path: root/src/mongo/s/commands/cluster_explain.cpp

/**
 *    Copyright (C) 2018-present MongoDB, Inc.
 *
 *    This program is free software: you can redistribute it and/or modify
 *    it under the terms of the Server Side Public License, version 1,
 *    as published by MongoDB, Inc.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    Server Side Public License for more details.
 *
 *    You should have received a copy of the Server Side Public License
 *    along with this program. If not, see
 *    <http://www.mongodb.com/licensing/server-side-public-license>.
 *
 *    As a special exception, the copyright holders give permission to link the
 *    code of portions of this program with the OpenSSL library under certain
 *    conditions as described in each individual source file and distribute
 *    linked combinations including the program with the OpenSSL library. You
 *    must comply with the Server Side Public License in all respects for
 *    all of the code used other than as permitted herein. If you modify file(s)
 *    with this exception, you may extend this exception to your version of the
 *    file(s), but you are not obligated to do so. If you do not wish to do so,
 *    delete this exception statement from your version. If you delete this
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 *    it in the license file.
 */

#include "mongo/platform/basic.h"

#include "mongo/bson/bsonmisc.h"
#include "mongo/db/command_generic_argument.h"
#include "mongo/db/commands.h"
#include "mongo/db/query/explain_common.h"
#include "mongo/rpc/get_status_from_command_result.h"
#include "mongo/s/client/shard_registry.h"
#include "mongo/s/commands/cluster_explain.h"
#include "mongo/s/grid.h"

namespace mongo {

using std::vector;

const char* ClusterExplain::kSingleShard = "SINGLE_SHARD";
const char* ClusterExplain::kMergeFromShards = "SHARD_MERGE";
const char* ClusterExplain::kMergeSortFromShards = "SHARD_MERGE_SORT";
const char* ClusterExplain::kWriteOnShards = "SHARD_WRITE";

namespace {

//
// BSON size limit management: these functions conditionally append to a
// BSON object or BSON array buidlder, depending on whether or not the
// maximum user size for a BSON object will be exceeded.
//

bool appendIfRoom(BSONObjBuilder* bob, const BSONObj& toAppend, StringData fieldName) {
    if ((bob->len() + toAppend.objsize()) < BSONObjMaxUserSize) {
        bob->append(fieldName, toAppend);
        return true;
    }

    // Unless 'bob' has already exceeded the max BSON user size, add a warning indicating
    // that data has been truncated.
    if (bob->len() < BSONObjMaxUserSize) {
        bob->append("warning", "output truncated due to nearing BSON max user size");
    }

    return false;
}

bool appendToArrayIfRoom(BSONArrayBuilder* arrayBuilder, const BSONElement& toAppend) {
    if ((arrayBuilder->len() + toAppend.size()) < BSONObjMaxUserSize) {
        arrayBuilder->append(toAppend);
        return true;
    }

    // Unless 'arrayBuilder' has already exceeded the max BSON user size, add a warning
    // indicating that data has been truncated.
    if (arrayBuilder->len() < BSONObjMaxUserSize) {
        arrayBuilder->append(BSON("warning"
                                  << "output truncated due to nearing BSON max user size"));
    }

    return false;
}

bool appendElementsIfRoom(BSONObjBuilder* bob, const BSONObj& toAppend) {
    if ((bob->len() + toAppend.objsize()) < BSONObjMaxUserSize) {
        bob->appendElements(toAppend);
        return true;
    }

    // Unless 'bob' has already exceeded the max BSON user size, add a warning indicating
    // that data has been truncated.
    if (bob->len() < BSONObjMaxUserSize) {
        bob->append("warning", "output truncated due to nearing BSON max user size");
    }

    return false;
}

void throwOnBadARSResponse(const AsyncRequestsSender::Response& arsResponse) {
    auto status = arsResponse.swResponse.isOK()
        ? getStatusFromCommandResult(arsResponse.swResponse.getValue().data)
        : arsResponse.swResponse.getStatus();
    uassertStatusOKWithContext(
        status, str::stream() << "Explain command on shard " << arsResponse.shardId << " failed");

    invariant(arsResponse.shardHostAndPort);
}

}  // namespace

// static
BSONObj ClusterExplain::wrapAsExplain(const BSONObj& cmdObj, ExplainOptions::Verbosity verbosity) {
    auto filtered = CommandHelpers::filterCommandRequestForPassthrough(cmdObj);
    BSONObjBuilder out;
    out.append("explain", filtered);
    out.append("verbosity", ExplainOptions::verbosityString(verbosity));

