SOLR-12160: Time Routed Alias (TRA) documentation

solr/solr-ref-guide/src/collections-api.adoc

@@ -480,26 +480,27 @@ http://localhost:8983/solr/admin/collections?action=DELETESHARD&collection=anoth
 [[createalias]]
 == CREATEALIAS: Create or Modify an Alias for a Collection
 
-The `CREATEALIAS` action will create a new alias pointing to one or more collections. If an alias by the same name
-already exists, this action will replace the existing alias, effectively acting like an atomic "MOVE" command. Aliases
-come in 2 flavors: routed and non-routed.
+The `CREATEALIAS` action will create a new alias pointing to one or more collections.
+Aliases come in 2 flavors: standard and routed.
 
-Non-routed aliases are simpler and can serve to rename a collection or to distribute queries across several collections.
-While it is possible to send updates to an alias spanning multiple collections, non-routed alias have no logic for
-distributing documents among the referenced collections so all updates will go to the first collection in the list.
+*Standard aliases* are simple:  CREATEALIAS registers the alias name with the names of one or more collections provided
+  by the command.
+If an existing alias exists, it is replaced/updated.
+A standard alias can serve to have the appearance of renaming a collection, and can be used to atomically swap
+which backing/underlying collection is "live" for various purposes.
+When Solr searches an alias pointing to multiple collections, Solr will search all shards of all the collections as an
+  aggregated whole.
+While it is possible to send updates to an alias spanning multiple collections, standard aliases have no logic for
+  distributing documents among the referenced collections so all updates will go to the first collection in the list.
 
 `/admin/collections?action=CREATEALIAS&name=_name_&collections=_collectionlist_`
 
-Routed aliases are more complex to configure, but include logic for automatically inspecting a field on the document
-and using the value in that field to select a destination collection. Additionally, the routed alias automates the
-partitioning of data across a series of collections by creating new collections periodically. This feature allows for
-indefinite indexing of data without degradation of performance otherwise experienced due to the continuous growth of
-an index. As new data arrives, a field on the document is inspected and the document is then potentially re-routed to
-another collection. The underlying collections can can be queried independently but usually the alias will be used.
-These collections are created automatically on the fly as new data arrives based on the parameters supplied in this
-command. For very high volume use cases or for use cases requiring only a limited span of data to be retained,
-collections older than a given age can be deleted. This delete of old collections only occurs if new documents are
-sent to the alias.
+*Routed aliases* are aliases with additional capabilities to act as a kind of super-collection -- routing
+  updates to the correct collection.
+Since the only routing strategy at present is time oriented, these are also called *Time Routed Aliases* (TRAs).
+A TRA manages an alias and a time sequential series of collections that it will both create and optionally delete on-demand.
+See <<time-routed-aliases.adoc#time-routed-aliases,Time Routed Aliases>> for some important high-level information
+  before getting started.
 
 NOTE: Presently this is only supported for temporal fields stored as a
 <<field-types-included-with-solr.adoc#field-types-included-with-solr,DatePointField or TrieDateField>> type. Other
@@ -541,15 +542,17 @@ requirements for collection naming.
 `async`::
 Request ID to track this action which will be <<Asynchronous Calls,processed asynchronously>>.
 
-==== Non-Routed Alias Parameters
+==== Standard Alias Parameters
 
 `collections`::
 A comma-separated list of collections to be aliased. The collections must already exist in the cluster.
-This parameter signals the creation of a simple (non-routed) alias. If it is present all routing parameters are
+This parameter signals the creation of a standard alias. If it is present all routing parameters are
 prohibited. If routing parameters are present this parameter is prohibited.
 
 ==== Routed Alias Parameters
 
+Most routed alias parameters become _alias properties_ that can subsequently be inspected and <<aliasprop,modified>>.
+
 `router.start`::
 The start date/time of data for this time routed alias in Solr's standard date/time format (i.e., ISO-8601 or "NOW"
 optionally with <<working-with-dates.adoc#date-math,date math>>).

solr/solr-ref-guide/src/how-solrcloud-works.adoc

@@ -1,5 +1,5 @@
 = How SolrCloud Works
-:page-children: shards-and-indexing-data-in-solrcloud, distributed-requests
+:page-children: shards-and-indexing-data-in-solrcloud, distributed-requests, time-routed-aliases
 // Licensed to the Apache Software Foundation (ASF) under one
 // or more contributor license agreements.  See the NOTICE file
 // distributed with this work for additional information
@@ -21,6 +21,7 @@ The following sections cover provide general information about how various SolrC
 
 * <<shards-and-indexing-data-in-solrcloud.adoc#shards-and-indexing-data-in-solrcloud,Shards and Indexing Data in SolrCloud>>
 * <<distributed-requests.adoc#distributed-requests,Distributed Requests>>
+* <<time-routed-aliases.adoc#time-routed-aliases,Time Routed Aliases>>
 
 If you are already familiar with SolrCloud concepts and basic functionality, you can skip to the section covering <<solrcloud-configuration-and-parameters.adoc#solrcloud-configuration-and-parameters,SolrCloud Configuration and Parameters>>.
 

