Airbnb’s web experience is powered by a Rails monolith, called Monorail, that talks to several different Java services. MySQL databases store business data and are partitioned by functionality, with messages and calendar management, for example, stored separately from the main booking flow in their own databases.

As traffic to the site continued growing, though, “one notable resource issue with MySQL databases [was] the increasing number of database connections from application servers.”

Airbnb uses AWS’s Relational Database Service (RDS) to power their MySQL instances, and “RDS uses the community edition of MySQL server, which employs a one-thread-per-connection model of connection management.” With Airbnb’s scale, this meant that their databases would hit the C10K problem, which states that “there is an upper bound in the number of connections that MySQL server can accept and serve without dramatically increasing the number of threads running, which severely degrades MySQL server performance.”

When an RDS MySQL server hits resource limits, users will have trouble connecting to the site.

MySQL does have dynamic thread pooling, but it’s only available in the enterprise edition; AWS MySQL RDS, though, doesn’t offer this feature, meaning Airbnb didn’t have access to dynamic thread pooling out-of-the-box.

After surveying several options, the team chose MariaDB MaxScale, which is “a MySQL database proxy that supports intelligent query routing in between client applications and a set of backend MySQL servers.”

Instead of using the MariaDB MaxScale off-the-shelf, however, they decided to fork it and implement their own version that would include connection pooling. Other MaxScale features, like request throttling and query blocklisting were implemented as well.

To enable horizontal scaling of the web application, the team deployed a MaxScale database proxy service in between app servers and MySQL servers. Through the service discovery system SmartStack, applications now “discover and connect to the database proxy service instead of the MySQL database,” allowing horizontal scaling to meet capacity demands.

Additionally, new Airbnb MaxScale proxy server instances can be launched to further enable horizontal scaling.