    // Propagate all generic arguments out of the inner command since the shards will only process
    // them at the top level.
    for (auto elem : filtered) {
        if (isGenericArgument(elem.fieldNameStringData())) {
            out.append(elem);
        }
    }

    return out.obj();
}

// static
void ClusterExplain::validateShardResponses(
    const vector<AsyncRequestsSender::Response>& shardResponses) {
    // Error we didn't get responses from any shards.
    uassert(ErrorCodes::InternalError, "No shards found for explain", !shardResponses.empty());

    // Count up the number of shards that have execution stats and all plan execution level
    // information.
    size_t numShardsExecStats = 0;
    size_t numShardsAllPlansStats = 0;

    // Check that the response from each shard has a true value for "ok" and has
    // the expected "queryPlanner" field.
    for (auto&& response : shardResponses) {
        throwOnBadARSResponse(response);

        auto responseData = response.swResponse.getValue().data;

        uassert(ErrorCodes::OperationFailed,
                str::stream() << "Explain command on shard " << response.shardId
                              << " failed, caused by: " << responseData,
                responseData["queryPlanner"].type() == Object);

        if (responseData.hasField("executionStats")) {
            numShardsExecStats++;
            BSONObj execStats = responseData["executionStats"].Obj();
            if (execStats.hasField("allPlansExecution")) {
                numShardsAllPlansStats++;
            }
        }
    }

    // Either all shards should have execution stats info, or none should.
    uassert(ErrorCodes::InternalError,
            str::stream() << "Only " << numShardsExecStats << "/" << shardResponses.size()
                          << " had executionStats explain information.",
            numShardsExecStats == 0 || numShardsExecStats == shardResponses.size());

    // Either all shards should have all plans execution stats, or none should.
    uassert(ErrorCodes::InternalError,
            str::stream() << "Only " << numShardsAllPlansStats << "/" << shardResponses.size()
                          << " had allPlansExecution explain information.",
            numShardsAllPlansStats == 0 || numShardsAllPlansStats == shardResponses.size());
}

// static
const char* ClusterExplain::getStageNameForReadOp(size_t numShards, const BSONObj& explainObj) {
    if (numShards == 1) {
        return kSingleShard;
    } else if (explainObj.hasField("sort")) {
        return kMergeSortFromShards;
    } else {
        return kMergeFromShards;
    }
}

// static
void ClusterExplain::buildPlannerInfo(OperationContext* opCtx,
                                      const vector<AsyncRequestsSender::Response>& shardResponses,
                                      const char* mongosStageName,
                                      BSONObjBuilder* out) {
    BSONObjBuilder queryPlannerBob(out->subobjStart("queryPlanner"));

    queryPlannerBob.appendNumber("mongosPlannerVersion", 1);
    BSONObjBuilder winningPlanBob(queryPlannerBob.subobjStart("winningPlan"));

    winningPlanBob.append("stage", mongosStageName);
    BSONArrayBuilder shardsBuilder(winningPlanBob.subarrayStart("shards"));
    for (size_t i = 0; i < shardResponses.size(); i++) {
        auto responseData = shardResponses[i].swResponse.getValue().data;

        BSONObjBuilder singleShardBob(shardsBuilder.subobjStart());

        BSONObj queryPlanner = responseData["queryPlanner"].Obj();
        BSONObj serverInfo = responseData["serverInfo"].Obj();

        singleShardBob.append("shardName", shardResponses[i].shardId.toString());
        {
            const auto shard = uassertStatusOK(
                Grid::get(opCtx)->shardRegistry()->getShard(opCtx, shardResponses[i].shardId));
            singleShardBob.append("connectionString", shard->getConnString().toString());
        }
        appendIfRoom(&singleShardBob, serverInfo, "serverInfo");
        appendElementsIfRoom(&singleShardBob, queryPlanner);

        singleShardBob.doneFast();
    }
    shardsBuilder.doneFast();
    winningPlanBob.doneFast();
    queryPlannerBob.doneFast();
}

// static
void ClusterExplain::buildExecStats(const vector<AsyncRequestsSender::Response>& shardResponses,
                                    const char* mongosStageName,
                                    long long millisElapsed,
                                    BSONObjBuilder* out) {
    auto firstShardResponseData = shardResponses[0].swResponse.getValue().data;
    if (!firstShardResponseData.hasField("executionStats")) {
        // The shards don't have execution stats info. Bail out without adding anything
        // to 'out'.
        return;
    }

