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Installations</a></div> + + <div class="row doc-sidebar-link"><a href="/docs/unreleased/administration/ssl">SSL</a></div> + + </div> + </div> + + + + + + + + + + + + + + + + + + + + + + <div class="panel-heading" role="tab" id="headingOne"> + <h4 class="panel-title"> + <a role="button" data-toggle="collapse" data-parent="#accordion" href="#collapsetroubleshooting" aria-expanded="false" aria-controls="collapsetroubleshooting"> + Troubleshooting + </a> + </h4> + </div> + <div id="collapsetroubleshooting" class="panel-collapse collapse" role="tabpanel" aria-labelledby="headingOne"> + <div class="panel-body"> + + + <div class="row doc-sidebar-link"><a href="/docs/unreleased/troubleshooting/overview">Overview</a></div> + + </div> + </div> + + + + </div> + </div> + </div> + <div class="col-md-9"> + + <p><a href="/docs/unreleased/">Accumulo unreleased docs</a> >> Getting started >> Table Design</p> + + + <h1>Table Design</h1> + + <h3 id="basic-table">Basic Table</h3> + +<p>Since Accumulo tables are sorted by row ID, each table can be thought of as being +indexed by the row ID. Lookups performed by row ID can be executed quickly, by doing +a binary search, first across the tablets, and then within a tablet. Clients should +choose a row ID carefully in order to support their desired application. A simple rule +is to select a unique identifier as the row ID for each entity to be stored and assign +all the other attributes to be tracked to be columns under this row ID. For example, +if we have the following data in a comma-separated file:</p> + +<div class="highlighter-rouge"><pre class="highlight"><code>userid,age,address,account-balance +</code></pre> +</div> + +<p>We might choose to store this data using the userid as the rowID, the column +name in the column family, and a blank column qualifier:</p> + +<div class="language-java highlighter-rouge"><pre class="highlight"><code><span class="n">Mutation</span> <span class="n">m</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Mutation</span><span class="o">(</span><span class="n">userid</span><span class="o">);</span> +<span class="kd">final</span> <span class="n">String</span> <span class="n">column_qualifier</span> <span class="o">=</span> <span class="s">""</span><span class="o">;</span> +<span class="n">m</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="s">"age"</span><span class="o">,</span> <span class="n">column_qualifier</span><span class="o">,</span> <span class="n">age</span><span class="o">);</span> +<span class="n">m</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="s">"address"</span><span class="o">,</span> <span class="n">column_qualifier</span><span class="o">,</span> <span class="n">address</span><span class="o">);</span> +<span class="n">m</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="s">"balance"</span><span class="o">,</span> <span class="n">column_qualifier</span><span class="o">,</span> <span class="n">account_balance</span><span class="o">);</span> + +<span class="n">writer</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">m</span><span class="o">);</span> +</code></pre> +</div> + +<p>We could then retrieve any of the columns for a specific userid by specifying the +userid as the range of a scanner and fetching specific columns:</p> + +<div class="language-java highlighter-rouge"><pre class="highlight"><code><span class="n">Range</span> <span class="n">r</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Range</span><span class="o">(</span><span class="n">userid</span><span class="o">,</span> <span class="n">userid</span><span class="o">);</span> <span class="c1">// single row</span> +<span class="n">Scanner</span> <span class="n">s</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="na">createScanner</span><span class="o">(</span><span class="s">"userdata"</span><span class="o">,</span> <span class="n">auths</span><span class="o">);</span> +<span class="n">s</span><span class="o">.</span><span class="na">setRange</span><span class="o">(</span><span class="n">r</span><span class="o">);</span> +<span class="n">s</span><span class="o">.</span><span class="na">fetchColumnFamily</span><span class="o">(</span><span class="k">new</span> <span class="n">Text</span><span class="o">(</span><span class="s">"age"</span><span class="o">));</span> + +<span class="k">for</span><span class="o">(</span><span class="n">Entry</span><span class="o"><</span><span class="n">Key</span><span class="o">,</span><span class="n">Value</span><span class="o">></span> <span class="n">entry</span> <span class="o">:</span> <span class="n">s</span><span class="o">)</span> <span class="o">{</span> + <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="n">entry</span><span class="o">.