cbaenziger commented on a change in pull request #496: HBASE-22625 documet use scan snapshot feature URL: https://github.com/apache/hbase/pull/496#discussion_r317322232
########## File path: src/main/asciidoc/_chapters/snapshot_scanner.adoc ########## @@ -0,0 +1,138 @@ +//// +/** + * + * Licensed to the Apache Software Foundation (ASF) under one + * or more contributor license agreements. See the NOTICE file + * distributed with this work for additional information + * regarding copyright ownership. The ASF licenses this file + * to you under the Apache License, Version 2.0 (the + * "License"); you may not use this file except in compliance + * with the License. You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, software + * distributed under the License is distributed on an "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + * See the License for the specific language governing permissions and + * limitations under the License. + */ +//// + +[[snapshot_scanner]] +== Scan over snapshot +:doctype: book +:numbered: +:toc: left +:icons: font +:experimental: +:toc: left +:source-language: java + +In HBase, scan a table costs many CPU, memory... resources. Luckily, HBase provides a TableSnapshotScanner and TableSnapshotInputFormat (introduced by link:https://issues.apache.org/jira/browse/HBASE-8369[HBASE-8369]), which performs a scan over snapshot files. +By this way, we can bypasse HBase servers, and access the underlying files directly to provide maximum performance. And can also be used with offline HBase with in-place or exported snapshot files. + +To read from snapshot files directly from the file system, the user must have sufficient permissions to access snapshot and reference data files. + +=== TableSnapshotScanner + +TableSnapshotScanner provide a way to do single client side scan over snapshot files. +When use TableSnapshotScanner, we must specify a temporary directory to copy the snapshot files into. Current user should have write permissions to this directory, and this should not be a subdirectory of rootdir. The scanner deletes the contents of the directory once the scanner is closed. + +.Use TableSnapshotScanner +==== +[source,java] +---- +Path restoreDir = new Path("XX"); // restore dir should not be a subdirectory HBase rootdir +Scan scan = new Scan(); +try (TableSnapshotScanner scanner = new TableSnapshotScanner(conf, restoreDir, snapshotName, scan)) { + Result result = scanner.next(); + while (result != null) { + ... + result = scanner.next(); + } +} +---- +==== + +=== TableSnapshotInputFormat +TableSnapshotInputFormat provide a way to scan over snapshot files in a MapReduce job. + +.Use TableSnapshotInputFormat +==== +[source,java] +---- +Job job = new Job(conf); +Path restoreDir = new Path("XX"); // restore dir should not be a subdirectory HBase rootdir +Scan scan = new Scan(); +TableMapReduceUtil.initTableSnapshotMapperJob(snapshotName, scan, MyTableMapper.class, MyMapKeyOutput.class, MyMapOutputValueWritable.class, job, true, restoreDir); +---- +==== + +=== Permission to access snapshot and data files +Generally, only the HBase owner or the HDFS admin have the permission to access hfiles. + +link:https://issues.apache.org/jira/browse/HBASE-18659[HBASE-18659] use HDFS ACLs to make HBase granted user have the permission to access the snapshot files. + +==== HDFS ACLs + +HDFS ACLs supports an "access ACL", which defines the rules to enforce during permission checks, and a "default ACL", which defines the ACL entries that new child files or sub-directories receive automatically during creation. +By HDFS ACLs, HBase sync granted users with read permission to files. + +==== Basic idea + +The HBase files are orginazed as the following ways: Review comment: orginazed->organized ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: [email protected] With regards, Apache Git Services
