Hive ingest patterns introduction
Understanding what does not work when designing Hive tables helps you understand recommended patterns discussed. You can avoid potential performance issues, and perhaps data loss.
Operations on non-ACID tables create a small file problem. Appending small, non-ACID files to the same partition or table generally prevents consolidation of files inside the partition directory. Consolidation happens only after an INSERT OVERWRITE to the table or partition.
Using classic INSERT OVERWRITE methods can lead to data loss. Data not picked up at beginning of INSERT OVERWRITE can be lost. INSERT OVERWRITES are not atomic, so for a time during that operation, there will be no data in the table when HMS is processing the data.
By contrast, ACID tables are consolidated through the compaction process. When you insert data into a non-ACID table before writing results to target partition or table, Hive tries to write to the file as if the file were new, and empty. An object storage failure, such as an HDFS failure, occurs as the file already exists because of a previous insert. Hive renames the file to copy 1 and tries again. If another failure occurs, which it will because copy 1 exists in that dir, it renames the file to copy 2.
An ACID anti-pattern is doing 1400 inserts a day to a relatively small table. Hive needs to iteract with the NameNode 1400 times, or more, just to insert a single file into a table. It must fail in each of the 1400 interations before finding a number that works. In addition to a small file problem, the thrashing activity overwhelms the NameNode.
- Extremely poor performance on the consumer side
- Many small files
- Enormous pressure on the metastore and filesystem
- More compute to handle queries
Avoid ingesting tables having numerous partitions and heavy append operations. Consolidate files. Doing so saves time and resources, and relieves stress on the NameNode. Running compaction of ACID tables achieves this consolidation and prevents these problems. Hive also collects ACID table statistics. All ACID operations are atomic.
When your data source cannot be altered, for example when the frequency of ingesting data is high, you need to keep the source data intact. You can use a sweep operation, or ingest table, to sweep data into an acid table. The ingest table can be an external table populated by NiFi, for example. NiFi reads data from Kafka and writes files to HDFS. Using a sweep table you append data to the ACID table on a less frequent basis. This technique helps you manage the operation. Read operations from consumer side go through the ACID table, which can be consumed efficiently.
When partitions make sense, design the partitions for the consumer, not for the ingest pattern to the final target acid table.