c# - 如何在第一个文档中恢复 MongoDB ChangeStream，而不仅仅是在我开始收听后进行更改

Question

我对这个应用程序的目标是创建监视数据库的逻辑，并在将文档添加到数据库时触发操作(例如发送电子邮件)。但是，由于第一次填充数据库时可能不会启动此应用程序，我如何手动创建一个指向添加到集合中的第一个文档的 ResumeToken，以便在第一次运行时，我可以从头开始并遍历更改，直到我到达终点。我认识到我需要存储来自 lastChangeStreamDocument 的 ResumeToken 以供将来重新启动，但我对“首次运行”场景感兴趣。我虽然 enumerator.Reset(); 是正确的选项，但它引发了一个异常，表明它不受支持。

我遵循了 https://github.com/mongodb/mongo-csharp-driver/blob/master/tests/MongoDB.Driver.Examples/ChangeStreamExamples.cs 中提供的测试并使用以下代码成功配置了 Change Stream

mongoClient = mongoClient ?? new MongoClient(ConnectionString);  //Create client object if it is null
IMongoDatabase sandboxDB = mongoClient.GetDatabase("SandboxDB");

var collection = sandboxDB.GetCollection<BsonDocument>("CollectionToMonitor");

try
{
    var cursor = collection.Watch();
    var enumerator = cursor.ToEnumerable().GetEnumerator();

    enumerator.MoveNext();  //Blocks until a record is UPDATED in the database
    var lastChangeStreamDocument = enumerator.Current;
    enumerator.Dispose();
    //lastChangeStreamDocument.FullDocument.Should().Be(document);

}
catch( Exception ex)
{
    Logger.WriteException(ex);
}

但是，使用此代码，enumerator.MoveNext() 行会阻塞，直到文档被更新，因此我只能在设置更改流后获取对更新文档的引用。

我想搜索 local.oplog 数据库并获取插入集合中的第一个文档的 UUID 并且成功了，但是，我没有看到将此引用转换为 ResumeToken 对象的方法可以喂 watch 方法。

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更新:

ResumeToken 似乎存储为 Base64，其中包含时间戳、o._id ObjectID 以及来自 oplog 条目的 ui UUID。我需要更多地遍历代码，但从源代码 ( https://github.com/mongodb/mongo/blob/c906f6357d22f66d58e3334868025069c62bd97b/src/mongo/db/pipeline/resume_token_test.cpp ) 看来，ResumeTokens 有不同的格式。有了这些信息，我希望可以构建自己的 ResumeToken 来匹配数据库期望的格式。

---

更新#2:

经过更多研究，我偶然发现了在 mongo 中解析 key_string 的代码 github.com/mongodb/mongo/src/mongo/db/storage/key_string.cpp .此文件包含 CType 的定义。我将 Base64 解码为字节数组，然后使用 CType 枚举定义，我能够更多地了解如何构建我自己的 ResumeToken。

考虑以下示例:更新文档后，我在 ChangeStream 上捕获了 ResumeToken。

glp9zsgAAAABRmRfaWQAZFp9zH40PyabFRwB/ABaEAQESw1YexhL967nKLXsT5Z+BA==

解码为字节数组:

82 5a 7d ce c8 00 00 00 01 46 64 5f 69 64 00 64 5a 7d cc 7e 34 3f 26 9b 15 1c 01 fc 00 5a 10 04 04 4b 0d 58 7b 18 4b f7 ae e7 28 b5 ec 4f 96 7e 04

我已经解码为:

//Timestamp (of oplog entry??)
82    //CType::TimeStamp
5a 7d ce c8 00 00 00 01   //It appears to be expecting a 64b number
//I'm not sure why the last byte 0x01 unless it has something to do with little/bit endian
//Matching oplog doc has { ts: TimeStamp(1518194376, 1) }
//  that integer converts to 0x5A7DCEC8

