Re: Will the order of message be guaranteed when using asyno IO?

Posted by Arvid Heise-3 on
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/Will-the-order-of-message-be-guaranteed-when-using-asyno-IO-tp37505p37524.html

Hi Lei,

there is no guarantee in the order of async tasks with asyncIO. It is the very nature of async programming that the tasks are potentially run in different threads and thus may end up overtaking each other.

With ordered async I/O, you only get the guarantee that results are fed back into Flink in the order the tasks were submitted. So if you use asyncIO to enrich your kafka records with SQL, the enriched kafka records stay in the same order as the initial kafka records.

If you absolutely want to try out async IO, then you should use optimistic locking techniques with some record versioning. For example, if you repeat the updates until they succeed, you could extend your example as follows:

update table table1 set a = 1, version = 2 where version = 1

update table table1 set a = 2, version = 3 where version = 2

The version number could be kept in the async Function (if you keyed by table) and added to your template.

However, I'd still recommend reviewing the architecture and maybe use a different sink, such as a key-value store, or try out a distributed SQL store instead.

On Wed, Aug 19, 2020 at 8:25 AM [hidden email] <[hidden email]> wrote:

Read kafka message and  write the message to MySQL

Writing to database is the bottleneck when too much message is sent to kafka with high throughput. 

So i want to change the operator to asynchronously. 

 public void asyncInvoke(ObjectNode node, ResultFuture<Integer> resultFuture) throws Exception {

    SqlProvider sqlProvider = new SqlProvider();

    String tableName = node.get("metadata").get("topic").asText();
    String sql = sqlProvider.getSql(node);
    CompletableFuture.supplyAsync(new Supplier<Integer>() {
        @Override
        public Integer get() {
            return Integer.valueOf(namedTemplate.update(sql, sqlProvider.getSqlParaMap()));
        }
    }).thenAccept((Integer res) -> {
        resultFuture.complete(Collections.singletonList(res));
    });

}

I want to know if there will be order issues. 

For example the kafaka message order is:

First:          update table table1 set a = 1

Second:     update table table1 set a = 2

It is possible that the final value in database is 1 instead of 2.

Thanks

Lei

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