= Updating Parts of Documents
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Once you have indexed the content you need in your Solr index, you will want to start thinking about your strategy for dealing with changes to those documents. Solr supports three approaches to updating documents that have only partially changed.

The first is _<<Atomic Updates,atomic updates>>_. This approach allows changing only one or more fields of a document without having to reindex the entire document.

The second approach is known as _<<In-Place Updates,in-place updates>>_. This approach is similar to atomic updates (is a subset of atomic updates in some sense), but can be used only for updating single valued non-indexed and non-stored docValue-based numeric fields.

The third approach is known as _<<Optimistic Concurrency,optimistic concurrency>>_ or _optimistic locking_. It is a feature of many NoSQL databases, and allows conditional updating a document based on its version. This approach includes semantics and rules for how to deal with version matches or mis-matches.

Atomic Updates (and in-place updates) and Optimistic Concurrency may be used as independent strategies for managing changes to documents, or they may be combined: you can use optimistic concurrency to conditionally apply an atomic update.

== Atomic Updates

Solr supports several modifiers that atomically update values of a document. This allows updating only specific fields, which can help speed indexing processes in an environment where speed of index additions is critical to the application.

To use atomic updates, add a modifier to the field that needs to be updated. The content can be updated, added to, or incrementally increased if the field has a numeric type.

`set`::
Set or replace the field value(s) with the specified value(s), or remove the values if 'null' or empty list is specified as the new value.
+
May be specified as a single value, or as a list for multiValued fields.

`add`::
Adds the specified values to a multiValued field. May be specified as a single value, or as a list.

`add-distinct`::
Adds the specified values to a multiValued field, only if not already present. May be specified as a single value, or as a list.

`remove`::
Removes (all occurrences of) the specified values from a multiValued field. May be specified as a single value, or as a list.

`removeregex`::
Removes all occurrences of the specified regex from a multiValued field. May be specified as a single value, or as a list.

`inc`::
Increments a numeric value by a specific amount. Must be specified as a single numeric value.

=== Field Storage

The core functionality of atomically updating a document requires that all fields in your schema must be configured as stored (`stored="true"`) or docValues (`docValues="true"`) except for fields which are `<copyField/>` destinations, which must be configured as `stored="false"`. Atomic updates are applied to the document represented by the existing stored field values. All data in copyField destinations fields must originate from ONLY copyField sources.

If `<copyField/>` destinations are configured as stored, then Solr will attempt to index both the current value of the field as well as an additional copy from any source fields. If such fields contain some information that comes from the indexing program and some information that comes from copyField, then the information which originally came from the indexing program will be lost when an atomic update is made.

There are other kinds of derived fields that must also be set so they aren't stored. Some spatial field types, such as BBoxField and LatLonType, use derived fields. CurrencyFieldType also uses derived fields. These types create additional fields which are normally specified by a dynamic field definition. That dynamic field definition must be not stored, or indexing will fail.

=== Example Updating Part of a Document

If the following document exists in our collection:

[source,json]
----
{"id":"mydoc",
 "price":10,
 "popularity":42,
 "categories":["kids"],
 "sub_categories":["under_5","under_10"],
 "promo_ids":["a123x"],
 "tags":["free_to_try","buy_now","clearance","on_sale"]
}
----

And we apply the following update command:

[source,json]
----
{"id":"mydoc",
 "price":{"set":99},
 "popularity":{"inc":20},
 "categories":{"add":["toys","games"]},
 "sub_categories":{"add-distinct":"under_10"},
 "promo_ids":{"remove":"a123x"},
 "tags":{"remove":["free_to_try","on_sale"]}
}
----

The resulting document in our collection will be:

[source,json]
----
{"id":"mydoc",
 "price":99,
 "popularity":62,
 "categories":["kids","toys","games"],
 "sub_categories":["under_5","under_10"],
 "tags":["buy_now","clearance"]
}
----

=== Updating Child Documents

Solr supports modifying, adding and removing child documents as part of atomic updates. +
Schema and configuration requirements are detailed in
<<updating-parts-of-documents#field-storage, Field Storage>> and <<indexing-nested-documents#schema-configuration, Indexing Nested Documents>>. +
Under the hood, Solr retrieves the whole nested structure, deletes the old documents,
and re-indexes the structure after applying the atomic update. +
Syntactically, nested/partial updates are very similar to a regular atomic update,
as demonstrated by the examples below.