//Unknown Object
46    //CType::Object
64 5f 69 64     //Either expecting a 32b value or null terminated
00    //Null terminator or divider

//Document ID
64    //CType::OID
5a 7d cc 7e 34 3f 26 9b 15 1c 01 fc  //o._id value from oplog entry
00    //OID expecting null terminated

//UUID
5a    //CType::BinData
10    //Length (16b)
04    //BinDataType of newUUID (from bsontypes.h)
04 4b 0d 58 7b 18 4b f7 ae e7 28 b5 ec 4f 96 7e  //UUID value from oplog entry
04    //Unknown byte. Perhaps end of ResumeToken, or end of UUID mark?

我现在遇到的问题是，如果我在一个集合中有很多 oplog 条目，并且我使用 oplog 中第一个条目的 ts、ui 和 o._id 值来构建我自己的 ResumeToken(硬编码未知 0x4664 5f69 6400 block 和结尾的 0x04 字节，然后服务器在设置 collection.Watch 时将其接受为有效的 ResumeToken。但是， enumerator.moveNext() 调用返回的文档总是返回第 3 个 oplog 条目而不是第 2 个!

在不知道该 12 字节 block 的用途，也不知道为什么我指向第 3 个条目而不是第 2 个条目的情况下，我很紧张在生产中依赖它。

---

更新 #3:

那些有问题的字节 block :

46 64 5f 69 64 00

0x46 = CType::Object
0x64 = d
0x5F = _
0x69 = i
0x64 = d
0x00 = NULL

以下字节 block 描述了受影响文档的 ObjectId，或者它的“_id”键。那么“d”字符的意义是什么？

Answer 1

在解决这个问题时，我一直在使用其他信息更新问题，现在我已经设法将其拼凑起来，因此它可以正常工作。

下面是我创建的代码:

1. 在 local.oplog 集合中找到命名空间的第一个条目
2. 从该 oplog 文档生成一个 ResumeToken(因此我们在第二个条目上恢复)
3. 测试这些功能的示例。

希望这段代码对其他尝试做同样事情的人有所帮助。

/// <summary>
/// Locates the first document for the given namespace in the local.oplog collection
/// </summary>
/// <param name="docNamespace">Namespace to search for</param>
/// <returns>First Document found in the local.oplog collection for the specified namespace</returns>
internal static BsonDocument GetFirstDocumentFromOpLog(string docNamespace)
{
    mongoClient = mongoClient ?? new MongoClient(ConnectionString);  //Create client object if it is null
    IMongoDatabase localDB = mongoClient.GetDatabase("local");
    var collection = localDB.GetCollection<BsonDocument>("oplog.rs");

    //Find the documents from the specified namespace (DatabaseName.CollectionName), that have an operation type of "insert" (The first entry to a collection must always be an insert)
    var filter = MongoDB.Bson.Serialization.BsonSerializer.Deserialize<BsonDocument>("{ $and: [ { 'ns': '" + docNamespace + "'}, { 'op': 'i'}] }");

    BsonDocument retDoc = null;
    try //to get the first document from the oplog entries
    {       
        retDoc = collection.Find<BsonDocument>(filter).First();
    }
    catch(Exception ex) { /*Logger.WriteException(ex);*/ }
    return retDoc;
}

/// <summary>
/// Takes a document from the OpLog and generates a ResumeToken
/// </summary>
/// <param name="firstDoc">BsonDocument from the local.oplog collection to base the ResumeToken on</param>
/// <returns>A ResumeToken that can be provided to a collection watch (ChangeStream) that points to the firstDoc provided</returns>
private static BsonDocument GetResumeTokenFromOpLogDoc(BsonDocument firstDoc)
{
    List<byte> hexVal = new List<byte>(34);

    //Insert Timestamp of document
    hexVal.Add(0x82);   //TimeStamp Tag
    byte[] docTimeStampByteArr = BitConverter.GetBytes(firstDoc["ts"].AsBsonTimestamp.Timestamp); //Timestamp is an integer, so we need to reverse it
    if (BitConverter.IsLittleEndian) { Array.Reverse(docTimeStampByteArr); }
    hexVal.AddRange(docTimeStampByteArr);

