feat(apollo-client): the spi of config service load balancer client

[Anilople]

What's the purpose of this PR

Let user can custom load balancer client of how to choose config service by spi.

Which issue(s) this PR fixes:

Fixes #3225

Brief changelog

• interface ConfigServiceLoadBalancerClient
• implementation RandomConfigServiceLoadBalancerClient
• implementation DefaultConfigServiceLoadBalancerClient extends RandomConfigServiceLoadBalancerClient
• use java.util.concurrent.ThreadLocalRandom instead of java.util.Random

Follow this checklist to help us incorporate your contribution quickly and easily:

• Read the Contributing Guide before making this pull request.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit tests to verify the code.
• Run mvn clean test to make sure this pull request doesn't break anything.
• Update the CHANGES log.

[Anilople]

todo:

• changelog
• document

[Anilople]

Should update the version of project before this pr merge?

apollo/pom.xml

Line 63 in 40772c4

<revision>2.0.1</revision>

[codecov-commenter]

Codecov Report

Merging #4394 (6ed0772) into master (2cc9fc8) will increase coverage by 0.00%.
The diff coverage is 92.30%.

@@            Coverage Diff            @@
##             master    #4394   +/-   ##
=========================================
  Coverage     53.20%   53.20%           
- Complexity     2677     2681    +4     
=========================================
  Files           489      490    +1     
  Lines         15291    15300    +9     
  Branches       1585     1586    +1     
=========================================
+ Hits           8135     8141    +6     
- Misses         6602     6604    +2     
- Partials        554      555    +1     

Impacted Files	Coverage Δ
...llo/spi/RandomConfigServiceLoadBalancerClient.java	87.50% <87.50%> (ø)
.../apollo/internals/RemoteConfigLongPollService.java	78.44% <100.00%> (+0.12%)	⬆️
...rk/apollo/spring/property/SpringValueRegistry.java	83.78% <0.00%> (-5.41%)	⬇️
.../framework/apollo/spring/property/SpringValue.java	87.71% <0.00%> (-1.76%)	⬇️
...ervice/service/ReleaseMessageServiceWithCache.java	87.05% <0.00%> (+1.17%)	⬆️

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[nobodyiam]

Should update the version of project before this pr merge?

apollo/pom.xml

Line 63 in 40772c4

<revision>2.0.1</revision>

Right, we could merge this pr once 2.0.1 is released.

[zouyx]

LGTM

[nobodyiam]

As 2.1.0 is released, please rebase the code and also update the CHANGES.md.

[nobodyiam]

BTW, is it necessary to also update the lb logic in RemoteConfigRepository?

[Anilople]

BTW, is it necessary to also update the lb logic in RemoteConfigRepository?

According the code

apollo/apollo-client/src/main/java/com/ctrip/framework/apollo/internals/RemoteConfigRepository.java

Lines 201 to 210 in 2cc9fc8

	for (int i = 0; i < maxRetries; i++) {
	List<ServiceDTO> randomConfigServices = Lists.newLinkedList(configServices);
	Collections.shuffle(randomConfigServices);
	//Access the server which notifies the client first
	if (m_longPollServiceDto.get() != null) {
	randomConfigServices.add(0, m_longPollServiceDto.getAndSet(null));
	}
	for (ServiceDTO configService : randomConfigServices) {
	if (onErrorSleepTime > 0) {

all ConfigServices will be accessed.

if we use random strategy to instead of it, maybe lose the correct one ConfigService?

[nobodyiam]

According the code
all ConfigServices will be accessed.

@Anilople Actually it's a random strategy to access the config service until some config service returns the config. So it won't access all config services unless every attempts to get the config fails. However, I think it may not be that necessary to customize its logic as it is only triggered when the config changes. So it is OK now.

[nobodyiam]

LGTM