Which way is better? Save a media file to MongoDB as array of bytes or as string?

I assume that you want to store the file inside a document.But have you considered using GridFS vs storing the file inside the document?

Like Liam pointed out, a MongoDB provides a blog-post on GridFS considerations here

One of the advantages in a project I'm working on is that no checking on file-sizes has to be done, and you can simply write and read the file in a binary stream.

From a performance perspective, saving and retrieving the file in binary form is faster than first serializing it to a string.In a test program, running against a MongoDb 3.2 database, saving a file in binary form in a document was up to 3 times faster than saving the file in a string-serialized form. Which is understandable, since the string-serialized form is simply 'more bytes' to save or read.

In the same test program a quick test was also performed against GridFS, but there you really have to play a round with the chunck-size to get to the best possible performance.

Below a code-dump for a very crude test program (note that you have to provide the right example.jpg yourself and that the database connection has been hard-coded.)

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;        }    }}