Advanced URL rewriting with Apache APISIX

I spoke at Swiss PgDay in Switzerland in late June. The talk was about how to create a no-code API with the famous PostgreSQL database, the related PostgREST, and Apache APISIX, of course. I already wrote about the idea in a previous post. However, I wanted to improve it, if only slightly.

PostgREST offers a powerful SELECT mechanism. To list all entities with a column equal to a value, you need the following command:

curl /products?id=eq.1

id is the column
eq.1 corresponds to the WHERE clause

In this case, the generated query is SELECT * FROM products WHERE id=1.

The query syntax is powerful and allows you to express complex queries. However, as an API designer, I want to avoid exposing users to this complexity. For example, a regular API can manage entities by their ID, e.g., /products/1. In turn, you'd expect PostgREST to be able to do the same with primary keys. Unfortunately, it doesn't treat primary keys any differently than regular columns. Apache APISIX to the rescue.

One of APISIX's best features is to rewrite requests, i.e., exposing /products/1 and forwarding /products?id=eq.1 to PostgREST. Let's do it.

First, we need to capture the ID of the path parameter. For this, we need to replace the regular radix tree router with the radix tree with a parameter router.

apisix:
    router:
        http: radixtree_uri_with_parameter

The next step is to rewrite the URL. We use the proxy-rewrite plugin for this on a /products/:id route. Unfortunately, using the :id parameter above in the regular expression is impossible. We need to copy it to a place that is accessible. To do that, before the rewriting, we can leverage the serverless-pre-function. With the plugin, you can write Lua code directly. It's an excellent alternative to a full-fledged plugin for short, straightforward snippets.

Here's the configuration:

curl -i http://localhost:9180/apisix/admin/plugin_configs/1 -X PUT -d '
{
  "plugins": {
    "serverless-pre-function": {
      "phase": "rewrite",
      "functions" : [
        "return function(_, ctx)
          ctx.var.product_id = ctx.curr_req_matched.id;         #1
        end"
      ]
    },
    "proxy-rewrite": {
      "uri": "/products?id=eq.$product_id"                      #2
    }
  }
}'

Copy the captured id variable to a place accessible to other plugins later on
Use it!

Thanks to my colleague Zeping for pointing out the solution to me!

You can expose the /products/1 REST-friendly URL and let APISIX rewrite it for PostgREST.

Conclusion

I've described using the proxy-rewrite plugin with a path variable in this post. You can reuse the same technique with multiple variables. Keep also in mind that the serverless plugin is a hidden jewel; it can help you with small Lua snippets before moving to a full-fledged plugin.

To go further:

Originally published at A Java Geek on July 14^th, 2024