pip-27: add PIP-27 for Proned Node (#132)

* pip-27: add PIP-27 for Pruned Node

* pip-27: update PIP-27

PIPs/pip-27.md

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+---
+pip: 27
+title: Pruned Node
+author: Pactus Development Team <[email protected]>
+discussions-to:
+status: Draft
+type: Standards Track
+category: Core
+created: 22-06-2024
+---
+
+## Abstract
+
+This proposal suggests supporting the "Pruned Node".
+A "Pruned Node" can validate all new blocks and transactions, but it doesn't keep all the historical data.
+Instead, it only retains the most recent part of the blockchain, deleting older data to save disk space.
+
+## Motivation
+
+Pruned nodes offer advantages such as saving storage space and syncing faster.
+They can verify new transactions and blocks without storing the entire blockchain history.
+Additionally, pruned nodes can download pruned data directly from a centralized server,
+speeding up the syncing process compared to downloading the entire blockchain history.
+
+## Specification
+
+### Pruning Config
+
+To support the Pruned Node feature, two questions should be answered:
+
+1. Is this node a Pruned Node or a Full Node?
+2. If it is a Pruned Node, how many blocks does the node need to keep or retain?
+
+To answer the first question, we can check if the genesis block or block number one exists.
+If it exists, the node is a "Full Node"; otherwise, the node is a "Pruned Node".
+
+For the second question, we introduce a new parameter in the `Config` and name it `RetentionDays`.
+This parameter indicates the number of days for which the node should keep or retain the blocks before pruning them.
+It is only applicable if the node is in Prune Mode.
+The minimum and default value is `10`.
+This means blocks older than 10 days will be removed from the store.
+There is no restriction for the maximum value.
+
+The config should have a private method to calculate the `RetentionBlocks` based on `RetentionDays`.
+The `RetentionBlocks` are the number of blocks that should be kept and not pruned.
+Given that each day has almost 8640 blocks, the number of blocks to keep is `RetentionBlocks = RetentionDays * 8640`.
+
+### PruneBlock Function
+
+The `PruneBlock` function removes a block and all transactions inside the block from the database.
+It is a private function and can't be accessed from outside the store.
+It accepts a `Batch` pointer and a block height, and returns a boolean indicating whether
+a block at the given height exists, and an error if there is any.
+The `PruneBlock` function first tries to retrieve the block at the given height and decode it.
+Once it is decoded, it iterates over all transactions and updates the batch
+by deleting the associated keys from the database.
+
+### Pruning Database
+
+A Full Node can convert to a Pruned Node by pruning the database and removing old blocks.
+Pruning the database is a time and resource-consuming process and should not be performed while the node is running.
+We need to add a new command named `prune` to `pactus-daemon` to prune an offline node.
+In the attachment to this proposal, there is a batch file for Windows users that can be used for pruning the node.
+
+Pruning the database works as follows:
+
+1. It checks if the `.pruning` file exists inside the database folder.
+   If it doesn't exist, it creates a `.pruning` file and
+   sets the content of the file to `LastBlockHeight - RetentionBlocks + 1`.
+2. It reads the content of the `.pruning` file and sets the pruning height to the content of the file minus 1.
+3. If the pruning height is less than or equal to zero, it exits.
+4. It updates the `.pruning` file and sets the pruning height.
+5. It calls the `PruneBlock` function and commits the batch.
+6. It repeats from step 2.
+
+### Pruning on New Block
+
+Once a new block is committed, the following operations should be performed:
+
+1. If the node is a Full Node, no action is needed.
+2. If the node is a Pruned Node, call the `PruneBlocks` function
+   with the block number equal to `LastBlockHeight - RetentionBlocks`.
+
+This process ensures that for each new block added,
+one old block will be removed, keeping the store blocks up to the `RetentionBlocks` number.
+
+### Importing Data
+
+Once a new node is initialized and before starting to sync with the network,
+it can download and import pruned data from a centralized server.
+This helps a pruned node sync faster.
+
+### Import Data in Pactus Daemon
+
+We need to add a new command named `import` to `pactus-daemon`.
+The `import` command shows a list of available databases that can be imported from a centralized server.
+Once the user chooses a database, it downloads and extracts it into the working directory.
+If a database already exists, it should show an error before downloading.
+
+### Import Data in Pactus GUI
+
+The GUI can import data once the node initialization is done and before syncing.
+It can ask users if they want to run a Full Node or a Pruned Node.
+If they want to run a Full Node, the procedure is the same as before.
+If they want to run a Pruned Node,
+a dialog will be shown with the server address and a list of available files to download.
+Once the file is selected and downloaded, it can be imported in the same manner as the `import` command.
+
+## Exporting Data
+
+To export data to a centralized server, we need to follow these procedures:
+
+1. The server needs to run a Pactus Pruned Node with `RetentionDays` set to `10`.
+2. At regular intervals, such as every week, the node will be stopped,
+   a copy of the `RetentionDays` folder will be obtained and compressed.
+3. The server will keep only the last 3 databases available for download and delete the rest.
+4. It will update a JSON object to show the available databases for download.
+
+## Backwards Compatibility
+
+No backward compatibility issues found.
+
+## Security Considerations
+
+A pruned node can fully verify new blocks without any issues.
+It retains more than 60,000 blocks, allowing it to calculate availability scores.
+Additionally, it can verify transaction lock-times since it has access to the last day's transactions.