Lab extension plots not showing up about dask-examples HOT 7 CLOSED

[bokeh] setting log level to: 'info'
connection.js:55 WebSocket connection to 'wss://hub.mybinder.org/user/mrocklin-dask-examples-pc2yzpdk/proxy/8787/workers/ws?bokeh-protocol-version=1.0&bokeh-session-id=g8vm0oUbBxywswUzxeq0KHgF4lrQHKD3ezTwSkp8Pet7' failed: Error during WebSocket handshake: Unexpected response code: 502
t.connect @ connection.js:55
connection.js:287 [bokeh] Failed to connect to Bokeh server Error: Could not open websocket
(anonymous) @ connection.js:287
connection.js:208 [bokeh] Lost websocket 0 connection, 1006 ()
connection.js:95 [bokeh] Websocket connection 0 disconnected, will not attempt to reconnect

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I'm not sure what the problem is just yet: I don't think it's changes to recent iframe-ing security policies in JupyterLab, since those are in the prerelease, rather than 0.35. Have there been any recent changes to distributed or the bokeh webserver that might result in a different security model?

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I see the same issue in dask/dask-examples master, which suggests to me that there may be some dependency that released a breaking change.

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Could also be related to a recent snafu with Tornado 6, which was released a couple of weeks ago.

Update: we have a winner, downgrading to notebook 5.7.4 and tornado 5.1.1 fixes the issue. Looks like we are not out of the woods with this compatibility issue yet :/

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Possible other related issues: jupyterhub/jupyter-server-proxy#109, jupyterhub/jupyter-server-proxy#115

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@mrocklin did dask have any troubles with tornado 6?

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A couple small ones, but we cleaned them up a release ago

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