Today we encountered slowness on MongoDB that caused all the infrastructure to get affected. The problem was that slowness on some specific mongo queries caused all the other queries to wait. YES we use index and YES we used explained on those queries and saw that those queries are using index. Now to mitigate the issue we had to kill very slow find queries until we fix the issue.

The function below kills slow queries:

function (sec) {db.currentOp()['inprog'].forEach(function (query) {     if (query.op !== 'query') { return; }      if (query.secs_running < sec) { return; }        print(['Killing query:', query.opid,             'which was running:', query.secs_running, 'sec.'].join('   '));     db.killOp(query.opid); })}

We need to save this query in mongo itself and run it directly. To save the above function in mongoDB use db.system.js.save:

db.system.js.save({_id:"kill_slow_queries", value:function (sec) {db.currentOp()['inprog'].forEach(function (query) {     if (query.op !== 'query') { return; }      if (query.secs_running < sec) { return; }        print(['Killing query:', query.opid,             'which   was running:', query.secs_running, 'sec.'].join(' '));     db.killOp(query.opid); })} })

I will explain the above function parts in a different post. Now you need to load server scripts and then run it:

db.loadServerScripts()
 kill_slow_queries(20)

The above query kills queries that has taken longer than 20s.

NOTE: you can create a shell script and run it periodically using crontab until you fix the slowness on your server.

#mongodb #mongo #function #kill_slow_queries #currentOp