    //Expecting UInt64, so make sure we added 8 bytes (likely only added 4)
    hexVal.AddRange(new byte[] { 0x00, 0x00, 0x00, 0x01 }); //Not sure why the last bytes is a 0x01, but it was present in observed ResumeTokens

    //Unknown Object observed in a ResumeToken
    //0x46 = CType::Object, followed by the string "d_id" NULL
    //This may be something that identifies that the following value is for the "_id" field of the ObjectID given next
    hexVal.AddRange(new byte[] { 0x46, 0x64, 0x5F, 0x69, 0x64, 0x00 }); //Unknown Object, expected to be 32 bits, with a 0x00 terminator

    //Insert OID (from 0._id.ObjectID)
    hexVal.Add(0x64);   //OID Tag
    byte[] docByteArr = firstDoc["o"]["_id"].AsObjectId.ToByteArray();
    hexVal.AddRange(docByteArr);
    hexVal.Add(0x00);   //End of OID

    //Insert UUID (from ui) as BinData
    hexVal.AddRange(new byte[] { 0x5a, 0x10, 0x04 });   //0x5A = BinData, 0x10 is Length (16 bytes), 0x04 is BinDataType (newUUID)
    hexVal.AddRange(firstDoc["ui"].AsByteArray);

    hexVal.Add(0x04);   //Unknown marker (maybe end of resumeToken since 0x04 == ASCII 'EOT')

    //Package the binary data into a BsonDocument with the key "_data" and the value as a Base64 encoded string
    BsonDocument retDoc = new BsonDocument("_data", new BsonBinaryData(hexVal.ToArray()));
    return retDoc;
}


/// <summary>
/// Example Code for setting up and resuming to the second doc
/// </summary>
internal static void MonitorChangeStream()
{
    mongoClient = mongoClient ?? new MongoClient(ConnectionString);  //Create client object if it is null
    IMongoDatabase sandboxDB = mongoClient.GetDatabase("SandboxDB");
    var collection = sandboxDB.GetCollection<BsonDocument>("CollectionToMonitor");

    var options = new ChangeStreamOptions();
    options.FullDocument = ChangeStreamFullDocumentOption.UpdateLookup;

    try
    {
        var pipeline = new EmptyPipelineDefinition<ChangeStreamDocument<BsonDocument>>().Match("{ operationType: { $in: [ 'replace', 'insert', 'update' ] } }");  //Works

        //Build ResumeToken from the first document in the oplog collection
        BsonDocument resumeTokenRefDoc = GetFirstDocumentFromOpLog(collection.CollectionNamespace.ToString());
        if (resumeTokenRefDoc != null)
        {
            BsonDocument docResumeToken = GetResumeTokenFromOpLogDoc(resumeTokenRefDoc);
            options.ResumeAfter = docResumeToken;
        }

        //Setup the ChangeStream/Watch Cursor
        var cursor = collection.Watch(pipeline, options);
        var enumerator = cursor.ToEnumerable().GetEnumerator();

        enumerator.MoveNext();  //Blocks until a record is UPDATEd, REPLACEd or INSERTed in the database (thanks to the pipeline arg), or returns the second entry (thanks to the ResumeToken that points to the first entry)

        ChangeStreamDocument<BsonDocument> lastChangeStreamDocument = enumerator.Current;
        //lastChangeStreamDocument is now pointing to the second entry in the oplog, or the just received entry
        //A loop can be setup to call enumerator.MoveNext() to step through each entry in the oplog history and to also receive new events

        enumerator.Dispose();   //Be sure to dispose of the enumerator when finished.
    }
    catch( Exception ex)
    {
        //Logger.WriteException(ex);
    }
}

如果大家对代码有什么改进的建议，请提出建议。我还在学习。