Using a Tailable Cursor

MongoDB offers the option to watch a capped collection for changes using a tailable cursor.

The code below “tails” a capped collection and outputs documents to the console as they are added. The method also handles the possibility of a dead cursor by tracking the field insertDate. New documents are added with increasing values of insertDate.

This version of the code uses the synchronous API to tail the capped collection:

private static void TailCollection(IMongoCollection<BsonDocument> collection)
{
    // Set lastInsertDate to the smallest value possible
    BsonValue lastInsertDate = BsonMinKey.Value;
    
    var options = new FindOptions<BsonDocument> 
    { 
        // Our cursor is a tailable cursor and informs the server to await
        CursorType = CursorType.TailableAwait
    };
    
    // Initially, we don't have a filter. An empty BsonDocument matches everything.
    BsonDocument filter = new BsonDocument();
    
    // NOTE: This loops forever. It would be prudent to provide some form of 
    // an escape condition based on your needs; e.g. the user presses a key.
    while (true)
    {
        // Start the cursor and wait for the initial response
        using (var cursor = collection.FindSync(filter, options))
        {
            foreach (var document in cursor.ToEnumerable())
            {
                // Set the last value we saw 
                lastInsertDate = document["insertDate"];
                
                // Write the document to the console.
                Console.WriteLine(document.ToString());
            }
        }

        // The tailable cursor died so loop through and restart it
        // Now, we want documents that are strictly greater than the last value we saw
        filter = new BsonDocument("$gt", new BsonDocument("insertDate", lastInsertDate));
    }
}

This version of the code uses the asynchronous API to tail the capped collection:

private static async Task TailCollectionAsync(IMongoCollection<BsonDocument> collection)
{
    // Set lastInsertDate to the smallest value possible
    BsonValue lastInsertDate = BsonMinKey.Value;
    
    var options = new FindOptions<BsonDocument> 
    { 
        // Our cursor is a tailable cursor and informs the server to await
        CursorType = CursorType.TailableAwait
    };
    
    // Initially, we don't have a filter. An empty BsonDocument matches everything.
    BsonDocument filter = new BsonDocument();
    
    // NOTE: This loops forever. It would be prudent to provide some form of 
    // an escape condition based on your needs; e.g. the user presses a key.
    while (true)
    {
        // Start the cursor and wait for the initial response
        using (var cursor = await collection.FindAsync(filter, options))
        {
            // This callback will get invoked with each new document found
            await cursor.ForEachAsync(document =>
            {
                // Set the last value we saw 
                lastInsertDate = document["insertDate"];
                
                // Write the document to the console.
                await Console.WriteLineAsync(document.ToString());
            });
        }

        // The tailable cursor died so loop through and restart it
        // Now, we want documents that are strictly greater than the last value we saw
        filter = new BsonDocument("$gt", new BsonDocument("insertDate", lastInsertDate));
    }
}

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