Routing in Kong Gateway

For each incoming request, Kong Gateway must determine which service gets to handle it based on the routes that are defined. With release 3.0, Kong Gateway introduced a router that can be running in two modes: the traditional_compat mode, which is configured like prior releases, and the expressions mode which uses a new configuration scheme. We recommend that new deployments use the expressions router as it is more powerful and expressive.

The default mode of the router is traditional_compat and the following sections describe how it operates. traditional_compat mode is designed to behave like the router in versions before Kong Gateway 3.x. For a description of the expressions mode, see How to Configure Routes using Expressions.

In general, the router orders all defined routes by their priority and uses the highest priority matching route to handle a request. If there are multiple matching routes with the same priority, it is not defined which of the matching routes will be used and Kong Gateway will use either of them according to how its internal data structures are organized.

If a route contains prefix or regular expression paths, the priority of the route will be calculated separately for each of the paths and requests will be routed accordingly.

In traditional_compat mode, the priority of a route is determined as follows, by the order of descending significance:

1. Priority points
2. Wildcard hosts
3. Header count
4. Regular expressions and prefix paths

Priority points

For the presence of each of a route’s methods, host, headers, and snis, a “priority point” will be added to the route. The number of “priority points” determines the overall order in which the routes will be considered. Routes with a higher “priority point” values will be considered before those with lower values. This means that if one route has methods defined, and second one has methods and headers defined, the second one will be considered before the first one.

Wildcard hosts

Among the routes with the same “priority point” value, those that have any wildcard host specification will be considered after routes that don’t have any wildcard host (or no host) specified.

Header count

The resulting groups are sorted so the routes with a higher number of specified headers have higher priority than those with a lower number of headers.

Within the resulting groups of routes with equal priority, the router sorts the routes as follows:

• Routes that have a regular expression path are considered first and are ordered by their regex_priority value. Routes with a higher regex_priority are considered before routes with lower regex_priority values.
• Routes that have no regular expression path are ordered by the length of their paths. Routes with longer paths are considered before routes with shorter paths.

For a route with multiple paths, each path will be considered separately for priority determination. Effectively, this means that separate routes exists for each of the paths.

Regular expressions used in routes use more resources to evaluate than simple prefix routes. If many regular expressions must be evaluated to route a request, the latency introduced by Kong Gateway can suffer and its CPU usage can increase. In installations with thousands of routes, replacing regular expression routes by simple prefix routes can improve throughput and latency of Kong Gateway. If regex must be used because an exact path match must be performed, using the expressions router will significantly improve Kong Gateway’s performance in this case.

Starting with version 3.0, Kong Gateway uses the regular expression engine shipped with the Rust programming language if the router is operating in expressions or traditional_compatible mode. Prior versions used the PCRE library to evaluate regular expression. While the two engines are largely compatible, subtle differences exist between the two. Refer to the documentation pertinent to the engine that you are using if you have problems getting regular expression routes to work.