router

Router

GOV.UK Router is an HTTP reverse proxy built on top of triemux.

How router loads routes

Router loads its routing table from Content Store's PostgreSQL database (or optionally from a flat file) into a trie data structure for fast path lookups.

Router can reload routes without restarting:

1. Automatically via PostgreSQL's LISTEN/NOTIFY mechanism
2. Periodic schedule
3. Manually via the API server

Internally these use a Go channel to send reload requests that causes Router to reload from content-store's PostgreSQL database.

Routes

Routes can be one of two types:

• exact: The path must match exactly (e.g. the exact route /government only matches a request for /government)
• prefix: The path prefix must match (e.g. the prefix route /government matches requests for /government, /government/policies, etc.)

The route type and URL path determine which route gets matched to a particular request.

Router maintains two separate tries:

1. Exact path matches
2. Prefix matches

Once a request comes in, Router uses the URL path to first check for an exact match, then falls back to the longest prefix match.

Suppose we have the following routes:

1. Prefix route on /foo
2. Exact route on /foo/bar
3. Exact route on /bar

Then Router will:

1. Returns 404 if a request is made for the children of an exact route (e.g. /bar/foo/).
2. Match on the prefix route if the request is made for /foo
3. Match on the exact route if the request is made for /foo/bar
4. Match on the prefix route if the request is made for /foo/bar/baz as there is no matching exact route

See route_selection_test.go for more cases.

Handling

Routes have a schemaName property:

1. Backend
2. Redirect
3. Gone

Once a request is matched to a route, Router uses the schemaName property to determine how the request should be handled.

There are 3 handler types to handle a request:

1. backend: Reverse proxies the request to a backend application server
2. redirect: Returns an HTTP 301 redirect to a new location
3. gone: Returns an HTTP 410 Gone response for deleted content

Note: some Gone routes are also handled by the backend handler.

Router otherwise:

• serves 503 if no routes are loaded
• serves 404 if the route can't be found

Redirect routes

Redirect routes have a flag that is used to determine whether the URL path in the request should be preserved.

If the source path is /source and the redirect target is /target then the target URL will preserve the path as follows:

https://source.example.com/target/path/subpath?q1=a&q2=b

Otherwise the URL will be:

https://source.example.com/target/

Redirect routes will only redirect to a lowercase route if the URL path is in all caps (e.g. /GOVERNMENT/GUIDANCE will redirect to /government/guidance).

For details on the route data structure and handler configuration, see docs/data-structure.md.

Request flow

graph LR;
    A[Fastly]-->B[Router Load Balancer];
    B[Router Load Balancer]-->C[Router nginx];
    C[Router nginx]-->D[Router];
    D[Router]-->E[Backend];

Router's load balancer adds the following headers:

1. X-Forwarded-For
2. X-Forwarded-Proto
3. X-Forwarded-Port

Router doesn't proxy redirect and gone routes to a backend but simply returns the response to the client.

Draft stack

The draft stack consists of 'draft' deployments of Router, content store and backends.

Here the request passes through an authenticating proxy before it hits draft router:

graph LR;
    A[Authenticating Proxy Load Balancer]-->B[Authenticating Proxy nginx];
    B[Authenticating Proxy nginx]-->C[Authenticating Proxy];
    C[Authenticating Proxy]-->D[Draft Router nginx];
    D[Draft Router nginx]-->E[Draft Router];
    E[Draft Router]-->F[Draft backend];

In addition to the headers added by the load balancer authenticating proxy adds the following headers:

1. X_GOVUK_AUTHENTICATED_USER_ORGANISATION
2. X_GOVUK_AUTHENTICATED_USER
3. X-Forwarded-Host replaces Host

As before draft router doesn't proxy redirect and gone routes to a backend.

