finishing the paper

publications/whitepaper/druid.tex

@@ -76,7 +76,7 @@ came to the conclusion that there was nothing in the open source world that
 could be fully leveraged for our requirements.
 
 We ended up creating Druid, an open-source, distributed, column-oriented,
-realtime analytical data store.  In many ways, Druid shares similarities with
+real-time analytical data store.  In many ways, Druid shares similarities with
 other OLAP systems \cite{oehler2012ibm, schrader2009oracle, lachev2005applied},
 interactive query systems \cite{melnik2010dremel}, main-memory databases
 \cite{farber2012sap}, and widely-known distributed data stores
@@ -413,7 +413,7 @@ distribution on historical nodes. The coordinator nodes tell historical nodes
 to load new data, drop outdated data, replicate data, and move data to load
 balance. Druid uses a multi-version concurrency control swapping protocol for
 managing immutable segments in order to maintain stable views.  If any
-immutable segment contains data that is wholly obseleted by newer segments, the
+immutable segment contains data that is wholly obsoleted by newer segments, the
 outdated segment is dropped from the cluster. Coordinator nodes undergo a
 leader-election process that determines a single node that runs the coordinator
 functionality. The remaining coordinator nodes act as redundant backups.
@@ -678,7 +678,7 @@ A sample count query over a week of data is as follows:
   "aggregations" : [{"type":"count", "name":"rows"}]
 }
 \end{verbatim}}
-The query shown above will return a count of the number of rows in the Wikipedia datasource
+The query shown above will return a count of the number of rows in the Wikipedia data source
 from 2013-01-01 to 2013-01-08, filtered for only those rows where the value of the ``page" dimension is 
 equal to ``Ke\$ha". The results will be bucketed by day and will be a JSON array of the following form:
 {\scriptsize\begin{verbatim}
@@ -780,11 +780,12 @@ involving all columns are very rare.
 
 A few notes about our results:
 \begin{itemize}[leftmargin=*,beginpenalty=5000,topsep=0pt]
-\item The results are from a ``hot" tier in our production cluster. We run
-several tiers of varying performance in production.
+\item The results are from a ``hot" tier in our production cluster. There were
+approximately 50 data sources in the tier and several hundred users issuing
+queries.
 
-\item There is approximately 10.5TB of RAM available in the ``hot" tier and
-approximately 10TB of segments loaded (including replication). Collectively,
+\item There was approximately 10.5TB of RAM available in the ``hot" tier and
+approximately 10TB of segments loaded. Collectively,
 there are about 50 billion Druid rows in this tier. Results for
 every data source are not shown.
 
@@ -799,10 +800,10 @@ Query latencies are shown in Figure~\ref{fig:query_latency} and the queries per
 minute are shown in Figure~\ref{fig:queries_per_min}. Across all the various
 data sources, average query latency is approximately 550 milliseconds, with
 90\% of queries returning in less than 1 second, 95\% in under 2 seconds, and
-99\% of queries returning in less than 10 seconds.
-Occasionally we observe spikes in latency, as observed on February 19,
-in which case network issues on the Memcached instances were compounded by very high
-query load on one of our largest datasources.
+99\% of queries returning in less than 10 seconds.  Occasionally we observe
+spikes in latency, as observed on February 19, in which case network issues on
+the Memcached instances were compounded by very high query load on one of our
+largest datasources.
 
 \begin{figure}
 \centering 
@@ -881,7 +882,7 @@ ingestion setup consists of 6 nodes, totalling 360GB of RAM and 96 cores
 (12 x Intel Xeon E5-2670).
 
 Note that in this setup, several other data sources were being ingested and
-many other Druid related ingestion tasks were running concurrently on those machines.
+many other Druid related ingestion tasks were running concurrently on the machines.
 
 Druid's data ingestion latency is heavily dependent on the complexity of the
 data set being ingested. The data complexity is determined by the number of
@@ -948,19 +949,19 @@ explore use case, the number of queries issued by a single user is much higher
 than in the reporting use case. Exploratory queries often involve progressively
 adding filters for the same time range to narrow down results. Users tend to
 explore short time intervals of recent data. In the generate report use case,
-users query for much longer data intervals, but users also already have the
-queries they want to issue in mind.
+users query for much longer data intervals, but users also already know the
+queries they want to issue.
 
 \paragraph{Multitenancy}
 Expensive concurrent queries can be problematic in a multitenant
-environment. Queries for large datasources may end up hitting every historical
+environment. Queries for large data sources may end up hitting every historical
 node in a cluster and consume all cluster resources. Smaller, cheaper queries
 may be blocked from executing in such cases. We introduced query prioritization
 to address these issues. Each historical node is able to prioritize which
 segments it needs to scan. Proper query planning is critical for production
 workloads. Thankfully, queries for a significant amount of data tend to be for
-reporting use cases, and users are not expecting the same level of
-interactivity as when they are querying to explore data. 
+reporting use cases and can be deprioritized. Users do not expect the same level of
+interactivity in this use case as when they are exploring data. 
 
 \paragraph{Node failures}
 Single node failures are common in distributed environments, but many nodes
@@ -979,7 +980,7 @@ center to fail. In such cases, new machines need to be provisioned. As long as
 deep storage is still available, cluster recovery time is network bound as
 historical nodes simply need to redownload every segment from deep storage. We
 have experienced such failures in the past, and the recovery time was around
-several hours in the AWS ecosystem on several TBs of data.
+several hours in the AWS ecosystem for several TBs of data.
 
 \subsection{Operational Monitoring}
 Proper monitoring is critical to run a large scale distributed cluster.