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+ <!-- Licensed to the Apache Software Foundation (ASF) under one
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+KIND, either express or implied. See the License for the
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+
+ <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>
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+ <li id="publishDate" class="pull-right">Last Published:
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