</span><span class="na">getValue</span><span class="o">().</span><span class="na">toString</span><span class="o">());</span> +<span class="o">}</span> +</code></pre> +</div> + +<h3 id="rowid-design">RowID Design</h3> + +<p>Often it is necessary to transform the rowID in order to have rows ordered in a way +that is optimal for anticipated access patterns. A good example of this is reversing +the order of components of internet domain names in order to group rows of the +same parent domain together:</p> + +<div class="highlighter-rouge"><pre class="highlight"><code>com.google.code +com.google.labs +com.google.mail +com.yahoo.mail +com.yahoo.research +</code></pre> +</div> + +<p>Some data may result in the creation of very large rows - rows with many columns. +In this case the table designer may wish to split up these rows for better load +balancing while keeping them sorted together for scanning purposes. This can be +done by appending a random substring at the end of the row:</p> + +<div class="highlighter-rouge"><pre class="highlight"><code>com.google.code_00 +com.google.code_01 +com.google.code_02 +com.google.labs_00 +com.google.mail_00 +com.google.mail_01 +</code></pre> +</div> + +<p>It could also be done by adding a string representation of some period of time such as date to the week +or month:</p> + +<div class="highlighter-rouge"><pre class="highlight"><code>com.google.code_201003 +com.google.code_201004 +com.google.code_201005 +com.google.labs_201003 +com.google.mail_201003 +com.google.mail_201004 +</code></pre> +</div> + +<p>Appending dates provides the additional capability of restricting a scan to a given +date range.</p> + +<h3 id="lexicoders">Lexicoders</h3> +<p>Since Keys in Accumulo are sorted lexicographically by default, itâs often useful to encode +common data types into a byte format in which their sort order corresponds to the sort order +in their native form. An example of this is encoding dates and numerical data so that they can +be better seeked or searched in ranges.</p> + +<p>The lexicoders are a standard and extensible way of encoding Java types. Hereâs an example +of a lexicoder that encodes a java Date object so that it sorts lexicographically:</p> + +<div class="language-java highlighter-rouge"><pre class="highlight"><code><span class="c1">// create new date lexicoder</span> +<span class="n">DateLexicoder</span> <span class="n">dateEncoder</span> <span class="o">=</span> <span class="k">new</span> <span class="n">DateLexicoder</span><span class="o">();</span> + +<span class="c1">// truncate time to hours</span> +<span class="kt">long</span> <span class="n">epoch</span> <span class="o">=</span> <span class="n">System</span><span class="o">.</span><span class="na">currentTimeMillis</span><span class="o">();</span> +<span class="n">Date</span> <span class="n">hour</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Date</span><span class="o">(</span><span class="n">epoch</span> <span class="o">-</span> <span class="o">(</span><span class="n">epoch</span> <span class="o">%</span> <span class="mi">3600000</span><span class="o">));</span> + +<span class="c1">// encode the rowId so that it is sorted lexicographically</span> +<span class="n">Mutation</span> <span class="n">mutation</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Mutation</span><span class="o">(</span><span class="n">dateEncoder</span><span class="o">.</span><span class="na">encode</span><span class="o">(</span><span class="n">hour</span><span class="o">));</span> +<span class="n">mutation</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="k">new</span> <span class="n">Text</span><span class="o">(</span><span class="s">"colf"</span><span class="o">),</span> <span class="k">new</span> <span class="n">Text</span><span class="o">(</span><span class="s">"colq"</span><span class="o">),</span> <span class="k">new</span> <span class="n">Value</span><span class="o">(</span><span class="k">new</span> <span class="kt">byte</span><span class="o">[]{}));</span> +</code></pre> +</div> + +<p>If we want to return the most recent date first, we can reverse the sort order +with the reverse lexicoder:</p> + +<div class="language-java highlighter-rouge"><pre class="highlight"><code><span class="c1">// create new date lexicoder and reverse lexicoder</span> +<span class="n">DateLexicoder</span> <span class="n">dateEncoder</span> <span class="o">=</span> <span class="k">new</span> <span class="n">DateLexicoder</span><span class="o">();</span> +<span class="n">ReverseLexicoder</span> <span class="n">reverseEncoder</span> <span class="o">=</span> <span class="k">new</span> <span class="n">ReverseLexicoder</span><span class="o">(</span><span class="n">dateEncoder</span><span class="o">);</span> + +<span class="c1">// truncate date to hours</span> +<span class="kt">long</span> <span class="n">epoch</span> <span class="o">=</span> <span class="n">System</span><span class="o">.