/**
 * serverStatus.metrics.repl.apply.batches.totalMillis is a cumulative measure of how much time a
 * node spends applying batches. This test checks that it includes the time spent waiting for
 * batches to finish by making sure that the timer is cumulative and monotonic. We cannot make
 * any assumptions about how long batches actually take as that can vary enormously between
 * machines and test runs.
 */

(function() {
"use strict";

// Gets the value of metrics.repl.apply.batches.totalMillis.
function getTotalMillis(node) {
    return assert.commandWorked(node.adminCommand({serverStatus: 1}))
        .metrics.repl.apply.batches.totalMillis;
}

// Do a bulk insert of documents as: {{key: 0}, {key: 1}, {key: 2}, ... , {key: num-1}}
function performBulkInsert(coll, key, num) {
    let bulk = coll.initializeUnorderedBulkOp();
    for (let i = 0; i < num; i++) {
        let doc = {};
        doc[key] = i;
        bulk.insert(doc);
    }
    assert.commandWorked(bulk.execute());
    rst.awaitReplication();
}

let name = "apply_batches_totalMillis";
let rst = new ReplSetTest({name: name, nodes: 2});
rst.startSet();
rst.initiate();

let primary = rst.getPrimary();
let secondary = rst.getSecondary();
let coll = primary.getDB(name)["foo"];

// Perform an initial write on the system and ensure steady state.
assert.commandWorked(coll.insert({init: 0}));
rst.awaitReplication();

let baseTime = getTotalMillis(secondary);
jsTestLog(`Base time recorded: ${baseTime}ms`);

// Introduce a small load and wait for it to be replicated.
performBulkInsert(coll, "small", 1000);

// Record the time spent applying the small load. The timer should not move backwards.
let timeAfterSmall = getTotalMillis(secondary);
jsTestLog(`Time recorded after smaller batch: ${timeAfterSmall}ms`);
assert.gte(timeAfterSmall, baseTime);

// Insert a significantly larger load.
performBulkInsert(coll, "large", 20000);

// Record the time spent applying the large load.
let timeAfterLarge = getTotalMillis(secondary);
jsTestLog(`Time recorded after larger batch: ${timeAfterLarge}ms`);
assert.gte(timeAfterLarge, timeAfterSmall);

rst.stopSet();
})();