    BSONObjBuilder executionStatsBob(out->subobjStart("executionStats"));

    // Collect summary stats from the shards.
    long long nReturned = 0;
    long long keysExamined = 0;
    long long docsExamined = 0;
    long long totalChildMillis = 0;
    for (size_t i = 0; i < shardResponses.size(); i++) {
        auto responseData = shardResponses[i].swResponse.getValue().data;
        BSONObj execStats = responseData["executionStats"].Obj();
        if (execStats.hasField("nReturned")) {
            nReturned += execStats["nReturned"].numberLong();
        }
        if (execStats.hasField("totalKeysExamined")) {
            keysExamined += execStats["totalKeysExamined"].numberLong();
        }
        if (execStats.hasField("totalDocsExamined")) {
            docsExamined += execStats["totalDocsExamined"].numberLong();
        }
        if (execStats.hasField("executionTimeMillis")) {
            totalChildMillis += execStats["executionTimeMillis"].numberLong();
        }
    }

    // Fill in top-level stats.
    executionStatsBob.appendNumber("nReturned", nReturned);
    executionStatsBob.appendNumber("executionTimeMillis", millisElapsed);
    executionStatsBob.appendNumber("totalKeysExamined", keysExamined);
    executionStatsBob.appendNumber("totalDocsExamined", docsExamined);

    // Fill in the tree of stages.
    BSONObjBuilder executionStagesBob(executionStatsBob.subobjStart("executionStages"));

    // Info for the root mongos stage.
    executionStagesBob.append("stage", mongosStageName);
    executionStatsBob.appendNumber("nReturned", nReturned);
    executionStatsBob.appendNumber("executionTimeMillis", millisElapsed);
    executionStatsBob.appendNumber("totalKeysExamined", keysExamined);
    executionStatsBob.appendNumber("totalDocsExamined", docsExamined);
    executionStagesBob.append("totalChildMillis", totalChildMillis);

    BSONArrayBuilder execShardsBuilder(executionStagesBob.subarrayStart("shards"));
    for (size_t i = 0; i < shardResponses.size(); i++) {
        auto responseData = shardResponses[i].swResponse.getValue().data;

        BSONObjBuilder singleShardBob(execShardsBuilder.subobjStart());
        BSONObj execStats = responseData["executionStats"].Obj();

        singleShardBob.append("shardName", shardResponses[i].shardId.toString());
        appendElementsIfRoom(&singleShardBob, execStats);
        singleShardBob.doneFast();
    }
    execShardsBuilder.doneFast();

    executionStagesBob.doneFast();

    if (!firstShardResponseData["executionStats"].Obj().hasField("allPlansExecution")) {
        // The shards don't have execution stats for all plans, so we're done.
        executionStatsBob.doneFast();
        return;
    }

    // Add the allPlans stats from each shard.
    BSONArrayBuilder allPlansExecBob(executionStatsBob.subarrayStart("allPlansExecution"));
    for (size_t i = 0; i < shardResponses.size(); i++) {
        auto responseData = shardResponses[i].swResponse.getValue().data;

        BSONObjBuilder singleShardBob(execShardsBuilder.subobjStart());
        singleShardBob.append("shardName", shardResponses[i].shardId.toString());

        BSONObj execStats = responseData["executionStats"].Obj();
        vector<BSONElement> allPlans = execStats["allPlansExecution"].Array();

        BSONArrayBuilder innerArrayBob(singleShardBob.subarrayStart("allPlans"));
        for (size_t j = 0; j < allPlans.size(); j++) {
            appendToArrayIfRoom(&innerArrayBob, allPlans[j]);
        }
        innerArrayBob.done();

        singleShardBob.doneFast();
    }
    allPlansExecBob.doneFast();

    executionStatsBob.doneFast();
}

// static
Status ClusterExplain::buildExplainResult(
    OperationContext* opCtx,
    const vector<AsyncRequestsSender::Response>& shardResponses,
    const char* mongosStageName,
    long long millisElapsed,
    const BSONObj& command,
    BSONObjBuilder* out) {
    // Explain only succeeds if all shards support the explain command.
    try {
        ClusterExplain::validateShardResponses(shardResponses);
    } catch (const DBException& ex) {
        return ex.toStatus();
    }

    buildPlannerInfo(opCtx, shardResponses, mongosStageName, out);
    buildExecStats(shardResponses, mongosStageName, millisElapsed, out);
    explain_common::generateServerInfo(out);
    appendIfRoom(out, command, "command");

    return Status::OK();
}


}  // namespace mongo