我正在尝试使用 Hector 插入记录，但不时出现此错误：
me.prettyprint.hector.api.exceptions.HUnavailableException: : May not be enough replicas present to handle consistency level.
at me.prettyprint.cassandra.service.ExceptionsTranslatorImpl.translate(ExceptionsTranslatorImpl.java:59)
at me.prettyprint.cassandra.connection.HConnectionManager.operateWithFailover(HConnectionManager.java:264)
at me.prettyprint.cassandra.model.ExecutingKeyspace.doExecuteOperation(ExecutingKeyspace.java:113)
at me.prettyprint.cassandra.model.MutatorImpl.execute(MutatorImpl.java:243)
at me.prettyprint.cassandra.service.template.AbstractColumnFamilyTemplate.executeBatch(AbstractColumnFamilyTemplate.java:115)
at me.prettyprint.cassandra.service.template.AbstractColumnFamilyTemplate.executeIfNotBatched(AbstractColumnFamilyTemplate.java:163)
at me.prettyprint.cassandra.service.template.ColumnFamilyTemplate.update(ColumnFamilyTemplate.java:69)
at ustocassandra.USToCassandraHector.consumer(USToCassandraHector.java:271)
at ustocassandra.USToCassandraHector.access$100(USToCassandraHector.java:41)
at ustocassandra.USToCassandraHector$2.run(USToCassandraHector.java:71)
at java.lang.Thread.run(Thread.java:724)
Caused by: UnavailableException()
at org.apache.cassandra.thrift.Cassandra$batch_mutate_result.read(Cassandra.java:20841)
at org.apache.thrift.TServiceClient.receiveBase(TServiceClient.java:78)
at org.apache.cassandra.thrift.Cassandra$Client.recv_batch_mutate(Cassandra.java:964)
at org.apache.cassandra.thrift.Cassandra$Client.batch_mutate(Cassandra.java:950)
at me.prettyprint.cassandra.model.MutatorImpl$3.execute(MutatorImpl.java:246)
at me.prettyprint.cassandra.model.MutatorImpl$3.execute(MutatorImpl.java:243)
at me.prettyprint.cassandra.service.Operation.executeAndSetResult(Operation.java:104)
at me.prettyprint.cassandra.connection.HConnectionManager.operateWithFailover(HConnectionManager.java:258)
... 9 more
我知道通常的解释是没有足够的节点，但事实并非如此。我的所有节点都已启动：
./nodetool ring
Note: Ownership information does not include topology; for complete information, specify a keyspace
Datacenter: DC1
==========
Address Rack Status State Load Owns Token
4611686018427388000
172.16.217.222 RAC1 Up Normal 353.36 MB 25.00% -9223372036854775808
172.16.217.223 RAC2 Up Normal 180.84 MB 25.00% -4611686018427388000
172.16.217.224 RAC3 Up Normal 260.34 MB 25.00% -2
172.16.217.225 RAC4 Up Normal 222.71 MB 25.00% 4611686018427388000
我正在插入带有 20 个线程的记录（也许我应该使用更少的线程？据我所知，在这种情况下，错误将是 Overloaded，而不是 Unavailable）。我正在使用 ONE 的写入一致性。我正在使用 AutoDiscoveryAtStartup 和 LeastActiveBalancingPolicy。复制因子为 2。
我正在使用 Cassandra 1.2.8（我尝试使用 2.0 并且它是一样的）。
错误不会从一开始就发生。我通常设法在收到错误之前插入大约 200 万条记录。我的代码设置为在发生错误时重试。经过几十次重试后，插入通常会成功。之后，它再次适用于数百万次插入，然后我再次收到错误并继续循环。
可能是因为我设置了 gc_grace = 60？无论如何，我不会每 60 秒收到一次错误，所以我认为这不是原因。
你能给我一些关于这个错误的原因和我应该怎么做的建议吗？
编辑：
'nodetool tpstats' 说我有一些消息被丢弃：
Message type Dropped
RANGE_SLICE 0
READ_REPAIR 0
BINARY 0
READ 0
MUTATION 11
_TRACE 0
我在日志文件中看到以下警告：
WARN [ScheduledTasks:1] 2013-09-30 09:20:16,633 GCInspector.java (line 136) Heap is 0.853986836999536 full. You may need to reduce memtable and/or cache sizes. Cassandra is now reducing cache sizes to free up memory. Adjust reduce_cache_sizes_at threshold in cassandra.yaml if you don't want Cassandra to do this automatically
WARN [ScheduledTasks:1] 2013-09-30 09:20:16,634 AutoSavingCache.java (line 185) Reducing KeyCache capacity from 1073741824 to 724 to reduce memory pressure
WARN [ScheduledTasks:1] 2013-09-30 09:20:16,634 GCInspector.java (line 142) Heap is 0.853986836999536 full. You may need to reduce memtable and/or cache sizes. Cassandra will now flush up to the two largest memtables to free up memory. Adjust flush_largest_memtables_at threshold in cassandra.yaml if you don't want Cassandra to do this automatically
WARN [ScheduledTasks:1] 2013-09-30 09:20:16,634 StorageService.java (line 3618) Flushing CFS(Keyspace='us', ColumnFamily='my_cf') to relieve memory pressure
这是 Hector 抛出 Unavailable 异常的确切时间。因此，这可能是与内存相关的问题。我想我会尝试警告所说的内容：减少内存表大小。