11. 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 Transactional Messaging and Bookmark Subscriptions chapter in the AMPS User Guide covers how to use the transaction log for historical replay, durable publish, and durable subscriptions. The Replication and High Availability chapter in the AMPS User Guide covers the use cases where a TransactionLog can be used to maximize the up-time of your AMPS instance.

Element	Description
`JournalDirectory`	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.
`JournalArchiveDirectory`	File system 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.
`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.
`FlushInterval`	The interval at which messages will be flushed the journal file during periods of slow activity. Default: 100ms Maximum: 100ms Minimum: 30us
`MetadataIndexing`	Specifies whether to create journal index files for the journal. When set to `persistent`, AMPS creates journal index files. When set to `transient`, AMPS does not create journal index files. When journal index files are not persisted, AMPS must rebuild the index from the journal during recovery. This requires AMPS to read the journal, which can increase the amount of time it takes for AMPS to start and begin accepting client connections. Default: persistent
`O_DIRECT`	Where supported, `O_DIRECT` will perform DMA directly from/to physical memory to a userspace 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. Default: enabled.
DEPRECATED `BatchSize`	This element is no longer necessary in releases of AMPS 4.0 and greater. If this element is present in the configuration, AMPS emits a deprecation warning and ignores the configured value.

Table 11.1: TransactionLog Configuration Parameters

Example 11.1 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 is going to contain only those messages which match the topic orders and also have a message type of fix. If, at any time, there is 40us of inactivity while there is data to be flushed to the journal file, AMPS will proactively flush the data to the file.

<AMPSConfig>
    ...

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

    ...
</AMPSConfig>

Example 11.1: Transaction Log Configuration Example