Chapter 15. Optimization Guidelines

There are numerous techniques you can use in order to ensure that Kodo operates in the fastest and most efficient manner. Following are some guidelines. Each describes what impact it will have on performance and scalability. Note that general guidelines regarding performance or scalability issues are just that - guidelines. Depending on the particular characteristics of your application, the optimal settings may be considerably different than what is outlined below.

In the following table, each row is labeled with a list of italicized keywords. These keywords identify what characteristics the row in question may improve upon. Many of the rows are marked with one or both of the performance and scalability labels. It is important to bear in mind the differences between performance and scalability (for the most part, we are referring to system-wide scalability, and not necessarily only scalability within a single JVM). The performance-related hints will probably improve the performance of your application for a given user load, whereas the scalability-related hints will probably increase the total number of users that your application can service. Sometimes, increasing performance will decrease scalability, and vice versa. Typically, options that reduce the amount of work done on the database server will improve scalability, whereas those that push more work onto the server will have a negative impact on scalability.

Table 15.1. Optimization Guidelines

Optimize database indexes

performance, scalability

The default set of indexes created by Kodo's mapping tool may not always be the most appropriate for your application. Manually setting indexes in your mapping metadata or manually manipulating database indexes to include frequently-queried fields (as well as dropping indexes on rarely-queried fields) can yield significant performance benefits.

A database must do extra work on insert, update, and delete to maintain an index. This extra work will benefit selects with WHERE clauses, which will execute much faster when the terms in the WHERE clause are appropriately indexed. So, for a read-mostly application, appropriate indexing will slow down updates (which are rare) but greatly accelerate reads. This means that the system as a whole will be faster, and also that the database will experience less load, meaning that the system will be more scalable.

Bear in mind that over-indexing is a bad thing, both for scalability and performance, especially for applications that perform lots of inserts, updates, or deletes.

Use the best JDBC driver

performance, scalability, reliability

The JDBC driver provided by the database vendor is not always the fastest and most efficient. Some JDBC drivers do not support features like batched statements, the lack of which can significantly slow down Kodo's data access and increase load on the database, reducing system performance and scalability.
JVM optimizations

performance, reliability

Manipulating various parameters of the Java Virtual Machine (such as hotspot compilation modes and the maximum memory) can result in performance improvements.
Use the data cache

performance, scalability

Using Kodo's data and query caching features can often result in a dramatic improvement in performance. Additionally, these caches can significantly reduce the amount of load on the database, increasing the scalability characteristics of your application.
Set LargeTransaction to true, or set PopulateDataCache to false

performance vs. scalability

When using Kodo's data caching features (available in Kodo Professional Edition and Enterprise Edition) in a transaction that will delete, modify, or create a very large number of objects you can set LargeTransaction to true and perform periodic flushes during your transaction to reduce its memory requirements. See the Javadoc: OpenJPAEntityManager.setLargeTransaction , KodoPersistenceManager.setLargeTransaction Note that transactions in large mode have to more aggressively flush items from the data cache.

If your transaction will visit objects that you know are very unlikely to be accessed by other transactions, for example an exhaustive report run only once a month, you can turn off population of the data cache so that the transaction doesn't fill the entire data cache with objects that won't be accessed again. Again, see the Javadoc: OpenJPAEntityManager.setPopulateDataCache , KodoPersistenceManager.setPopulateDataCache

Disable logging, performance tracking


Developer options such as verbose logging and the JDBC performance tracker can result in serious performance hits for your application. Before evaluating Kodo's performance, these options should all be disabled.
Use the Kodo Profiler


Take advantage of the Kodo Profiler described in Chapter 13, Profiling to discover where your application is spending the most time, and to recognize misconfigured fetch groups.
Set IgnoreChanges to true, or set FlushBeforeQueries to true

performance vs. scalability

When both the kodo.IgnoreChanges and kodo.FlushBeforeQueries properties are set to false, Kodo needs to consider in-memory dirty instances during queries. This can sometimes result in Kodo needing to evaluate the entire extent objects in order to return the correct query results, which can have drastic performance consequences. If it is appropriate for your application, configuring FlushBeforeQueries to automatically flush before queries involving dirty objects will ensure that this never happens. Setting IgnoreChanges to false will result in a small performance hit even if FlushBeforeQueries is true, as incremental flushing is not as efficient overall as delaying all flushing to a single operation during commit. This is because incrementally flushing decreases Kodo's ability to maximize statement batching, and increases resource utilization.

