by Oren Eini

posted on: August 04, 2021

In this episode of Coffee with Pros , we have special guest speaker Oren Eini. He is the CEO of RavenDB, a NoSQL Distributed Database that's Fully Transactional (ACID) both across your database and throughout your database cluster. RavenDB Cloud is the Managed Cloud Service (DBaaS) for easy use.

by Oren Eini

posted on: August 03, 2021

I’m teaching a course in university, which gives me some interesting perspective into the mind of new people who join our profession. One of the biggest differences that I noticed was with the approach to software architecture and maintenance concerns.  Frankly, some of the the exercises that I had to review made my eyes bleed a little (the students got full marks, because the point was getting things done, not code quality). I talked with the students about the topic and I realized that I have a very different perspective on software development and architecture than they have.The codebase that I work with the most is RavenDB, I have been working on the project for the past 12 years, with some pieces of code going back closer to two decades. In contrast, my rule for giving tasks for students is that I can complete the task in under two hours from an empty slate. Part and parcel of the way that I’m thinking about software is the realization that any piece of code that I’ll write is going to be maintained for a long period of time. A student writing code for a course doesn’t have that approach, in fact, it is rare that they use the same code across semesters. That lead to seeing a lot of practices as unnecessary or superfluous. Even some of the things that I consider as the very basic (source control, tests, build scripts) are things that the students didn’t even encounter up to this point (years 2 and 3 for most of the people I interact with) and they may very well get a degree with no real exposure for those concerns.Most tasks in university are well scoped, clear and they are known to be feasible within the given time frame. Most of the tasks outside of university are anything but. That got me thinking about how you can get a student to realize the value inherent in industry best practices, and the only real way to do that is to immerse them in a big project, something that has been around for at least 3 – 5 years. Ideally, you could have some project that the students will do throughout the degree, but that requires a massive amount of coordination upfront. It is likely not feasible outside of specific fields. If you are learning to be a programmer in the gaming industry, maybe you can do something like produce a game throughout the degree, but my guess is that this is still not possible. A better alternative would be to give students the chance to work with a large project, maybe even contributing code to it. The problem there is that having a whole class start randomly sending pull requests to a project is likely to cause some heartburn to the maintenance staff. What was your experience when moving from single use, transient projects to projects that are expected to run for decades? Not as a single running instance, just a project that is going to be kept alive for a long while…

by Andrew Lock

posted on: August 03, 2021

A deep dive on StringBuilder - Part 4

by Oren Eini

posted on: August 02, 2021

I got an interesting scenario that I thought would make for a good blog post. Given a social network similar to Twitter, how would you deal with the social aspect of it? In other words, how would you model the following relationship between users.In pretty much all social media platforms, the following relationship is unidirectional. I’m following The Wheel Of Time, but it isn’t following me back. That is a very important aspect for modeling as well as the maximum number of accounts that you can follow. Both Facebook and Twitter will limit the number of accounts that you can follow to 5,000. All of those gives us a pretty good operating environment. To model the following relationship in a social network I’ll use the following option: This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters Show hidden characters public record class FollowingRelationship (string UserId, DateTime StartedFollowingAt); public record class Fellowship (string UserId, FollowingRelationship[] Following); /* // As JSON "UserId": "ayende", "Following": [ { "UserId": "ravendb", "StartedFollowingAt": "2009-01-01" }, { "UserId": "TheWheelOfTime", "StartedFollowingAt": "2021-08-01" } ] */ view raw social.cs hosted with ❤ by GitHub In other words, the recommended approach is to store all the accounts that a user follows in a single document. The fact that an account is limited to following a maximum of 5,000 accounts gives us a hard limit on the size of that document (and most accounts will follow far less).Note that the code above is also pretty much the simplest possible option for how to record the following relationship. In this case, the simplest and best option are one and the same. Let’s consider what operations we need to apply on the following relationship:Find all the followers of a particular account. from Fellowships where Following[].UserId = $userIdFind the number of followers per account: from Fellowships group by Following[].UserId  where Following[].UserId = $userId select count(), Following[].UserId Finding all the users that a particular user is following is simply loading the document and looking at the Following property, so I’m skipping that. The key here is that this is going to be simple to build and use, scalable because different users can start following accounts independently from one another and doesn’t require to do anything fancy.

by Gérald Barré

posted on: August 02, 2021

To upload a file you can use the FileUpload control. This control generates a <input type="file"> element and allows you to upload a file. To get a good user experience, you also need to support dropping files from the explorer or pasting images from the clipboard. The final result should loo

