Materialized Path Category Hierarchy

The nested categories schema design pattern targets the hierarchical tree structures traditionally found in a product catalog on an e-commerce website.

A Category Hierarchy Example

We'll will be presenting a pattern called the materialized path pattern. This pattern refers to categories by a file system like path concept /monitors/lg/led.

Schema Observations

  • Allows for efficient retrieval of a category's direct children or subtree.
  • Very flexible.
  • Makes it easy to allow products to belong to multiple categories.
  • Easily adaptable to increased performance using an index.
  • Relies on regular expressions, making it more complicated (a wrong regular expression can easily cause a full collection scan)

Schema

Lets look at an example category document.

{
    "_id": ObjectId("54fd7392742abeef6186a68e")
  , "name": "embedded"
  , "parent": "/electronics"
  , "category": "/electronics/embedded"
}

Schema

Schema Attribute Description
name Name of the category
parent The parent category path. If the parent is the root it will be /
category The current category path

Lets insert a stripped down product document.

{
    "_id": ObjectId("54fd7392742abeef6186a68e")
  , "name": "Arduino"
  , "cost": 125
  , "currency": "USD"
  , "categories": ["/electronics/embedded"]
}
Schema attribute Description
name Name of the product
cost The cost of the product
currency The product currency
categories An embedded array of category paths for this product. A product can belong to one or more categories

Operations

Find the direct ancestors of a specified category

We want to retrieve all the direct children categories of the /electronics category.

var col = db.getSisterDB("catalog").categories;
var categories = col.find({parent: /^\/electronics$/});

Example 1: Fetch direct children of /electronics category

Notice the regular expression /^\/electronics$/ that we're using. This translates as all documents, where the field parent starts with /electronics and ends with /electronics.

Find the subtree of a specific category

We want to retrieve the entire subtree of categories for the /electronics category.

var col = db.getSisterDB("catalog").categories;
var categories = col.find({parent: /^\/electronics/});

Example 2: Fetch subtree of /electronics category

Notice the regular expression /^\/electronics/ that we're using. This regular expression matches all documents where the field parent starts with /^\/electronics/. The effect is such that the query will return all the categories in the subtree below /electronics.

Find all products by category

It's fairly straightforward to query for products by a specific category, thanks to the embedded categories field.

var col = db.getSisterDB("catalog").products;
var products = col.find({categories: "/electronics/arduino"});

Example 3: Fetch all products of /electronics/arduino category

Find all the categories in the parent path for a specific category

To get all the categories in the parent path for a specific category, we need to do some preprocessing of the path for the specific category.

var elements = '/electronics/embedded/arduino'.split('/');
var paths = [];

for(var i = 0; i < elements.length - 1; i++) {
  elements.pop();
  paths.push(elements.join('/'));
}

paths.push('/');
paths.reverse();

var col = db.getSisterDB("catalog").categories;
var products = col.find({ name: { $in: paths } });

Example 4: Fetch root path of /electronics/embedded/arduino

In the example above, we created all the combinations of paths possible from the originating path /electronics/embedded/arduino by creating an array containing all the paths from the route the parent category of our provided path. In this case that becomes the array [ '/', '/electronics', '/electronics/embedded' ].

Find all the products for a category's direct children

If we wish to find all the products for a specific category's direct children, we have to first retrieve the category and then perform an $in query on the products collection.

var c = db.getSisterDB("catalog").categories;
var p = db.getSisterDB("catalog").products;
var categories = c.find({parent: /^\/electronics$/}).map(function(x) {
      return x.category;
  });
var products = p.find({ categories: { $in: categories } });

Example 5: Fetch products of direct category children

Find all the products in a category subtree

In order to locate all the products for a specific category's subtree, we need to first retrieve the category and then perform an $in query on the products collection.

var c = db.getSisterDB("catalog").categories;
var p = db.getSisterDB("catalog").products;
var categories = c.find({parent: /^\/electronics/}).map(function(x) {
      return x.category;
  });
var products = p.find({ categories: { $in: categories } });

Example 6: Fetch products of category subtree

Indexes

We retrieve categories by the parent and category fields. Let's ensure we can index on the category field as well as the parent field for the categories collection.

var col = db.getSisterDB("catalog").categories;
col.ensureIndex({category:1});

Example 7: Create the category field index

We also need to create an index for the parent field.

var col = db.getSisterDB("catalog").categories;
col.ensureIndex({parent:1});

Example 8: Create the parent field index

For the products collection we retrieve products by using the categories field. So, let's ensure we have an index for the categories field.

var col = db.getSisterDB("catalog").products;
col.ensureIndex({categories:1});

Example 9: Create the categories field index

Covered Index Queries

Covered index queries are queries that can be answered using only the information stored in the index. Basically MongoDB answers the query using the fields stored in an index but never actually materializes the document into memory. We could create a covered index for categories to allow for quick retrieval of a specific category path.

var col = db.getSisterDB("catalog").categories;
col.ensureIndex({parent:1, name: 1})

Example 10: Adding a covered index

The Index {parent:1, name:1} is a compound index and will contain both the parent and name fields, and can cover queries containing both those fields.

