c# - 哪种方式更好？将媒体文件作为字节数组还是字符串保存到 MongoDB？

Question

我正在 MongoDB 中将媒体文件(图片、PDF 等)保存为字节数组。我看到人们通过将字节数组编码和解码为字符串来保存它的示例。有什么不同？也许性能上有区别？那么哪种方式更好呢？

我注意到，当文件保存为字节数组时，Mongo Management Studio 打开集合的时间比保存为字符串时的时间长

Answer 1

我假设您想将文件存储在文档中。但是您是否考虑过使用 GridFS 与将文件存储在文档中？

就像 Liam 指出的那样，MongoDB 提供了一篇关于 GridFS 注意事项的博客文章 here

我正在从事的项目的优点之一是无需检查文件大小，您可以简单地在二进制流中写入和读取文件。

从性能角度来看，以二进制形式保存和检索文件比首先将其序列化为字符串要快。在针对 MongoDb 3.2 数据库运行的测试程序中，以二进制形式在文档中保存文件比以字符串序列化形式保存文件快 3 倍。这是可以理解的，因为字符串序列化形式只是要保存或读取“更多字节”。

在同一个测试程序中，还针对 GridFS 进行了快速测试，但是您确实必须使用 block 大小进行一轮测试才能获得最佳性能。

下面是一个非常粗糙的测试程序的代码转储(请注意，您必须自己提供正确的example.jpg，并且数据库连接已被硬编码。)

class Program
{
    static bool keepRunning;
    static string fileName = "example.jpg";
    static int numDocs = 571;
    static IMongoDatabase mongoDb;

    static void Main(string[] args)
    {
        Console.CancelKeyPress += delegate
        {
            Exit();
        };

        keepRunning = true;

        SetupMongoDb();

        var fileBytes = File.ReadAllBytes(fileName);
        Console.WriteLine($"Picturesize in bytes: {fileBytes.Length}");

        ClearCollections();

        Console.WriteLine($"Saving {numDocs} pictures to the database.");

        Console.WriteLine("\nStart Saving in Binary Mode.");
        Stopwatch binaryStopWatch = Stopwatch.StartNew();
        SaveBinaryBased(numDocs, fileBytes);
        binaryStopWatch.Stop();
        Console.WriteLine("Done Saving in Binary Mode.");

        Console.WriteLine("\nStart Saving in String-based Mode.");
        Stopwatch stringStopWatch = Stopwatch.StartNew();
        SaveStringBased(numDocs, fileBytes);
        stringStopWatch.Stop();
        Console.WriteLine("Done Saving in String-based Mode.");

        Console.WriteLine("\nTime Report Saving");
        Console.WriteLine($"   * Total Time Binary for {numDocs} records: {binaryStopWatch.ElapsedMilliseconds} ms.");
        Console.WriteLine($"   * Total Time String for {numDocs} records: {stringStopWatch.ElapsedMilliseconds} ms.");

        Console.WriteLine("\nCollection Statistics:");
        Statistics("binaryPics");
        Statistics("stringBasedPics");

        Console.WriteLine("\nTest Retrieval:");
        Console.WriteLine("\nStart Retrieving from binary collection.");
        binaryStopWatch.Restart();
        RetrieveBinary();
        binaryStopWatch.Stop();
        Console.WriteLine("Done Retrieving from binary collection.");

        Console.WriteLine("\nStart Retrieving from string-based collection.");
        stringStopWatch.Restart();
        RetrieveString();
        stringStopWatch.Stop();
        Console.WriteLine("Done Retrieving from string-based collection.");

        Console.WriteLine("\nTime Report Retrieving:");
        Console.WriteLine($"   * Total Time Binary for retrieving {numDocs} records: {binaryStopWatch.ElapsedMilliseconds} ms.");
        Console.WriteLine($"   * Total Time String for retrieving {numDocs} records: {stringStopWatch.ElapsedMilliseconds} ms.");

        ClearGridFS();
        Console.WriteLine($"\nStart saving {numDocs} files to GridFS:");
        binaryStopWatch.Restart();
        SaveFilesToGridFS(numDocs, fileBytes);
        binaryStopWatch.Stop();
        Console.WriteLine($"Saved {numDocs} files to GridFS in {binaryStopWatch.ElapsedMilliseconds} ms.");

