporting a development of QlikView BI projects to Qlik Sense.

Qlik Sense is a new product and the main interface to communicate with it is through WebSockets (Qlik specifically claim that any client capable to communicate through WebSockets could use their Qlik Sense API)

I have some experience with Dart WebSockets before and was relatively confident that in that area I should not find big troubles. But in fact I could not connect to QlikSense services with dart:io WebSockets. 

Initially, I could not find any reasons for that and tried all sorts of solutions. Strange fact for me was that I could successfully connect to QlikSense services from within dart web applications (using dart:html WebSocket implementation)

That was not very helpful for me though as I have to create CLI utility, not the Web interface.

So my temporary solution is to write tiny transparent nodejs WebSocket proxy. From dart script I interact with that proxy, and that proxy in turn interacts with Qlik Sense service. That works for me now on development stage, but for deployment that would be great burden indeed.

So I spent some time with WireShark to find what is different in WebSocket handshaking of nodejs (successful) and dart:io (unsuccessful) communication. For now I'm almost totally sure that difference and reason why dart:io WebSocket connections break with error is that dart:io header keys are always converted to lower case.

Successful communication looks like: 

    GET /app/%3Ftransient%3D HTTP/1.1

    Connection: Upgrade

    Upgrade: websocket

    Host: 192.168.188.10

    Sec-WebSocket-Version: 13

    Sec-WebSocket-Key: MTMtMTQ0OTMzNjYzNzY0NQ==

    Content-Type: application/json

    Cookie: X-Qlik-Session=70a54c5b-a08b-4665-a0d7-1a39871ebea9; Path=/; HttpOnly; Secure

    Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits

    HTTP/1.1 101 Switching Protocols

    Upgrade: websocket

    Connection: Upgrade

    Sec-WebSocket-Accept: Tb5mxSaTiFApEyYAAv/ci7AMOzI=

    Access-Control-Allow-Origin: ws://192.168.188.10

Unsuccessful communication looks like:

    GET /app/%3Ftransient%3D HTTP/1.1

    user-agent: Dart/1.14 (dart:io)

    connection: Upgrade

    cache-control: no-cache

    accept-encoding: gzip

    cookie: X-Qlik-Session=70a54c5b-a08b-4665-a0d7-1a39871ebea9; Path=/; HttpOnly; Secure

    content-length: 0

    sec-websocket-version: 13

    host: 192.168.188.10

    sec-websocket-extensions: permessage-deflate; client_max_window_bits

    content-type: application/json

    sec-websocket-key: lVH5YxHL8QjS7ddjIJ/Ohg==

    upgrade: websocket

    HTTP/1.1 404 Not Found

    Content-Length: 0

    Access-Control-Allow-Origin: ws://192.168.188.10

I did not find any way to pass intact (not converted to lower case) headers to dart:io WebSocket or HttpClient. So finally I've tried raw Socket and with it (writing to it capitalized Connection and Upgrade as header keys) I've managed to get switching protocol response from the Qlik Sense WebSocket server. So apparently from that I can proceed to create WebSocket from manually upgraded WebSocket.

But I wonder now - is Qlik Sense WebSocket server implementation is wrong to reject dart:io client WebSocket connections? Or dart:io WebSockets are somehow not strictly adhere to specification?

Dart:io WebSocket client successfully connects to http://www.websocket.org/ sample server, to dart:io WebSocket servers, to nodejs ws server - but how many `Qlik Sense like` not compatible with dart:io WebSocket servers are in the wild?

Communication example on Wikipedia looks more like nodejs version (with capitalized keys)

    GET /chat HTTP/1.1

    Host: server.example.com

    Upgrade: websocket

    Connection: Upgrade

    Sec-WebSocket-Key: x3JJHMbDL1EzLkh9GBhXDw==

    Sec-WebSocket-Protocol: chat, superchat

    Sec-WebSocket-Version: 13

    Origin: http://example.com

     

[Vadim Tsushko]

I've created a https://github.com/dart-lang/sdk/issues/25120 for that too

[Søren]

Thanks for the report, however the current behaviour is by design.

The HTTP/1.1 specification section 4.2 clearly states that the field names in HTTP headers are case-insensitive. Early on in the design of the Dart HTTP library we decided to make all header fields lowercase, and not try to neither keep the passed-in case nor produce the Xxx-Yyyy-Zzz format.

Btw. the HTTP/2 specification went with all lowercase headers.

I have updated the issue https://github.com/dart-lang/sdk/issues/25120 with this information.

Regards,

Søren

---

Søren Gjesse

Software Engineer, Google Denmark

CVR nr. 28 86 69 84

[Vadim Tsushko]

Thank for update. I underestand the reason and can see that it is valid.

Unfortunately it mean that porting some stuff from nodejs (or dotnet in that case) equivalient to dart is not feasible to me. Actually I think I would upgrade WebSockets manually in that case but next thing I discover - I cannot pass client certificate to Qlik Sense in a way that was done in nodejs or dotnet.

So I probably stay put with deploing nodejs proxy service.

вторник, 8 декабря 2015 г., 13:15:47 UTC+5 пользователь Søren написал: