Function secondarySelector

src/sdam/server_selection.ts:285–376

(
  readPreference: ReadPreference,
  topologyDescription: TopologyDescription,
  servers: ServerDescription[],
  deprioritized: DeprioritizedServers
)

}

function secondarySelector(
  readPreference: ReadPreference,
  topologyDescription: TopologyDescription,
  servers: ServerDescription[],
  deprioritized: DeprioritizedServers
) {
  const mode = readPreference.mode;
  switch (mode) {
    case 'primary':
      // Note: no need to filter for deprioritized servers.  A replica set has only one primary; that means that
      // we are in one of two scenarios:
      // 1. deprioritized servers is empty - return the primary.
      // 2. deprioritized servers contains the primary - return the primary.
      return servers.filter(primaryFilter);
    case 'primaryPreferred': {
      const primary = servers.filter(primaryFilter);

      // If there is a primary and it is not deprioritized, use the primary.  Otherwise,
      // check for secondaries.
      const eligiblePrimary = primary.filter(isDeprioritizedFactory(deprioritized));
      if (eligiblePrimary.length) {
        return eligiblePrimary;
      }

      // If we make it here, we either have:
      // 1. a deprioritized primary
      // 2. no eligible primary
      // secondaries take precedence of deprioritized primaries.
      const secondaries = tagSetReducer(
        readPreference,
        maxStalenessReducer(readPreference, topologyDescription, servers.filter(secondaryFilter))
      );

      const eligibleSecondaries = secondaries.filter(isDeprioritizedFactory(deprioritized));
      if (eligibleSecondaries.length) {
        return latencyWindowReducer(topologyDescription, eligibleSecondaries);
      }

      // if we make it here, we have no primaries or secondaries that not deprioritized.
      // prefer the primary (which may not exist, if the topology has no primary).
      // otherwise, return the secondaries (which also may not exist, but there is nothing else to check here).
      return primary.length ? primary : latencyWindowReducer(topologyDescription, secondaries);
    }
    case 'nearest': {
      const eligible = filterDeprioritized(
        tagSetReducer(
          readPreference,
          maxStalenessReducer(readPreference, topologyDescription, servers.filter(nearestFilter))
        ),
        deprioritized
      );
      return latencyWindowReducer(topologyDescription, eligible);
    }
    case 'secondary':
    case 'secondaryPreferred': {
      const secondaries = tagSetReducer(
        readPreference,
        maxStalenessReducer(readPreference, topologyDescription, servers.filter(secondaryFilter))