Nginx

Router runs an nginx instance that proxies traffic to Router. The configuration for both live and draft stack live in govuk-helm-charts

The nginx instance also provides:

1. Healthcheck endpoints
2. Static error pages
3. robots.txt and humans.txt
4. Google Search Console verification files
5. Licensify endpoint

It also sets and hides some HTTP headers.

API server

Router runs two HTTP servers:

1. Public server (default :8080) for handling requests
2. API server (default :8081) for admin operations

The API server exposes the following routes inside the cluster:

1. /reload
2. /healthcheck
3. /memory-stats
4. /metrics

Configuration

Router is configured via environment variables:

Backend applications are configured with BACKEND_URL_<backend_id> environment variables:

export BACKEND_URL_frontend=http://localhost:3000
export BACKEND_URL_publisher=http://localhost:3001

Routes reference these backends by their ID (e.g., "frontend", "publisher").

Serving routes from a flat file

When ROUTER_ROUTES_FILE is set, Router will load routes from the specified JSONL file (one JSON object per line). Router will also no longer load routes from PostgreSQL, and periodic route updates are disabled.

Example file:

{"BackendID":"frontend","IncomingPath":"/government","RouteType":"prefix","RedirectTo":null,"SegmentsMode":null,"SchemaName":null,"Details":null}
{"BackendID":null,"IncomingPath":"/old-page","RouteType":"exact","RedirectTo":"/new-page","SegmentsMode":"ignore","SchemaName":"redirect","Details":null}
{"BackendID":null,"IncomingPath":"/deleted","RouteType":"exact","RedirectTo":null,"SegmentsMode":null,"SchemaName":"gone","Details":null}

You can export routes from PostgreSQL to a JSONL file using:

ROUTER_ROUTES_FILE=/path/to/routes.jsonl ./router -export-routes

This can be used to continue serving routes when Content Store's database is down for maintenance.

For details on how to configure Router to load from a file see docs/how-to-serve-routes-from-flat-file.md.

Technical documentation

Recommended reading: How to Write Go Code

Run the test suite

Checks run automatically on GitHub on PR and push. For faster feedback, you can run the tests locally.

The lint check uses golangci-lint, which you can install via Homebrew or your favourite package manager:

brew install golangci-lint

You can run all tests (some of which need Docker installed) by running:

make test

You can also run just the unit tests or just the integration tests, using the unit_tests and integration_tests targets. The unit tests don't need Docker.

The trie and triemux packages have unit tests. To run these on their own:

go test -bench=. ./trie ./triemux

The integration tests need Docker in order to run PostgreSQL. They are intended to cover Router's overall request handling, error reporting and performance.

You can use --ginkgo.focus <partial regex> to run a subset of the integration tests, for example:

go test ./integration_tests -v --ginkgo.focus 'redirect should preserve the query string'

See Site tests on how to run the site tests.

Debug output

To see debug messages when running tests, set both the ROUTER_DEBUG and ROUTER_DEBUG_TESTS environment variables:

ROUTER_DEBUG=1 ROUTER_DEBUG_TESTS=1 make test

Update the dependencies

This project uses Go Modules to vendor its dependencies. To update the dependencies:

1. Update all the dependencies, including test dependencies, in your working copy:

   make update_deps
   

2. Check for any errors and commit.

   git commit -- go.{mod,sum} vendor
   

3. Run the Router test suite. If you need to fix a failing test, keep your changes in separate commits to the go get / go mod commit.

4. Run the tests for all dependencies:

   go test all
   

   • If there are failures, look into each one and determine whether it needs fixing.
   • If anything under vendor/ needs changing then either raise a PR with the upstream project or revert to a set of versions that work together. Only go get and go mod should touch files in vendor/.

5. Raise a PR.

Further documentation

• Data structure
• Original thinking behind the router
• Example of adding a metric using the Go prometheus client library
• Site tests
• Triemux
• Trie

Team

GOV.UK Platform Engineering team looks after this repo. If you're inside GDS, you can find us in #govuk-platform-engineering or view our kanban board.

Licence

MIT License