</span><span class="na">currentTimeMillis</span><span class="o">();</span> +<span class="n">Date</span> <span class="n">hour</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Date</span><span class="o">(</span><span class="n">epoch</span> <span class="o">-</span> <span class="o">(</span><span class="n">epoch</span> <span class="o">%</span> <span class="mi">3600000</span><span class="o">));</span> + +<span class="c1">// encode the rowId so that it sorts in reverse lexicographic order</span> +<span class="n">Mutation</span> <span class="n">mutation</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Mutation</span><span class="o">(</span><span class="n">reverseEncoder</span><span class="o">.</span><span class="na">encode</span><span class="o">(</span><span class="n">hour</span><span class="o">));</span> +<span class="n">mutation</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="k">new</span> <span class="n">Text</span><span class="o">(</span><span class="s">"colf"</span><span class="o">),</span> <span class="k">new</span> <span class="n">Text</span><span class="o">(</span><span class="s">"colq"</span><span class="o">),</span> <span class="k">new</span> <span class="n">Value</span><span class="o">(</span><span class="k">new</span> <span class="kt">byte</span><span class="o">[]{}));</span> +</code></pre> +</div> + +<h3 id="indexing">Indexing</h3> + +<p>In order to support lookups via more than one attribute of an entity, additional +indexes can be built. However, because Accumulo tables can support any number of +columns without specifying them beforehand, a single additional index will often +suffice for supporting lookups of records in the main table. Here, the index has, as +the rowID, the Value or Term from the main table, the column families are the same, +and the column qualifier of the index table contains the rowID from the main table.</p> + +<table> + <thead> + <tr> + <th>RowID</th> + <th>Column Family</th> + <th>Column Qualifier</th> + <th>Value</th> + </tr> + </thead> + <tbody> + <tr> + <td>Term</td> + <td>Field Name</td> + <td>MainRowID</td> + <td> </td> + </tr> + </tbody> +</table> + +<p>Note: We store rowIDs in the column qualifier rather than the Value so that we can +have more than one rowID associated with a particular term within the index. If we +stored this in the Value we would only see one of the rows in which the value +appears since Accumulo is configured by default to return the one most recent +value associated with a key.</p> + +<p>Lookups can then be done by scanning the Index Table first for occurrences of the +desired values in the columns specified, which returns a list of row ID from the main +table. These can then be used to retrieve each matching record, in their entirety, or a +subset of their columns, from the Main Table.</p> + +<p>To support efficient lookups of multiple rowIDs from the same table, the Accumulo +client library provides a BatchScanner. Users specify a set of Ranges to the +BatchScanner, which performs the lookups in multiple threads to multiple servers +and returns an Iterator over all the rows retrieved. The rows returned are NOT in +sorted order, as is the case with the basic Scanner interface.</p> + +<div class="language-java highlighter-rouge"><pre class="highlight"><code><span class="c1">// first we scan the index for IDs of rows matching our query</span> +<span class="n">Text</span> <span class="n">term</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Text</span><span class="o">(</span><span class="s">"mySearchTerm"</span><span class="o">);</span> + +<span class="n">HashSet</span><span class="o"><</span><span class="n">Range</span><span class="o">></span> <span class="n">matchingRows</span> <span class="o">=</span> <span class="k">new</span> <span class="n">HashSet</span><span class="o"><</span><span class="n">Range</span><span class="o">>();</span> + +<span class="n">Scanner</span> <span class="n">indexScanner</span> <span class="o">=</span> <span class="n">createScanner</span><span class="o">(</span><span class="s">"index"</span><span class="o">,</span> <span class="n">auths</span><span class="o">);</span> +<span class="n">indexScanner</span><span class="o">.