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62 lines
3.0 KiB
Markdown
62 lines
3.0 KiB
Markdown
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# The Current/Future Status of ZMQ in Monero
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## ZMQ Pub/Sub
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Client `ZMQ_SUB` sockets must "subscribe" to topics before it receives any data.
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This allows filtering on the server side, so network traffic is reduced. Monero
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allows for filtering on: (1) format, (2) context, and (3) event.
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* **format** refers to the _wire_ format (i.e. JSON) used to send event
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information.
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* **context** allows for a reduction in fields for the event, so the
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daemon doesn't waste cycles serializing fields that get ignored.
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* **event** refers to status changes occurring within the daemon (i.e. new
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block to main chain).
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* Formats:
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* `json`
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* Contexts:
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* `full` - the entire block or transaction is transmitted (the hash can be
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computed remotely).
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* `minimal` - the bare minimum for a remote client to react to an event is
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sent.
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* Events:
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* `chain_main` - changes to the primary/main blockchain.
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* `txpool_add` - new _publicly visible_ transactions in the mempool.
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Includes previously unseen transactions in a block but _not_ the
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`miner_tx`. Does not "re-publish" after a reorg. Includes `do_not_relay`
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transactions.
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The subscription topics are formatted as `format-context-event`, with prefix
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matching supported by both Monero and ZMQ. The `format`, `context` and `event`
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will _never_ have hyphens or colons in their name. For example, subscribing to
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`json-minimal-chain_main` will send minimal information in JSON when changes
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to the main/primary blockchain occur. Whereas, subscribing to `json-minimal`
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will send minimal information in JSON on all available events supported by the
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daemon.
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The Monero daemon will ensure that events prefixed by `chain` will be sent in
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"chain-order" - the `prev_id` (hash) field will _always_ refer to a previous
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block. On rollbacks/reorgs, the event will reference an earlier block in the
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chain instead of the last block. The Monero daemon also ensures that
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`txpool_add` events are sent before `chain_*` events - the `chain_*` messages
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will only serialize miner transactions since the other transactions were
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previously published via `txpool_add`. This prevents transactions from being
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serialized twice, even when the transaction was first observed in a block.
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ZMQ Pub/Sub will drop messages if the network is congested, so the above rules
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for send order are used for detecting lost messages. A missing gap in `height`
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or `prev_id` for `chain_*` events indicates a lost pub message. Missing
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`txpool_add` messages can only be detected at the next `chain_` message.
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Since blockchain events can be dropped, clients will likely want to have a
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timeout against `chain_main` events. The `GetLastBlockHeader` RPC is useful
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for checking the current chain state. Dropped messages should be rare in most
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conditions.
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The Monero daemon will send a `txpool_add` pub exactly once for each
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transaction, even after a reorg or restarts. Clients should use the
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`GetTransactionPool` after a reorg to get all transactions that have been put
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back into the tx pool or been invalidated due to a double-spend.
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