ACID ingest patterns
Understanding Hive ACID ingest patterns helps you adopt one that fits best. You gain an understanding of how to build a pipeline that keeps the original data and builds or updates a more efficient table for recurring READ operations.
Although Hive handles compactions, micro-batching appends, and Hive streaming writes, you still have to avoid inserting small records into ACID tables. Using ACID does not correct a bad ingest design. If you perform micro-inserts and create many delta directories, at some point the compaction system, and other components, such as NameNodes, have to reconcile the delta files. Eventually, compaction consolidates files, but if you have hundreds of these delta files before compaction even starts, Hive needs to work hard in the background. Heavy compute resources and metastore resources are needed.
The data you ingest into ACID tables using the following pattern must be of a reasonable size.
ACID pattern 1
ACID pattern 1 characteristics are:
Handles compactions in the background, but you still have to understand the impact of deltas
Performs well for micro batch appends and Hive streaming
Works best when you partition on business need, not ingestion
Frequently partitioned to optimize file size and access (pruning)
Supports adding a batch-id field to record ingest events
Not designed for online transaction processing (OLTP)
Consider how quickly you need to access the data. If you need immediate access to data, look at how many queries your organization actually issues to access data received within the last 5 minutes, for example. You might realize that rarely do you need access your data so quickly, but if not, consider other technologies, such as Impala with Kudu or HBase.
If you need to track batch operations, for example by associating a batch ID with every record, add a second-level partition element. Add a batch field to the table with the batch ID. You can perform delete operations against ACID tables to remove a batch and replay it. ORC and CRUD functionality repair that table based on a replay or removal of an insert.
Hive does not satisfy OLTP requirements.
ACID pattern 2
ACID pattern 2 has the following characteristics:
- Designed to be business-, not ingest-centric.
- Supports highly granular partitions, for example YY-MM-DD-HH vs YY-MM.
- Achieves efficient content size per partition to reduce file counts.
Beware of dynamic partitions and avoid cross partition distribution of data. If your design requires Hive to cross partitions unnecessarily when you insert data into a table, collapse year-month-day-hour partitions down to year-month if possible.
ACID pattern 3
To build a pipeline that keeps the original data and builds or updates a more efficient table for recurring READ operations, use the ACID pattern 3.
ACID pattern 3 represents the sweep process. This pattern keeps track of historical changes. You can lose information that has business value if you do not keep track the original transactional elements. Consider having an ingest table with original values and also a change data capture (CDC) table. For example, if you have 2 million customers making thousands of changes a day to an ACID table target, you lose all the thrashing that might have happened if you do not capture changes.
Using a sweep process not only consolidates files to eleviate ACID performance problems, but also supports data change analytics. If a consumer changes their address frequently, say 50 times a day, perhaps fraud is indicated. The cost of space you need for historical data if often worth the expense.
A portion of the sweep pattern, shown below, looks similar to the classic ingest pattern. You use a non-acid table with partitions. Instead of inserting data into a non-ACID table every 15 minutes, for example, you instead sweep data from the ingest table into the ACID table every hour or two. You use the ACID table as your consumer table, which has collapsed partitions. Hive performs compaction on the ACID table.
Another approach is to use an ACID table as the staging place for ingesting data or other data pipelines for writing and aggregating data. Turn off auto-compaction, or raise thresholds, to enjoy tranaction isolation of your streaming with no overhead.
In summary, use ACID pattern 3 as follows:
- Use a non-ACID table to stage micro batches to a final ACID table.
- Use more aggregate partition strategy (if required) on the final table (YYMM vs. YYMMDD_HH).
- Allow ACID to undergo compaction.
- Direct consumers to the final ACID table for better performance and less system resource overhead.
- Include a batch ID in the schema to support rollback.
ACID CDC pattern
The ACID CDC pattern has the following characteristics:
- Used for the initial seed of new records.
- Used for follow-up updates and inserts.
- Used for follow-up deletes.
- ACID deltas track updates and deletes, rolled up in SELECT.
- Relys on major compaction to reconcile updates and deletes.
For changing dimension tables, the ACID CDC pattern supports a large partition, late arriving data, or perhaps no partition at all. Using the ACID CDC pattern, you might have just a consumer table. Inserts occur, and then updates to the inserts. All changes are captured as delta records. A major compaction reconciles inserts and updates to give you a new base. The next update creates another delta in this case.
When you insert into an ACID table, you must deduplicate the records before making insertions, or deduplicate the records before updates. You cannot skip the deduplication process because there is no enforceable constraint primary key that takes care of eliminating duplicate copies of data. If you put a record in a transactional table three times before it is pushed into an ACID table, the record comes into Hive three times: first as an insert, next as an update, and finally as another update before you push your changes into the ACID table through the merge process. You must reconcile inserts and updates before the merge to make sure you get only one operation; otherwise, you get duplicate records.