Implement while loop in Reactor to fetch latest Elasticsearch index

My index name in reactive elastic search are as follow:

logs-2020.08.18
logs-2020.08.17
logs-2020.08.16

It will be created daily basis.

I wanna fetch the latest index name and get the logs using reactiveElasticsearchClient or spring data.
Is it possible?

I tried the following way in my spring webflux application:

I have the below code snippet to find index availability:

public Flux<Log> getLogFromLatestIndex(String serialId) {
    Calendar cal = Calendar.getInstance();
    String currentIndex = StringUtils.EMPTY;
    boolean indexExists = false;
    while (!indexExists) {
        currentIndex = String.format("logs-%s”, format(cal.getTime(), "yyyy.MM.dd"));
        indexExists = isIndexExists(currentIndex).block();
        cal.add(Calendar.DATE, -1); // Decrease day 1 until you find index
    }

    SearchQuery searchQuery = new NativeSearchQueryBuilder()
            .withQuery(matchQuery("serialId", serialId))
            .withIndices(currentIndex)
            .build();

    return reactiveElasticsearchTemplate.find(searchQuery, Log.class);
}

public Mono<Boolean> isIndexExists(String indexName) {
    return reactiveElasticsearchClient.indices().existsIndex(new GetIndexRequest().indices(indexName));
}

How do I fetch boolean value without using block here

indexExists = isIndexExists(currentIndex).block();

Obviously I will get the below error:

java.lang.IllegalStateException: block()/blockFirst()/blockLast() are blocking, which is not supported in thread reactor-http-nio-2

You can use Flux.generate and (take/skip)(Until/While) to do a while loop in reactor.

Notes:

• replaced Calendar with LocalDate as that is immutable and fits better with functional/reactive programming.
• the isIndexExists method returns a Tuple to have the reference for the index name but obviously it can be replaced with some more descriptive class as needed

public Flux<Log> getLog(String serialId) {
    return Flux.generate(LocalDate::now, this::generateNextDate)
               .map(day -> String.format("logs-%s", day.format(DateTimeFormatter.ofPattern("yyyy.MM.dd"))))
               .concatMap(this::isIndexExists)
               .skipUntil(Tuple2::getT2) // check index exists boolean and drop non-existing ones
               .next() // takes first existing
               .flatMapMany(tuple -> findLogs(tuple.getT1(), serialId));
}

private LocalDate generateNextDate(LocalDate currentDay, SynchronousSink<LocalDate> sink) {
    sink.next(currentDay);
    return currentDay.minusDays(1);
}

private Mono<Tuple2<String, Boolean>> isIndexExists(String indexName) {
    return reactiveElasticsearchClient.indices().existsIndex(new GetIndexRequest().indices(indexName))
            .map(exists -> Tuples.of(indexName, exists));
}

private Flux<Log> findLogs(String index, String serialId) {
    // your other ES query here
}

application.yml

spring.data.elasticsearch.client.reactive.endpoints: ip1:9200,ip2:9200,ip3:9200
spring.data.elasticsearch.rest.uris: ip1:9200,ip2:9200,ip3:9200

Configuration.java

@Configuration
public class ElasticSearchConfig extends AbstractReactiveElasticsearchConfiguration {

    @Value("${spring.data.elasticsearch.client.reactive.endpoints}")
    private String elasticSearchEndpoint;

    @Override
    public ReactiveElasticsearchClient reactiveElasticsearchClient() {
        final ClientConfiguration clientConfiguration = ClientConfiguration.builder()
            .connectedTo(elasticSearchEndpoint.split(","))
            .withWebClientConfigurer(webClient -> {
                ExchangeStrategies exchangeStrategies = ExchangeStrategies.builder()
                        .codecs(configurer -> configurer.defaultCodecs()
                                .maxInMemorySize(-1))
                        .build();
                return webClient.mutate().exchangeStrategies(exchangeStrategies).build();
            })
            .build();
    return ReactiveRestClients.create(clientConfiguration);

    }
}

Controller.java

@GetMapping("/getLog/{serialId}")
public Flux<Log> getLog(@PathVariable String serialId) {
    return loggerService.getLog(serialId);
}

Rest are all your code. I am just printing index name inside map. Although I am having index logs-2020.08.21 in elastic search, it is keep on printing indices like logs-2020.08.20, logs-2020.08.19, logs-2020.08.18 and so on, eventually throws error.

Note: When I tried with single ip in application.yml, I got the same error.

public Flux<Log> getLog(String serialId) {
    return Flux.generate(LocalDate::now, this::generateNextDate)
           .map(day -> {
                System.out.println(String.format("logs-%s", day.format(DateTimeFormatter.ofPattern("yyyy.MM.dd"))));
                return String.format("logs-%s", day.format(DateTimeFormatter.ofPattern("yyyy.MM.dd")));
            })               
           .flatMap(this::isIndexExists)
           .skipUntil(Tuple2::getT2) // check index exists boolean and drop non-existing ones
           .next() // takes first existing
           .flatMapMany(tuple -> findLogs(tuple.getT1(), serialId));
}

private LocalDate generateNextDate(LocalDate currentDay, SynchronousSink<LocalDate> sink) {
    sink.next(currentDay);
    return currentDay.minusDays(1);
}

private Mono<Tuple2<String, Boolean>> isIndexExists(String indexName) {
    return reactiveElasticsearchClient.indices().existsIndex(new GetIndexRequest().indices(indexName))
        .map(exists -> Tuples.of(indexName, exists));
}

private Flux<Log> findLogs(String index, String serialId) {
    // your other ES query here
}