Note that the default setting of FlushBeforeQueries is with-connection, which means that data will be flushed only if a dedicated connection is already in use by the EntityManager or PersistenceManager. So, the default value may not be appropriate for you.

Setting IgnoreChanges to true will help performance, since dirty objects can be ignored for queries, meaning that incremental flushing or client-side processing is not necessary. It will also improve scalability, since overall database server usage is diminished. On the other hand, setting IgnoreChanges to false will have a negative impact on scalability, even when using automatic flushing before queries, since more operations will be performed on the database server.

Configure kodo.ConnectionRetainMode appropriately

performance vs. scalability

The ConnectionRetainMode configuration option controls when Kodo will obtain a connection, and how long it will hold that connection. The optimal settings for this option will vary considerably depending on the particular behavior of your application. You may even benefit from using different retain modes for different parts of your application.

The default setting of on-demand minimizes the amount of time that Kodo holds onto a datastore connection. This is generally the best option from a scalability standpoind, as database resources are held for a minimal amount of time. However, if your connection pool is overly small relative to the number of concurrent sessions that need access to the database, or if your DataSource is not efficient at managing its pool, then this default value could cause undesirable pool contention.

Ensure that batch updates are available

performance, scalability

When performing bulk inserts, updates, or deletes, Kodo will use batched statements. If this feature is not available in your JDBC driver, then Kodo will need to issue multiple SQL statements instead of a single batch statement.
Use flat inheritance

performance, scalability vs. disk space

Mapping inheritance hierarchies to a single database table is faster for most operations than other strategies employing multiple tables. If it is appropriate for your application, you should use this strategy whenever possible.

However, this strategy will require more disk space on the database side. Disk space is relatively inexpensive, but if your object model is particularly large, it can become a factor.

High sequence increment

performance, scalability

For applications that perform large bulk inserts, the retrieval of sequence numbers can be a bottleneck. Increasing sequence increments and using table-based rather than native database sequences can reduce or eliminate this bottleneck. In some cases, implementing your own sequence factory can further optimize sequence number retrieval.
Use optimistic transactions

performance, scalability

Using datastore transactions translates into pessimistic database row locking, which can be a performance hit (depending on the database). If appropriate for your application, optimistic transactions are typically faster than datastore transactions.

Optimistic transactions provide the same transactional guarantees as datastore transactions, except that you must handle a potential optimistic verification exception at the end of a transaction instead of assuming that a transaction will successfully complete. In many applications, it is unlikely that different concurrent transactions will operate on the same set of data at the same time, so optimistic verification increases the concurrency, and therefore both the performance and scalability characteristics, of the application. A common approach to handling optimistic verification exceptions is to simply present the end user with the fact that concurrent modifications happened, and require that the user redo any work.

Use query aggregates and projections

performance, scalability

Using aggregates to compute reporting data on the database server can drastically speed up queries. Similarly, using projections when you are interested in specific object fields or relations rather than the entire object state can reduce the amount of data Kodo must transfer from the database to your application.
Always close resources


Under certain settings, EntityManagers, PersistenceManagers, Extent iterators, and Query results may be backed by resources in the database.

For example, if you have configured Kodo to use scrollable cursors and lazy object instantiation by default, each query result will hold open a ResultSet object, which, in turn, will hold open a Statement object (preventing it from being re-used). Garbage collection will clean up these resources, so it is never necessary to explicitly close them, but it is always faster if it is done at the application level.

Optimize connection pool settings

performance, scalability

Kodo's built-in connection pool's default settings may not be optimal for all applications. For applications that instantiate and close many EntityManagers or PersistenceManagers (such as a web application), increasing the size of the connection pool will reduce the overhead of waiting on free connections or opening new connections.

You may want to tune the prepared statement pool size with the connection pool size.