by Oren Eini

posted on: July 30, 2021

The Open Closed Principle is part of the SOLID principles. It isn’t new or anything exciting, but I wanted to discuss this today in the context of using that not as a code artifact but as part of your overall architecture.The Open Closed Principle states that the code should be opened for extension, but closed for modification. That is a fancy way to say that you should spend most of your time writing new code, not modifying old code. Old code is something that is known to be working, it is stable (hopefully), but messing around with old code can break that. Adding new code, on the other hand, carry far less risk. You may break the new things, but the old stuff will continue to work. There is also another aspect to this, to successfully add new code to a project, you should have a structure that support that. In other words, you typically have very small core of functionality and then the entire system is built on top of this.  Probably the best example of systems that follow the Open Closed Principle is the vast majority of PHP applications. Hold up,I can hear you say. Did you just called out PHP as an architectural best practice? Indeed I did, and more than that, the more basic the PHP application in question, the closer it is to the ideal of Open Closed Principle.Consider how you’ll typically add a feature to a PHP application. You’ll create a new script file and write the functionality there. You might need to add links to that (or you already have this happen automatically), but that is about it. You aren’t modifying existing code, you are adding new one. The rest of the system just know how to respond to that and handle that appropriately. Your shared component might be the site’s menu, a site map and the like. Adding a new functionality may occasionally involve adding a link to a new page, but for the most parts, all of those operations are safe, they are isolated and independent from one another.In C#, on the other hand, you can do the same by adding a new class to a project. It isn’t at the same level of not even touching anything else, since it all compiles to a single binary, but the situation is roughly the same. That is the Open Closed Principle when it applies to the code inside your application. What happens when you try to apply the same principle to your overall architecture?I think that Terraform is a great example of doing just that. They have a plugin system that they built, which spawns a new process (so completely independent) and then connect to it via gRPC. Adding a new plugin to Terraform doesn’t involve modifying any code (you do have to update some configuration, but even that can be automated away). You can write everything using separate systems, runtime and versions quite easily.If we push the idea a bit further, we’ll discover that Open Closed Principle at the architecture level is the Service Oriented Architecture.  Note that I explicitly don’t count Microservices in this role, because they are usually intermixed (yes, I know they aren’t supposed to, I’m talking about what is). In those situations, adding a new feature to the system would involve adding a new service. For example, in a banking system, if you want to add a new feature to classify fraudulent transactions, how would you do it?One way is to go to the transaction processing code and write something like: This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters Show hidden characters public void processTransactions(HttpServletRequest request, HttpServletResponse response) { String src = request.getParameter("from"); String dst = request.getParameter("to"); int amountInCents = Integer.parseInt(request.getParameter("amount")); /// new code here TransactionProcessingFactory fact = new TransactionProcessingFactory(); // will throw if fraud fact.createProcessor().process(new Transaction(src, dst, amountInCents/1.0)); // new code here /* ** Rest of the actual functionality **/ } view raw txn_proc.java hosted with ❤ by GitHub That, of course, would mean that you are going to have to modify existing code, that is not a good idea. Welcome to six months of meeting about when you can deploy your changes to the code. On the other hand, applying the Open Closed Principle to the architecture, we won’t ever touch the actual system that process transactions. Instead, we’ll use a side channel. Transactions will be written to a queue and we’ll be able to add listeners to the queue. In such a way, we’ll have the ability to add additional processing seamlessly. Another fraud system will just have to listen to the stream of messages and react accordingly.Note that there is a big difference here, however, unlike with modifying the code directly, we can no longer just throw an exception to stop the process. By the time that we process the message, the transaction has already been applied. That requires that we’ll build the system in such a way that there are ways to stop transactions after the fact (maybe by actually submitting them to the central bank after a certain amount of time, or releasing them to the system only after all the configured endpoints authorized it).At the architecture level, we are intentionally building something that is initially more complex, because we have to take into account asynchronous operations and work that happens out of band, including work that we couldn’t expect. In the context of a bank, that means that we need to provide the mechanisms for future code to intervene. For example, we may not know what we’ll want the additional code to do, but we’ll have a way to do things like pause a transaction for manual review, add additional fees, raise alerts, etc.  Those are the capabilities of the system, and the additional behavior would be policy around building that. There are other things that make this very attractive, you don’t have to run everything at the same time, you can independently upgrade different pieces and you have clear lines of demarcation between the different pieces of your system.

by Oren Eini

posted on: July 29, 2021

From a conceptual model, a thread and a task are very similar. That is very much by design, since the Task is meant to allow you to work with asynchronous code while maintaining the illusion that you are running in a sequential manner. It is tempting to think about this in terms of the Task actually executing the work, but that isn’t actually the case.The Task doesn’t represent the execution of whatever asynchronous process is running, the Task represent a ticket that will be punched when the asynchronous process is done. Consider the case of going to a restaurant and asking for a table, if there isn’t an available table, you cannot be seated. What the restaurant will do is hand you a pager that will buzz when the table is ready. In the same sense, a Task is just such a token. The restaurant pager doesn’t means that someone is actively clearing a table for you. It is just something that will buzz when a table is ready.A code sample may make things clearer: This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters Show hidden characters public class Restaurant { private int _occupancy; private List<(TaskCompletionSource<object>, int)> _waiters = new List<(TaskCompletionSource<object>, int)>(); public Task GetTable(int numberOfDiners) { lock (this) { if(_occupancy+ numberOfDiners < 60) { _occupancy += numberOfDiners; return Task.CompletedTask; } var tcs = new TaskCompletionSource<object>(); _waiters.Add((tcs, numberOfDiners)); _waiters.Sort((x, y) => x.Item2.CompareTo(y.Item2)); return tcs.Task; } } ​ public void ReleaseTable(int numberOfDiners) { lock (this) { _occupancy -= numberOfDiners; while (_waiters.Count > 0) { if(_waiters[0].Item2 + _occupancy > 60) break; var tcs = _waiters[0].Item1; _occupancy += _waiters[0].Item2; _waiters.RemoveAt(0); tcs.SetResult(null); } } } } ​ view raw Restaurant.cs hosted with ❤ by GitHub In this case, we are manually coordinating the Task using its completion source and you can see that the Task instance that was handed when trying to get a table doesn’t actually start anything. It is simply waiting to be raised when called.That is an important aspect of how System.Threading.Tasks.Task works, because it is usually in contrast to the mental model in our head.

