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Beautiful Data by Toby Segaran, Jeff Hammerbacher

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Complex Queries

As web applications become more complex and interesting, they need to retrieve and combine information from the database in new and different ways. Next, we examine how to support those queries at a massive scale.

The Challenge

Our system is optimized for queries that touch one or just a few records. In particular, we can look up records by primary key; once we know Alice's username, it is straightforward to determine which partition contains her profile record and read it while loading her page. Also, our system can store data as hash-partitioned or range-partitioned tables. For range-partitioned tables, we can conduct range scans over ordered ranges of primary keys. For example, we might store Alice's friends list by having one record per connection, where the primary key of each connection is the pair of user IDs for Alice and the friend (Table 4-7).

Table 4-7. Friends table

User1

User2

Alice

Bob

Alice

Charles

Alice

Dave

In a range-partitioned table, all of the records prefixed with "Alice" will be clustered, and a short-range scan will be able to pick them up.

Now imagine that we want to add another feature to our social network site. Users can post photos and then comment on one another's photos. Alice might comment on Bob's photo, Charles's photo, and Dave's photo. When we display a photo, we want to show the set of comments associated with that photo. We also want to show Alice the set of comments she has made on other people's photos. We specify the primary key of the comments table as (PhotoID,CommentID) and store it as an ordered YDOT table (Table 4-8), so that all comments for the same photo are clustered and can be retrieved by a range query.

Table 4-8. Photo comments table

PhotoID

CommentID

Comment

Commenter

Photo123

18

Cool

Mary

Photo123

22

Pretty

Alice

Photo123

29

Interesting

Charles

How can we collect the set of comments that Alice has made? We have to perform a join between Alice's profile record (which contains her username as a key) and the comment records (which have Alice's username as a foreign key). Because of our scale-out architecture, data is partitioned across many servers, so computing the join can require accessing many servers. This expensive operation drives up the latency of requests, both because multiple servers must be contacted and because a single query generates a great deal of server load (which slows down other requests).

Another type of query that can be expensive to compute in a scale-out system is group-by-aggregate queries. Imagine that users specify hobbies, and we want to count the number of users who have each hobby so that we can show Alice which hobbies are most popular. Such a query requires scanning all of the data and maintaining counts. The table scan will place prohibitive load on the system and certainly cannot be done synchronously, as Alice's page will take forever to load.

These examples show that while point lookups and range scans can be executed quickly, more expensive join and aggregation queries cannot be executed synchronously.

Our Approach

Our key principle for handling expensive operations is to do them asynchronously, but expensive queries cannot really be handled this way; we do not want to make Alice come back repeatedly to check whether the asynchronous query collecting all of her comments has completed.

Materialized views (Agarawal et al. 2009) can, however, be maintained asynchronously, and when Alice logs in she can quickly (and synchronously) query the view.[6] Although an asynchronously maintained view can be stale compared to the base data, the application already must be built to cope with stale replicas, so dealing with stale view data is usually acceptable. In fact, we treat a materialized view as a special kind of replica that both replicates and transforms data. By using the same mechanism that updates replicas to also update views, we ensure that views have similar reliability and consistency guarantees as replicated base data, without having to design and implement a second mechanism.

Even though view maintenance is done in the background, we still want to make it cheap. If view maintenance takes too many system resources, it will either disrupt synchronous read and write requests (adding latency to every query), or we will have to throttle it to run slowly, at which point the view will be so stale as to possibly be unusable. Thus, we have to find ways to make view maintenance efficient. Consider the earlier example where we want to show Alice all of the comments she has made on other people's photos. We will create a materialized view where comment data is reorganized to be clustered by the foreign key (username of the commenter) rather than the primary key. Then, all of the comments made by Alice will be clustered together. We can also place Alice's profile record in the view, keyed by her username, so that her profile and her comments are clustered. Computing the key/foreign key join is as easy as scanning the set of view records prefixed with "Alice", and then joining them. The result is shown in Table 4-9.

Table 4-9. Co-clustering joining profile and comment records

Alice

West

32

Alice Smith

← Profile record

Alice

Photo123

22

Pretty

← Comment records

Alice

Photo203

43

Nice

Alice

Photo418

33

OK

 

Note that we do not prejoin the profile and comment records in the view. By merely co-locating records that would join, we make join maintenance cheap: whenever there is an update to a base record, we only have to update a single view record, even if that view record would join with multiple other records.

How can we store profile and comment records in the same table? In a traditional database it would be difficult, since the two records have different schemas. However, a core feature of PNUTS is its ability to represent flexible schemas. Different records in the same table can have different sets of attributes. This feature is very useful in web applications since web data is often sparse; a database of items for sale will have different attributes (e.g., color, weight, RAM, flavor) depending on what kind of item it is. It turns out that flexible schemas are also key to implementing materialized join views so that we can colocate joining records from different tables.

The asynchronous view approach is useful for helping to answer other kinds of queries as well. A group-by-aggregation query can be effectively answered by a materialized view that has pregrouped, and maybe even preaggregated, the data. There are even "simple" queries, such as a selection over a nonprimary key attribute, that can be most effectively answered by a materialized view. Consider a query for users who live in Sunnyvale, California. Since our user table is keyed by username, this query normally requires an expensive table scan. However, we can use the materialized view mechanism to build a secondary index over the "location" field of the table, store the index in an ordered YDOT table, and then conduct a range scan over the "Sunnyvale, California" index records to answer our query (Table 4-10).

Table 4-10. Location index

Location

Username

Sunnyvale, CA

Alice

Sunnyvale, CA

Mary

Sunnyvale, CA

Steve

Sunnyvale, CA

Zach

As with materialized views in other systems, we can create them effectively only if we know in advance what kinds of queries to expect. Luckily, in web-serving workloads, the queries are usually templates known in advance with specific parameters (such as the location or username) bound at runtime. As such, application developers know in advance which queries are complex enough to require materializing a view. To ask ad hoc queries over data stored in PNUTS, developers have to use our plug-ins to pull data out of our system into a compute grid running Hadoop, the open source implementation of MapReduce.

Once we have a few different mechanisms for handling complex queries, it will be useful to implement a query planner to help execute queries effectively. A planner helps remove some of the burden from the application developer, who can write declarative queries without worrying too much about how they will be executed. However, an effective query planner at our scale will require sophisticated statistics collection, load monitoring, network monitoring, and a variety of other mechanisms to make sure the planner has enough information about all the possible bottlenecks in the system to make the most effective query plan.



[6] Materialized views are not currently in the production version of the system.

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