Ho aggiornato la domanda con informazioni aggiuntive mentre lavoravo su questo e ora sono riuscito a metterla a posto del tutto, in modo che funzioni.
Di seguito è riportato il codice che ho creato per:
- Trova la prima voce di uno spazio dei nomi nella raccolta local.oplog
- Genera un ResumeToken da quel documento oplog (quindi riprendiamo dalla seconda voce)
- Esempio per testare queste funzioni.
Si spera che questo codice sia utile per altri che stanno tentando di fare lo stesso.
/// <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);
}
}
Se qualcuno ha suggerimenti su miglioramenti al codice, si prega di offrire suggerimenti. Sto ancora imparando.