Let's rewrite the query of all the categories just below the top level /electronics and look at the explain plan.

var col = db.getSisterDB("catalog").categories;
col.find({parent: /^\/electronics$/}, {_id:0, parent:1, name:1}).explain();

Example 11: Query using covered index

This should return a document result that contains a field indexOnly that is set to true indicating that the query can be answered by only using the index. You will have to give up the _id field for this query. In many cases covered index queries can provide a significant performance boost over normal queries as it avoids accessing the documents during the read operation.

Knowing this we can rewrite the query for the direct sub categories of the path /electronics.

var col = db.getSisterDB("catalog").categories;
var categories = col.find({parent: /^\/electronics/}, {_id:0, parent:1, name:1});

Example 12: Sub categories using covered index

Scaling

Secondary Reads

In the case of a product catalog, secondaries might help the application scale its reads, given that most of the workload is read only. This allows the application to leverage multiple secondaries to increase read throughput. The trade off is that it might take some time for product document changes to replicate from the primary to the secondaries causing some stale reads. This might not be a problem but it's important to be aware of the trade-off.

Sharding

Sharding the product category may not be effective and in most circumstances sharding will not add much in the way of performance as the workload is mostly read only. It could be better to approach scaling using secondary reads.

Performance

A simple exploration of the performance on a single machine with MongoDb 3.0 shows the difference between MMAP and WiredTiger for a narrow simulation using the schema simulation framework mongodb-schema-simulator.

Scenarios

https://github.com/christkv/mongodb-schema-simulator/blob/master/examples/scripts/single_or_replset/materialized_path_category_hierarchy/retrieve_direct_child_categories_scenario.js

https://github.com/christkv/mongodb-schema-simulator/blob/master/examples/scripts/single_or_replset/materialized_path_category_hierarchy/retrieve_entire_sub_tree_by_category_scenario.js

MongoDb runs locally on a MacBook Pro Retina 2015 with ssd and 16 gb ram. The simulation runs with the following parameters against a single mongodb instance under osx 10.10 Yosemite.

Retrieve direct category children categories

Parameters Value
processes 4
poolSize per process 50
type linear
Resolution in milliseconds 1000
Iterations run 25
Number of users reading 1000
Execution strategy slicetime

Retrieve category subtree

Parameters Value
processes 4
poolSize per process 50
type linear
Resolution in milliseconds 1000
Iterations run 25
Number of users reading 1000
Execution strategy slicetime

MMAP

The MMAP engine is run using the default settings on MongoDB 3.0.1.

Direct Category Children

direct children results

Statistics Value
Runtime 31.773 seconds
Mean 0.954 milliseconds
Standard Deviation 0.357 milliseconds
75 percentile 1.162 milliseconds
95 percentile 1.630 milliseconds
99 percentile 1.998 milliseconds
Minimum 0.394 milliseconds
Maximum 7.196 milliseconds

Category Subtree

category subtree results

Statistics Value
Runtime 31.617 seconds
Mean 0.963 milliseconds
Standard Deviation 0.413 milliseconds
75 percentile 1.159 milliseconds
95 percentile 1.669 milliseconds
99 percentile 2.104 milliseconds
Minimum 0.379 milliseconds
Maximum 8.818 milliseconds

WiredTiger

The WiredTiger engine is run using the default settings on MongoDB 3.0.1.

Direct Category Children

direct children results

Statistics Value
Runtime 31.187 seconds
Mean 0.998 milliseconds
Standard Deviation 0.417 milliseconds
75 percentile 1.187 milliseconds
95 percentile 1.874 milliseconds
99 percentile 2.247 milliseconds
Minimum 0.399 milliseconds
Maximum 7.093 milliseconds

Category Subtree

category subtree results

Statistics Value
Runtime 30.584 seconds
Mean 0.992 milliseconds
Standard Deviation 0.495 milliseconds
75 percentile 1.183 milliseconds
95 percentile 1.886 milliseconds
99 percentile 2.386 milliseconds
Minimum 0.394 milliseconds
Maximum 19.589 milliseconds

As this is a read only workload the two storage engines are close in performance for the amount of load we applied to the MongoDB server instance.

Notes

One of the things to remember is that regular expression queries can only leverage indexes when they are case insensitive and convert to a simple start to end string match that can leverage an index. A more complex regular expression will cause a full index scan.

The following query will correctly use the index.

col.find({parent: /^\/electronics/}, {_id:0, parent:1, name:1}).explain();

Example 1: A document containing the metadata schema

The next query will cause an index scan due to it being a case insensitive regular expression.

col.find({parent: /^\/electronics/i}, {_id:0, parent:1, name:1}).explain();

Example 1: A document containing the metadata schema

If you can live with not retrieving the whole document for your category tree queries the covered indexes will allow you to avoid materializing the actual category documents into memory as the queries can be answered with the fields stored in the index itself.

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