Apache Solr Reference Guide Covering Apache Solr 6.0

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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:
1.
2.
3.
4.
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 documented added with out needing to make a
redundant /get request.
For example...
Apache Solr Reference Guide 6.0
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