一、概念
什么是ACID?
ACID就是常见数据库事务的四大特性：
Atomicity(原子性)、Consistency（一致性）、Isolation（隔离性）、Durability（持久性）。
但是我们知道hive是在hdfs上以文件存储，只支持文件的新增和删除，不支持delete和update。
从Hive 0.13开始，Hive可以支持行级别的ACID语义。
事务表：支持原子性update操作的Hive表
事务表的2种类型：
1.ORC格式，支持CRUD操作(增删改查)
2.TextFile文件格式，只符合insert-only特性
存储在事务表中的数据会被分成两种类型的文件：
1.base文件，用来存放平常的数据
2.delta文件，用来存储新增、更新、删除的数据。每一个事务处理数据的结果都会单独新建一个delta文件夹用来存储数据。
二、表类型和ACID特性对照图
表类型 | ACID | 文件格式 | 插入 | 更新/删除 |
|---|---|---|---|---|
托管表：CRUD事务 | 是 | ORC | 是 | 是 |
托管表：仅插入式事务 | 是 | 任意格式 | 是 | 没有 |
托管表：临时 | 没有 | 任意格式 | 是 | 没有 |
外部表 | 没有 | 任意格式 | 是 | 没有 |
三、事务表的适用场景
事务表适用于数仓中的行级数据更新删除需求比较频繁的表
如果事务表太多，并且存在大量的更新操作，metastore后台启动的合并线程会定期的提交MapReduce Job，也会一定程度上增重集群的负担
注意事项
1.不支持 BEGIN、COMMIT、ROLLBACK 等语句，所有的语句都是自动提交
2.仅支持ORC格式
四、开启事务的配置
客户端方面的修改 会话中设置或者修改HiveServer2的配置文件：
hive.support.concurrency – true hive.enforce.bucketing – true (Hive 2.0之后就不用专门设置了) hive.exec.dynamic.partition.mode – nonstrict hive.txn.manager – org.apache.hadoop.hive.ql.lockmgr.DbTxnManager
服务端方面 主要修改MetaStore上的配置
hive.compactor.initiator.on – true hive.compactor.worker.threads – 压缩任务的数量
五、参数详解
Configuration key | Values | Location | Notes |
|---|---|---|---|
hive.txn.manager | Default: org.apache.hadoop.hive.ql.lockmgr.DummyTxnManager Value required for transactions: org.apache.hadoop.hive.ql.lockmgr.DbTxnManager | Client/ HiveServer2 | DummyTxnManager replicates pre Hive-0.13 behavior and provides no transactions. |
hive.txn.strict.locking.mode | Default: true | Client/ HiveServer2 | In strict mode non-ACID resources use standard R/W lock semantics, e.g. INSERT will acquire exclusive lock. In non-strict mode, for non-ACID resources, INSERT will only acquire shared lock, which allows two concurrent writes to the same partition but still lets lock manager prevent DROP TABLE etc. when the table is being written to (as of Hive 2.2.0). |
hive.txn.timeout | Default: 300 | Client/ HiveServer2/ Metastore | Time after which transactions are declared aborted if the client has not sent a heartbeat, in seconds. It's critical that this property has the same value for all components/services.5 |
hive.txn.heartbeat.threadpool.size | Default: 5 | Client/ HiveServer2 | The number of threads to use for heartbeating (as of Hive 1.3.0 and 2.0.0). |
hive.timedout.txn.reaper.start | Default: 100s | Metastore | Time delay of first reaper (the process which aborts timed-out transactions) run after the metastore starts (as of Hive 1.3.0). Controls AcidHouseKeeperServcie above. |
hive.timedout.txn.reaper.interval | Default: 180s | Metastore | Time interval describing how often the reaper (the process which aborts timed-out transactions) runs (as of Hive 1.3.0). Controls AcidHouseKeeperServcie above. |
hive.txn.max.open.batch | Default: 1000 | Client | Maximum number of transactions that can be fetched in one call to open_txns().1 |
hive.max.open.txns | Default: 100000 | HiveServer2/ Metastore | Maximum number of open transactions. If current open transactions reach this limit, future open transaction requests will be rejected, until the number goes below the limit. (As of Hive 1.3.0 and 2.1.0.) |
hive.count.open.txns.interval | Default: 1s | HiveServer2/ Metastore | Time in seconds between checks to count open transactions (as of Hive 1.3.0 and 2.1.0). |
hive.txn.retryable.sqlex.regex | Default: "" (empty string) | HiveServer2/ Metastore | Comma separated list of regular expression patterns for SQL state, error code, and error message of retryable SQLExceptions, that's suitable for the Hive metastore database (as of Hive 1.3.0 and 2.1.0). For an example, see Configuration Properties. |
