Transaction Log

AMPS includes the ability to record and replay messages. This capability can be used by applications for durable subscriptions, reliable publish, and historical replay. The AMPS transaction log is also the foundation of the high availability features in AMPS. To enable message recording and replay, configure a TransactionLog to keep a journal of messages published to an AMPS instance.

The Record and Replay Messages section of the AMPS User Guide covers how to use the transaction log for historical replay, durable publish, and durable subscriptions.

The AMPS User Guide sections on Replicating Messages Between Instances and Highly Available AMPS Installations cover how AMPS uses the TransactionLog (and other features) in high availability.

Element	Description
JournalDirectory (required)	Filesystem location where journal files will be stored. This is the directory where AMPS will create new journal files as messages are recorded to the transaction log. A journal directory should be dedicated to a single instance of AMPS.
JournalArchiveDirectory	Filesystem location where journal files are archived. The archive directory is intended to allow older files to be stored on a higher-capacity (but potentially slower) device. Journal files in this directory are part of the transaction log. AMPS can replicate from these files, provide bookmark replay from these files, distribute queue messages from these files, and so on. Specifying that the JournalArchiveDirectory and the JournalDirectory are on the same storage device is not recommended. A journal archive directory should be dedicated to a single instance of AMPS. Use AMPS actions to move files from the JournalDirectory to the JournalArchiveDirectory.
PreallocatedJournalFiles	The number of journal files AMPS will create as part of the server startup. Default: 2 Minimum: 1
JournalSize	Sets the target size for AMPS to use when calculating the size of journal files. AMPS allocates journal files based on the size of an internal buffer. This option sets the target size for the journal file: AMPS will use the smallest file size that is an even multiple of the internal buffer without going under the specified JournalSize. Notice that AMPS does not grow journal files once they are allocated. When a journal file is full, AMPS uses the next journal file. AMPS accepts MinJournalSize as a synonym for JournalSize. Default: 1GB Minimum: 10M
Topic	The topic to include in the transaction log. When no Topic is specified, AMPS initializes transaction log management for the instance, but does not persist messages. If a Topic is specified, then all messages which match exactly the specified topic or regular expression will be included in the transaction log. If you want all topics of a specific message type to be persisted, use the regular expression .* for the name of the topic. Multiple Topic elements can be included in a TransactionLog element. See the following table for the elements included within a Topic element. There is no default for this element. If no Topic elements are configured, the transaction log will record no messages.
FlushInterval	AMPS batches writes to the transaction log to optimize for maximum sustained throughput. If a batch is not full within a certain period of time, AMPS will write the partially-filled batch to the transaction log so that the messages can be replicated, delivered to subscribers, and so on. The interval at which messages will be flushed to the journal file during periods of slow activity is the FlushInterval. Setting this explicitly has the potential to reduce latency during periods of low traffic, at the risk of somewhat lower performance during periods of higher traffic. Reducing this interval below the default may produce larger journal files during periods of low traffic, since AMPS may write more partial batches to the transaction log. 60East recommends leaving this option at the default unless the application is intended to optimize for low data rates and testing at production volumes on the storage that will be used for production demonstrates a performance advantage from reducing the interval. Default: 100ms Maximum: 1000ms Minimum: 1us
O_DIRECT	Where supported, O_DIRECT will perform DMA directly from/to physical memory to a user space buffer. Having this enabled can improve AMPS performance, however not all devices support O_DIRECT. When O_DIRECT is disabled, data loss can occur because the operating system can acknowledge that data has been written to the device before the actual write has happened. 60East does not recommend disabling O_DIRECT unless the device that holds the transaction log does not support O_DIRECT. Default: enabled
InactiveClientAckExpiration	Sets the amount of time to retain records for an inactive publisher. On recovery, AMPS will remove the clients.ack entry for any publisher that has not published a message for longer than the interval set in this option. This option can help reduce clients.ack growth for installations that have large numbers of short-lived publishers. If this option is specified, the interval should be longer than the expected time that a given publisher would be inactive and longer than the time that messages from that publisher would be retained. For example, if an application has publishers that are active at the end of every week and the installation retains journals for 14 days, the interval should be longer than 14 days. Default: Retain records indefinitely.
CompressedJournalCacheMemoryLimit	Sets the maximum amount of server memory to use for caching compressed journal files when one or more bookmark replays is using a compressed journal file. Increasing this setting can improve performance when replaying bookmark subscriptions from compressed journals, in exchange for temporarily consuming more memory to service those replays. Default: 10% of server memory or 10GB, whichever is lower.

The Topic element in a TransactionLog can contain the following elements:

Element	Description
Name (required)	The name of the topic to record. This element can be a literal name or a regular expression. To capture logical topics stored in a physical SOW topic (that is, to capture a State of the World Topic that uses a Pattern element), the Topic directive should match the Name of the physical topic. (The names of the logical topics within the physical topic do not matter in this case). This element is required. There is no default for this element.
MessageType (required)	The message type of the topic. This must be one of the message types loaded by default or a message type declared in the configuration file. This element is required. There is no default for this element.

The following example demonstrates a transaction log where the journal file will be written to ./amps/journal. When AMPS starts, a single journal file will be pre-allocated as noted by the PreallocatedJournalFiles setting; and when the first journal file is completely full, a new journal file will be created. This journal will contain those messages which match the topic orders and also have a message type of fix and any messages published to a topic that contains the string LOGGED_ of a message type of json.

<AMPSConfig>
    ...

    <TransactionLog>
        <JournalDirectory>./amps/journal/</JournalDirectory>
        <PreallocatedJournalFiles>1</PreallocatedJournalFiles>
        <JournalSize>10MB</JournalSize>
        <Topic>
            <Name>orders</Name>
            <MessageType>nvfix</MessageType>
        </Topic>
        <Topic>
            <Name>LOGGED_.*</Name>
            <MessageType>json</MessageType>
        </Topic>
    </TransactionLog>

    ...
</AMPSConfig>