Ayende @ Rahien

Oren Eini aka Ayende Rahien CEO of Hibernating Rhinos LTD, which develops RavenDB, a NoSQL Open Source Document Database.

You can reach me by:

oren@ravendb.net

+972 52-548-6969

, @ Q j

Posts: 6,868 | Comments: 49,216

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time to read 6 min | 1018 words

I got a great comment on my previous post about using Map/Reduce indexes in RavenDB for event sourcing. The question was how to handle time sensitive events or ordered events in this manner. The simple answer is that you can’t, RavenDB intentionally don’t expose anything about the ordering of the documents to the index. In fact, given the distributed nature of RavenDB, even the notion of ordering documents by time become really hard.

But before we close the question as “cannot do that by design", let’s see why we want to do something like that. Sometimes, this really is just the developer wanting to do things in the way they are used to and there is no need for actually enforcing the ordering of documents. But in other cases, you want to do this because there is a business meaning behind these events. In those cases, however, you need to handle several things that are a lot more complex than they appear. Because you may be informed of an event long after that actually happened, and you need to handle that.

Our example for this post is going to be mortgage payments. This is a good example of a system where time matters. If you don’t pay your payments on time, that matters. So let’s see how we can model this as an event based system, shall we?

A mortgage goes through several stages, but the only two that are of interest for us right now are:

  • Approval – when the terms of the loan are set (how much money, what is the collateral, the APR, etc).
  • Withdrawal – when money is actually withdrawn, which may happen in installments.

Depending on the terms of the mortgage, we need to compute how much money should be paid on a monthly basis. This depend on a lot of factors, for example, if the principle is tied to some base line, changes to the base line will change the amount of the principle. If only some of the amount was withdrawn, if there are late fees, balloon payment, etc. Because of that, on a monthly basis, we are going to run a computation for the expected amount due for the next month.

And, obviously, we have the actual payments that are being made.

Here is what the (highly simplified) structure looks like:

image

This includes all the details about the mortgage, how much was approved, the APR, etc.

The following is what the expected amount to be paid looks like:

image

And here we have the actual payment:

image

All pretty much bare bones, but sufficient to explain what is going on here.

With that in place, let’s see how we can actually make use of it, shall we?

Here are the expected payments:

image

Here are the mortgage payments:

image

The first thing we want to do is to aggregate the relevant operations on a monthly basis, since this is how mortgages usually work. I’m going to use a map reduce index to do so, and as usual in this series of post, we’ll use JavaScript indexes to do the deed.

Unlike previous examples, now we have real business logic in the index. Most specifically, funds allocations for partial payments. If the amount of money paid is less than the expected amount, we first apply it to the interest, and only then to the principle.

Here are the results of this index:

image

You can clearly see that mistake that were made in the payments. On March, the amount due for the loan increased (took another installment from the mortgage) but the payments were made on the old amount.

We aren’t done yet, though. So far we have the status of the mortgage on a monthly basis, but we want to have a global view of the mortgage. In order to do that, we need to take a few steps. First, we need to define an Output Collection for the index, that will allow us to further process the results on this index.

In order to compute the current status of the mortgage, we aggregate both the mortgage status over time and the amount paid by the bank for the mortgage, so we have the following index:

Which gives us the following output:

image

As you can see, we have a PastDue marker on the loan. At this point, we can make another payment on the mortgage, to close the missing amount, like so:

image

This will update the monthly mortgage status and then the overall status. Of course, in a real system (I mentioned this is highly simplified, right?) we’ll need to take into account payments made in one time but applied to different times (which we can handle by an AppliedTo property) and a lot of the actual core logic isn’t in indexes. Please don’t do mortgage logic in RavenDB indexes, that stuff deserve its own handling, in your own code. And most certainly don’t do that in JavaScript. The idea behind this post is to explore how we can handle non trivial event projection using RavenDB. The example was chosen because I assume most people will be familiar with it and it wasn’t immediately obvious how to go about actually solving it.