        Console.WriteLine($"\nStart retrieving {numDocs} files from GridFS:");
        binaryStopWatch.Restart();
        RetrieveFromGridFS();
        binaryStopWatch.Stop();
        Console.WriteLine($"Retrieved {numDocs} files from GridFS in {binaryStopWatch.ElapsedMilliseconds} ms.");

        while (keepRunning)
        {
            Thread.Sleep(500);
        }
    }

    private static void Exit()
    {
        keepRunning = false;
    }

    private static void ClearCollections()
    {
        var collectionBin = mongoDb.GetCollection<BsonDocument>("binaryPics");
        var collectionString = mongoDb.GetCollection<BsonDocument>("stringBasedPics");

        collectionBin.DeleteMany(new BsonDocument());
        collectionString.DeleteMany(new BsonDocument());
    }

    private static void SetupMongoDb()
    {
        string hostName = "localhost";
        int portNumber = 27017;
        string databaseName = "exampleSerialization";

        var clientSettings = new MongoClientSettings()
        {
            Server = new MongoServerAddress(hostName, portNumber),
            MinConnectionPoolSize = 1,
            MaxConnectionPoolSize = 1500,
            ConnectTimeout = new TimeSpan(0, 0, 30),
            SocketTimeout = new TimeSpan(0, 1, 30),
            WaitQueueTimeout = new TimeSpan(0, 1, 0)
        };

        mongoDb = new MongoClient(clientSettings).GetDatabase(databaseName);
    }

    private static void SaveBinaryBased(int numDocuments, byte[] content)
    {
        var collection = mongoDb.GetCollection<BsonDocument>("binaryPics");

        BsonDocument baseDoc = new BsonDocument();
        baseDoc.SetElement(new BsonElement("jpgContent", content));

        for (int i = 0; i < numDocs; ++i)
        {
            baseDoc.SetElement(new BsonElement("_id", Guid.NewGuid()));
            baseDoc.SetElement(new BsonElement("filename", fileName));
            baseDoc.SetElement(new BsonElement("title", $"picture number {i}"));
            collection.InsertOne(baseDoc);
        }
    }

    private static void SaveStringBased(int numDocuments, byte[] content)
    {
        var collection = mongoDb.GetCollection<BsonDocument>("stringBasedPics");

        BsonDocument baseDoc = new BsonDocument();
        baseDoc.SetElement(new BsonElement("jpgStringContent", System.Text.Encoding.UTF8.GetString(content)));

        for (int i = 0; i < numDocs; ++i)
        {
            baseDoc.SetElement(new BsonElement("_id", Guid.NewGuid()));
            baseDoc.SetElement(new BsonElement("filename", fileName));
            baseDoc.SetElement(new BsonElement("title", $"picture number {i}"));
            collection.InsertOne(baseDoc);
        }
    }

    private static void Statistics(string collectionName)
    {
        new BsonDocument { { "collstats", collectionName } };
        var command = new BsonDocumentCommand<BsonDocument>(new BsonDocument { { "collstats", collectionName } });
        var stats = mongoDb.RunCommand(command);

        Console.WriteLine($"  * Collection      : {collectionName}");
        Console.WriteLine($"  * Count           : {stats["count"].AsInt32} documents");
        Console.WriteLine($"  * Average Doc Size: {stats["avgObjSize"].AsInt32} bytes");
        Console.WriteLine($"  * Total Storage   : {stats["storageSize"].AsInt32} bytes");
        Console.WriteLine("\n");
    }

    private static void RetrieveBinary()
    {
        var collection = mongoDb.GetCollection<BsonDocument>("binaryPics");
        var docs = collection.Find(new BsonDocument()).ToEnumerable();

        foreach (var doc in docs)
        {
            byte[] fileArray = doc.GetElement("jpgContent").Value.AsByteArray;
            // we can simulate that we do something with the results but that's not the purpose of this experiment
            fileArray = null;
        }
    }

    private static void RetrieveString()
    {
        var collection = mongoDb.GetCollection<BsonDocument>("stringBasedPics");
        var docs = collection.Find(new BsonDocument()).ToEnumerable();

        foreach (var doc in docs)
        {
            // Simply get the string, we don't want to hit the performance test
            // with a conversion to a byte array
            string result = doc.GetElement("jpgStringContent").Value.AsString;
        }
    }

    private static void SaveFilesToGridFS(int numFiles, byte[] content)
    {
        var bucket = new GridFSBucket(mongoDb, new GridFSBucketOptions
        {
            BucketName = "pictures"
        });

        for (int i = 0; i < numFiles; ++i)
        {
            string targetFileName = $"{fileName.Substring(0, fileName.Length - ".jpg".Length)}{i}.jpg";
            int chunkSize = content.Length <= 1048576 ? 51200 : 1048576;
            bucket.UploadFromBytes(targetFileName, content, new GridFSUploadOptions { ChunkSizeBytes = chunkSize });
        }
    }

    private static void ClearGridFS()
    {
        var bucket = new GridFSBucket(mongoDb, new GridFSBucketOptions { BucketName = "pictures" });
        bucket.Drop();
    }

    private static void RetrieveFromGridFS()
    {
        var bucket = new GridFSBucket(mongoDb, new GridFSBucketOptions { BucketName = "pictures" });
        var filesIds = mongoDb.GetCollection<BsonDocument>("pictures.files").Find(new BsonDocument()).ToEnumerable().Select(doc => doc.GetElement("_id").Value);

        foreach (var id in filesIds)
        {
            var fileBytes = bucket.DownloadAsBytes(id);
            fileBytes = null;
        }
    }
}