ottobackwards commented on a change in pull request #1571: METRON-2313 - Added
additional documentation on SOLR performance, cha…
URL: https://github.com/apache/metron/pull/1571#discussion_r349934427
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File path: metron-platform/metron-solr/metron-solr-common/README.md
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@@ -72,6 +69,89 @@ via the global config. The following settings are possible
as part of the globa
* `httpBasicAuthPassword` : Basic auth password
* `solr.ssl.checkPeerName` : Check peer name
+### Performance: Server-side versus Client-side commits
+It is important to note that SOLR is not a ACID compliant database, in
particular there is no isolation between transactions.
+This has a major impact on performance if client-side commits are configured,
as a commit causes the entire collection to check-pointed
+and written to disk, pausing any other client writing data to the same
collection.
+
+In Metron, it is possible that dozens of storm spouts are writing data to the
same SOLR collection simultaneously.
+Each of these spouts triggering a client-side commit on the same SOLR
collection can have a catastrophic effect on performance.
+
+SOLR can manage this issue by removing the responsibility of committing data
from the clients, and letting the server
+trigger regular commits on a collection to flush data to disk. Because this
is server-side as opposed to client-side functionality, it
+ is controlled via the following parameters in each Collection's
`solrconfig.xml` configuration file:
+
+* autoCommit : Also called a 'hard' autocommit, this regularly synchronizes
ingested data to disk to guarantee data persistence.
+* autoSoftCommit : Allows data to become visible to searchers without
requiring an expensive commit to disk.
+
+`openSearcher=true` is an expensive (hard) autoCommit option that triggers
written data to be merged into on-disk indexes and become visible to searchers.
+It is the equivalent of a soft and hard commit performed at the same time. It
is rarely used for Near Real Time (NRT) search scenarios
+
+
+The standard mantra for configuring SOLR for Near Real-Time Search is:
+
+* autoCommit (with `openSearcher=false`) for persisting data,
+* autoSoftCommit for making data visible.
+
+These functions can (and nearly always do) have different time periods
configured for them.
+For example:
+* `autoCommit` (with `openSearcher=false` and `maxTime=30000` milliseconds) to
persist data,
+* `autoSoftCommit` (with `maxTime=120000` milliseconds) to make newly ingested
data visible.
+
+### Managing the risk of data loss
+Experienced admins at this stage would now be asking the question as to what
happens if SOLR crashes before a hard commit occurs to
+persist the data. While SOLR does write data to its transaction log as soon
as it is received, it does not fsync this log to disk until
+a hard commit is requested. Thus a hardware crash could technically risk the
data collected up to the interval that hard autoCommits
+are configured for.
+
+If the potential for data loss is unacceptable to the business then a common
architecture used to manage the risk of
+data loss is to have one or more replicas of each SOLR collection. When
ingesting data into a collection that has replicas, SOLR will
+not return a result to the client until the data has been passed to each
replica in a collection. Replicas immediately
+return acknowledgement as soon as the data is stored in local memory buffers.
The configured autoCommit/autoSoftCommit intervals
+later process and store the data on the replica in exactly the same way it is
processed and stored on the primary node.
+
+So if you are using collection replicas you are protected against individual
machine failures
+by the fact that your data is present in the main memory (immediately) and
disks (after an interval of time) of other replicas.
+This type of architecture is similar to how other distributed systems like
Kafka manages the performance/reliability trade-offs
+
+### Disabling client-side commits in Metron SOLR
+To disable client-side commits in Metron's Storm spouts, make sure
`solr.commitPerBatch = false` is set in Metron's
+global json configuration section. For details on how to change Metron's
global configuration, please refer to the documentation
+for Metron's `zk_load_config.sh` script.
+
+
+### Configuring server-side commits
Review comment:
Maybe we can explicitly state that these instructions are known to work and
tried with version 7.x? of SOLR.
Since we can't keep up with the latest?
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