printSecondaryReplicationInfo as JSON output

Wernfried_Domscheit · April 2, 2022, 10:08pm

When you run rs.printSecondaryReplicationInfo(); then the output is like this:

source: d-mipmdb-cfg-02:27019
	syncedTo: Sat Apr 02 2022 20:01:01 GMT+0200 (W. Europe Daylight Time)
	0 secs (0 hrs) behind the primary 
source: d-mipmdb-cfg-03:27019
	syncedTo: Sat Apr 02 2022 20:00:58 GMT+0200 (W. Europe Daylight Time)
	3 secs (0 hrs) behind the primary

The documentation says “Use rs.printSecondaryReplicationInfo() for manual inspection, and rs.status() in scripts.” but it does not tell me directly where to see this information.

I guess with rs.status() you get the same information with a script like this:

const members = rs.status().members
const primary = members.filter(x=> x.stateStr == "PRIMARY").shift().optimeDate;
members.filter(x=> x.stateStr == "SECONDARY").map(x=> {return `${x.name} is ${(primary - x.optimeDate)/1000} Seconds behind the primary`});

[
	"d-mipmdb-cfg-02:27019 is 2 Seconds behind the primary",
	"d-mipmdb-cfg-03:27019 is 3 Seconds behind the primary"
]

Is this correct or do I need to run something else?

Kind Regards
Wernfried

Stennie · April 3, 2022, 12:49am

Hi @Wernfried_Domscheit,

MongoDB shell helpers derive replication lag from rs.status(), so your approach looks correct.

The implementations in the new and legacy MongoDB shells may be helpful references, although they have additional logic and error handling to try to cover broader usage scenarios than your example.

`mongosh` (`mongodb-js/mongosh`):

The new MongoDB shell uses the MongoDB Node.js driver and TypeScript:

github.com

mongodb-js/mongosh/blob/v1.3.1/packages/shell-api/src/database.ts#L1271

      
        
            @apiVersions([1])
            async printShardingStatus(verbose = false): Promise<CommandResult> {
              this._emitDatabaseApiCall('printShardingStatus', { verbose });
              const result = await getPrintableShardStatus(await getConfigDB(this), verbose);
              return new CommandResult('StatsResult', result);
            }
            
            
@returnsPromise
            @topologies([Topologies.ReplSet])
            @apiVersions([])
            async printSecondaryReplicationInfo(): Promise<CommandResult> {
              let startOptimeDate = null;
              const local = this.getSiblingDB('local');
            
            
  if (await local.getCollection('system.replset').countDocuments({}) !== 0) {
                const status = await this._runAdminCommand({ 'replSetGetStatus': 1 });
                // get primary
                let primary = null;
                for (const member of status.members) {
                  if (member.state === 1) {
                    primary = member;

`mongo` shell (`mongodb/mongo/src/mongo/shell`)

The legacy mongo shell uses an embedded JavaScript engine:

github.com

mongodb/mongo/blob/r5.0.0/src/mongo/shell/db.js#L1047

      
        
                print("oplog last event time:   " + result.tLast);
                print("now:                     " + result.now);
            };
            
            
DB.prototype.printSlaveReplicationInfo = function() {
                print(
                    "WARNING: printSlaveReplicationInfo is deprecated and may be removed in the next major release. Please use printSecondaryReplicationInfo instead.");
                this.printSecondaryReplicationInfo();
            };
            
            
DB.prototype.printSecondaryReplicationInfo = function() {
                var startOptimeDate = null;
                var primary = null;
            
            
    function getReplLag(st) {
                    assert(startOptimeDate, "how could this be null (getReplLag startOptimeDate)");
                    print("\tsyncedTo: " + st.toString());
                    var ago = (startOptimeDate - st) / 1000;
                    var hrs = Math.round(ago / 36) / 100;
                    var suffix = "";
                    if (primary) {

Regards,
Stennie

printSecondaryReplicationInfo as JSON output

mongosh (mongodb-js/mongosh):

mongo shell (mongodb/mongo/src/mongo/shell)

`mongosh` (`mongodb-js/mongosh`):

`mongo` shell (`mongodb/mongo/src/mongo/shell`)