Added design docs about the batching

doc/BATCHING.md

@@ -0,0 +1,50 @@
+# How Batching works in Cyphernode
+
+Details on how batching was implemented in Cyphernode.
+
+## Glossary
+
+A Batcher is a batching template with corresponding past batched transactions and a queue of outputs waiting to be batched in the next batch transaction.
+
+A batched transaction is a transaction that combines multiple recipients in one transaction with multiple outputs, instead of using multiple individual transactions.
+
+An ongoing batch is a batcher with its queued outputs waiting to be part of the next batch transaction.  There's no associated txid yet.
+
+## Entities
+
+### Database
+
+See [Cyphernode's Entity-Relation Model](../proxy_docker/app/data/cyphernode.sql).
+
+- `batcher`: batching template.  The conf_target is the default confTarget that will be used when creating the batch transaction if no confTarget is supplied to batchspend that would override it.
+  - id: autoincrementing primary key
+  - label: optional unique label to be used on subsequent calls instead of using the id
+  - conf_target: optional default confTarget to be used when creating the batched transaction
+- `recipient`: a batch output.  Minimally requires the destination address and the amount.
+  - id: autoincrementing primary key
+  - address: destination Bitcoin address
+  - amount: amount to be sent, in BTC
+  - tx_id: foreign key on the tx table, the actual transaction if created
+  - webhook_url: optional URL that you want Cyphernode to call back when the batch transaction is broadcast
+  - batcher_id: foreign key on the batcher table, the corresponding batching template for this recipient
+  - label: an optional label for this output.
+- `tx`: a transaction.  The information about a broadcast Bitcoin transaction.
+
+### Good to know
+
+- There is a default batcher created on installation time, with id 1, label "default" and conf_target 6.
+- When a recipient has no tx_id, it means it is waiting for the next batch.
+- When a recipian has a batcher_id, it means it is part of a past or ongoing batch.
+- When a batch transaction is broadcast, the webhook_url of each included recipient will be called by Cyphernode, if present, with information about the batched transaction in the POSTed body.
+- Cyphernode knows when a callback webhook didn't work.  It will retry the callback when a new blocks is mined, until it works.
+
+## 2nd layer: the Batcher cypherapp
+
+The Cyphernode's base functionalities for batching is pretty basic.  Instead of adding complex batching features to the Cyphernode API, we decided to develop a [CypherApp](CYPHERAPPS.md).
+
+The [Batcher](https://github.com/SatoshiPortal/batcher) cypherapp will take care of the following tasks:
+
+- Merging same destination outputs into one, adding the amounts and calling the different webhook URLs on batch execution.
+- Scheduling the batches.
+- Executing the batches when an amount threshold has been reached.
+- Hiding Cyphernode complexity by dealing only with a `batchRequestId` and a `batchId`.