If you want to play with this, you can import the following file (Settings > Import Data) to get the documents and index definitions.

time to read 3 min | 500 words

In the previous post I talked about how to use a map reduce index to aggregate events into a final model. You can see this on the right. This is an interesting use case of indexing, and it can consolidate a lot of complexity into a single place, at which point you can utilize additional tooling available inside of RavenDB.

As a reminder, you can get the dump of the database that you can import into your own copy of RavenDB (or our live demo instance) if you want to follow along with this post.

Starting from the previous index, all we need to do is edit the index definition and set the Output Collection, like so:

image

What does this do? This tell RavenDB that in addition to indexing the data, it should also take the output of the index and create new documents from it in the ShoppingCarts collection. Here is what these documents look like:

image

You can see at the bottom that this document is flagged as artificial and coming from an index. The document id is a hash of the reduce key, so changes to the same cart will always go to this document.

What is important about this feature is that once the result of the index is a document, we can operate it using all the usual tools for indexes. For example, we might want to create another index on top of the shopping carts, like the following example:

In this case, we are building another aggregation. Taking all the paid shopping carts and computing the total sales per product from these. Note that we are now operating on top of our event streams but are able to extract second level aggregation from the data.

Of course, normal indexes on top of the artificial ShoppingCarts allow you to do things like: “Show me my previous orders”. In essence, you are using the events for your writes, define the aggregation to the final model in an index and then RavenDB take care of the read model.

Some other options to pay attention to is the not doing the read model and the full work on the same database instance as your events. Instead, you can output the documents to a collection and then use RavenDB’s native ETL capabilities to push them to another database (which can be another RavenDB instance or a relational database) for further processing.

The end result is a system that is built on dynamic data flow. Add an event to the system, the index will go through it, aggregate it with other events on the same root and output it to a document, at which point more indexes will pick it up and do further work, ETL will push it to other databases, subscriptions can start operation on it, etc.

time to read 5 min | 875 words

In this post, I want to take the notion of doing computation inside RavenDB’s indexes to the next stage. So far, we talked only about indexes that work on a single document at a time, but that is just the tip of the iceberg of what you can do with indexes inside RavenDB. What I want to talk about today is the ability to do computations over multiple documents and aggregate them. The obvious example is in the following RQL query:

image

That is easy to understand, it is simple aggregation of data. But it can get a lot more interesting. To start with, you can add your own aggregation logic in here, which open up some interesting ideas. Event Sourcing, for example, is basically a set of events on a subject that are aggregated into the final model. Probably the classiest example of event sourcing is the shopping cart example. In such a model, we have the following events:

  • AddItemToCart
  • RemoveItemFromCart
  • PayForCart

Here what these look like, in document form:

image

We add a couple of items to the cart, remove excess quantity and pay for the whole thing. Pretty simple model, right? But how does this relate to indexing in RavenDB?

Well, the problem here is that we don’t have a complete view of the shopping cart. We know what the actions were, but not what its current state is. This is where our index come into play, let’s see how it works.

The final result of the cart should be something like this:

image

Let’s see how we get there, shall we?

We’ll start by processing the add to cart events, like so:

As you can see, the map phase here build the relevant parts of the end model directly. But we still need to complete the work by doing the aggregation. This is done on the reduce phase, like so:

Most of the code here is to deal with merging of products from multiple add actions, but even that should be pretty simple. You can see that there is a business rule here. The customer will be paying the minimum price they encountered throughout the process of building their shopping cart.

Next, let’s handle the removal of items from the cart, which is done in two steps. First, we map the remove events:

There are a few things to note here, the quantity is negative, and the price is zeroed, that necessitate changes in the reduce as well. Here they are:

As you can see, we now only get the cheapest price, above zero, and we’ll remove empty items from the cart. The final step we have to take is handle the payment events. We’ll start with the map first, obviously.

Note that we added a new field to the output. Just like we set the Products fields in the pay for cart map to empty array, we need to update the rest of the maps to include a Paid: {} to match the structure. This is because all the maps (and the reduce) in an index must output the same shape out.

