Gateway caching proposal (#1773)

design/gateway/gateway_caching.md

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+# Ten Gateway Caching Design
+
+## 1. Why cache requests on Gateway?
+
+Currently, all `eth_` requests that hit the gateway are forwarded to the Ten Nodes and are executed by the Ten enclave. 
+This is not ideal since there is only one Sequencer and it can be overloaded with requests
+and there can be timeouts or errors in the sequencer.
+
+To solve this problem, we can cache the responses of the `eth_` requests on the Gateway.
+Not all requests can be cached, that is why I analyzed the Ethereum JSON-RPC Specification and formed two groups of requests that can be cached and those that cannot be cached.
+For some methods, I used a rule of thumb since results can be cached for a certain period of time, but quickly become outdated.
+
+
+Cacheable request methods are:
+
+- `eth_accounts`
+- `eth_chainID`
+- `eth_coinbase`
+- `eth_getBlockByHash`
+- `eth_getBlockByNumber`
+- `eth_getBlockRecipients`
+- `eth_getBlockTransactionCountByHash`
+- `eth_getBlockTransactionCountByNumber`
+- `eth_getCode`
+- `eth_getStorageAt`
+- `eth_getTransactionByBlockHashAndIndex`
+- `eth_getTransactionByBlockNumberAndIndex`
+- `eth_getTransactionByHash`
+- `eth_getTransactionReceipt`
+- `eth_getUncleCountByBlockHash`
+- `eth_getUncleCountByBlockNumber`
+- `eth_maxPriorityFeePerGas`
+- `eth_sign`
+- `eth_signTransaction`
+
+Methods cachable for a short period of time (TTL until next batch):
+
+- `eth_getBalance`
+- `eth_blockNumber`
+- `eth_call`
+- `eth_createAccessList`
+- `eth_estimateGas`
+- `eth_feeHistory`
+- `eth_getProof`
+- `eth_gasPrice`
+- `eth_getFilterChanges`
+- `eth_getFilterLogs`
+- `eth_getTransactionCount`
+- `eth_newBlockFilter`
+- `eth_newFilter`
+- `eth_newPendingTransactionFilter`
+- `eth_sendRawTransaction`
+- `eth_sendTransaction`
+- `eth_syncing`
+- `eth_uninstallFilter`
+
+### Expiration time
+Some cacheable methods will always produce the same result and can be cached indefinitely, while others will have a short expiration time.
+Even for those that can be cached indefinitely I don't think it's a good idea to cache them for a long time,
+since a large percentage of the requests will probably be requested only once
+and caching will only consume memory and don't provide any benefit. 
+As far as I can see it can help
+to cache the results for shorter amount of time to help reduce the load on the sequencer / help in case of DDoS attacks
+(but we need to do also other things to prevent them).
+
+
+
+## 3. Implementation
+
+For the implementation we can use some of the Go libraries for caching, such as: https://github.com/patrickmn/go-cache.
+All the requests come from a single point in the code, so we can easily add caching to the gateway.
+
+
+
+## Pros and cos of caching
+I want to present also some negative effects of caching, so we can make a better decision.
+
+### Pros
+- it can reduce the load on the sequencer
+- cah help if there is a spike of requests (e.g. DDoS attack / high load from specific user)
+
+### Cons
+- caching can consume additional memory
+- caching can lead to outdated results (only in some methods)
+- it is not a good way to prevent DDoS attacks, since users can easily request non-cacheable methods