Use detached state managers


Attaching and even persisting instances can be more efficient when your detached objects use detached state managers. By default, Kodo does not use detached state managers when serializing an instance across tiers. See Section 11.1.3, “Defining the Detached Object Graph” for how to force Kodo to use detached state managers across tiers, and for other options for more efficient attachment.

The downside of using a detached state manager across tiers is that your enhanced persistent classes and the Kodo libraries must be available on the client tier.

Utilize the EntityManager and PersistenceManager caches

performance, scalability

When possible and appropriate, re-using EntityManagers and PersistenceManagers and setting the RetainState configuration option to true may result in significant performance gains, since the manager's built-in object cache will be used.
Enable multithreaded operation only when necessary


Kodo respects the kodo.Multithreaded option in that it does not impose as much synchronization overhead for applications that do not set this value to true. If your application is guaranteed to only use single-threaded access to Kodo resources and persistent objects, leaving this option as false will reduce synchronization overhead, and may result in a modest performance increase.
Enable large data set handling

performance, scalability

If you execute queries that return large numbers of objects or have relations (collections or maps) that are large, and if you often only access parts of these data sets, enabling large result set handling where appropriate can dramatically speed up your application, since Kodo will bring the data sets into memory from the database only as necessary.
Disable large data set handling

performance, scalability

If you have enabled scrollable result sets and on-demand loading but do you not require it, consider disabling it again. Some JDBC drivers and databases (SQLServer for example) are much slower when used with scrolling result sets.
Use short discriminator values, or turn off the discriminator

performance, scalability

The default discriminator strategy of storing the class name in the discriminator column is quite robust, in that it can handle any class and needs no configuration, but the downside of this robustness is that it puts a relatively lengthy string into each row of the database. With a little application-specific configuration, you can easily reduce this to a single character or integer. This can result in significant performance gains when dealing with many small objects, since the subclass indicator data can become a significant proportion of the data transferred between the JVM and the database.

Alternately, if certain persistent classes in your application do not make use of inheritance, then you can disable the discriminator for these classes altogether.

Use the DynamicSchemaFactory

performance, validation

If you are using a kodo.jdbc.SchemaFactory setting of something other than the default of dynamic, consider switching back. While other factories can ensure that object-relational mapping information is valid when a persistent class is first used, this can be a slow process. Though the validation is only performed once for each class, switching back to the DynamicSchemaFactory can reduce the warm-up time for your application.
Do not use XA transactions

performance, scalability

XA transactions can be orders of magnitude slower than standard transactions. Unless distributed transaction functionality is required by your application, use standard transactions.

Recall that XA transactions are distinct from managed transactions - managed transaction services such as that provided by EJB declarative transactions can be used both with XA and non-XA transactions. XA transactions should only be used when a given business transaction involves multiple different transactional resources (an Oracle database and an IBM transactional message queue, for example).

Use Sets instead of List/Collections

performance, scalability

There is a small amount of extra overhead for Kodo to maintain collections where each element is not guaranteed to be unique. If your application does not require duplicates for a collection, you should always declare your fields to be of type Set, SortedSet, HashSet, or TreeSet.
Use query parameters instead of encoding search data in filter strings


If your queries depend on parameter data only known at runtime, you should use query parameters rather than dynamically building different query strings. Kodo performs aggressive caching of query compilation data, and the effectiveness of this cache is diminished if multiple query filters are used where a single one could have sufficed.
Tune your fetch groups appropriately

performance, scalability

The fetch groups used when loading an object control how much data is eagerly loaded, and by extension, which fields must be lazily loaded at a future time. The ideal fetch group configuration loads all the data that is needed in one fetch, and no extra fields - this minimizes both the amount of data transferred from the database, and the number of trips to the database.

If extra fields are specified in the fetch groups (in particular, large fields such as binary data, or relations to other persistence-capable objects), then network overhead (for the extra data) and database processing (for any necessary additional joins) will hurt your application's performance. If too few fields are specified in the fetch groups, then Kodo will have to make additional trips to the database to load additional fields as necessary.

Use eager fetching

performance, scalability

Using eager fetching when loading subclass data or traversing relations for each instance in a large collection of results can speed up data loading by orders of magnitude.


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