Function maxStalenessReducer

src/sdam/server_selection.ts:119–181

* Reduces the passed in array of servers by the rules of the "Max Staleness" specification * found here: * * @see https://github.com/mongodb/specifications/blob/master/source/max-staleness/max-staleness.md * * @param readPreference - The read preference providing max staleness guidance * @para

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

 * @returns The list of servers that satisfy the requirements of max staleness
 */
function maxStalenessReducer(
  readPreference: ReadPreference,
  topologyDescription: TopologyDescription,
  servers: ServerDescription[]
): ServerDescription[] {
  if (readPreference.maxStalenessSeconds == null || readPreference.maxStalenessSeconds < 0) {
    return servers;
  }

  const maxStaleness = readPreference.maxStalenessSeconds;
  const maxStalenessVariance =
    (topologyDescription.heartbeatFrequencyMS + IDLE_WRITE_PERIOD) / 1000;
  if (maxStaleness < maxStalenessVariance) {
    throw new MongoInvalidArgumentError(
      `Option "maxStalenessSeconds" must be at least ${maxStalenessVariance} seconds`
    );
  }

  if (maxStaleness < SMALLEST_MAX_STALENESS_SECONDS) {
    throw new MongoInvalidArgumentError(
      `Option "maxStalenessSeconds" must be at least ${SMALLEST_MAX_STALENESS_SECONDS} seconds`
    );
  }

  if (topologyDescription.type === TopologyType.ReplicaSetWithPrimary) {
    const primary: ServerDescription = Array.from(topologyDescription.servers.values()).filter(
      primaryFilter
    )[0];

    return servers.filter((server: ServerDescription) => {
      const stalenessMS =
        server.lastUpdateTime -
        server.lastWriteDate -
        (primary.lastUpdateTime - primary.lastWriteDate) +
        topologyDescription.heartbeatFrequencyMS;

      const staleness = stalenessMS / 1000;
      const maxStalenessSeconds = readPreference.maxStalenessSeconds ?? 0;
      return staleness <= maxStalenessSeconds;
    });
  }

  if (topologyDescription.type === TopologyType.ReplicaSetNoPrimary) {
    if (servers.length === 0) {
      return servers;
    }

    const sMax = servers.reduce((max: ServerDescription, s: ServerDescription) =>
      s.lastWriteDate > max.lastWriteDate ? s : max
    );

    return servers.filter((server: ServerDescription) => {
      const stalenessMS =
        sMax.lastWriteDate - server.lastWriteDate + topologyDescription.heartbeatFrequencyMS;

      const staleness = stalenessMS / 1000;
      const maxStalenessSeconds = readPreference.maxStalenessSeconds ?? 0;
      return staleness <= maxStalenessSeconds;