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Wez Furlong
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TL;DR: you can easily halve your system performance by logging headers vs. logging meta. This is one of those things that is easy to overlook or forget, but: whenever you need to operate on the message data, rather than its metadata, the aggregate cost is high. In this case, we were recently troubleshooting a system where the CPU was bogged down and we traced it to the logging configuration: a number of message headers were being logged in a configuration that made heavy use of throttles and limits in its traffic shaping, and thus had a large number of Delayed and TransientFailure events being written to the logs. When logging headers, each one of those events requires loading the message from the spool and parsing out the headers. When the average message size is ~100KB this imposes a notable overhead on the CPU and IO utilization of the system. What we recommend instead of logging headers directly is capturing the information that you want to log into the message metadata at the time that the message is received. The message meta is usually already loaded, but is also typically much smaller and easier to decode than the full message content in the cases where it is not loaded. As a result, it is much cheaper to log meta than to log headers. This commit adds some warnings and cross links to help folks be aware of this, and to generally navigate related meta and logging topics more easily via tags.
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message:set_meta(KEY, VALUE)
Messages are associated with some metadata. You can think of this metadata as being equivalent to a JSON object.
The set_meta method allows you to set a field of that object to a value
that you specify.
You can assign any value that is serializable as a JSON:
-- set foo='bar', a string value
msg:set_meta('foo', 'bar')
-- set foo=123, a numeric value
msg:set_meta('foo', 123)
-- set foo=true, a boolean value
msg:set_meta('foo', true)
-- set foo={key="value"}, an object value
msg:set_meta('foo', { key = 'value' })
You can retrieve a metadata value via message:get_meta.
Pre-defined meta values
The following meta values are unique to the message scope:
<style> table tbody tr td:nth-of-type(2) { white-space: nowrap; } </style>| Scope | Name | Purpose | Since |
|---|---|---|---|
| Message | queue |
specify the name of the queue to which the message will be queued. Must be a string value. | |
| Message | tenant |
specify the name/identifier of the tenant, if any. Must be a string value. | |
| Message | campaign |
specify the name/identifier of the campaign. Must be a string value. | |
| Message | routing_domain |
Overrides the domain of the recipient domain for routing purposes. | {{since('2023.08.22-4d895015', inline=True)}} |
!!! Note Additional metadata is available in the message scope that is copied in from the connection scope, for a full list of all available metadata, see the Predefined Metadata page.