mongodb/node-mongodb-native · error · MongoInvalidArgumentError

Connection pool minimum size must not be greater than…

Q: What causes "Connection pool minimum size must not be greater than maximum pool size" in mongodb/node-mongodb-native?

Ensure minPoolSize <= maxPoolSize in the connection string and/or options.

Error message

Connection pool minimum size must not be greater than maximum pool size

What it means

A MongoInvalidArgumentError thrown in the ConnectionPool constructor when minPoolSize exceeds maxPoolSize. The pool enforces this invariant immediately at construction so an invalid configuration never reaches runtime. The defaults are minPoolSize=0 and maxPoolSize=100, so this only triggers when both are explicitly misconfigured.

Solutions

Ensure minPoolSize <= maxPoolSize in the connection string and/or options.
Set maxPoolSize to at least the expected concurrent operation count.
Use minPoolSize=0 for serverless/short-lived environments.
Validate pool options at application startup with a config schema.

Example fix

// before
const client = new MongoClient(uri + '?minPoolSize=50&maxPoolSize=10');

// after
const client = new MongoClient(uri + '?minPoolSize=10&maxPoolSize=50');

Defensive patterns

Strategy: validation

Validate before calling

const minPoolSize = 10;
const maxPoolSize = 50;
if (minPoolSize > maxPoolSize) {
  throw new Error('minPoolSize must be <= maxPoolSize');
}
const client = new MongoClient(uri, { minPoolSize, maxPoolSize });

Prevention

Validate pool options in a config schema at startup.
Use minPoolSize=0 for serverless/short-lived environments.
Keep maxPoolSize >= expected peak concurrency.

When it happens

Trigger: Constructing a MongoClient with minPoolSize greater than maxPoolSize in the connection string or options (e.g., ?minPoolSize=50&maxPoolSize=10).

Common situations: Copy-paste errors in connection string; tuning pool sizes for load and inverting the values; setting maxPoolSize low for a serverless/function environment while keeping a high minPoolSize from a previous config.

Related errors

AI-assisted analysis of mongodb/node-mongodb-native@dce7939f86 (2026-08-11). Data as JSON: /api/errors/094dff75db8bae0e. Report an issue: GitHub.

Appendix: source

Thrown at src/cmap/connection_pool.ts:211

  constructor(server: Server, options: ConnectionPoolOptions) {
    super();
    this.on('error', noop);

    this.options = Object.freeze({
      connectionType: Connection,
      ...options,
      maxPoolSize: options.maxPoolSize ?? 100,
      minPoolSize: options.minPoolSize ?? 0,
      maxConnecting: options.maxConnecting ?? 2,
      maxIdleTimeMS: options.maxIdleTimeMS ?? 0,
      waitQueueTimeoutMS: options.waitQueueTimeoutMS ?? 0,
      minPoolSizeCheckFrequencyMS: options.minPoolSizeCheckFrequencyMS ?? 100,
      autoEncrypter: options.autoEncrypter
    });

    if (this.options.minPoolSize > this.options.maxPoolSize) {
      throw new MongoInvalidArgumentError(
        'Connection pool minimum size must not be greater than maximum pool size'
      );
    }

    this.poolState = PoolState.paused;
    this.server = server;
    this.connections = new List();
    this.pending = 0;
    this.checkedOut = new Set();
    this.minPoolSizeTimer = undefined;
    this.generation = 0;
    this.serviceGenerations = new Map();
    this.connectionCounter = makeCounter(1);
    this.cancellationToken = new CancellationToken();
    this.cancellationToken.setMaxListeners(Infinity);
    this.waitQueue = new List();
    this.metrics = new ConnectionPoolMetrics();
    this.processingWaitQueue = false;

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