by Steve Gordon

posted on: July 28, 2021

Part Three:  How Appending Works and the StringBuilder Expands So far in this series, we’ve learned when we should consider using StringBuilder in our code and learned about the memory overhead of using a StringBuilder. It’s now time to learn how the StringBuilder can “expand” its capacity and support appending string data efficiently. As with […]

by Oren Eini

posted on: July 28, 2021

If you build any kind of non trivial system, one of the absolutely best things that you can do for the long term health of your system is to move all significant processing to sit behind a queue.  That is one of those things that is going to pay massive dividends down the line as your system grows. Basically, any time that you want to do something that isn’t “pull data and show it to the user” or “store the data immediately”, throwing a queue at the problem is going to make things easier in the long run.Yes, this is a bold statement, and I’m sure that you can see that this may be abused. Nevertheless, if you’ll follow this one rule, even if you misuse it, you are likely going to be better off than if you don’t. I should probably explain.When I’m talking about using a queue, I’m talking about moving actual processing of an operation from the request handler (controller / action / web sever ) to popping a message from a queue and processing that. The actual queue implementation (SQS, Kafka, MSMQ, ring buffer) doesn’t actually matter. It also doesn’t matter if you are writing to the queue in the same process and machine or a distributed system. What matter is that you can created a break in the system between three very important aspects of command processing:Accepting a request.Processing the request.Sending the result of the request back.A system without a queue will do all of that inline, in this manner: This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters Show hidden characters @app.route('/purchase-policy') def purchase_policy(): user_id = request.args.get("user_id") # load the user's details # process the policy, evaluate terms return "Thank you for your purchase", 201 view raw inline.py hosted with ❤ by GitHub What is the problem here? If the processing of the request is complex or takes some time, you have an inherent clock here. At some point, the client is going to timeout on the request, which lead to things like this:On the other hand, if you put a queue in the middle, this looks like this: This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters Show hidden characters @app.route('/purchase-policy') def purchase_policy(): req_id = uuid.uuidv4() user_id = request.args.get("user_id") other_details = ... # get them somehow put_on_queue(req_id, user_id, other_details); return req_id, 202 # accepted, not processed @app.route('/purchase-policy-status') def purchase_policy_status(): req_id = request.args.get("req_id") if load_req(req_id) is None: return "Please wait, processing", 203 return "Thank you for your purchase", 201 # created # called somewhere else, not part of the request processing def queue_callback(msg): # load the user's details # process the policy, evaluate terms save_data(msg.req_id) view raw queued.py hosted with ❤ by GitHub Note that there is a separation in the processing of the request and sending the accepted answer to the customer.What is the impact of this change? Well, it is a bit more complex to manage in the user interface. Instead of getting the response for the request immediately, we have to fetch it in a separate operation. I’m typically really strict on policing the number of remote calls, why am I advocating for an architectural pattern that requires more remote calls?The answer is that we build, from the first step, the ability of the system to delay processing. The user interface no longer attempts to pretend that the system reacts instantly, and have far more freedom to change what we do behind the scenes.Just putting the operation on a queue gives us the ability to shift the processing, which means that we can:Maintain speedy responses and responsive system to the users.Can easily bridge spikes in the system by having the queue flatten them.Scale up the processing of the operations without needing to do anything in the front end.Go from a local to a distribute mechanism without changing the overall architecture (that holds even if you previously held the queue in memory and processed that with a separate thread). Monitor the size of the queue to get a really good indication about where we are at in terms of load.Gain the ability to push updates to the backend seamlessly.At more advanced levels:Can copy message to an audit log, which gives great debugging abilities.Can retry messages that failed.There are whole patterns of message based operations that are available to you, but the things that I list above are what you get almost for free. The reason that I think you should do that upfront is that your entire system would already be used to that. Your UI (and users’ expectations) would already be set to handle potential delays. That gives you a far better system down the line. And you can play games on the front end to present the illusion that operations are accepted on the server (in pending status) without compromising the health of the system.In short, for the health of your system, put a queue on that, your future self will thank you later.One final word of warning, this apply to operations, not queries. Don’t bother putting queries through the queue unless they are intended to be very long lived / complex ones.

by Vladimir Khorikov

posted on: July 27, 2021

There’s an interesting controversy between two DDD topics: the Specification pattern and the Always-Valid domain model.