tendermint/docs/architecture/adr-002-event-subscription.md

ADR 2: Event Subscription

Context

In the light client (or any other client), the user may want to subscribe to a subset of transactions (rather than all of them) using /subscribe?event=X. For example, I want to subscribe for all transactions associated with a particular account. Same for fetching. The user may want to fetch transactions based on some filter (rather than fetching all the blocks). For example, I want to get all transactions for a particular account in the last two weeks (tx's block time >= '2017-06-05').

Now you can't even subscribe to "all txs" in Tendermint.

The goal is a simple and easy to use API for doing that.

Decision

ABCI app return tags with a DeliverTx response inside the data field (for now, later we may create a separate field). Tags is a list of key-value pairs, protobuf encoded.

Example data:

{
  "abci.account.name": "Igor",
  "abci.account.address": "0xdeadbeef",
  "tx.gas": 7
}

Subscribing for transactions events

If the user wants to receive only a subset of transactions, ABCI-app must return a list of tags with a DeliverTx response. These tags will be parsed and matched with the current queries (subscribers). If the query matches the tags, subscriber will get the transaction event.

/subscribe?query="tm.event = Tx AND tx.hash = AB0023433CF0334223212243BDD AND abci.account.invoice.number = 22"

A new package must be developed to replace the current events package. It will allow clients to subscribe to a different types of events in the future:

/subscribe?query="abci.account.invoice.number = 22"
/subscribe?query="abci.account.invoice.owner CONTAINS Igor"

Fetching transactions

This is a bit tricky because a) we want to support a number of indexers, all of which have a different API b) we don't know whenever tags will be sufficient for the most apps (I guess we'll see).

/txs/search?query="tx.hash = AB0023433CF0334223212243BDD AND abci.account.owner CONTAINS Igor"
/txs/search?query="abci.account.owner = Igor"

For historic queries we will need a indexing storage (Postgres, SQLite, ...).

Issues

• https://github.com/tendermint/tendermint/issues/376
• https://github.com/tendermint/tendermint/issues/287
• https://github.com/tendermint/tendermint/issues/525 (related)

Status

Implemented

Consequences

Positive

• same format for event notifications and search APIs
• powerful enough query

Negative

• performance of the match function (where we have too many queries / subscribers)
• there is an issue where there are too many txs in the DB

Neutral