</span><span class="na">setRange</span><span class="o">(</span><span class="k">new</span> <span class="n">Range</span><span class="o">(</span><span class="n">term</span><span class="o">,</span> <span class="n">term</span><span class="o">));</span> + +<span class="c1">// we retrieve the matching rowIDs and create a set of ranges</span> +<span class="k">for</span><span class="o">(</span><span class="n">Entry</span><span class="o"><</span><span class="n">Key</span><span class="o">,</span><span class="n">Value</span><span class="o">></span> <span class="n">entry</span> <span class="o">:</span> <span class="n">indexScanner</span><span class="o">)</span> <span class="o">{</span> + <span class="n">matchingRows</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="k">new</span> <span class="n">Range</span><span class="o">(</span><span class="n">entry</span><span class="o">.</span><span class="na">getKey</span><span class="o">().</span><span class="na">getColumnQualifier</span><span class="o">()));</span> +<span class="o">}</span> + +<span class="c1">// now we pass the set of rowIDs to the batch scanner to retrieve them</span> +<span class="n">BatchScanner</span> <span class="n">bscan</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="na">createBatchScanner</span><span class="o">(</span><span class="s">"table"</span><span class="o">,</span> <span class="n">auths</span><span class="o">,</span> <span class="mi">10</span><span class="o">);</span> +<span class="n">bscan</span><span class="o">.</span><span class="na">setRanges</span><span class="o">(</span><span class="n">matchingRows</span><span class="o">);</span> +<span class="n">bscan</span><span class="o">.</span><span class="na">fetchColumnFamily</span><span class="o">(</span><span class="k">new</span> <span class="n">Text</span><span class="o">(</span><span class="s">"attributes"</span><span class="o">));</span> + +<span class="k">for</span><span class="o">(</span><span class="n">Entry</span><span class="o"><</span><span class="n">Key</span><span class="o">,</span><span class="n">Value</span><span class="o">></span> <span class="n">entry</span> <span class="o">:</span> <span class="n">bscan</span><span class="o">)</span> <span class="o">{</span> + <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="n">entry</span><span class="o">.</span><span class="na">getValue</span><span class="o">());</span> +<span class="o">}</span> +</code></pre> +</div> + +<p>One advantage of the dynamic schema capabilities of Accumulo is that different +fields may be indexed into the same physical table. However, it may be necessary to +create different index tables if the terms must be formatted differently in order to +maintain proper sort order. For example, real numbers must be formatted +differently than their usual notation in order to be sorted correctly. In these cases, +usually one index per unique data type will suffice.</p> + +<h3 id="entity-attribute-and-graph-tables">Entity-Attribute and Graph Tables</h3> + +<p>Accumulo is ideal for storing entities and their attributes, especially of the +attributes are sparse. It is often useful to join several datasets together on common +entities within the same table. This can allow for the representation of graphs, +including nodes, their attributes, and connections to other nodes.</p> + +<p>Rather than storing individual events, Entity-Attribute or Graph tables store +aggregate information about the entities involved in the events and the +relationships between entities. This is often preferrable when single events arenât +very useful and when a continuously updated summarization is desired.</p> + +<p>The physical schema for an entity-attribute or graph table is as follows:</p> + +<table> + <thead> + <tr> + <th>RowID</th> + <th>Column Family</th> + <th>Column Qualifier</th> + <th>Value</th> + </tr> + </thead> + <tbody> + <tr> + <td>EntityID</td> + <td>Attribute Name</td> + <td>Attribute Value</td> + <td>Weight</td> + </tr> + <tr> + <td>EntityID</td> + <td>Edge Type</td> + <td>Related EntityID</td> + <td>Weight</td> + </tr> + </tbody> +</table> + +<p>For example, to keep track of employees, managers and products the following +entity-attribute table could be used. Note that the weights are not always necessary +and are set to 0 when not used.