const { Gauge } = require('prom-client')

function monitor(mongoClient) {
  const labelNames = ['mongo_server']
  const poolSize = new Gauge({
    name: 'mongo_connection_pool_size',
    help: 'number of connections in the connection pool',
    labelNames,
    // Use this one metric's collect() to set all metrics' values.
    collect,
  })
  const availableConnections = new Gauge({
    name: 'mongo_connection_pool_available',
    help: 'number of connections that are not busy',
    labelNames,
  })
  const waitQueueSize = new Gauge({
    name: 'mongo_connection_pool_waiting',
    help: 'number of operations waiting for an available connection',
    labelNames,
  })
  const maxPoolSize = new Gauge({
    name: 'mongo_connection_pool_max',
    help: 'max size for the connection pool',
    labelNames,
  })

  function collect() {
    // Reset all gauges in case they contain values for servers that
    // disappeared
    poolSize.reset()
    availableConnections.reset()
    waitQueueSize.reset()
    maxPoolSize.reset()

    const servers = mongoClient.topology?.s?.servers
    if (servers != null) {
      for (const [address, server] of servers) {
        const pool = server.s?.pool
        if (pool == null) {
          continue
        }

        const labels = { mongo_server: address }
        poolSize.set(labels, pool.totalConnectionCount)
        availableConnections.set(labels, pool.availableConnectionCount)
        waitQueueSize.set(labels, pool.waitQueueSize)
        maxPoolSize.set(labels, pool.options.maxPoolSize)
      }
    }
  }
}

module.exports = { monitor }