To avoid the anticipated limitations to our disk-based cache, we will now build our cache on top of an existing database system. When crawling, we may need to cache massive amounts of data and will not need any complex joins, so we will use a NoSQL database, which is easier to scale than a traditional relational database. Specifically, our cache will use MongoDB, which is currently the most popular NoSQL database.
NoSQL stands for Not Only SQL and is a relatively new approach to database design. The traditional relational model used a fixed schema and splits the data into tables. However, with large datasets, the data is too big for a single server and needs to be scaled across multiple servers. This does not fit well with the relational model because, when querying multiple tables, the data will not necessarily be available on the same server. NoSQL databases, on the other hand, are generally schemaless and designed from the start to shard seamlessly across...