When We Should Retry in RestTemplate

Overview

API request using RestTemplate in Spring is not always successful for various exceptions. So we can consider “retrying request” in some exceptions.

RestException	Retry
ResourceAccessException	O
HTTPServerErrorException	O
HTTPClientErrorException	O/X
UnknownHTTPStatusCodeException	X

@retryable for Retry

Can use @retryable for retry in Spring. It is ok to code oneself by Java but using annotation is more clear and easy to read.

@Retryable(value = {ResourceAccessException.class, HTTPServerErrorException.class}, maxAttempts = 2)
public void updateDB() throws ResourceAccessException, HTTPServerErrorException {
  post("update_data");
}

Spring-Retryでリトライ処理の実装

Exception List in RestTemplate

Using RestTemplate in Spring can throw RestClientException. And RestClientException has some child exceptions. So…

Have to consider belows when retry

• ResourceAccessException
• HTTPServerErrorException
• HTTPClientErrorException
• UnknownHTTPStatusCodeException

RestTemplateが投げる例外クラスまとめ

Retry in each Exception

ResourceAccessException

Had better retry when ResourceAccessException occur for it thrown when an I/O error occurs like time out. Many practices do retry this exception occurs.
→ spring-docs-ResourceAccessException

HTTPServerErrorException

Had better retry when HTTPServerErrorException occur for it thrown when an HTTP 5xx is received.

Here are 5xx Responses and had better retry when first request failed except 505.

Code	Status	Note
500	Internal Server Error	General server issue going on right now
503	Service Unavailable	server is temporarily unable to handle the request for being overloaded or down for maintenance
504	Gateway Timeout	one server did not receive a timely response from another server
505	HTTP Version Not Supported	server can emit if it doesn’t support the major HTTP version the client used to make the request

spring-docs-HttpServerErrorException

HTTPClientErrorException

not retry, add validation

Had better NOT retry when HTTPClientErrorException occur for it thrown when an HTTP 4xx is received. 4xx is caused by wrong request from client. Unless client fix wrong request, same 4xx will return even though how many request you send.

Here are 4xx Responses

Code	Status	Note
400	Bad Request	server was unable to process the request sent by the client due to invalid syntax
401	Unauthorized	request has not been applied because it lacks valid authentication credentials for the target resource
403	Forbidden	access to the requested (valid) URL by the client is Forbidden for some reason
404	Not Found	browser was able to communicate with a given server, but the server could not find what was requested
405	Method Not Allowed	server can emit if it doesn’t support HTTP method is simply not supported

About HTTP 400, can prevent it by adding validation to request parameter.

ApiRequestParams params = new ApiRequestParams();
BindingResult bindingResult = new DataBinder(params).getBindingResult();

// call api

spring-docs-HttpServerErrorException

may ok to retry when 4xx but…

Except HTTP 400, other 4xx errors are hardly happen. HTTP 400 itself will not happen if you add validation to request parameter. So retry when HTTPClientErrorException may ok in many cases but have to consider one more when doing like this.

UnknownHTTPStatusCodeException

Had better NOT retry when UnknownHTTPStatusCodeException occur for it thrown when an unknown (or custom) HTTP status code is received. Have to check the cause of this exception before you try to retry.