185 lines
7.5 KiB
Markdown
185 lines
7.5 KiB
Markdown
# Paging
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Apache ActiveMQ Artemis transparently supports huge queues containing millions
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of messages while the server is running with limited memory.
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In such a situation it's not possible to store all of the queues in memory at
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any one time, so Apache ActiveMQ Artemis transparently *pages* messages into
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and out of memory as they are needed, thus allowing massive queues with a low
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memory footprint.
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Apache ActiveMQ Artemis will start paging messages to disk, when the size of
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all messages in memory for an address exceeds a configured maximum size.
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The default configuration from Artemis has destinations with paging.
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## Page Files
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Messages are stored per address on the file system. Each address has an
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individual folder where messages are stored in multiple files (page files).
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Each file will contain messages up to a max configured size
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(`page-size-bytes`). The system will navigate the files as needed, and it
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will remove the page file as soon as all the messages are acknowledged up to
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that point.
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Browsers will read through the page-cursor system.
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Consumers with selectors will also navigate through the page-files and it will
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ignore messages that don't match the criteria.
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> *Warning:*
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>
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> When you have a queue, and consumers filtering the queue with a very
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> restrictive selector you may get into a situation where you won't be able to
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> read more data from paging until you consume messages from the queue.
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>
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> Example: in one consumer you make a selector as 'color="red"' but you only
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> have one color red 1 millions messages after blue, you won't be able to
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> consume red until you consume blue ones.
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>
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> This is different to browsing as we will "browse" the entire queue looking
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> for messages and while we "depage" messages while feeding the queue.
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### Configuration
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You can configure the location of the paging folder in `broker.xml`.
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- `paging-directory` Where page files are stored. Apache ActiveMQ Artemis will
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create one folder for each address being paged under this configured
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location. Default is `data/paging`.
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## Paging Mode
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As soon as messages delivered to an address exceed the configured size,
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that address alone goes into page mode.
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> **Note:**
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>
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> Paging is done individually per address. If you configure a max-size-bytes
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> for an address, that means each matching address will have a maximum size
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> that you specified. It DOES NOT mean that the total overall size of all
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> matching addresses is limited to max-size-bytes.
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### Configuration
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Configuration is done at the address settings in `broker.xml`.
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```xml
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<address-settings>
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<address-setting match="jms.someaddress">
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<max-size-bytes>104857600</max-size-bytes>
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<page-size-bytes>10485760</page-size-bytes>
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<address-full-policy>PAGE</address-full-policy>
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</address-setting>
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</address-settings>
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```
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> **Note:**
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> The [management-address](management.md#configuring-management)
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> settings cannot be changed or overridden ie management
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> messages aren't allowed to page/block/fail and are considered
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> an internal broker management mechanism.
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> The memory occupation of the [management-address](management.md#configuring-management)
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> is not considered while evaluating if [global-max-size](#global-max-size)
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> is hit and can't cause other non-management addresses to trigger a
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> configured `address-full-policy`.
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This is the list of available parameters on the address settings.
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Property Name|Description|Default
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---|---|---
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`max-size-bytes`|What's the max memory the address could have before entering on page mode.|-1 (disabled)
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`page-size-bytes`|The size of each page file used on the paging system|10MB
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`address-full-policy`|This must be set to `PAGE` for paging to enable. If the value is `PAGE` then further messages will be paged to disk. If the value is `DROP` then further messages will be silently dropped. If the value is `FAIL` then the messages will be dropped and the client message producers will receive an exception. If the value is `BLOCK` then client message producers will block when they try and send further messages.|`PAGE`
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`page-max-cache-size`|The system will keep up to `page-max-cache-size` page files in memory to optimize IO during paging navigation.|5
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## Global Max Size
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Beyond the `max-size-bytes` on the address you can also set the global-max-size
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on the main configuration. If you set `max-size-bytes` = `-1` on paging the
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`global-max-size` can still be used.
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When you have more messages than what is configured `global-max-size` any new
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produced message will make that destination to go through its paging policy.
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`global-max-size` is calculated as half of the max memory available to the Java
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Virtual Machine, unless specified on the `broker.xml` configuration.
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## Dropping messages
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Instead of paging messages when the max size is reached, an address can also be
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configured to just drop messages when the address is full.
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To do this just set the `address-full-policy` to `DROP` in the address settings
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## Dropping messages and throwing an exception to producers
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Instead of paging messages when the max size is reached, an address can also be
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configured to drop messages and also throw an exception on the client-side when
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the address is full.
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To do this just set the `address-full-policy` to `FAIL` in the address settings
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## Blocking producers
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Instead of paging messages when the max size is reached, an address can also be
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configured to block producers from sending further messages when the address is
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full, thus preventing the memory being exhausted on the server.
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When memory is freed up on the server, producers will automatically unblock and
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be able to continue sending.
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To do this just set the `address-full-policy` to `BLOCK` in the address
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settings
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In the default configuration, all addresses are configured to block producers
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after 10 MiB of data are in the address.
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## Caution with Addresses with Multiple Multicast Queues
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When a message is routed to an address that has multiple multicast queues bound
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to it, e.g. a JMS subscription in a Topic, there is only 1 copy of the message
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in memory. Each queue only deals with a reference to this. Because of this the
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memory is only freed up once all queues referencing the message have delivered
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it.
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If you have a single lazy subscription, the entire address will suffer IO
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performance hit as all the queues will have messages being sent through an
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extra storage on the paging system.
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For example:
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- An address has 10 multicast queues
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- One of the queues does not deliver its messages (maybe because of a
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slow consumer).
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- Messages continually arrive at the address and paging is started.
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- The other 9 queues are empty even though messages have been sent.
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In this example all the other 9 queues will be consuming messages from the page
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system. This may cause performance issues if this is an undesirable state.
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## Max Disk Usage
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The System will perform scans on the disk to determine if the disk is beyond a
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configured limit. These are configured through `max-disk-usage` in percentage.
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Once that limit is reached any message will be blocked. (unless the protocol
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doesn't support flow control on which case there will be an exception thrown
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and the connection for those clients dropped).
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## Page Sync Timeout
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The pages are synced periodically and the sync period is configured through
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`page-sync-timeout` in nanoseconds. When using NIO journal, by default has
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the same value of `journal-buffer-timeout`. When using ASYNCIO, the default
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should be `3333333`.
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## Example
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See the [Paging Example](examples.md#paging) which shows how to use paging with
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Apache ActiveMQ Artemis.
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