[NOTE]
====
.\_route_ Param
To ensure each nested update is routed to its respective shard,
`\_route_` param must be set to the root document's ID when the
update does not have that root document.
====

If the following document exists in our collection:

[source,json]
----
{
 "id":"mydoc",
 "product":"T-Shirt",
 "stock": {
           "id":"mydoc2",
           "color":"red",
           "size": ["L"]
           }
}
----

And we apply the following update command:

[source,json]
----
{
 "id":"mydoc",
 "stock": {
           "add":
                 {
                  "id":"mydoc3",
                  "color":"blue",
                  "size": ["M"]
                 }
          }
}
----

The resulting document in our collection will be:

[source,json]
----
{
 "id":"mydoc",
 "product":"T-Shirt",
 "stock": [{
            "id":"mydoc2",
            "color":"red",
            "size": ["L"]
           },
           {
            "id":"mydoc3",
            "color":"blue",
            "size": ["M"]
           }]
}
----

Documents inside nested structures can also be updated.
These type of updates require setting the `\_route_` set to the root document's ID

If we send this update, setting `\_route_`=mydoc

[source,json]
----
{
 "id":"mydoc2",
 "size": {"add": ["S"]}
}
----

The resulting document in our collection will be:

[source,json]
----
{
 "id":"mydoc",
 "product":"T-Shirt",
 "stock": [{
            "id":"mydoc2",
            "color":"red",
            "size": ["L", "S"]
           },
           {
            "id":"mydoc3",
            "color":"blue",
            "size": ["M"]
           }]
}
----


== In-Place Updates

In-place updates are very similar to atomic updates; in some sense, this is a subset of atomic updates. In regular atomic updates, the entire document is reindexed internally during the application of the update. However, in this approach, only the fields to be updated are affected and the rest of the documents are not reindexed internally. Hence, the efficiency of updating in-place is unaffected by the size of the documents that are updated (i.e., number of fields, size of fields, etc.). Apart from these internal differences, there is no functional difference between atomic updates and in-place updates.

An atomic update operation is performed using this approach only when the fields to be updated meet these three conditions:

* are non-indexed (`indexed="false"`), non-stored (`stored="false"`), single valued (`multiValued="false"`) numeric docValues (`docValues="true"`) fields;
* the `\_version_` field is also a non-indexed, non-stored single valued docValues field; and,
* copy targets of updated fields, if any, are also non-indexed, non-stored single valued numeric docValues fields.

To use in-place updates, add a modifier to the field that needs to be updated. The content can be updated or incrementally increased.

`set`::
Set or replace the field value(s) with the specified value(s). May be specified as a single value.

`inc`::
Increments a numeric value by a specific amount. Must be specified as a single numeric value.

=== In-Place Update Example

If the price and popularity fields are defined in the schema as:

`<field name="price" type="float" indexed="false" stored="false" docValues="true"/>`

`<field name="popularity" type="float" indexed="false" stored="false" docValues="true"/>`

If the following document exists in our collection:

[source,json]
----
{
 "id":"mydoc",
 "price":10,
 "popularity":42,
 "categories":["kids"],
 "promo_ids":["a123x"],
 "tags":["free_to_try","buy_now","clearance","on_sale"]
}
----

And we apply the following update command:

[source,json]
----
{
 "id":"mydoc",
 "price":{"set":99},
 "popularity":{"inc":20}
}
----

The resulting document in our collection will be:

[source,json]
----
{
 "id":"mydoc",
 "price":99,
 "popularity":62,
 "categories":["kids"],
 "promo_ids":["a123x"],
 "tags":["free_to_try","buy_now","clearance","on_sale"]
}
----

== Optimistic Concurrency

Optimistic Concurrency is a feature of Solr that can be used by client applications which update/replace documents to ensure that the document they are replacing/updating has not been concurrently modified by another client application. This feature works by requiring a `\_version_` field on all documents in the index, and comparing that to a `\_version_` specified as part of the update command. By default, Solr's Schema includes a `\_version_` field, and this field is automatically added to each new document.

