I am following Livy programmatic API page https://livy.incubator.apache.org/docs/latest/programmatic-api.html to submit a job to existing session.

String clientURL = String.format("http://%s:%s/sessions/%s", ip, port, session);
    System.out.println(clientURL);
    LivyClient client = new LivyClientBuilder()
            .setURI(new URI(clientURL))
            .build();
client.submit(new MyJob(index, query))

The clientURL is http://myip:8998/sessions/1

But my code fails with stacktrace:

Failed to handleRequest java.lang.RuntimeException: java.io.IOException: Not Found: {"msg":"Session '1' not found."}java.lang.RuntimeException: java.io.IOException: Not Found: {"msg":"Session '1' not found."}
at org.apache.livy.client.http.HttpClient.propagate(HttpClient.java:182)
at org.apache.livy.client.http.HttpClient.<init>(HttpClient.java:82)
at org.apache.livy.client.http.HttpClientFactory.createClient(HttpClientFactory.java:37)
at org.apache.livy.LivyClientBuilder.build(LivyClientBuilder.java:142)

I read your livy code, I am quite confused by this code line 73. Why it calls

conn.post(null, SessionInfo.class, "/%d/connect", sessionId);

What is this connect endpoint?

My session was created by very easy python code from lambda. I even post 1 simple statement after it so I know the session was created.

    host = 'http://{0}:8998'.format(ip)
    data = {'kind': 'pyspark', 'numExecutors': 200, 
    'conf': 
        {
        'spark.submit.deployMode':'client',
        'spark.driver.memory': '10g',
        'spark.executor.memory': '2g',
        'spark.executor.cores': '2'}}
    headers = {'Content-Type': 'application/json'}
    r = requests.post(host + '/sessions', data=json.dumps(data), headers=headers)

I can not find this connect endpoint from anywhere https://livy.incubator.apache.org/docs/latest/rest-api.html.