I'm trying to load test a few pure SQL query on my local SQL Server (I'm not sure this is the best way to test performance, but this would give me a rough estimate). I'm using Java's PreparedStatement and MS JDBC for SQL Server, and I instanciate one hundred Thread with a new `Connection` in each one.

I had a SQL Server Express edition (11.0.210) already installed. Unfortunately, Express edition won't run multiple query in parallel (as will the Pro edition), so my test wouldn't be quite usefull. 

A colleague told me I could use SQL Developer 2014. I had some difficulties setting it up, so I might have forgotten something. I started using it with sysadmin (sa) account. 

Making a single `Connection` with JDBC and querying works as expected.

---

Now, I'm trying to create two `Connection`:

```
String SQLSERVER_CONNECTION_STRING = "jdbc:sqlserver://localhost\\SQLDEVELOPPER:55372";
Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");
Connection c1 = DriverManager.getConnection(SQLSERVER_CONNECTION_STRING, USER, USER_PWD);
Connection c2 = DriverManager.getConnection(SQLSERVER_CONNECTION_STRING, USER, USER_PWD);

```

First I get this message on `stderr`:

```
nov. 08, 2017 10:41:00 AM com.microsoft.sqlserver.jdbc.SQLServerConnection Prelogin
AVERTISSEMENT: ConnectionID:1 ClientConnectionId: bfb1d22d-a5fb-4f74-9be8-60fb91c9d701 Prelogin error: host localhost port 55372 Error reading prelogin response: Software caused connection abort: recv failed ClientConnectionId:bfb1d22d-a5fb-4f74-9be8-60fb91c9d701

```

And get this error 16 times:

```
nov. 08, 2017 10:41:01 AM com.microsoft.sqlserver.jdbc.SQLServerConnection Prelogin
AVERTISSEMENT: ConnectionID:1 ClientConnectionId: fcd64c62-ba9a-48ee-b195-11307e7bad30 Prelogin error: host localhost port 55372 Error reading prelogin response: Une connexion existante a dû être fermée par l’hôte distant ClientConnectionId:fcd64c62-ba9a-48ee-b195-11307e7bad30

```

Translated: "An existing connection had to be closed by distant host" (I read it first as "An existing connection might have been closed by distant host", but as @fab's comment points out, it might be wrong)

Then this stacktrace:

```
Exception in thread "main" java.lang.RuntimeException: com.microsoft.sqlserver.jdbc.SQLServerException: Une connexion existante a dû être fermée par l’hôte distant ClientConnectionId:0fa4b1cb-ae03-407a-bcc3-54a0cd85005c
    at fr.aso.sandbox.utils.SQLServerConnectionManager.getConnection(SQLServerConnectionManager.java:24)
    at fr.aso.sandbox.myComp.myComp.test.KeycopterTest.doubleConnectionThreadTest(KeycopterTest.java:78)
    at fr.aso.sandbox.myComp.myComp.test.KeycopterTest.main(KeycopterTest.java:49)
Caused by: com.microsoft.sqlserver.jdbc.SQLServerException: Une connexion existante a dû être fermée par l’hôte distant ClientConnectionId:0fa4b1cb-ae03-407a-bcc3-54a0cd85005c
    at com.microsoft.sqlserver.jdbc.SQLServerConnection.terminate(SQLServerConnection.java:2397)
    at com.microsoft.sqlserver.jdbc.SQLServerConnection.terminate(SQLServerConnection.java:2384)
    at com.microsoft.sqlserver.jdbc.TDSChannel.read(IOBuffer.java:1884)
    at com.microsoft.sqlserver.jdbc.SQLServerConnection.Prelogin(SQLServerConnection.java:2137)
    at com.microsoft.sqlserver.jdbc.SQLServerConnection.connectHelper(SQLServerConnection.java:1973)
    at com.microsoft.sqlserver.jdbc.SQLServerConnection.login(SQLServerConnection.java:1628)
    at com.microsoft.sqlserver.jdbc.SQLServerConnection.connectInternal(SQLServerConnection.java:1459)
    at com.microsoft.sqlserver.jdbc.SQLServerConnection.connect(SQLServerConnection.java:773)
    at com.microsoft.sqlserver.jdbc.SQLServerDriver.connect(SQLServerDriver.java:1168)
    at java.sql.DriverManager.getConnection(DriverManager.java:664)
    at java.sql.DriverManager.getConnection(DriverManager.java:247)
    at fr.aso.sandbox.utils.SQLServerConnectionManager.getConnection(SQLServerConnectionManager.java:22)
    ... 2 more

```

---

I've found SO questions and answers about this, but for most of them were about a single connection not working. 

Here, the problem occurs when I try 2 or more connections with the same user, So I have ruled out the "TCP not enabled" and "Firewall block sql server" problems.

SQL Developer version is :

```
Microsoft SQL Server 2014 (SP2) (KB3171021) - 12.0.5000.0 (X64) 
    Jun 17 2016 19:14:09 
    Copyright (c) Microsoft Corporation
    Developer Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1)

```

Because I could make multiple time the same connection on SQL Express, I believe this is a SQL Server configuration problem, but couldn't find anything related.

Did I miss something somewhere ? Is there a way to get a more explicit error ?