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POPerformance problems when using MongoDB Aggregation Framework through C#
 

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  1. POPerformance problems when using MongoDB Aggregation Framework through C#
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    <p>I'm having performance problems when using the MongoDB Aggregation Framework through C#. An aggregation which works fast through Mongo shell takes forever when executed with C#.</p> <p>Before trying to call the framework through C#, I executed the following aggregation through Mongo shell to check that everything works:</p> <pre><code>db.runCommand( { aggregate: "actions", pipeline : [ { $match : { CustomerAppId : "f5357224-b1a8-4f1a-8ea2-a06a00ca597a", ActionName : "install"}}, { $group : { _id : { CustomerAppId:"$CustomerAppId",ActionDate:"$ActionDate" }, count : { $sum : 1 } }} ] }); </code></pre> <p>The script <strong>executed in &lt; 500ms</strong> and returns the expected around 200 results (The CustomerAppId is defined as a string in the database. It's not possible to use GUIDs with aggregation framework.).</p> <p>Then, I ported the same script to C#:</p> <pre><code>var pipeline = new BsonArray { new BsonDocument { { "$match", new BsonDocument { {"CustomerAppId", "f5357224-b1a8-4f1a-8ea2-a06a00ca597a"}, {"ActionName", "install"} } }, { "$group", new BsonDocument { { "_id", new BsonDocument { { "CustomerAppId","$CustomerAppId" }, { "ActionName","$ActionName" } } }, { "Count", new BsonDocument { { "$sum", 1 } } } } } } }; var command = new CommandDocument { { "aggregate", "actions" }, { "pipeline", pipeline } }; </code></pre> <p>(Please let me know if there's an easier way to write the aggregation in C# :) )</p> <p>Which I'm executing like this:</p> <pre><code>var result = db.RunCommand(command); </code></pre> <p>The problem is that <strong>it kills the server</strong>: The CPU and mem usage go way up. When I check db.currentOp(), I can see the aggregate operation but I eventually have to kill it using db.killOp(1281546):</p> <pre><code>"opid" : 1281546, "active" : true, "secs_running" : 294, "op" : "query", "ns" : "database.actions", "query" : { "aggregate" : "actions", "pipeline" : [ { "$match" : { "CustomerAppId" : "f5357224-b1a8-4f1a-8ea2-a06a00ca597a", "ActionName" : "install" }, "$group" : { "_id" : { "CustomerAppId" : "$CustomerAppId", "ActionName" : "$ActionName" }, "Count" : { "$sum" : 1 } } } ] }, </code></pre> <p>To me the operation looks completely fine and similar to the script I run directly from mongo shell. It feels like running the aggregation through C# causes the MongoDB to miss the index and it's doing a table scan for all the ~6 million documents in the collection. </p> <p>Any ideas? </p> <p><strong>Update: Logs</strong></p> <p>Thanks to cirrus' suggestion, I enabled the verbose logging and then used tail to get the queries. And <strong>they are different</strong>! So I think there is something wrong in my C# port. Any ideas on how to format the query correctly?</p> <p>The query when executed through <strong>shell</strong>:</p> <pre><code>Mon Oct 8 15:00:13 [conn1] run command database.$cmd { aggregate: "actions", pipeline: [ { $match: { CustomerAppId: "f5357224-b1a8-4f1a-8ea2-a06a00ca597a", ActionName: "install" } }, { $group: { _id: { CustomerAppId: "$CustomerAppId", ActionDate: "$ActionDate" }, count: { $sum: 1.0 } } } ] } Mon Oct 8 15:00:13 [conn1] command database.$cmd command: { aggregate: "actions", pipeline: [ { $match: { CustomerAppId: "f5357224-b1a8-4f1a-8ea2-a06a00ca597a", ActionName: "install" } }, { $group: { _id: { CustomerAppId: "$CustomerAppId", ActionDate: "$ActionDate" }, count: { $sum: 1.0 } } } ] } ntoreturn:1 keyUpdates:0 locks(micros) r:27944 reslen:12705 29ms </code></pre> <p>And the query when executed through C#:</p> <pre><code>Mon Oct 8 15:00:16 [conn8] run command database.$cmd { aggregate: "actions", pipeline: [ { $match: { CustomerAppId: "f5357224-b1a8-4f1a-8ea2-a06a00ca597a", ActionName: "install" }, $group: { _id: { CustomerAppId: "$CustomerAppId", ActionDate: "$ActionDate" }, Count: { $sum: 1 } } } ] } </code></pre> <p>Second line is missing, I suppose because the query doesn't finish.</p> <p>And here are the logs again for easier comparison. Script is up, C# down:</p> <pre><code>Mon Oct 8 15:00:13 [conn1] run command database.$cmd { aggregate: "actions", pipeline: [ { $match: { CustomerAppId: "f5357224-b1a8-4f1a-8ea2-a06a00ca597a", ActionName: "install" } }, { $group: { _id: { CustomerAppId: "$CustomerAppId", ActionDate: "$ActionDate" }, count: { $sum: 1.0 } } } ] } Mon Oct 8 15:00:16 [conn8] run command database.$cmd { aggregate: "actions", pipeline: [ { $match: { CustomerAppId: "f5357224-b1a8-4f1a-8ea2-a06a00ca597a", ActionName: "install" }, $group: { _id: { CustomerAppId: "$CustomerAppId", ActionDate: "$ActionDate" }, Count: { $sum: 1 } } } ] } </code></pre>
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    1. USMikael Koskinen
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      1. POPerformance problems when using MongoDB Aggregation Framework through C#
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    1. COYou know you've used ActionDate in the C# version as opposed to ActionName in the shell version right? Beyond that, turn logging on with "verbose=true" in your config file and tail.exe the log file. It will show you the actual query it's executing on the DB.
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    2. COActually no! I swear I triple checked that things match but I still managed to miss it. But unfortunately the execution is still slow. The index contains ActionName, CustomerAppId and ActionDate. Grouping from shell with either AppId&Date or AppId&ActionName is fast, but from the code they are both slow. Thanks for the tip on the verbose. I'll have to check that out.
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      1. POPerformance problems when using MongoDB Aggregation Framework through C#
      1. USMikael Koskinen
    3. CODon't forget you can upvote helpful comments as well as answers ;)
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      1. UScirrus
 

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