Add tips for rewriting legacy services (#2429)
This commit is contained in:
Paul Melnikow
GitHub
239
doc/rewriting-services.md
Normal file
239
doc/rewriting-services.md
Normal file
@@ -0,0 +1,239 @@
|
||||
# Tips for rewriting legacy services
|
||||
|
||||
## First, write some tests
|
||||
|
||||
If service tests don’t exist for the legacy service, stop and write them first.
|
||||
It’s recommended to PR these separately. If there’s some test coverage, it’s
|
||||
probably fine to move right ahead and add more in the process. Make sure the
|
||||
tests are passing, though.
|
||||
|
||||
## Organization
|
||||
|
||||
1. When there’s a single legacy service that handles lots of different things
|
||||
(e.g. version, license, and downloads), it should be split into three separate
|
||||
service classes and placed in three separate files, e.g.:
|
||||
|
||||
- `example-version.service.js`
|
||||
- `example-license.service.js`
|
||||
- `example-downloads.service.js`
|
||||
|
||||
2. When a badge offers different variants of basically the same thing, it’s okay
|
||||
to put them in the same service class. For example, daily/weekly/monthly/total
|
||||
downloads can go in one badge, and star rating vs point rating vs rating count
|
||||
can go in one badge, and same with various kinds of detail about a pull request.
|
||||
The hard limit (as of now anyway) is *one category per service class*.
|
||||
|
||||
3. If the tests haven’t been split up, split them up too and make sure they
|
||||
still pass.
|
||||
|
||||
## Get the route working
|
||||
|
||||
1. Disable the legacy service by adding a `return` at the top of
|
||||
`registerLegacyRouteHandler()`.
|
||||
|
||||
2. Set up the route for one of the badges. First determine if you can express
|
||||
the route using a `pattern`. A `pattern` (e.g. `pattern: ':param1/:param2'`) is
|
||||
the simplest way to declare the route, also the most readable, and will be
|
||||
useful for displaying a badge builder with fields in the front end and
|
||||
generating badge URLs programmatically. One limitation to keep in mind is that,
|
||||
at present, the trailing parameter of a pattern can't be optional. If the last
|
||||
part of a route is optional, like a branch, you will need to use a `format`
|
||||
regex string (e.g. `format: '([^/]+/[^/]+)'`).
|
||||
|
||||
3. When creating the initial route, you can stub out the service. A minimal
|
||||
service extends BaseJsonService (or BaseService, or one of the others), and
|
||||
defines `route()` and `handle()`. `defaultBadgeData` is optional but suggested:
|
||||
|
||||
```js
|
||||
const BaseJsonService = require('../base-json')
|
||||
|
||||
class ExampleDownloads extends BaseJsonService {
|
||||
static get route() {
|
||||
return {
|
||||
base: 'example/d',
|
||||
pattern: ':param1/:param2',
|
||||
}
|
||||
}
|
||||
|
||||
static defaultBadgeData() {
|
||||
return { label: 'downloads' } // or whatever
|
||||
}
|
||||
|
||||
async handle({ param1, param2 }) {
|
||||
return { message: 'hello' }
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
4. We don’t have really good tools for debugging matches, so the best you can do
|
||||
is run a subset of your tests. To run a single service test, add `.only()`
|
||||
somewhere in the chain, and run `npm run test:services:trace — —only=example`.
|
||||
|
||||
```js
|
||||
t.create('build status')
|
||||
.get('/pip.json')
|
||||
.only() // Prevent this ServiceTester from running its other tests.
|
||||
.expectJSONTypes(
|
||||
Joi.object().keys({
|
||||
name: 'docs',
|
||||
value: Joi.alternatives().try(isBuildStatus, Joi.equal('unknown')),
|
||||
})
|
||||
)
|
||||
```
|
||||
|
||||
5. Presumably the test will fail, though by examining the copious output, you
|
||||
can confirm the route was matched and the named parameters mapped successfully.
|
||||
Since you'll have just run the tests on the old code (right?) you'll know you
|
||||
haven't inadvertently changed the route (an easy mistake to make).
|
||||
|
||||
6. If the legacy service had a base URL and you've changed it, you’ll need to
|
||||
update the tests _and_ the examples. Take care to do that.
|
||||
|
||||
## Implement `render()` and `handle()`
|
||||
|
||||
Once the route is working, fill out `render()` and `handle()`.
|
||||
|
||||
1. If there’s a single service, you can implement fetch as a method or a
|
||||
function at the top of the file. If there are more than one service which share
|
||||
fetching code, you can put the fetch function in `example-common.js` like this
|
||||
one for github:
|
||||
|
||||
<details>
|
||||
|
||||
```js
|
||||
const Joi = require('joi')
|
||||
const { errorMessagesFor } = require('./github-helpers')
|
||||
|
||||
const issueSchema = Joi.object({
|
||||
head: Joi.object({
|
||||
sha: Joi.string().required(),
|
||||
}).required(),
|
||||
}).required()
|
||||
|
||||
async function fetchIssue(serviceInstance, { user, repo, number }) {
|
||||
return serviceInstance._requestJson({
|
||||
schema: issueSchema,
|
||||
url: `/repos/${user}/${repo}/pulls/${number}`,
|
||||
errorMessages: errorMessagesFor('pull request or repo not found'),
|
||||
})
|
||||
}
|
||||
|
||||
module.exports = {
|
||||
fetchIssue,
|
||||
}
|
||||
```
|
||||
|
||||
</details>
|
||||
|
||||
or create an abstract superclass like **PypiBase**:
|
||||
|
||||
<details>
|
||||
|
||||
```js
|
||||
const Joi = require('joi')
|
||||
const BaseJsonService = require('../base-json')
|
||||
|
||||
const schema = Joi.object({
|
||||
info: Joi.object({
|
||||
...
