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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. --> + + <div class="section"> +<h2><a name="About_this_Document"></a>About this Document</h2> + +<p>Apache HBase (TM) is not an ACID compliant database. However, it does guarantee certain specific + properties.</p> + +<p>This specification enumerates the ACID properties of HBase.</p> + </div> + +<div class="section"> +<h2><a name="Definitions"></a>Definitions</h2> + +<p>For the sake of common vocabulary, we define the following terms:</p> + +<dl> + +<dt>Atomicity</dt> + +<dd>an operation is atomic if it either completes entirely or not at all</dd> + + +<dt>Consistency</dt> + +<dd> + all actions cause the table to transition from one valid state directly to another + (eg a row will not disappear during an update, etc) + </dd> + + +<dt>Isolation</dt> + +<dd> + an operation is isolated if it appears to complete independently of any other concurrent transaction + </dd> + + +<dt>Durability</dt> + +<dd>any update that reports "successful" to the client will not be lost</dd> + + +<dt>Visibility</dt> + +<dd>an update is considered visible if any subsequent read will see the update as having been committed</dd> + </dl> + +<p> + The terms <i>must</i> and <i>may</i> are used as specified by RFC 2119. + In short, the word "must" implies that, if some case exists where the statement + is not true, it is a bug. The word "may" implies that, even if the guarantee + is provided in a current release, users should not rely on it. + </p> + </div> + +<div class="section"> +<h2><a name="APIs_to_consider"></a>APIs to consider</h2> + +<ul> + +<li>Read APIs + +<ul> + +<li>get</li> + +<li>scan</li> + </ul> + </li> + +<li>Write APIs</li> + +<ul> + +<li>put</li> + +<li>batch put</li> + +<li>delete</li> + </ul> + +<li>Combination (read-modify-write) APIs</li> + +<ul> + +<li>incrementColumnValue</li> + +<li>checkAndPut</li> + </ul> + </ul> + </div> + + +<div class="section"> +<h2><a name="Guarantees_Provided"></a>Guarantees Provided</h2> + + +<div class="section"> +<h2><a name="Atomicity"></a>Atomicity</h2> + + +<ol style="list-style-type: decimal"> + +<li>All mutations are atomic within a row. Any put will either wholly succeed or wholly fail.[3]</li> + +<ol style="list-style-type: decimal"> + +<li>An operation that returns a "success" code has completely succeeded.</li> + +<li>An operation that returns a "failure" code has completely failed.</li> + +<li>An operation that times out may have succeeded and may have failed. However, + it will not have partially succeeded or failed.</li> + </ol> + +<li> This is true even if the mutation crosses multiple column families within a row.</li> + +<li> APIs that mutate several rows will _not_ be atomic across the multiple rows. + For example, a multiput that operates on rows 'a','b', and 'c' may return having + mutated some but not all of the rows. In such cases, these APIs will return a list + of success codes, each of which may be succeeded, failed, or timed out as described above.</li> + +<li> The checkAndPut API happens atomically like the typical compareAndSet (CAS) operation + found in many hardware architectures.</li> + +<li> The order of mutations is seen to happen in a well-defined order for each row, with no + interleaving. For example, if one writer issues the mutation "a=1,b=1,c=1" and + another writer issues the mutation "a=2,b=2,c=2", the row must either + be "a=1,b=1,c=1" or "a=2,b=2,c=2" and must <i>not</i> be something + like "a=1,b=2,c=1".</li> + +<ol style="list-style-type: decimal"> + +<li>Please note that this is not true _across rows_ for multirow batch mutations.</li> + </ol> + </ol> + </div> + +<div class="section"> +<h2><a name="Consistency_and_Isolation"></a>Consistency and Isolation</h2> + +<ol style="list-style-type: decimal"> + +<li>All rows returned via any access API will consist of a complete row that existed at + some point in the table's history.</li> + +<li>This is true across column families - i.e a get of a full row that occurs concurrent + with some mutations 1,2,3,4,5 will return a complete row that existed at some point in time + between mutation i and i+1 for some i between 1 and 5.</li> + +<li>The state of a row will only move forward through the history of edits to it.</li> + </ol> + + +<div class="section"> +<h2><a name="Consistency_of_Scans"></a>Consistency of Scans</h2> + +<p> + A scan is <b>not</b> a consistent view of a table. Scans do + <b>not</b> exhibit <i>snapshot isolation</i>. + </p> + +<p> + Rather, scans have the following properties: + </p> + + +<ol style="list-style-type: decimal"> + +<li> + Any row returned by the scan will be a consistent view (i.e. that version + of the complete row existed at some point in time) [1] + </li> + +<li> + A scan will always reflect a view of the data <i>at least as new as</i> + the beginning of the scan. This satisfies the visibility guarantees + enumerated below.</li> + +<ol style="list-style-type: decimal"> + +<li>For example, if client A writes data X and then communicates via a side + channel to client B, any scans started by client B will contain data at least + as new as X.