add indexing proposal [ci skip]

docs/architecture/adr-002.md

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+# ADR 2: Indexing
+
+## 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.
+
+![Tx Send Flow Diagram](img/tags1.png)
+
+## 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:
+
+```json
+{
+  "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/basecoin/issues/91
+- https://github.com/tendermint/tendermint/issues/376
+- https://github.com/tendermint/tendermint/issues/287
+- https://github.com/tendermint/tendermint/issues/525 (related)
+
+## Status
+
+proposed
+
+## 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