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