hive.compactor.initiator.on | Default: false Value required for transactions: true (for exactly one instance of the Thrift metastore service) | Metastore | Whether to run the initiator and cleaner threads on this metastore instance. Prior to Hive 1.3.0 it's critical that this is enabled on exactly one standalone metastore service instance (not enforced yet). As of Hive 1.3.0 this property may be enabled on any number of standalone metastore instances. |
hive.compactor.worker.threads | Default: 0 Value required for transactions: > 0 on at least one instance of the Thrift metastore service | Metastore | How many compactor worker threads to run on this metastore instance.2 |
hive.compactor.worker.timeout | Default: 86400 | Metastore | Time in seconds after which a compaction job will be declared failed and the compaction re-queued. |
hive.compactor.cleaner.run.interval | Default: 5000 | Metastore | Time in milliseconds between runs of the cleaner thread. (Hive 0.14.0 and later.) |
hive.compactor.check.interval | Default: 300 | Metastore | Time in seconds between checks to see if any tables or partitions need to be compacted.3 |
hive.compactor.delta.num.threshold | Default: 10 | Metastore | Number of delta directories in a table or partition that will trigger a minor compaction. |
hive.compactor.delta.pct.threshold | Default: 0.1 | Metastore | Percentage (fractional) size of the delta files relative to the base that will trigger a major compaction. 1 = 100%, so the default 0.1 = 10%. |
hive.compactor.abortedtxn.threshold | Default: 1000 | Metastore | Number of aborted transactions involving a given table or partition that will trigger a major compaction. |
hive.compactor.aborted.txn.time.threshold | Default: 12h | Metastore | Age of table/partition's oldest aborted transaction when compaction will be triggered. Default time unit is: hours. Set to a negative number to disable. |
hive.compactor.max.num.delta | Default: 500 | Metastore | Maximum number of delta files that the compactor will attempt to handle in a single job (as of Hive 1.3.0).4 |
hive.compactor.job.queue | Default: "" (empty string) | Metastore | Used to specify name of Hadoop queue to which Compaction jobs will be submitted. Set to empty string to let Hadoop choose the queue (as of Hive 1.3.0). |
Compaction History | |||
hive.compactor.history.retention.succeeded | Default: 3 | Metastore | Number of successful compaction entries to retain in history (per partition). |
hive.compactor.history.retention.failed | Default: 3 | Metastore | Number of failed compaction entries to retain in history (per partition). |
hive.compactor.history.retention.attempted | Default: 2 | Metastore | Number of attempted compaction entries to retain in history (per partition). |
hive.compactor.initiator.failed.compacts.threshold | Default: 2 | Metastore | Number of of consecutive failed compactions for a given partition after which the Initiator will stop attempting to schedule compactions automatically. It is still possible to use ALTER TABLE to initiate compaction. Once a manually initiated compaction succeeds auto initiated compactions will resume. Note that this must be less than hive.compactor.history.retention.failed. |
hive.compactor.history.reaper.interval | Default: 2m | Metastore | Controls how often the process to purge historical record of compactions runs. |