In general, using optimistic concurrency involves the following work flow:

. A client reads a document. In Solr, one might retrieve the document with the `/get` handler to be sure to have the latest version.
. A client changes the document locally.
. The client resubmits the changed document to Solr, for example, perhaps with the `/update` handler.
. If there is a version conflict (HTTP error code 409), the client starts the process over.

When the client resubmits a changed document to Solr, the `\_version_` can be included with the update to invoke optimistic concurrency control. Specific semantics are used to define when the document should be updated or when to report a conflict.

* If the content in the `\_version_` field is greater than '1' (i.e., '12345'), then the `\_version_` in the document must match the `\_version_` in the index.
* If the content in the `\_version_` field is equal to '1', then the document must simply exist. In this case, no version matching occurs, but if the document does not exist, the updates will be rejected.
* If the content in the `\_version_` field is less than '0' (i.e., '-1'), then the document must *not* exist. In this case, no version matching occurs, but if the document exists, the updates will be rejected.
* If the content in the `\_version_` field is equal to '0', then it doesn't matter if the versions match or if the document exists or not. If it exists, it will be overwritten; if it does not exist, it will be added.

If the document being updated does not include the `\_version_` field, and atomic updates are not being used, the document will be treated by normal Solr rules, which is usually to discard the previous version.

When using Optimistic Concurrency, clients can include an optional `versions=true` request parameter to indicate that the _new_ versions of the documents being added should be included in the response. This allows clients to immediately know what the `\_version_` is of every document added without needing to make a redundant <<realtime-get.adoc#realtime-get,`/get` request>>.

Following are some examples using `versions=true` in queries:

[source,bash]
----
$ curl -X POST -H 'Content-Type: application/json' 'http://localhost:8983/solr/techproducts/update?versions=true' --data-binary '
[ { "id" : "aaa" },
  { "id" : "bbb" } ]'
----
[source,json]
----
{"responseHeader":{"status":0,"QTime":6},
 "adds":["aaa",1498562471222312960,
         "bbb",1498562471225458688]}
----

In this example, we have added 2 documents "aaa" and "bbb". Because we added `versions=true` to the request, the response shows the document version for each document.

[source,bash]
----
$ curl -X POST -H 'Content-Type: application/json' 'http://localhost:8983/solr/techproducts/update?_version_=999999&versions=true' --data-binary '
[{ "id" : "aaa",
   "foo_s" : "update attempt with wrong existing version" }]'
----
[source,json]
----
{"responseHeader":{"status":409,"QTime":3},
 "error":{"msg":"version conflict for aaa expected=999999 actual=1498562471222312960",
          "code":409}}
----


In this example, we've attempted to update document "aaa" but specified the wrong version in the request: `_version_=999999` doesn't match the document version we just got when we added the document. We get an error in response.

[source,bash]
----
$ curl -X POST -H 'Content-Type: application/json' 'http://localhost:8983/solr/techproducts/update?_version_=1498562471222312960&versions=true&commit=true' --data-binary '
[{ "id" : "aaa",
   "foo_s" : "update attempt with correct existing version" }]'
----
[source,json]
----
{"responseHeader":{"status":0,"QTime":5},
 "adds":["aaa",1498562624496861184]}
----

Now we've sent an update with a value for `\_version_` that matches the value in the index, and it succeeds. Because we included `versions=true` to the update request, the response includes a different value for the `\_version_` field.