solr/solr-ref-guide/src/time-routed-aliases.adoc

@@ -0,0 +1,95 @@
+= Time Routed Aliases
+
+Time Routed Aliases (TRAs) is a SolrCloud feature that manages an alias and a time sequential series of collections.
+It automatically creates new collections and (optionally) deletes old ones as it routes documents to the correct
+  collection based on its timestamp.
+This approach allows for indefinite indexing of data without degradation of performance otherwise experienced due to the
+  continuous growth of a single index.
+If you need to store a lot of timestamped data in Solr, such as logs or IoT sensor data, then this feature probably
+  makes more sense than creating one sharded hash-routed collection.
+
+== How it works
+
+First you create a time routed aliases using the <<collections-api.adoc#createalias,CREATEALIAS>> command with some
+  router settings.
+Most of the settings are editable at a later time using the <<collections-api.adoc#aliasprop,ALIASPROP>> command.
+The first collection will be created automatically, along with an alias pointing to it.
+Each underlying Solr "core" in a collection that is a member of a TRA has a special core property referencing the alias.
+The name of each collection is comprised of the TRA name and the start timestamp (UTC), with trailing zeros and symbols
+  truncated.
+Ideally, as a user of this feature, you needn't concern yourself with the particulars of the collection naming pattern
+  since both queries and updates may be done via the alias.
+
+When adding data, you should usually direct documents to the alias (e.g. reference the alias name instead of any collection).
+The Solr server and CloudSolrClient will direct an update request to the first collection that an alias points to.
+The collections list for a TRA is always reverse sorted, and thus the connection path of the request will route to the
+  lead collection.  Using CloudSolrClient is preferable as it can reduce the number of underlying physical HTTP requests by one.
+If you know that a particular set of documents to be delivered is going to a particular older collection then you could
+  direct it there at the client side as an optimization but it's not necessary.  CloudSolrClient does not (yet) do this.
+
+When processing an update for a TRA, Solr initializes its
+  <<update-request-processors.adoc#update-request-processors,UpdateRequestProcessor>> chain as usual, but
+  when DistributedUpdateProcessor (DUP) initializes, it detects that the update targets a TRA and injects
+  TimeRoutedUpdateProcessor (TRUP) in front of itself.
+TRUP, in coordination with the Overseer, is the main part of a TRA, and must immediately precede DUP. It is not
+  possible to configure custom chains with other types of UpdateRequestProcessors between TRUP and DUP.
+TRUP first reads TRA configuration from the alias properties when it is initialized.  As it sees each document, it checks for
+  changes to TRA properties, updates its cached configuration if needed and then determines which collection the
+  document belongs to:
+
+* If TRUP needs to send it to a time segment represented by a collection other than the one that
+  the client chose to communicate with, then it will do so using mechanisms shared with DUP.
+  Once the document is forwarded to the correct collection (i.e. the correct TRA time segment), it skips directly to
+  DUP on the target collection and continues normally, potentially being routed again to the correct shard & replica
+  within the target collection.
+
+* If it belongs in the current collection (which is usually the case if processing events as they occur), the document
+  passes through to DUP. DUP does it's normal collection-level processing that may involve routing the document
+  to another shard & replica.
+
+* If the time stamp on the document is more recent than the most recent TRA segment, then a new collection needs to be
+  added at the front of the TRA.
+  TRUP will create this collection, add it to the alias and then forward the document to the collection it just created.
+  This can happen recursively if more than one collection needs to be created.
+  Each time a new collection is added, the oldest collections in the TRA are examined for possible deletion, if that has
+    been configured.
+  All this happens synchronously, potentially adding seconds to the update request and indexing latency.
+
+Any other type of update like a commit or delete is routed by TRUP to all collections.
+Generally speaking, this is not a performance concern. When Solr receives a delete or commit wherein nothing is deleted
+or nothing needs to be committed, then it's pretty cheap.
+
+== Improvement Possibilities
+
+This is a new feature of SolrCloud that can be expected to be improved.
+Some _potential_ areas for improvement that _are not implemented yet_ are:
+
+* Searches with time filters should only go to applicable collections.
+
+* Collections ought to be constrained by their size instead of or in addition to time.
+  Based on the underlying design, this would only apply to the lead collection.
+
+* Ways to automatically optimize (or reduce the resources of) older collections that aren't expected to receive more
+  updates, and might have less search demand.
+
+* New collections ought to be created preemptively, as an option, to avoid delaying a document that does not yet have
+  a collection to go to.
+
+* CloudSolrClient could route documents to the correct collection based on a timestamp instead always picking the
+  latest.
+
+== Limitations & Assumptions
+
+* Only *time* routed aliases are supported.  If you instead have some other sequential number, you could fake it
+  as a time (e.g. convert to a timestamp assuming some epoch and increment).
+  The smallest possible interval is one second.
+  No other routing scheme is supported, although this feature was developed with considerations that it could be
+  extended/improved to other schemes.
+
+* The underlying collections form a contiguous sequence without gaps.  This will not be suitable when there are
+  large gaps in the underlying data, as Solr will insist that there be a collection for each increment.  This
+  is due in part on Solr calculating the end time of each interval collection based on the timestamp of
+  the next collection, since it is otherwise not stored in any way.
+
+* Avoid sending updates to the oldest collection if you have also configured that old collections should be
+  automatically deleted.  It could lead to exceptions bubbling back to the indexing client.