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# Relayer
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## Prerequisites Readings
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- [IBC Overview](./overview.md) {prereq}
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- [Events](https://github.com/cosmos/cosmos-sdk/blob/master/docs/core/events.md) {prereq}
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## Events
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Events are emitted for every transaction processed by the base application to indicate the execution
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of some logic clients may want to be aware of. This is extremely useful when relaying IBC packets.
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Any message that uses IBC will emit events for the corresponding TAO logic executed as defined in
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the [IBC events spec](https://github.com/cosmos/ibc-go/blob/main/modules/core/spec/06_events.md).
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In the Cosmos SDK, it can be assumed that for every message there is an event emitted with the type `message`,
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attribute key `action`, and an attribute value representing the type of message sent
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(`channel_open_init` would be the attribute value for `MsgChannelOpenInit`). If a relayer queries
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for transaction events, it can split message events using this event Type/Attribute Key pair.
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The Event Type `message` with the Attribute Key `module` may be emitted multiple times for a single
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message due to application callbacks. It can be assumed that any TAO logic executed will result in
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a module event emission with the attribute value `ibc_<submodulename>` (02-client emits `ibc_client`).
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### Subscribing with Tendermint
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Calling the Tendermint RPC method `Subscribe` via [Tendermint's Websocket](https://docs.tendermint.com/master/rpc/) will return events using
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Tendermint's internal representation of them. Instead of receiving back a list of events as they
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were emitted, Tendermint will return the type `map[string][]string` which maps a string in the
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form `<event_type>.<attribute_key>` to `attribute_value`. This causes extraction of the event
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ordering to be non-trivial, but still possible.
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A relayer should use the `message.action` key to extract the number of messages in the transaction
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and the type of IBC transactions sent. For every IBC transaction within the string array for
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`message.action`, the necessary information should be extracted from the other event fields. If
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`send_packet` appears at index 2 in the value for `message.action`, a relayer will need to use the
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value at index 2 of the key `send_packet.packet_sequence`. This process should be repeated for each
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piece of information needed to relay a packet.
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## Example Implementations
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- [Golang Relayer](https://github.com/iqlusioninc/relayer)
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- [Hermes](https://github.com/informalsystems/ibc-rs/tree/master/relayer)
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- [Typescript Relayer](https://github.com/confio/ts-relayer)
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