</p> + +<table> + <thead> + <tr> + <th>RowID</th> + <th>Column Family</th> + <th>Column Qualifier</th> + <th>Value</th> + </tr> + </thead> + <tbody> + <tr> + <td>E001</td> + <td>name</td> + <td>bob</td> + <td>0</td> + </tr> + <tr> + <td>E001</td> + <td>department</td> + <td>sales</td> + <td>0</td> + </tr> + <tr> + <td>E001</td> + <td>hire_date</td> + <td>20030102</td> + <td>0</td> + </tr> + <tr> + <td>E001</td> + <td>units_sold</td> + <td>P001</td> + <td>780</td> + </tr> + <tr> + <td>E002</td> + <td>name</td> + <td>george</td> + <td>0</td> + </tr> + <tr> + <td>E002</td> + <td>department</td> + <td>sales</td> + <td>0</td> + </tr> + <tr> + <td>E002</td> + <td>manager_of</td> + <td>E001</td> + <td>0</td> + </tr> + <tr> + <td>E002</td> + <td>manager_of</td> + <td>E003</td> + <td>0</td> + </tr> + <tr> + <td>E003</td> + <td>name</td> + <td>harry</td> + <td>0</td> + </tr> + <tr> + <td>E003</td> + <td>department</td> + <td>accounts_recv</td> + <td>0</td> + </tr> + <tr> + <td>E003</td> + <td>hire_date</td> + <td>20000405</td> + <td>0</td> + </tr> + <tr> + <td>E003</td> + <td>units_sold</td> + <td>P002</td> + <td>566</td> + </tr> + <tr> + <td>E003</td> + <td>units_sold</td> + <td>P001</td> + <td>232</td> + </tr> + <tr> + <td>P001</td> + <td>product_name</td> + <td>nike_airs</td> + <td>0</td> + </tr> + <tr> + <td>P001</td> + <td>product_type</td> + <td>shoe</td> + <td>0</td> + </tr> + <tr> + <td>P001</td> + <td>in_stock</td> + <td>germany</td> + <td>900</td> + </tr> + <tr> + <td>P001</td> + <td>in_stock</td> + <td>brazil</td> + <td>200</td> + </tr> + <tr> + <td>P002</td> + <td>product_name</td> + <td>basic_jacket</td> + <td>0</td> + </tr> + <tr> + <td>P002</td> + <td>product_type</td> + <td>clothing</td> + <td>0</td> + </tr> + <tr> + <td>P002</td> + <td>in_stock</td> + <td>usa</td> + <td>3454</td> + </tr> + <tr> + <td>P002</td> + <td>in_stock</td> + <td>germany</td> + <td>700</td> + </tr> + </tbody> +</table> + +<p>To allow efficient updating of edge weights, an aggregating iterator can be +configured to add the value of all mutations applied with the same key. These types +of tables can easily be created from raw events by simply extracting the entities, +attributes, and relationships from individual events and inserting the keys into +Accumulo each with a count of 1. The aggregating iterator will take care of +maintaining the edge weights.</p> + +<h3 id="document-partitioned-indexing">Document-Partitioned Indexing</h3> + +<p>Using a simple index as described above works well when looking for records that +match one of a set of given criteria. When looking for records that match more than +one criterion simultaneously, such as when looking for documents that contain all of +the words âtheâ and âwhiteâ and âhouseâ, there are several issues.</p> + +<p>First is that the set of all records matching any one of the search terms must be sent +to the client, which incurs a lot of network traffic. The second problem is that the +client is responsible for performing set intersection on the sets of records returned +to eliminate all but the records matching all search terms. The memory of the client +may easily be overwhelmed during this operation.</p> + +<p>For these reasons Accumulo includes support for a scheme known as sharded +indexing, in which these set operations can be performed at the TabletServers and +decisions about which records to include in the result set can be made without +incurring network traffic.</p> + +<p>This is accomplished via partitioning records into bins that each reside on at most +one TabletServer, and then creating an index of terms per record within each bin as +follows:</p> + +<table> + <thead> + <tr> + <th>RowID</th> + <th>Column Family</th> + <th>Column Qualifier</th> + <th>Value</th> + </tr> + </thead> + <tbody> + <tr> + <td>BinID</td> + <td>Term</td> + <td>DocID</td> + <td>Weight</td> + </tr> + </tbody> +</table> + +<p>Documents or records are mapped into bins by a user-defined ingest application. By +storing the BinID as the RowID we ensure that all the information for a particular +bin is contained in a single tablet and hosted on a single TabletServer since +Accumulo never splits rows across tablets. Storing the Terms as column families +serves to enable fast lookups of all the documents within this bin that contain the +given term.</p> + +<p>Finally, we perform set intersection operations on the TabletServer via a special +iterator called the Intersecting Iterator. Since documents are partitioned into many +bins, a search of all documents must search every bin. We can use the BatchScanner +to scan all bins in parallel. The Intersecting Iterator should be enabled on a +BatchScanner within user query code as follows:</p> + +<div class="language-java highlighter-rouge"><pre class="highlight"><code><span class="n">Text</span><span class="o">[]</span> <span class="n">terms</span> <span class="o">=</span> <span class="o">{</span><span class="k">new</span> <span class="n">Text</span><span class="o">(</span><span class="s">"the"</span><span class="o">),</span> <span class="k">new</span> <span class="n">Text</span><span class="o">(</span><span class="s">"white"</span><span class="o">),</span> <span class="k">new</span> <span