[BUG] Batches are executed out of order when using DNS connections

[yangbodong22011]

Backgroud

Our user told us that when they use batch on our cloud (AlibabaCloud), their commands came out of order. code is below

namespace RedisTest;

using StackExchange.Redis;
using System;
using System.Collections.Generic;
using System.Linq;

public class RedisClient
{
    class Program
    {
        static void Main(string[] args)
        {
            ConfigurationOptions configurationOptions = ConfigurationOptions.Parse("r-bpxxxxpd.redis.rds.aliyuncs.com:6379,password=xxx,connectTimeout=2000");
            ConnectionMultiplexer redisConn = ConnectionMultiplexer.Connect(configurationOptions);
            var db = redisConn.GetDatabase();
            var batch = db.CreateBatch();
            batch.KeyDeleteAsync("testhash"); // UNLINK
            var keyval = new List<HashEntry>(10);
            for (int i = 0; i < 10; i++) {
                keyval.Add(new HashEntry(i, i));
            }
            batch.HashSetAsync("testhash", keyval.ToArray()); // HMSET
            batch.KeyExpireAsync("testhash", TimeSpan.FromSeconds(100)); // EXPIRE
            batch.Execute();
        }
    }
}

Ideally, they would get the following sequence:

• UNLINK
• HMSET
• EXPIRE

But sometimes, the following sequence is produced:

• UNLINK
• EXPIRE
• HMSET

Going a step further, we found that UNLINK and EXPIRE are sent over one TCP connection, but HMSET uses another TCP connection.

Why are there two links? We found that when connecting through a domain name, there will be two links in ConnectionMultiplexer#ServerSnapshot (one is the domain name, and the other is the IP resolved by the domain name (obtained through cluster nodes)). Therefore, two links will be returned randomly in AnyServer, causing commands to be assigned to different connections.

How to reproduction

1. Find a server with DNS (I can provide Alibaba Cloud test environment for free)
2. Run the above command

How to fix

During the initialization process, Lettuce will generate the result of the domain name as a URI, but will change the domain name to an alias. Please refer to redis/lettuce@16f9e75

Version

• StackExchange.Redis: latest
• .Net: I reproduced this issue under 6.0 and 8.0, but it has nothing to do with the .net version.

[shaofing]

@mgravell Can you fix it ?

[mgravell]

This is ... odd; short term, switching up to transactions will enforce single connection, but... this is odd; I'd have to investigate, which I can add to the list, but I can't guarantee a "today" thing

[worming004]

As both theses lines are not awaited

batch.HashSetAsync("testhash", keyval.ToArray()); // HMSET
batch.KeyExpireAsync("testhash", TimeSpan.FromSeconds(100)); // EXPIRE

There is no garantee of order. Just probability that line 1 would be executed earlier, but not garantee. Right ?

Edit: didn't see it is about batch. My bad, my remark is irrelevant

[mgravell]

It is reasonable to expect this to occur in order; what is described is definitely not "working as intended", but it doesn't sound trivial - it will need some investigation

[yangbodong22011]

@mgravell I sent you the account and password of the test instance environment via google mail.