Consuming REST Services

Marc Kapke

kapkema@gmail.com

Monolith

• Most, if not all, of system on single server
• DB Access
• Some RPC (SOAP,etc)

Microservices

• System domain distributed across several servers
• Very dependent on Server to Server communication
• Some DB Access - but most data fetched from other Domain services

Rise of REST

• Existing protocols (SOAP, etc) were to heavy-weight
  • Bloated messages
  • Message processing overhead
  • Lots of configuration

Why REST took off

• Standard HTTP
• Cachable
• Fast and efficient
  • No additional message processing
  • Smaller message formats

Spring RestTemplate

• RestTemplate is Spring’s HTTP Client
  • Central class for synchronous http access
  • Supports all major REST Verbs
    • (GET, PUT, POST, DELETE, HEAD, OPTIONS)
  • Simply provide a Url
  • Returns an Http Response with message body

Serialization

• Response can be retrieved as JSON String
  • String jsonString = restTemplate.getForObject("http://myurl.com", String)
• Response can be serialized as POJO
  • MyDomainClass responseBody = restTemplate.getForEntity("http://myurl.com", MyDomainClass)

Response types

• ForEntity methods return ResponseEntity
  • ResponseEntity myEntity = restTemplate.getForEntity("http://myurl.com", Map)
• ForObject methods return Object only
  • Map myResponse = restTemplate.getForObject("http://myurl.com", Map)

Configurable and Injectable

• Add RestTemplate bean to Autowire
• Add ClientHttpRequestFactory when building RestTemplate to configure

@Bean
RestTemplate restTemplate(RestTemplateBuilder builder) {
    builder.requestFactory(getClientHttpRequestFactory())
    return builder.build()
}


private static ClientHttpRequestFactory getClientHttpRequestFactory() {
    HttpComponentsClientHttpRequestFactory clientHttpRequestFactory = new HttpComponentsClientHttpRequestFactory()
    clientHttpRequestFactory.setConnectTimeout(200)
    clientHttpRequestFactory.setConnectionRequestTimeout(100)
    clientHttpRequestFactory.setReadTimeout(3000)
    return clientHttpRequestFactory
}

Example GET Request

  @Autowired
  RestTemplate restTemplate

  ResponseEntity getArticles(String category, Long time) {
    try {
      restTemplate.getForEntity("https://api.nytimes.com/svc/mostpopular/v2/mostviewed/${category}/${time}.json?api-key=<key>&offset=0", Map)
    }catch (HttpClientErrorException ex){
      return new ResponseEntity([error: ex.message], ex.statusCode)
    }
  }

Testing 3rd party dependencies

• Not reliable
  • Could be down
  • May not be accessible from all test environments
  • Can’t trigger error states

Mock 3rd party dependencies

• Consistent responses
• Reliably trigger request/response scenarios
  • Bad request
  • Internal Server Error
  • etc…

Http layer is tricky to mock

• Want to mock at lowest level possible
• Spring provides MockRestServiceServer to handle this
  • Replaces RestTemplate at runtime with Mock implementation
  • Setup Mock responses to return during test execution
  • Returned just like actual response

Example Test

@Autowired
RestTemplate restTemplate

@Autowired
TestRestTemplate testRestTemplate

MockRestServiceServer mockServer

def "get articles"() {
  setup:
  mockServer = MockRestServiceServer.createServer(restTemplate)

  mockServer.expect(requestTo(new StringContains("/svc/mostpopular/v2/mostviewed/food/1.json"))).andExpect(method(HttpMethod.GET))
            .andRespond(withSuccess("{}", MediaType.APPLICATION_JSON_UTF8))

  when:
  def responseEntity = testRestTemplate.getForEntity("/articles/food/1", Map)

  then:
  mockServer.verify()


  responseEntity.body == [:]
  responseEntity.statusCode == HttpStatus.OK
}