class="n">Text</span><span class="o">(</span><span class="s">"house"</span><span class="o">)};</span> + +<span class="n">BatchScanner</span> <span class="n">bscan</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="na">createBatchScanner</span><span class="o">(</span><span class="n">table</span><span class="o">,</span> <span class="n">auths</span><span class="o">,</span> <span class="mi">20</span><span class="o">);</span> + +<span class="n">IteratorSetting</span> <span class="n">iter</span> <span class="o">=</span> <span class="k">new</span> <span class="n">IteratorSetting</span><span class="o">(</span><span class="mi">20</span><span class="o">,</span> <span class="s">"ii"</span><span class="o">,</span> <span class="n">IntersectingIterator</span><span class="o">.</span><span class="na">class</span><span class="o">);</span> +<span class="n">IntersectingIterator</span><span class="o">.</span><span class="na">setColumnFamilies</span><span class="o">(</span><span class="n">iter</span><span class="o">,</span> <span class="n">terms</span><span class="o">);</span> + +<span class="n">bscan</span><span class="o">.</span><span class="na">addScanIterator</span><span class="o">(</span><span class="n">iter</span><span class="o">);</span> +<span class="n">bscan</span><span class="o">.</span><span class="na">setRanges</span><span class="o">(</span><span class="n">Collections</span><span class="o">.</span><span class="na">singleton</span><span class="o">(</span><span class="k">new</span> <span class="n">Range</span><span class="o">()));</span> + +<span class="k">for</span><span class="o">(</span><span class="n">Entry</span><span class="o"><</span><span class="n">Key</span><span class="o">,</span><span class="n">Value</span><span class="o">></span> <span class="n">entry</span> <span class="o">:</span> <span class="n">bscan</span><span class="o">)</span> <span class="o">{</span> + <span class="n">System</span><span class="o">.</span><span class="na">out</span><span class="o">.</span><span class="na">println</span><span class="o">(</span><span class="s">" "</span> <span class="o">+</span> <span class="n">entry</span><span class="o">.</span><span class="na">getKey</span><span class="o">().</span><span class="na">getColumnQualifier</span><span class="o">());</span> +<span class="o">}</span> +</code></pre> +</div> + +<p>This code effectively has the BatchScanner scan all tablets of a table, looking for +documents that match all the given terms. Because all tablets are being scanned for +every query, each query is more expensive than other Accumulo scans, which +typically involve a small number of TabletServers. This reduces the number of +concurrent queries supported and is subject to what is known as the âstragglerâ +problem in which every query runs as slow as the slowest server participating.</p> + +<p>Of course, fast servers will return their results to the client which can display them +to the user immediately while they wait for the rest of the results to arrive. If the +results are unordered this is quite effective as the first results to arrive are as good +as any others to the user.</p> + + </div> +</div> + + </div> + + +<footer> + + <p><a href="https://www.apache.org/foundation/contributing";><img src="https://www.apache.org/images/SupportApache-small.png"; alt="Support the ASF" id="asf-logo" height="100" /></a></p> + + <p>Copyright © 2011-2017 The Apache Software Foundation. Licensed under the <a href="https://www.apache.org/licenses/LICENSE-2.0";>Apache License, Version 2.0</a>.</p> + +</footer> + + + </div> + </div> + </div> +</body> +</html>
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Testing</a></div> + + <div class="row doc-sidebar-link"><a href="/docs/unreleased/development/development_tools">Development Tools</a></div> + + <div class="row doc-sidebar-link"><a href="/docs/unreleased/development/sampling">Sampling</a></div> + + <div class="row doc-sidebar-link"><a href="/docs/unreleased/development/summaries">Summary Statistics</a></div> + + <div class="row doc-sidebar-link"><a href="/docs/unreleased/development/security">Security</a></div> + + <div class="row doc-sidebar-link"><a href="/docs/unreleased/development/high_speed_ingest">High-Speed Ingest</a></div> + + <div class="row doc-sidebar-link"><a href="/docs/unreleased/development/analytics">Analytics</a></div> + + </div> + </div> + + + + + + + + + + + + <div class="panel-heading" role="tab" id="headingOne"> + <h4 class="panel-title"> + <a role="button" data-toggle="collapse" data-parent="#accordion" href="#collapseadministration" aria-expanded="false" aria-controls="collapseadministration"> + Administration 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