Intermittent Communications link failure with Cloud SQL - google-app-engine

Question

I'm using jmeter to stress test a GAE web service which uses CloudSQL and I'm getting intermittent communications link failure exceptions.
I've tried using direct connections and a connection pool, and I see exceptions in either scenario. The exceptions increase as the number of requests per second increase.
Note that we are using the highest tier of cloud sql, D32 and the tests are well under the max 3200 connections.
Here's a stack trace for reference:
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
at sun.reflect.GeneratedConstructorAccessor48.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:33)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1117)
at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:350)
at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2413)
at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2450)
at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2235)
at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:818)
at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:46)
at sun.reflect.GeneratedConstructorAccessor46.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:33)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:404)
at com.mysql.jdbc.GoogleNonRegisteringDriver$JdbcWrapper.getInstance(GoogleNonRegisteringDriver.java:276)
at com.mysql.jdbc.GoogleNonRegisteringDriver.connect(GoogleNonRegisteringDriver.java:246)
at java.sql.DriverManager.getConnection(DriverManager.java:571)
at java.sql.DriverManager.getConnection(DriverManager.java:215)
Update: I changed the connection pool settings to maxActive = 5 and maxIdle = 5 and the intermittent communications link exceptions went away. Note that I've tried commons dbcp and tomcat dbcp. I'm now seeing the following exceptions in the logs:
Caused by: java.sql.SQLException: java.lang.SecurityException: Unable to access gatherPerformanceMetrics
Caused by: java.sql.SQLException: java.lang.SecurityException: Unable to access includeThreadDumpInDeadlockExceptions
Caused by: java.sql.SQLException: java.lang.SecurityException: Unable to access nullNamePatternMatchesAll

Answer 1

From https://cloud.google.com/appengine/docs/java/cloud-sql/#Java_Size_and_access_limits
"Each App Engine instance cannot have more than 12 concurrent connections to a Google Cloud SQL instance."
Can you tell more about the test set-up? How many requests is jmeter sending to appengine and how many connections does the app instance open for each of those requests?

Answer 2

To everyone who are looking for why you might be getting "com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure" on a connection.
Make sure your IP is allowed if you are calling from a test server!
I was testing at a friends house, and this unhelpful error kept showing up.