[source,bash]
----
$ curl 'http://localhost:8983/solr/techproducts/query?q=*:*&fl=id,_version_'
----

[source,json]
----
{
  "responseHeader":{
    "status":0,
    "QTime":5,
    "params":{
      "fl":"id,_version_",
      "q":"*:*"}},
  "response":{"numFound":2,"start":0,"docs":[
      {
        "id":"bbb",
        "_version_":1498562471225458688},
      {
        "id":"aaa",
        "_version_":1498562624496861184}]
  }}
----

Finally, we can issue a query that requests the `\_version_` field be included in the response, and we can see that for the two documents in our example index.

For more information, please also see Yonik Seeley's presentation on https://www.youtube.com/watch?v=WYVM6Wz-XTw[NoSQL features in Solr 4] from Apache Lucene EuroCon 2012.

== Document Centric Versioning Constraints

Optimistic Concurrency is extremely powerful, and works very efficiently because it uses an internally assigned, globally unique values for the `\_version_` field.
However, in some situations users may want to configure their own document specific version field, where the version values are assigned on a per-document basis by an external system, and have Solr reject updates that attempt to replace a document with an "older" version.
In situations like this the {solr-javadocs}/solr-core/org/apache/solr/update/processor/DocBasedVersionConstraintsProcessorFactory.html[`DocBasedVersionConstraintsProcessorFactory`] can be useful.

The basic usage of `DocBasedVersionConstraintsProcessorFactory` is to configure it in `solrconfig.xml` as part of the <<update-request-processors.adoc#update-request-processor-configuration,UpdateRequestProcessorChain>> and specify the name of your custom `versionField` in your schema that should be checked when validating updates:

[source,xml]
----
<processor class="solr.DocBasedVersionConstraintsProcessorFactory">
  <str name="versionField">my_version_l</str>
</processor>
----

Note that `versionField` is a comma delimited list of fields to check for version numbers.
Once configured, this update processor will reject (HTTP error code 409) any attempt to update an existing document where the value of the `my_version_l` field in the "new" document is not greater then the value of that field in the existing document.

.versionField vs `\_version_`
[IMPORTANT]
====
The `\_version_` field used by Solr for its normal optimistic concurrency also has important semantics in how updates are distributed to replicas in SolrCloud, and *MUST* be assigned internally by Solr. Users can not re-purpose that field and specify it as the `versionField` for use in the `DocBasedVersionConstraintsProcessorFactory` configuration.
====

`DocBasedVersionConstraintsProcessorFactory` supports the following additional configuration parameters, which are all optional:

`ignoreOldUpdates`::
A boolean option which defaults to `false`. If set to `true`, the update will be silently ignored (and return a status 200 to the client) instead of rejecting updates where the `versionField` is too low.

`deleteVersionParam`::
A String parameter that can be specified to indicate that this processor should also inspect Delete By Id commands.
+
The value of this option should be the name of a request parameter that the processor will consider mandatory for all attempts to Delete By Id, and must be be used by clients to specify a value for the `versionField` which is greater then the existing value of the document to be deleted.
+
When using this request parameter, any Delete By Id command with a high enough document version number to succeed will be internally converted into an Add Document command that replaces the existing document with a new one which is empty except for the Unique Key and `versionField` to keeping a record of the deleted version so future Add Document commands will fail if their "new" version is not high enough.
+
If `versionField` is specified as a list, then this parameter too must be specified as a comma delimited list of the same size so that the parameters correspond with the fields.

`supportMissingVersionOnOldDocs`::
This boolean parameter defaults to `false`, but if set to `true` allows any documents written *before* this feature is enabled, and which are missing the `versionField`, to be overwritten.

Please consult the {solr-javadocs}/solr-core/org/apache/solr/update/processor/DocBasedVersionConstraintsProcessorFactory.html[DocBasedVersionConstraintsProcessorFactory javadocs] and https://git1-us-west.apache.org/repos/asf?p=lucene-solr.git;a=blob;f=solr/core/src/test-files/solr/collection1/conf/solrconfig-externalversionconstraint.xml;hb=HEAD[test solrconfig.xml file] for additional information and example usages.