And now we can update the reduce accordingly. Here is the third version:

This is almost there, but we still need to do a bit more work to get the final output right. To make things interesting, I changed things up a bit and here is how we are paying for this cart:

image

And here is the final version of the reduce:

And the output of this for is:

image

You can see that this is a bit different from what I originally envisioned it. This is mostly because I’m bad at JavaScript and likely took many shortcuts along the way to make things easy for myself. Basically, I was easier to do the internal grouping using an object than using arrays.

Some final thoughts:

  • A shopping cart is usually going to be fairly small with a few dozens of events in the common case. This method works great for this, but it will also scale nicely if you need to aggregate over tens of thousands of events.
  • A key concept here is that the reduce portion is called recursively on all the items, incrementally building the data until we can’t reduce it any further. That means that the output we have get should also serve as the input to the reduce. This take some getting used to, but it is a very powerful technique.
  • The output of the index is a complete model, which you can use inside your system. I the next post, I’ll discuss how we can more fully flesh this out.

If you want to play with this, you can get the dump of the database that you can import into your own copy of RavenDB (or our live demo instance).

time to read 3 min | 505 words

Computation during indexes open up some nice  features when we are talking about data modeling and working with your data. In this post, I want to discuss predicting the future with it. Let’s see how we can do that, shall we?

Consider the following document, representing a (simplified) customer model:

image

We have a customer that is making monthly payments. This is a pretty straightforward model, right?

We can do a lot with this kind of data. We can obviously compute the lifetime value of a customer, based on how much they paid us. We already did something very similar in a previous post, so that isn’t very interesting.

What is interesting is looking into the future. Let’s see how we can start simple, but figuring out what is the next charge rate for this customer. For now, the logic is about as simple as it can be. Monthly customers pay by month, basically. Here is the index:

image

I’m using Linq instead of JS here because I’m dealing with dates and JS support for dates is… poor.

As you can see, we are simply looking at the last date and the subscription, figuring out how much we paid the last three times and use that as the expected next payment amount. That can allow us to do nice things, obviously. We can now do queries on the future. So finding out how many customers will (probably) pay us more than 100$ on the 1st of Feb both easy and cheap.

We can actually take this further, though. Instead of using a simple index, we can use a map/reduce one. Here is what this looks like:

image

And the reduce:

image

This may seem a bit dense at first, so let’s de-cypher it, shall we?

We take the last payment date and compute the average of the last three payments, just as we did before. The fun part now is that we don’t compute just the single next payment, but the next three. We then output all the payments, both existing (that already happened) and projected (that will happen in the future) from the map function. The reduce function is a lot simpler, and simply sum up the amounts per month.

This allows us to effectively project data into the future, and this map reduce index can be used to calculate expected income. Note that this is aggregated across all customers, so we can get a pretty good picture of what is going to happen.

A real system would probably have some uncertainty factor, but that touches on business strategy more than modeling, so I don’t think we need to go into that here.

time to read 4 min | 613 words

imageIn my last post on the topic, I showed how we can define a simple computation during the indexing process. That was easy enough, for sure, but it turns out that there are quite a few use cases for this feature that go quite far from what you would expect. For example, we can use this feature as part of defining and working with business rules in our domain.

For example, let’s say that we have some logic that determine whatever a product is offered with a warranty (and for how long that warranty is valid). This is an important piece of information, obviously, but it is the kind of thing that changes on a fairly regular basis. For example, consider the following feature description:

As a user, I want to be able to see the offered warranty on the products, as well as to filter searches based on the warranty status.

Warranty rules are:

  • For new products made in house, full warranty for 24 months.
  • For new products from 3rd parties, parts only warranty for 6 months.
  • Refurbished products by us, full warranty, for half of new warranty duration.
  • Refurbished 3rd parties products, parts only warranty, 3 months.
  • Used products, parts only, 1 month.

Just from reading the description, you can see that this is a business rule, which means that it is subject to many changes over time. We can obviously create a couple of fields on the document to hold the warranty information, but that means that whenever the warranty rules change, we’ll have to go through all of them again. We’ll also need to ensure that any business logic that touches the document will re-run the logic to apply the warranty computation (to be fair, these sort of things are usually done as a subscription in RavenDB, which alleviate that need).

