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Chapter 24. Paging

24.1. Page Files
24.2. Configuration
24.3. Paging Mode
24.3.1. Configuration
24.4. Dropping messages
24.5. Blocking producers
24.6. Caution with Addresses with Multiple Queues
24.7. Example

HornetQ transparently supports huge queues containing millions of messages while the server is running with limited memory.

In such a situation it's not possible to store all of the queues in memory at any one time, so HornetQ transparently pages messages into and out of memory as they are needed, thus allowing massive queues with a low memory footprint.

HornetQ will start paging messages to disk, when the size of all messages in memory for an address exceeds a configured maximum size.

By default, HornetQ does not page messages - this must be explicitly configured to activate it.

Messages are stored per address on the file system. Each address has an individual folder where messages are stored in multiple files (page files). Each file will contain messages up to a max configured size (page-size-bytes). The system will navigate on the files as needed, and it will remove the page file as soon as all the messages are acknowledged up to that point.

Browsers will read through the page-cursor system.

Consumers with selectors will also navigate through the page-files and it will ignore messages that don't match the criteria.

You can configure the location of the paging folder

Global paging parameters are specified on the main configuration file (hornetq-configuration.xml).

<configuration xmlns="urn:hornetq"
            xsi:schemaLocation="urn:hornetq /schema/hornetq-configuration.xsd">

As soon as messages delivered to an address exceed the configured size, that address alone goes into page mode.

Instead of paging messages when the max size is reached, an address can also be configured to just drop messages when the address is full.

To do this just set the address-full-policy to DROP in the address settings

Instead of paging messages when the max size is reached, an address can also be configured to block producers from sending further messages when the address is full, thus preventing the memory being exhausted on the server.

When memory is freed up on the server, producers will automatically unblock and be able to continue sending.

To do this just set the address-full-policy to BLOCK in the address settings

In the default configuration, all addresses are configured to block producers after 10 MiB of data are in the address.

When a message is routed to an address that has multiple queues bound to it, e.g. a JMS subscription in a Topic, there is only 1 copy of the message in memory. Each queue only deals with a reference to this. Because of this the memory is only freed up once all queues referencing the message have delivered it.

If you have a single lazy subscription, the entire address will suffer IO performance hit as all the queues will have messages being sent through an extra storage on the paging system.

For example:

In this example all the other 9 queues will be consuming messages from the page system. This may cause performance issues if this is an undesirable state.

See Section 11.1.41, “Paging” for an example which shows how to use paging with HornetQ.