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# Best Practices when Using Jackrabbit Oak

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## Session Management
### Session refresh behavior

Oak is based on the MVCC model where each session starts with a snapshot
view of the repository. Concurrent changes from other sessions *are not
visible* to a session until it gets refreshed. A session can be refreshed
either explicitly by calling the ``refresh()`` method or implicitly by
direct-to-workspace methods or by the auto-refresh mode. Also observation
event delivery causes a session to be refreshed.

By default the auto-refresh mode automatically refreshes all sessions that
have been idle for more than one second, and it's also possible to
explicitly set the auto-refresh parameters. A typical approach would be
for long-lived admin sessions to set the auto-refresh mode to keep the
session always up to date with latest changes from the repository.

### Pattern: One session for one request/operation

One of the key patterns targeted by Oak is a web application that serves
HTTP requests. The recommended way to handle such cases is to use a
separate session for each HTTP request, and never to refresh that session.

### Anti pattern: concurrent session access

Oak is designed to be virtually lock free as long as sessions are not shared
across threads. Don't access the same session instance concurrently from
multiple threads. When doing so Oak will protect its internal data structures
from becoming corrupted but will not make any guarantees beyond that. In
particular violating clients might suffer from lock contentions or deadlocks.

If Oak detects concurrent write access to a session it will log a warning. 
For concurrent read access the warning will only be logged if `DEBUG` level 
is enabled for `org.apache.jackrabbit.oak.jcr.delegate.SessionDelegate`.
In this case the stack trace of the other session involved will also be 
logged. For efficiency reasons the stack trace will not be logged if 
`DEBUG` level is not enabled.

## Content Modelling
### Large number of direct child node

Oak scales to large number of direct child nodes of a node as long as those
are *not* orderable. For orderable child nodes Oak keeps the order in an
internal property, which will lead to a performance degradation when the list
grows too large. For such scenarios Oak provides the ``oak:Unstructured`` node
type, which is equivalent to ``nt:unstructured`` except that it is not orderable.

### Large Multi Value Property

Using nodes with large multi value property would not scale well. Depending on 
NodeStore it might hit some size limit restriction also. For e.g. with 
DocumentMK the MVP would be stored in the backing Document which on Mongo has a 
16MB limit.

More efficient alternatives to large MVPs include:
* store the list of values in a binary property
* use a [PropertySequence](https://jackrabbit.apache.org/api/trunk/org/apache/jackrabbit/commons/flat/PropertySequence.html) available in jackrabbit-commons (JCR-2688)

### Inlining large binaries

Most of the `BlobStore` provide an option to inline small binary content as part of 
node property itself. For example `FileDataStore` supports `minRecordLength` property.
If that is set to say 4096 then any binary with size less than 4kb would be stored
as part of node data itself and not in BlobStore.

It is recommended to not set very high value for this as depending on implementation it
might hit some limit causing the commit to fail. For e.g. the SegmentNodeStore enforces a limit of
8k for any inlined binary value. Further this would also lead to repository growth as
by default when binaries are stored in BlobStore then they are deduplicated.

### Creating files

The default node type provided by JCR 1.0 to model file structure using
`nt:file` is to add `jcr:content` child with type `nt:resource`, which makes
that content referenceable.

If the file has no need to be referenceable it is recommended to use the
node type `oak:Resource` instead and add the mixin type `mix:referenceble`
only upon demand (see [OAK-4567](https://issues.apache.org/jira/browse/OAK-4567))

## Hierarchy Operations
### Tree traversal

As explained in [Understanding the node state model](https://jackrabbit.apache.org/oak/docs/architecture/nodestate.html), Oak stores content in a tree hierarchy. 
Considering that, when traversing the path to access parent or child nodes, even though being equivalent operations, 
it is preferable to use JCR Node API instead of Session API. The reason behind is that session API uses an absolute path, 
and to get to the desired parent or child node, all ancestor nodes will have to be traversed before reaching the target node. 
Traversal for each ancestor node includes building the node state and associating it with 
TreePermission (check [Permission Evaluation in Detail](https://jackrabbit.apache.org/oak/docs/security/permission/evaluation.html)), 
where this is not needed when using Node API and relative paths.
```
Node c = session.getNode("/a/b/c");
Node d = null;

// get the child node
d = session.getNode("/a/b/c/d");
d = c.getNode("d");                             // preferred way to fetch the child node

// get the parent node
c = session.getNode("/a/b/c");
c = d.getParent();                              // preferred way to fetch the parent node
```
## Security
- [Best Practices for Authorization](security/authorization/bestpractices.html)

## Misc
### Don't use Thread.interrupt()

`Thread.interrupt()` can severely impact or even stop the repository. The reason for 
this is that Oak internally uses various classes from the `nio` package that implement 
`InterruptibleChannel`, which are [asynchronously closed](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/channels/InterruptibleChannel.html) 
when receiving an `InterruptedException` while blocked on IO. See [OAK-2609](https://issues.apache.org/jira/browse/OAK-2609).  

### Avoid or minimize conflicts
To reduce the possiblity of having errors like `OakState0001: Unresolved conflicts in ...`:

1. Make sure you always release the session by calling session.logout(). If possible, avoid long-running sessions. If they are required (e.g. for observation) make sure 
to always call session.refresh(false) before applying changes or session.refresh(true) before saving the changes. 

2. Enable the DEBUG level for `org.apache.jackrabbit.oak.plugins.commit.MergingNodeStateDiff` and `org.apache.jackrabbit.oak.plugins.commit.ConflictValidator` loggers if you want 
to have more information on the circumstances of a conflict that happened in a point of time.

3. Write your own conflict handler and add it when configuring your Oak or WhiteBoard instances. Only if you know what you are doing (i.e. you know how to resolve 
the conflict in each one of the possible situations). By default, the [AnnotatingConflictHandler](https://jackrabbit.apache.org/oak/docs/apidocs/org/apache/jackrabbit/oak/plugins/commit/AnnotatingConflictHandler.html) instance will discard your changes 
and your commit will fail. If persisting changes fails with a conflict and you cannot lose them, refactor your code such that you can retry after having called session.refresh(false).
Check the source code of [JcrLastModifiedConflictHandler](https://jackrabbit.apache.org/oak/docs/apidocs/org/apache/jackrabbit/oak/plugins/commit/JcrLastModifiedConflictHandler.html) for an example of a conflict handler.