keith-turner commented on a change in pull request #282:
URL: https://github.com/apache/accumulo-website/pull/282#discussion_r632613635
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File path: _docs-2/administration/compaction.md
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@@ -62,6 +68,151 @@ For more information see the javadoc for {% jlink
org.apache.accumulo.core.spi.c
The names of the compaction services and executors are used for logging and
metrics.
+## External Compactions
+
+In Accumulo 2.1 we introduced a new optional feature that allows compactions
to run
+outside of the Tablet Server. External compactions introduces two new server
processes
+in an Accumulo deployment:
+
+ * *Compactor*: Accumulo process that runs external compactions and is
started with the name of a queue for which it will perform compactions. In a
typical deployment there will be many of these processes running, some for
queue A, queue B, etc. This process will only run a single compaction at a
time and will communicate with the Compaction Coordinator to get a compaction
job and report its status.
+
+ * *Compaction Coordinator*: a process that manages the compaction queues for
all external compactions in the system and assigns compaction tasks to
Compactors. In a typical deployment there will be one instance of this process
in use at a time with a backup process waiting to become primary (much like the
primary and secondary manager processes). This process communicates with the
TabletServers to get external compaction job information and report back their
status.
+
+### Configuration
+
+Configuration for external compactions is very similar to the internal
compaction example above.
+In the example below we create a Compaction Service `cs1` and configure it
with an externalQueue
+named `DCQ1`. We then define the Compaction Dispatcher on table `testTable`
and configure the
+table to use the `cs1` Compaction Service for planning and executing
compactions.
+
+```
+config -s
tserver.compaction.major.service.cs1.planner=org.apache.accumulo.core.spi.compaction.DefaultCompactionPlanner
+config -s
'tserver.compaction.major.service.cs1.planner.opts.executors=[{"name":"all","externalQueue":"DCQ1"}]'
+config -t testTable -s
table.compaction.dispatcher=org.apache.accumulo.core.spi.compaction.SimpleCompactionDispatcher
+config -t testTable -s table.compaction.dispatcher.opts.service=cs1
+```
+
+### Overview
+
+The CompactionCoordinator is responsible for managing the global external
compaction work queue. For each external compaction queue, the tablet server
will maintain an in memory priority queue of the tablets loaded on it that
require external compactions. The coordinator polls all tservers to get summary
information about their external compaction queues to combine the summary
information to determine which tablet server to contact next to get work. The
coordinator does not maintain per tablet information, it only maintains enough
information to allow it know which tablet server to contact next for a given
queue. The tablet server will then know what specific tablet in that queue
needs to compact.
Review comment:
There are two types of information in the metadata table related to
external compactions.
The first type of metadata is stored under a tablets row and contains
information about running external compactions. Tablets are authorities for
this information and are the only ones to read/write it, the coordinator does
not.
The second type of information in the metadata table is stored under the
~ecomp prefix and contains information about completed or failed compactions.
This information is written by the coordinator and deleted by tablets upon
successful commit of an external compaction or deleted by the coordinator when
it detects a completed compaction for a tablet that no longer exists. The
primary purpose of this information is to allow the coordinator to persist
information about completed compactions for tablets that are temporarily
offline so that it can notify them later.
Compactors reserver and commit external compactions via the coordinator
(which in turn talks to tservers). During this process they pass back and
forth information about specific extents. However for the purpose of finding
the next tserver to reserve an external compaction form, the coordinator does
not maintain per tablet information. Rather it maintains per tserver summary
information that helps it find the next tserver to contact. The summary
information is managed by
[QueueSummaries.java](https://github.com/apache/accumulo/blob/327a48f4cf8d09a006cc137d3505bfc644e93994/server/compaction-coordinator/src/main/java/org/apache/accumulo/coordinator/QueueSummaries.java#L38)
within the coordinator.
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