</li> + +<li>A scan _must_ reflect all mutations committed prior to the construction + of the scanner, and _may_ reflect some mutations committed subsequent to the + construction of the scanner.</li> + +<li>Scans must include <i>all</i> data written prior to the scan (except in + the case where data is subsequently mutated, in which case it _may_ reflect + the mutation)</li> + </ol> + </ol> + +<p> + Those familiar with relational databases will recognize this isolation level as "read committed". + </p> + +<p> + Please note that the guarantees listed above regarding scanner consistency + are referring to "transaction commit time", not the "timestamp" + field of each cell. That is to say, a scanner started at time <i>t</i> may see edits + with a timestamp value greater than <i>t</i>, if those edits were committed with a + "forward dated" timestamp before the scanner was constructed. + </p> + </div> + </div> + +<div class="section"> +<h2><a name="Visibility"></a>Visibility</h2> + +<ol style="list-style-type: decimal"> + +<li> When a client receives a "success" response for any mutation, that + mutation is immediately visible to both that client and any client with whom it + later communicates through side channels. [3]</li> + +<li> A row must never exhibit so-called "time-travel" properties. That + is to say, if a series of mutations moves a row sequentially through a series of + states, any sequence of concurrent reads will return a subsequence of those states.</li> + +<ol style="list-style-type: decimal"> + +<li>For example, if a row's cells are mutated using the "incrementColumnValue" + API, a client must never see the value of any cell decrease.</li> + +<li>This is true regardless of which read API is used to read back the mutation.</li> + </ol> + +<li> Any version of a cell that has been returned to a read operation is guaranteed to + be durably stored.</li> + </ol> + + </div> + +<div class="section"> +<h2><a name="Durability"></a>Durability</h2> + +<ol style="list-style-type: decimal"> + +<li> All visible data is also durable data. That is to say, a read will never return + data that has not been made durable on disk[2]</li> + +<li> Any operation that returns a "success" code (eg does not throw an exception) + will be made durable.[3]</li> + +<li> Any operation that returns a "failure" code will not be made durable + (subject to the Atomicity guarantees above)</li> + +<li> All reasonable failure scenarios will not affect any of the guarantees of this document.</li> + + </ol> + </div> + +<div class="section"> +<h2><a name="Tunability"></a>Tunability</h2> + +<p>All of the above guarantees must be possible within Apache HBase. For users who would like to trade + off some guarantees for performance, HBase may offer several tuning options. For example:</p> + +<ul> + +<li>Visibility may be tuned on a per-read basis to allow stale reads or time travel.</li> + +<li>Durability may be tuned to only flush data to disk on a periodic basis</li> + </ul> + </div> + </div> + +<div class="section"> +<h2><a name="More_Information"></a>More Information</h2> + +<p> + For more information, see the <a href="book.html#client">client architecture</a> or <a href="book.html#datamodel">data model</a> sections in the Apache HBase Reference Guide. + </p> + </div> + + +<div class="section"> +<h2><a name="Footnotes"></a>Footnotes</h2> + +<p>[1] A consistent view is not guaranteed intra-row scanning -- i.e. fetching a portion of + a row in one RPC then going back to fetch another portion of the row in a subsequent RPC. + Intra-row scanning happens when you set a limit on how many values to return per Scan#next + (See <a class="externalLink" href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/client/Scan.html#setBatch(int)">Scan#setBatch(int)</a>). + </p> + + +<p>[2] In the context of Apache HBase, "durably on disk" implies an hflush() call on the transaction + log. This does not actually imply an fsync() to magnetic media, but rather just that the data has been + written to the OS cache on all replicas of the log. In the case of a full datacenter power loss, it is + possible that the edits are not truly durable.</p> + +<p>[3] Puts will either wholly succeed or wholly fail, provided that they are actually sent + to the RegionServer. If the writebuffer is used, Puts will not be sent until the writebuffer is filled + or it is explicitly flushed.</p> + + </div> + + + + </div> + </div> + + <hr/> + + <footer> + <div class="container"> + <div class="row"> + <p >Copyright © 2007–2018 + <a href="https://www.apache.org/";>The Apache Software Foundation</a>. + All rights reserved. + + <li id="publishDate" class="pull-right">Last Published: 2018-07-10</li> + </p> + </div> + + <p id="poweredBy" class="pull-right"> + <a href="http://maven.apache.org/"; title="Built by Maven" class="poweredBy"> + <img class="builtBy" alt="Built by Maven" src="./images/logos/maven-feather.png" /> + </a> + </p> + + </div> + </footer> + </body> +</html>
