Randomizing db names for each test

[k2xl]

Wanted to know if there were any downsides to this approach as well as what I am missing in implementing unique tests appropriately. I've been trying to optimize our test suite runtime to see how fast I can have it run through all the tests. We are using nextjs + jest + mongodb-memory-server (v 9.1.3)

Note that each test has been written to assume the db is starting from the same state.

Originally, I had on setup() for each test call a dbConnect() function which basically does a

const replSetOptions = {
          replSet: {
            count: 1,
          },

        };
        
global.mongoMemoryServer = await MongoMemoryReplSet.create(replSetOptions);

However, I realized that every call to MongoMemoryReplSet.create() was taking about ~125-150ms for each test. So I was thinking of alternative approaches and thought to try using a global method per recommended in the docs

// globalSetup.ts
import { MongoMemoryReplSet } from 'mongodb-memory-server';

module.exports = async () => {
  const mongoServer = await MongoMemoryReplSet.create({
    replSet: { count: 1 },
  });
  const mongoUri = mongoServer.getUri();

  // Set the Mongo URI for the tests to use
  process.env.MONGODB_TEST_URI = mongoUri;
  console.log('\nMONGODB_TEST_URI', process.env.MONGODB_TEST_URI);

  // Add mongoServer to the global config so we can tear it down later
  global.__MONGOSERVER__ = mongoServer;
};

Then in dbConnect() basically mongoose.connect to process.env.MONGODB_TEST_URI

However, the issue I had is that the db isn't actually cleared out between each run. When I tried to dropDatabase() right after connect() the issue was that parallel tests that were running would crossover on top of each other (i.e. setting maxWorkers to 2 or more)

I came up with a hack which kind of works (though I get sometimes flaky port is in use as well as random test timeouts)

uri = process.env.MONGODB_TEST_URI;
        // now set the uri to point to a randomally generated database name
        // this is so that we can run tests in parallel
        const randomDbName = 't_' + new Types.ObjectId().toString(); // objectId should be unique between tests since it is a mix of timestamp and randomness...

        uri = uri.replace('/?', '/' + randomDbName + '?');
...
 // later ... mongoose.connect(uri as string, options).then((mongoose) => { ...

Basically the idea is that each test gets their own 'database' to test with... So they don't overlap with each other. The good news is that the tests definitely run faster now (a decent ~15% off our test suite time) -

However, as I mentioned I still sometimes (like 1/6 times maybe) get random tests that just... "hang" and timeout. So I feel that I missed doing this the correct way.

[hasezoey]

However, I realized that every call to MongoMemoryReplSet.create() was taking about ~125-150ms for each test

this is to be expected, especially for repl-sets, because each create (/start) actually starts a binary, and then waits until it gets a Ready for connections message and then resolved, for repl-sets, in addition to waiting for all specified server ready for connections, it also needs to set-up the replset-config and then wait for a instance is now Primary message before resolving.
this is generally one of the reasons why i dont recommend having a new server for each test (not meaning test-suite/file here).

another reason on why i dont recommend it is because of the lack of port locking, see #827 for some more info

I came up with a hack which kind of works (though I get sometimes flaky port is in use

i am not sure why you would still get port in use errors, if you still use the global method

as well as random test timeouts)

without logs and use-case i cannot say much in-detail but this may be related that the instance(s) are "overloaded" (depending on your machine and number of tests and complexity running) or I/O bound (when using wiredTiger)