Without further ado, here is the index to implement the logic above:

You can now query over the warranty types and it’s duration, project them from the index, etc. Whenever a document is updates, we’ll re-compute the warranty status and update the index.

This saves you from having additional fields in your model and greatly diminish the cost of queries that need to filter on warranty or its duration (since you don’t need to do this computation during the query, only once, during indexing).

If the business rule definition changes, you can update the index definition and RavenDB will effectively roll out your change to the entire dataset. That is nice, but even though I’m writing about cool RavenDB features, there are some words of cautions that I want to mention.

Putting queryable business rules in the database can greatly ease your life, but be wary of putting too much business logic in there. In general, you want your business logic to reside right next to the rest of your application code, not running in a different server in a mode that is much harder to debug, version and diagnose. And if the level of complexity involved in the business rule exceed some level (hard to define, but easy to know when you hit it), you should probably move from defining the business rules in an index to a subscription.

A RavenDB subscription allow you to get all changes to documents and apply your own logic in response. This is a reliable way to process data in RavenDB, this runs in your own code, under your own terms, so it can enjoy all the usual benefits of… well, being your code, and not mine. You can read more about them in this post and of course, the documentation.

time to read 4 min | 734 words

imageThe title of this post is pretty strange, I admit. Usually, when we think about modeling, we think about our data. If it is a relational database, this mostly mean the structure of your tables and the relations between them. When using a document database, this means the shape of your documents. But in both cases, indexes are there merely to speed things up. Oh, a particular important query may need an index, and that may impact how you lay out the data, but these are relatively rare cases. In relational databases and most non relational ones, indexes do not play any major role in data modeling decisions.

This isn’t the case for RavenDB. In RavenDB, an index doesn’t exist merely to organize the data in a way that make it easier for the database to search for it. An index is actually able to modify and transform the data, on the current document or full related data from related documents. A map/reduce index is even able aggregate data from multiple documents as part of the indexing process. I’ll touch on the last one in more depth later in this series, first, let’s tackle the more obvious parts. Because I want to show off some of the new features, I’m going to use JS for most of the indexes definitions in these psots, but you can do the same using Linq / C# as well, obviously.

When brain storming for this post, I got so many good ideas about the kind of non obvious things that you can do with RavenDB’s indexes that a single post has transformed into a series and I got two pages of notes to go through. Almost all of those ideas are basically some form of computation during indexing, but applied in novel manners to give you a lot of flexibility and power.

RavenDB prefers to have more work to do during indexing (which is batched and happen on the background) than during query time. This means that we can push a lot more work into the background and just let RavenDB handle it for us. Let’s start from what is probably the most basic example of computation during query, the Order’s Total. Consider the following document:

image

As you can see, we have the Order document and the list of the line items in this order. What we don’t have here is the total order value.

Now, actually computing how much you are going to pay for an order is complex. You have to take into account taxation, promotions, discounts, shipping costs, etc. That isn’t something that you can do trivially, but it does serve to make an excellent simple example and similar requirements exists in many fields.

We can obvious add an Total field to the order, but then we have to make sure that we update it whenever we update the order. This is possible, but if we have multiple clients consuming the data, this can be fairly hard to do. Instead, we can place the logic to compute the property in the index itself. Here is how it would look like:

image

The same index in JavaScript is almost identical:

In this case, they are very similar, but as the complexity grow, I find it is usually easier to express logic as a JavaScript index rather than use a single (complex) Linq expression.

Such an index give us a computed field, Total, that has the total value of an order. We can query on this field, sort it and even project it back (if this field is stored). It allow us to always have the most up to date value and have the database take care of computing it.

This basic technique can be applied in many different ways and affect the way we shape and model our data. Currently I have at least three more posts planned for this series, and I would love to hear your feedback. Both on the kind of stuff you would like me to talk about and the kind of indexes you are using RavenDB and how it impacted your data modeling.

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