reitit/doc/performance.md

# Performance

There are many great routing libraries for Clojure(Script), but not many are optimized for perf. Reitit tries to be both great in features and really fast. Originally the routing was adopted from [Pedestal](http://pedestal.io/), but it has been since mostly rewritten.

### Rationale

* Multiple routing algorithms, select for for a given route tree
* Route flattening and re-ordering
* Managed mutability over Immutability
* Precompute/compile as much as possible (matches, middleware, routes)
* Use abstractions that enable JVM optimizations
* Use small functions to enable JVM Inlining
* Protocols over Multimethods
* Records over Maps
* Always be measuring
* Don't trust the (micro-)benchmarks

### Performance guides

Some things related to performance:

* avoid wildcard-routes - it's an order of magnitude slower to match than non-wildcard routes
* it's ok to mix non-wildcard and wildcard routes in a same routing tree as long as you don't disable the [conflict resolution](basics/route_conflicts.md) => if no conflicting routes are found, a `:mixed-router` can be created, which collects all non-wildcard routes into a separate fast subrouter.

### Does routing performance matter?

Well, it depends. Some tested routing libs seem to spend more time resolving the routes than it takes to encode & decode a 1k JSON payload. For busy sites, this actually matters.

### Example

The routing sample taken from [bide](https://github.com/funcool/bide) README, run with a Late 2013 MacBook Pro, with the `perf` profile:

```clj
(require '[reitit.core :as r])
(require '[criterium.core :as cc])

(def routes
  (r/router
    [["/auth/login" :auth/login]
     ["/auth/recovery/token/:token" :auth/recovery]
     ["/workspace/:project/:page" :workspace/page]]))

;; Execution time mean : 3.488297 µs -> 286M ops/sec
(cc/quick-bench
  (dotimes [_ 1000]
    (r/match-by-path routes "/auth/login")))

;; Execution time mean : 692.905995 µs -> 1.4M ops/sec
(cc/quick-bench
  (dotimes [_ 1000]
    (r/match-by-path routes "/workspace/1/1")))
```

### Is that good?

Based on some [quick perf tests](https://github.com/metosin/reitit/tree/master/perf-test/clj/reitit), the first lookup is two orders of magnitude faster than other tested Clojure routing libraries. The second lookup is 3-18x faster.

But, most micro-benchmarks lie. For example, Pedestal is always matching the `:request-method` which means it does more work. With real life routing trees, the differences are most likely more subtle, or some other lib might be actually faster.

### So why test?

Real value of perf tests is to get a internal baseline to optimize against. Also, to ensure that new features don't regress the performance.

It might be interesting to look out of the box and compare the fast Clojure routing libs to routers in other languages, like the [routers in Go](https://github.com/julienschmidt/go-http-routing-benchmark).

### Roadmap

Currently, the non-wildcard routes are already really fast to match, but wildcard routes use only a naive linear scan. Plan is to add a optimized [Trie](https://en.wikipedia.org/wiki/Trie)-based router. See
[httprouter](https://github.com/julienschmidt/httprouter#how-does-it-work) and [Pedestal](https://github.com/pedestal/pedestal/pull/330) for details.

PRs welcome.