|
||||
}).required()
|
||||
}).required()
|
||||
|
||||
module.exports = class PypiBase extends BaseJsonService {
|
||||
static buildRoute(base) {
|
||||
return {
|
||||
base,
|
||||
pattern: ':egg*',
|
||||
}
|
||||
}
|
||||
|
||||
async fetch({ egg }) {
|
||||
return this._requestJson({
|
||||
schema,
|
||||
url: `https://pypi.org/pypi/${egg}/json`,
|
||||
errorMessages: { 404: 'package or version not found' },
|
||||
})
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
</details>
|
||||
|
||||
2. Validation should be handled using Joi. Save this for last. While you're
|
||||
getting things working, you can use `const schema = Joi.any()`, which matches
|
||||
anything.
|
||||
|
||||
3. Substitution of default values should also be handled by Joi, using
|
||||
`.default()`.
|
||||
|
||||
5. To keep with the design pattern of `render()`, formatting concerns, including
|
||||
concatenation and color computation, should be dealt with inside `render()`.
|
||||
This helps avoid static examples falling out of sync with the implementation.
|
||||
|
||||
## Error handling
|
||||
|
||||
BaseService includes built-in runtime error handling. Error classes are defined
|
||||
in `services/errors.js`. Request code and validation code will throw a runtime
|
||||
error, which will then bubble up to BaseService, which then renders an error
|
||||
badge. The cases covered by built-in error handling need not be tested in each
|
||||
service, and existing tests should be removed.
|
||||
|
||||
1. If an external server can't be reached or returns a 5xx status code,
|
||||
`_requestJson()` along with code in `lib/error-helper.js` will bubble up an
|
||||
**Inaccessible** error.
|
||||
|
||||
2. If a response does not match the schema, `validate()` will bubble up an
|
||||
**InvalidResponse** error which will display **invalid response data**.
|
||||
|
||||
Error handling can also be customized by the service. Alternate messages
|
||||
corresponding to HTTP status codes can be specified in the `errorMessages`
|
||||
parameter to `_requestJson()` etc.
|
||||
|
||||
For the not found case, a service test should establish that the API is doing
|
||||
what we expect. If the API returns a 404 error, code in `lib/error-helper.js`
|
||||
will automatically throw a **NotFound** error. The error message can, and
|
||||
generally should be customized to display something more specific like
|
||||
**package not found** or **room not found**.
|
||||
|
||||
Not all services return a 404 response in the not found case. Sometimes a
|
||||
different status code is returned.
|
||||
|
||||
Sometimes a 200 response must be examined to distinguish the not found case from a success case. This can be handled in either of two ways:
|
||||
|
||||
- Write a schema which accommodates both the success and error cases.
|
||||
- Write the schema for the success case. Pass `schema: Joi.any()` to
|
||||
`_requestJson()`. Manually check for the error case, then invoke
|
||||
`_validate()` with the success-case schema.
|
||||
|
||||
In either case, the service should throw e.g
|
||||
`new NotFound({ prettyMessage: 'package not found' })`.
|
||||
|
||||
## Convert the examples
|
||||
|
||||
1. Convert all the examples to `pattern`, `namedParams`, and `staticExample`. In some cases you can use the `pattern` inherited from `route`, though in other cases you may need to specify a pattern in the example. For example, when showing download badges for several periods, you may want to render the example with an explicit `dt` instead of `:which`. You will also need to specify a pattern for badges that use a `format` regex in the route.
|
||||
|
||||
2. Open the frontend and check that the static preview badges look good.
|
||||
Remember, none of them are live.
|
||||
|
||||
3. Open up the prepared example URLs in their own tabs, and make sure they work correctly.
|
||||
|
||||
## Validation
|
||||
|
||||
When it's time to add the schema, refer to the Joi API docs:
|
||||
https://github.com/hapijs/joi/blob/master/API.md
|
||||
|
||||
## Housekeeping
|
||||
|
||||
Switch to `createServiceTester`:
|
||||
|
||||
```js
|
||||
const t = require('../create-service-tester')()
|
||||
```
|
||||
|
||||
This may require updating the URLs, which will be relative to the service's base
|
||||
URL. When using `createServiceTester`, services need to be specified using
|
||||
the non-case-sensitive service class name, or a trailing substring (e.g.
|
||||
`AppveyorTests` or `appveyor`).
|
||||
|
||||
Do this last. Since it involves changing test URLs, and you don't want to
|
||||
accidentally change them.
|
||||
Reference in New Issue
Block a user