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| # JetStream Per-Message TTL | ||
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| | Metadata | Value | | ||
| |----------|---------------------------| | ||
| | Date | 2024-07-11 | | ||
| | Author | @ripienaar | | ||
| | Status | Approved | | ||
| | Tags | jetstream, client, server | | ||
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| ## Context and motivation | ||
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| Streams support a one-size-fits-all approach to message TTL based on the MaxAge setting. This causes any message in the | ||
| Stream to expire at that age. | ||
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| There are numerous uses for a per-message version of this limit, some listed below: | ||
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| * KV tombstones are a problem in that they forever clog up the buckets with noise, these could have a TTL to make them expire once not useful anymore | ||
| * Server-applied limits can result in tombstones with a short per message TTL so that consumers can be notified of limits being processed. Useful in KV watch scenarios being notified about TTL removals | ||
| * A stream may have a general MaxAge but some messages may have infinite retention, think a schema or type hints in a KV bucket that is forever while general keys have TTLs | ||
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| Related issues [#3268](https://github.com/nats-io/nats-server/issues/3268) | ||
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| ## Per-Message TTL | ||
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| We will allow a message to supply a TTL using a header called `Nats-TTL` followed by the duration as seconds. | ||
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| The duration will be used by the server to calculate the deadline for removing the message based on its Stream | ||
| timestamp and the stated duration. | ||
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| The TTL may not exceed the Stream MaxAge to give administrators a hard cap for the purposes of managing resources. | ||
| The shortest allowed TTL would be 1 second. | ||
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| A TTL of zero will be ignored, any other unparsable value will result in a error reported in the Pub Ack and the message | ||
| being discarded. | ||
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| Setting the header `Nats-No-Expire` to `1` will result in a message that will never be expired. | ||
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| ## Limit Tombstones | ||
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| Several scenarios for server-created tombstones can be imagined, the most often requested one though is when MaxAge | ||
| removes last value (ie. the current value) for a Key. | ||
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| In this case when the server removes a message and the message is the last in the subject it would place a message | ||
| with a TTL matching the Stream configuration value. The following headers would be placed: | ||
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| ``` | ||
| Nats-Applied-Limit: MaxAge | ||
| Nats-TTL: 1 | ||
| ``` | ||
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| The `Nats-Limit-Applied` field is there to support future expansion of this feature. | ||
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| This behaviour is off by default unless opted in on the Stream Configuration. | ||
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| ## Publish Acknowledgements | ||
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| We could optionally extend the `PubAck` as follows: | ||
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| ```golang | ||
| type PubAck struct { | ||
| MsgTTL uint64 `json:"msg_ttl,omitempty"` | ||
| } | ||
| ``` | ||
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| This gives clients a chance to confirm, without Stream Info or should the Stream be edited after Info, if the TTL | ||
| got applied. | ||
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| ## Stream Configuration | ||
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| Weather or not a stream support this behavior should be a configuration opt-in. We want clients to definitely know | ||
| when this is supported which the opt-in approach with a boolean on the configuration would make clear. | ||
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| We have to assume someone will want to create a replication topology where at some point in the topology these tombstone | ||
| type messages are retained for an audit trail. So a Stream with this feature enabled can replicate to one with it | ||
| disabled and all the messages that would have been TTLed will be retained. | ||
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| ```golang | ||
| type StreamConfig struct { | ||
| // AllowMsgTTL allows header initiated per-message TTLs | ||
| AllowMsgTTL bool `json:"allow_msg_ttl"` | ||
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| // LimitsTTL activates writing of messages when limits are applied with a specific TTL | ||
| LimitsTTL time.Duration `json:"limits_ttl"` | ||
| } | ||
| ``` |