[HBASE-13071] Hbase Streaming Scan Feature

• Type: New Feature
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.0.0
• Hadoop Flags: Reviewed

Release Note

MOTIVATION

A pipelined scan API is introduced for speeding up applications that combine massive data traversal with compute-intensive processing. Traditional HBase scans save network trips through prefetching the data to the client side cache. However, they prefetch synchronously: the fetch request to regionserver is invoked only when the entire cache is consumed. This leads to a stop-and-wait access pattern, in which the client stalls until the next chunk of data is fetched. Applications that do significant processing can benefit from background data prefetching, which eliminates this bottleneck. The pipelined scan implementation overlaps the cache population at the client side with application processing. Namely, it issues a new scan RPC when the iteration retrieves 50% of the cache. If the application processing (that is, the time between invocations of next()) is substantial, the new chunk of data will be available before the previous one is exhausted, and the client will not experience any delay. Ideally, the prefetch and the processing times should be balanced.

API AND CONFIGURATION

Asynchronous scanning can be configured either globally for all tables and scans, or on per-scan basis via a new Scan class API.

Configuration in hbase-site.xml: hbase.client.scanner.async.prefetch, default false:

 <property>
   <name>hbase.client.scanner.async.prefetch</name>
   <value>true</value>
 </property>

API - Scan#setAsyncPrefetch(boolean)

      Scan scan = new Scan();
      scan.setCaching(1000);
      scan.setMaxResultSize(BIG_SIZE);
      scan.setAsyncPrefetch(true);
        ...
      ResultScanner scanner = table.getScanner(scan);

IMPLEMENTATION NOTES

Pipelined scan is implemented by a new ClientAsyncPrefetchScanner class, which is fully API-compatible with the synchronous ClientSimpleScanner. ClientAsyncPrefetchScanner is not instantiated in case of small (Scan#setSmall) and reversed (Scan#setReversed) scanners. The application is responsible for setting the prefetch size in a way that the prefetch time and the processing times are balanced. Note that due to double buffering, the client side cache can use twice as much memory as the synchronous scanner.

Generally, this feature will put more load on the server (higher fetch rate -- which is the whole point). Also, YMMV.

• Assignee: Eshcar Hillel
• Reporter: Eshcar Hillel
• Created: 19/Feb/15 12:49
• Updated: 23/Jun/22 18:44
• Resolved: 13/May/15 15:29

Description

A scan operation iterates over all rows of a table or a subrange of the table. The synchronous nature in which the data is served at the client side hinders the speed the application traverses the data: it increases the overall processing time, and may cause a great variance in the times the application waits for the next piece of data.

The scanner next() method at the client side invokes an RPC to the regionserver and then stores the results in a cache. The application can specify how many rows will be transmitted per RPC; by default this is set to 100 rows.
The cache can be considered as a producer-consumer queue, where the hbase client pushes the data to the queue and the application consumes it. Currently this queue is synchronous, i.e., blocking. More specifically, when the application consumed all the data from the cache — so the cache is empty — the hbase client retrieves additional data from the server and re-fills the cache with new data. During this time the application is blocked.

Under the assumption that the application processing time can be balanced by the time it takes to retrieve the data, an asynchronous approach can reduce the time the application is waiting for data.

We attach a design document.
We also have a patch that is based on a private branch, and some evaluation results of this code.

Issue Links

• is duplicated by: HBASE-8691 High-Throughput Streaming Scan API
• is related to: HBASE-11544 [Ergonomics] hbase.client.scanner.caching is dogged and will try to return batch even if it means OOME
• is related to: HBASE-13082 Coarsen StoreScanner locks to RegionScanner
• is related to: HBASE-13090 Progress heartbeats for long running scanners
• is related to: HBASE-12994 Improve network utilization for scanning RPC requests by preloading the next set of results on the server
• relates to: HBASE-13719 Asynchronous scanner -- cache size-in-bytes bug fix
• links to: Link to multi-step measurements extension of YCSB
• links to: review board